Effects of the global and regional climate events of the Eocene on the Pan-American provinces of marine molluscs

Effects of the global and regional climate events of the Eocene on the Pan-American provinces of marine molluscs | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Research Article Effects of the global and regional climate events of the Eocene on the Pan-American provinces of marine molluscs Priscila Morales-Ortega, Gerardo Gonzalez-Barba This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-5278171/v1 This work is licensed under a CC BY 4.0 License Status: Posted Version 1 posted You are reading this latest preprint version Abstract The phylum Mollusca is a well-studied group in the Eocene epoch due to the radiation and extensive redistribution of families globally. However, these phenomena were “interrupted” by progressive and abrupt events in the geological, environmental, and climatic global conditions during the Early Eocene (hyperthermal), Middle Eocene (cooling trend and sudden warming), and Late Eocene (‘cooling’). In recent years, attempts have been made to correlate faunal associations of molluscs to understand regional paleoenvironmental changes and changes in the geographical redistribution of species. In this study, we correlate the faunistic assemblages of marine molluscs among four provinces: Northeastern Pacific Ocean Province (NEPP), Southeastern Pacific Ocean Province (SEPP), Caribbean Province (CP or Caribbean), and Gulf Province (GP or Gulf). The provinces of the northern hemisphere (NEPP, GP, and CP) reflected a closer interprovincial relationship. The Tethys current, at least during the Ypresian and Lutetian stages, allowed this faunal exchange, where the 'Central American seaway' was crucial. Moreover, our study shows that the maximum marine biodiversity of molluscs coincides with hyperthermal events. For the NEPP during the Ypresian (Early Eocene Climatic Optimum), while for the GP during the Bartonian (Middle Eocene Climatic Optimum). The Priabonian of the NEPP was a period of redistribution, possibly due to the dynamics of the surface currents of the northwestern Pacific. Meanwhile, cooling caused a significant reduction of taxa in the GP. Moreover, the SEPP during the Bartonian and Priabonian is correlated to the provinces of the northern hemisphere, which could be explained by the displacement of the South American plate towards the north and by the persistence of affinity of the Tethys fauna (Tethys Realm). America Eocene province climate change regional events Tethys Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Figure 7 Figure 8 Introduction The Eocene was one of the most active epochs in Earth's geological history. Climatic events during this epoch are known to be associated with the movement of tectonic plates, shifting ocean currents, significant changes in atmospheric circulation, alterations in biogeochemical cycles, and even variation in orbital cycles; all of these variations resulted in a constantly changing and highly variable biosphere (Coxall & Pearson, 2007 ; Collins, 2011 ; Westerhold & Röhl, 2012; Gebhardt et al., 2013 ). The phylum Mollusca is one of the most studied groups of the Eocene, as there was radiation and a wide distribution of families globally. However, this was "interrupted" by progressive and abrupt events in geological, environmental and climatic conditions during the Early Eocene (hyperthermal), Middle Eocene (cooling trend and sudden warming), and Late Eocene (‘cooling’) (Piccoli & Savazzi, 1983 ; McGowran et al., 2000 ; Oleinik, 2001 ; Squires, 2003 ; Lozouet, 2014 ; Das & Halder, 2018 ; Morales-Ortega & González-Barba, 2018 ; Hutchinson et al., 2021 ). These events affected the marine biota; hence, in recent years, attempts have been made to correlate the faunal assemblages of molluscs from different parts of the world to understand regional palaeoenvironmental changes and shifts in the geographical redistribution of species (Coxall & Pearson, 2007 ; Das & Halder, 2018 ; Foster et al., 2020 ). Thus, the main aim of our study was to evaluate the relationship among faunal assemblages of molluscs reported in different formations of the American continent in order to identify the interprovincial relationship and the effects caused by regional and global climatic-environmental events during the Eocene. Climate events The Early Eocene was characterised by reaching the highest mean annual temperatures of the entire Cenozoic Era, with relatively low pole-to-pole temperature gradients and high precipitation in an ice-free world (Zachos et al., 2001 ). From ≈ 55 to ≈ 52 Ma, there was a series of short-term changes in the carbon isotope composition of the ocean due to the release of carbon from the ocean to the atmosphere, which caused an increase in ocean surface temperature of ≈ 4 to 8°C (Galeotti et al., 2010 ). These short-term hyperthermal events are known as the Paleocene-Eocene Thermal Maximum (PETM) and the Eocene Thermal Maximum 2 and 3 (ETM2 and ETM3) (Lunt et al., 2011 ; Thomas et al., 2018 ). The most recently accepted hypothesis is that orbital parameters such as eccentricity and obliquity may have triggered ETM2 and ETM3 (Galeotti et al., 2010 ; Lunt et al., 2011 ). These thermal events significantly perturbed planktonic and benthic foraminifera; however, the regional and global impact on other marine fauna remains unknown (Galeotti et al., 2010 ; Lunt et al., 2011 ; Sexton et al., 2011 ). Moreover, the Middle Eocene marked the transition from a warm to a cold climate (‘warmhouse to coolhouse’; ≈49 Ma) (Bohaty et al., 2009 ; Westerhold et al., 2020 ). Carbon and oxygen isotopes indicate a shift towards global cooling caused by a significant decrease in greenhouse gases (mainly CO2), a change in ocean circulation patterns, and global heat transport, which resulted in a cold climate by the end of the Eocene (Speelman et al., 2009 ; Straume et al., 2022 ). However, during this global cooling trend, there was a significant reversal of warming in the Bartonian, known as the Middle Eocene Climatic Optimum (MECO) (Bohaty & Zachos, 2003 ). The MECO is considered a warming period due to the increased atmospheric CO2 with a maximum of 4,000 ppm, the highest amount of CO2 detected during the epoch (Bohaty & Zachos, 2003 ; Pearson, 2010 ). The increase in this gas was driven by the rate of expansion of the ocean floor, the metamorphic decarbonisation reactions between Australia and Antarctica, and the increase in volcanism in the region, in addition to continental drift and the collision of India with Asia (formation of the Himalayas). However, the exact timing of the release of atmospheric CO2 is still unknown (Bohaty & Zachos, 2003 ). Additionally, an increase in the surface temperature of the Tethys Sea, ranging from 32 to 36°C, was reported (Cramwinckel et al., 2023 ), as well as acidification of the deep sea. The MECO event is estimated to have lasted ≈ 500 kyr, with a maximum warming period of < 100 kyr (Bohaty et al., 2009 ; Spofforth et al., 2010 ). After the MECO, oxygen isotopic records indicate a return to cooling at ≈ 40 Ma (Bohaty & Zachos, 2003 ). Undoubtedly, the climatological transition from the late Eocene to the early Oligocene is the most prominent within the Cenozoic (Hutchinson et al., 2021 ). This change was primarily marked by (1) the cooling of Antarctica due to tectonic plate reorganisation and related changes in ocean circulation controlling poleward heat transport and (2) a threshold response to decreasing atmospheric CO2, which determined the Earth's modern glacial climate (Coxall & Pearson, 2007 ; Gebhardt, 2013; Hutchinson et al., 2021 ; Straume et al., 2022 ). This cooling brought significant climatic and geographical changes, while the flora and fauna recorded a global shift towards species more adapted to the cold climate (Fenero et al., 2010 ; Hutchinson et al., 2021 ). However, the fossil record documents a gradual turnover pattern indicating an adjustment to food and nutrient availability changes, habitat, and climatic regime (Coxall & Pearson, 2007 ). Material and methods One of our main objectives was to record the mollusc species of the classes Bivalvia, Gastropoda, Cephalopoda, and Scaphopoda reported in various Pan-American lithostratigraphic formations. Thus, we reviewed faunal lists of molluscs from the United States of America (USA) (West Coast and Gulf Coast), Mexico (Baja California Sur, Chiapas, Tamaulipas, and Nuevo Leon), Central America (Jamaica), and Peru (north and south-central). Record of taxa and localities In this study, we formulated a database to organise and analyse the data obtained. We designed the database entirely without any previously recorded data or elements. Each item recorded in the database was analysed and classified under certain conditions. For taxa : we reviewed each faunal list to obtain the most significant quantity and quality of records for genera and species. We compared each taxon with other faunal lists or databases to recognise whether or not that genera or species is catalogued as a valid taxon. We excluded genera and/or species with a question mark ( ? ); genera or species affinis ( aff .) or cofer ( cf ./ cfr .); genera or subgenera currently in doubt or not accepted (WoRMS, https://www.marinespecies.org ); genera or species with doubt in the age range; and genera or species with erroneous locality data. For localities : to corroborate locality data such as coordinates, depositional environments, and age range, we used databases such as Paleobiology DataBase ( https://paleobiodb.org/#/ ), Mindat – The Hudson Institute of Mineralogy ( https://www.mindat.org ), and NGMDB-USGS – National Geologic Map Database Project (NGMDB: https://ngmdb.usgs.gov/Geolex ) in collaboration with the United States Geological Survey (USGS) and the Association of American State Geologists (AASG). Data analysis We used 'PaleoTax' ( http://paleotax.de ) and PAST ( https://past.en.lo4d.com ) software for data analysis. PaleoTax is a universal information system and a valuable tool for taxonomic work. The system facilitates the management of a large number of taxa so that each species, genus, family or order can be correlated with locality, lithostratigraphic formation, depositional environments, provincial data and more, depending on the quantity and quality of the information recorded in this database. In addition, PaleoTax has other tools such as PaleoTax/Map ( https://www.paleotax.de/map ), with which we were able to generate maps of the localities (we also used CorelDRAW, 2021 to detail figures) and PaleoTax/Graph ( https://www.paleotax.de/pgraph ), with which we generated the correlation matrices, which we exported to the PAST software for analysis. In PAST, we used the Jaccard similarity analysis, a qualitative or incidence index that gauges the similarity between the provinces evaluated. This analysis is based on incidence data and does not include shared absences, as it considers that comparing the absence of a species in two different areas does not provide relevant information (Flores-Contreras & Luna-Reyes, 2017 ). The index values range from 0, when no shared species exist between provinces and age intervals, to 1, when stations have the same species composition. Similarity values were grouped by the UPGMA (unweighted pair-group method using arithmetic averages) (Reyes & Torres-Florez, 2009 ; Flores-Contreras & Luna-Reyes, 2017 ). Results We reviewed 73 faunal lists for the study region, with records from 1915 to 2023. From these lists, 1,479 species, 557 genera, 188 families, 35 orders, 1,625 localities, and 12,626 occurrences were obtained (Figs. 1 , 2 ). We recorded taxonomic data, locality(s), depositional environments, and age intervals (Ypresian, Lutetian, Bartonian, and Priabonian) for each genus and/or species. Likewise, we separated the taxa into four paleobiogeographic provinces based on the studies of Piccolli & Savazzi, 1983; Oleinik, 2001 ; Squires, 2003 ; and Das & Halder, 2018 as follows: Northeastern Pacific Ocean Province (NEPP): Washington, Oregon, California, USA; and Baja California Sur, Mexico. Southeastern Pacific Ocean Province (SEPP): Peru. Caribbean Province (CP or Caribbean): Chiapas, Mexico, and Jamaica. Gulf Province (GP or Gulf): Texas, Louisiana, Mississippi, Alabama and Florida, USA; and Nuevo Leon and Tamaulipas, Mexico. It is essential to mention that the four provinces were limited by age interval; for example, the Gulf Province was divided into four: GP-Ypresian, GP-Lutetian, GP-Bartonian, and GP-Priabonian. Therefore, we correlated 22 provinces for the similarity analysis, of which 15 (except CP-Bartonian, no data) presented a well-defined age interval (Ypresian, Lutetian, Bartonian, and Priabonian) (Fig. 3 , bold and underlined letters). In contrast, seven provinces showed an extended age interval; for example, GP-Ypresian-Lutetian, GP-Lutetian-Priabonian, etc. (see Fig. 3 , single letters). We used the 22 provinces for the analysis to ensure greater data representativeness. In the similarity analysis, we obtained a dendrogram from which five groups could be defined (Fig. 3 ). Also, we described in detail the percentages of species per class (Tab. I; Figs. 4 , 7 ) and the number of formations and localities per province and age range (Tab. I). Table I. Overview of the five defined clusters. Each cluster indicates the number of formations per province and stage. Furthermore, the percentage of species per class, province, and stage. Provinces: northeastern Pacific Ocean Province (NEPP), southeastern Pacific Ocean Province (SEPP), Gulf Province (Gulf of GP), and Caribbean Province (Caribbean or CP). Stages: Ypr, Ypresian; Lut, Lutetian; Bar, Bartonian; and Pri, Priabonian. Fms: Formations. Class: Bi, Bivalvia; Ce: Cephalopoda; Ga: Gastropoda; and Sca: Scaphopoda. Cluster Provinces No. of Fms. % of species per class 1 GP-Pri 6 Bi: 23.7; Ce: 1.2; Ga: 73; & Sca: 2.4 GP-Lut 9 Bi: 27.8; Ce: 1.6; Ga: 69.3; & Sca: 1.3 2 NEPP-Pri 10 Bi: 39.9; Ce: <1; Ga: 57.8; & Sca: 1.4 NEPP-Bar 5 Bi: 37.5; Ce: 1.6; Ga: 59.3; & Sca: 1.6 NEPP-Lut 12 Bi: 38.7; Ce: 1.8; Ga: 57.7; & Sca: 1.8 NEPP-Ypr 19 Bi: 34.8; Ce: <1; Ga: 63.1; & Sca: 1.6 3 GP-Bar 9 Bi: 22.9; Ce: 1.1; Ga: 74.9; & Sca: 1.1 CP-Lut 2 Ga: 100 GP-Ypr 7 Bi: 26.5; Ce: <2; Ga: 69.8; & Sca: 1.7 4 SEPP-Pri 3 Bi: 30.2; Ce: 2.9; & Ga: 67.1 SEPP-Bar 4 Bi: 30.9; Ce: 2.8; Ga: 64.8; & Sca: 1.4 5 SEPP-Lut 3 Bi: 42.9; Ce: 14.3; & Ga: 42.9 SEPP-Ypr 2 Bi: 37.5; Ce: 12.5; & Ga: 50 Discussion The dendrogram (read from right to left, with a similarity level of 0.5) allowed us to identify and infer what the faunal exchange was like during the epoch and how global and regional geological, environmental and climatic events caused changes in the distribution of the species (Morales-Ortega et al., 2015 , 2016 ; Das & Halder, 2018 ). According to the reading, we discuss the clusters from right to left, from group number five to one (Fig. 3 ). Southeastern Pacific Ocean Province, Ypresian and Lutetian (cluster five). Group five is composed of the SEPP-Ypresian and SEPP-Lutetian-Bartonian. Despite the level of similarity excludes SEPP-Lutetian and SEPP-Ypresian-Lutetian, the latter age intervals are related between them. Thus, we propose that SEPP-Ypresian and SEPP-Lutetian were correlated (Fig. 3 ). In Peru (the only country included in the SEPP), the Paleogene was widely distributed, although in isolated basins controlled by megaregional geological features ('Andean magmatic arc') and regional faults and suture zones (Monges, 1995 ). It is recognised that during the Peruvian Cenozoic, prolonged floods (transgressions) resulted in bioclastic and silty sandstone deposits (DeVries, 2019 ). Specifically, we report the Caballas and Negritos Formations with an age range of the Ypresian; and the Pale Greda, Paracas and Caballas Formations for the Lutetian (Fig. 4 ). In addition, we reported the genera Ostrea , Potamides , Turritella and Clavilithes , at least for the Caballas Formation, which marks a transition from fluvial and alluvial plain environments to marine paleoenvironments. In this sense, the presence of oysters and potamids is consistent with the interpretation of a brackish water environment and the deposition environments reported in our study, which are summarised in coastal habitats such as plains, estuaries and lagoons (Fig. 4 ) (DeVries, 2017 ; 2019 ). However, DeVries ( 2019 ) mentions that the majority of mollusc species from the Caballas Formation exhibit a pattern of mixed affinities with faunas from the Early Eocene and Late Cretaceous in northern Peru, which is the reason why this fauna is considered to be of an Early Paleogene age. It is not easy for us to determine whether the faunal assemblages reported in our study are from the appointed time, given that we did not obtain sufficient data, as only eight genera were recorded, including eight species for the SEPP-Ypresian and seven genera with seven species from the SEPP-Lutetian (Fig. 4 ), and the age was not specified either (this should be done in future research). Therefore, we consider that these faunas are from the early Cenozoic era, as proposed by DeVries ( 2019 ). Southeastern Pacific Ocean Province, Bartonian and Priabonian (cluster four) Most of the continental masses detected at present were identifiable at the beginning of the Cenozoic (66 Ma). However, their relative positions and shapes changed until they reached their present geographical configuration (Ubilla et al., 2009 ). The South American plate shifted both longitudinally, which allowed the expansion of the Atlantic and at the same time stimulated the 'Andean orogeny', and latitudinally, which led to the formation of the Caribbean Island arc and the opening of the Drake Passage (≈ 45 Ma) (Ramos, 2009 ; Ghiglione, 2016 ). In northwest Peru, the Talara Basin contains one of South America's most extensive Paleogene sequences (≈ 4,500 m). Towards the boundary of the Middle Eocene to Late Eocene, several sequences of marine origin have been recognized within the basin, such as the Talara Formation (Middle Eocene), recorded as a deep marine sequence, and the Sandstone-Talara (Middle Eocene), Verdun and Chira (Upper Eocene) Formations, with characteristics of deltaic systems and shallow marine sequences. Similarly, the Paracas Formation (Middle Eocene), south Peru (Ica), is recognized as a marine sequence (Fig. 5 ) (Monges, 1995 ). The marine sedimentary sequences may be a consequence of the sea level variations that occurred during the Middle and Late Eocene, which are related to tectonic development, and also of a ≈ 40 m rise in sea level under near ice-free conditions during the MECO (≈ 40.1 Ma) and a fall of the sea level of ≈ 20 m (39.5 Ma) (Monges, 1995 ; Bohaty & Zachos, 2003 ; Miller et al., 2020 ). In addition, a drop in sea level during the Late Eocene (≈ 40 m) has also been recorded, rising again in almost ice-free conditions (≈ 35 Ma) (sea-level data from Ocean Drilling Program -ODP- sites 1209 -Western Pacific- and 1218 -Eastern Pacific-) (Miller et al., 2020 , p. 6). All of these variations may have favoured transition and coastal plains, the inner and outer shelves, and deep environments (deep subtidal and submarine fans), allowing the presence of molluscs (Fig. 5 ). Moreover, it is known that the Tethys currents allowed faunal exchange between the Atlantic and Pacific oceans. The movement of the South American plate towards the north may have favoured the “vicinity” of species from the Caribbean and Gulf provinces (see Fig. 3 ). With the dendrogram obtained in our study, we confirm the hypothesis that the surface currents of the Tethys Ocean allowed a faunal exchange across the Atlantic, as this ocean was narrower in the past, and faunas represent a constant process in the equatorial area (Piccoli & Savazzi, 1983 ; Morales-Ortega et al., 2015 ). Das & Halder ( 2018 ) stated that the western Atlantic provinces, e.g. the Caribbean Northern South American Province (Colombia, Peru, Venezuela, Jamaica, Trinidad & Tobago, Panama, and Haiti) and the Gulf Coastal Province (USA: Texas, Arkansas, Alabama, Tennessee, Louisiana, Mississippi, Georgia, Florida, North Carolina, South Carolina, Virginia, Maryland, New Jersey) exhibit a remarkable similarity in generic composition throughout the Eocene. Coincidingly, our work proposes that SEPP and GP depict a similar relationship. However, it is worth noting that this affinity was stronger during the Bartonian and Priabonian periods in Peru. Likewise, the Peruvian faunas reported for the Middle Eocene (SEPP-Bartonian) and Late Eocene (SEPP-Priabonian) are related to the tropical and subtropical belts of Tethys (Tethys Realm) (Piccoli & Savazzi, 1983 ; Lozouet, 2014 ; Das & Halder, 2018 ; DeVries, 2019 ). This is confirmed by the number of genera and species of molluscs reported in our study (Fig. 5 ), which mainly represent shallow water environments and warm to subtropical seas typical of Tethys (Das & Halder, 2018 ). Gulf Province, Ypresian and Bartonian; and its proximity to the Caribbean Province during the Lutetian (cluster three) The Gulf Province (GP or Gulf) and the Caribbean Province (CP or Caribbean) are known for their high interprovincial similarity (Das & Halder, 2018 ). This similarity reflects their geographic proximity, and the genera and species reported in our work are known to exhibit faunal similarities with those of the Mediterranean and Central Europe. These results indicate and reinforce the idea that the transatlantic Tethys current drove the planktonic larval transport (Das & Halder, 2018 ). At present, major surface currents are known to transport marine fauna from the eastern Atlantic westward across the mid-Atlantic, developing some resemblance between the faunas of Western Europe and Eastern America (Silva et al., 2014 ; Cox, 2020). Some genera reflecting the wide dispersal during the Eocene are the bivalve Venericor ; the gastropods Cornulina , Raphitoma , Hastula , Turricula , Coronia , Eopleurotoma and Surculites ; and the cuttlefish Belosaepia of the family Belosaepiidae (Paleontology Database, 2024 ). The environments recorded in our study are mostly shallow; the lithostratigraphic formations contain calcareous sands, marls, silts, and clays, both lithified and non-lithified (Fig. 6 ). These fine-grained deposits that accumulate considerably slowly contain organic materials such as phosphate and glauconite (minerals deposited in a shallow marine sedimentary environment), which are related to transgressions (maximum flooding surfaces) (Fig. 6 ) (Garrison, 2009 ). In this sense, global sea-level changes estimated during the Early and Middle Eocene could have favoured this type of shallow environment. Additionally, the GP is located in a passive margin, allowing a slow sedimentary deposition (Garrison, 2009 ; Miller et al., 2020 ; Zhao et al., 2022 ). The assemblages of molluscs and sedimentary sequences reported in our work are some of the consequences of changes in sea level, which allowed the dispersal of species and occupation of new niches during the Ypresian and Bartonian in the GP and the Lutetian in the CP (Fig. 6 ). However, it is interesting to note that the fauna is related to the hyperthermal events of the Ypresian (EECO) and the Bartonian (MECO), especially in the GP (Lunt et al., 2011 ; Thomas et al., 2018 ; Foster et al., 2020 ). Superimposed on the long-term warming events (estimated increase of ≈ 6°C on the Earth's surface), EECO and MECO are short-duration intervals recognized as hyperthermal. One hyperthermal event is the PETM (marking the beginning of the Eocene), and the other two hyperthermal events are known as ETM2 (≈ 54 Ma) and ETM3 (≈ 53 Ma) during the Early Eocene. These hyperthermals are characterised by injections of carbon into the atmosphere, ocean acidification and variable increases in sea surface temperature (Zachos et al., 2008 ; Lunt et al., 2011 ; Foster et al., 2020 ). Foster and collaborators (2020) mentioned that the environmental changes associated with hyperthermal events were not harmful to shallow-water mollusc faunas despite increased temperatures, acidification, and deoxygenation. Instead, the authors inferred that these hyperthermal events produced slight changes, which could have been reversible and short-lived. In our study, we reported 117 genera and 171 species for the PG-Ypresian, and 179 genera and 390 species for the GP-Bartonian (Fig. 1 ). The highest mean annual temperatures for the entire Cenozoic Era have been recorded in both age ranges (Pearson & Palmer, 2000 ). However, the taxonomic richness was not "affected" by the hyperthermal events, especially during the Bartonian, which could be considered as the maximum marine biodiversity stage of the Gulf Coastal Plain, in contrast with the maximum marine biodiversity reported for the Paris Basin, which occurred in the Lutetian (Huyghe et al., 2012 ). Despite the CP-Lutetian (proposed in our work) could be considered the maximum marine biodiversity of molluscs (19 genera and 22 species, see Fig. 1 ), more taxonomic data and the inclusion of other Caribbean countries are needed in future research as we only recorded data from the sedimentary sequences of Chiapas, Mexico and Saint James, Jamaica (Fig. 6 ). Similarly, regarding GP, we attribute the GP-Bartonian to be the maximum marine biodiversity of molluscs, although it is worth noting that the Early Eocene hyperthermal events (Ypresian) did not observe “negative effects” on the reported fauna. Instead, the molluscs resisted the hostile conditions (mainly of the PETM), which allowed unoccupied areas to be recolonised once the environmental conditions improved (Foster et al., 2020 ). Geochemical studies have suggested that during the PETM (similar to ETM2 and ETM3 but at different intensities), there was a reduction in calcite compensation depth (CCD) and ocean surface acidification (Foster et al., 2020 ). In this sense, the lysocline reduction could explain the extinction of deep-sea benthic foraminifera (Molina, 2007 ; Speijer et al., 2012 ), while acidification would have harmed the metamorphosis, growth and survival of planktonic mollusc larvae (Talmage & Gobler, 2009 ). However, the molluscs reported in our study for the GP-Ypresian mostly suggest a shallow water environment (Fig. 6 ). Thus, it is possible that these environments were not affected by CCD and acidification and that molluscs were able to cope with the acidification of the oceans by regulating the pH and the chemistry of carbonates in their habitats (Foster et al., 2020 ). As mentioned earlier, after the hyperthermal events of the Early Eocene, the development of the epoch was led by a global cooling trend until the sudden warming of the Bartonian (MECO) (Boscolo-Galazzo et al., 2014 ; Savian et al., 2014 ). The MECO resembled its predecessor events from the Early Eocene, with increased atmospheric carbon dioxide (Bohaty et al., 2009 ). However, this hyperthermal did not impact the species richness of the GP-Bartonian; in fact, this stage is considered the maximum marine biodiversity of the province (Fig. 1 ). As with the Early Eocene, in the Bartonian (late Middle Eocene), the temperature rise, ocean acidification and deoxygenation were not detrimental, with which we persist in the idea that shallow marine environments "did not undergo detrimental changes" for the fauna (Fig. 6 ). Furthermore, a sea level rise of ≈ 40 m (≈ 40.1 Ma) (Miller et al., 2020 ) has been recorded during this period. This rise triggered transgressions (presence of glauconite) (Fig. 6 ) and may have favoured the development of shallow shelves, allowing remarkable species diversification without leading to long-term changes in faunal diversity or composition at the genus level (Fig. 1 ) (Huyghe et al., 2012 ; Das & Halder, 2018 ; Foster et al., 2020 ). It is important to note the noticeable increase in GP-Bartonian species, which could be explained by species stratigraphic ranges being short and concomitantly exhibiting higher turnover rates across the age range, suggesting that Eocene hyperthermal events impacted benthic mollusc turnover rates at the species level, but not at the genus level (see Fig. 1 ) (Foster et al., 2020 ). The hyperthermal events during the Early and Middle Eocene did not have a “negative effect” on molluscan assemblages; instead, these benthic communities (genera) were warm-water tolerant and thus tolerated the high temperatures (Fig. 6 ). Since the Mesozoic greenhouse periods, the mollusc faunal assemblages of the Gulf Coastal Plain have been dominated by taxa that tolerate high temperatures. Indeed, most of these molluscs are associated with warm-temperate climates (Fig. 6 ) and tend to inhabit broader niches than tropical species, which explains the low impact of hyperthermal events (Foster et al., 2020 ). Finally, in the Middle Eocene, the biodiversity of marine molluscs significantly increased, particularly in the Paris Basin (middle Lutetian) and Gulf Province (Bartonian). The temperate conditions of the long-term relatively stable cooling trend during this period enabled the increase in marine mollusc biodiversity. In addition, the hyperthermal events allowed for greater thermal tolerance and plasticity among the genera (Huyghe et al., 2012 ; Foster et al., 2020 ). Future studies could address dated diversification and extinction rates of representative taxa of each biogeographic province under a phylogenetic context to further evaluate the biodiversity patterns of marine molluscs. Northeastern Pacific Ocean Province (cluster two) Based on the dendrogram obtained in our study, the NEEP showed the highest similarity (0.675). The clustering of the Caribbean-Priabonian in group two is further evidence of faunal exchange (Fig. 3 ). The Tethys Current provided a faunal migratory route between the Atlantic and the Indo-West Pacific through the 'Central American seaway'. The presence of taxa with Tethyan affinity reflects this connectivity, although this current was almost closed by the end of the Eocene (Oleinik, 2001 ; Morales-Ortega et al., 2015 ; Zhao et al., 2022 ). With our findings, we can infer that the marine molluscs from this area exhibit a certain resemblance to the faunas reported from Western Europe and Eastern America, at least concerning the taxa reported from the Ypresian (mainly) and Lutetian of the NEPP (Silva et al., 2014 ). This high interconnectivity is reflected with some bivalve genera such as Anodontia , Chama , † Cubitostrea , † Gryphaeostrea , Plicatula and Varicorbula ; the gastropod genera Athleta , Bathytoma , Campanile , † Clavilithes , Conomitra , † Crommium , Cypraea , † Cypraedia , † Domenginella , † Eocithara , † Eocypraea , † Eosurcula , Euspira , Galeodea , Gemmula , Harpa , † Keepingia , Latirus , Mesalia , Metula , Oliva , † Platyoptera , † Pleurofusia , Puncturella , Ringicula , † Strepsidura , † Surculites , † Trypanotoma , Turbonilla , † Velates and Xenophora ; and the cephalopod genera † Aturia , † Eutrephoceras and Nautilus (Fig. 7 ). In group two, the NEPP-Ypresian represents the maximum marine biodiversity of molluscs (Fig. 7 ), contrary to what is reflected in the Paris Basin (middle Lutetian) (Huyghe et al., 2012 ) and the Gulf Province (Bartonian) (our study). This maximum biodiversity (275 species, see Fig. 1 ) could be explained by the more significant number of lithostratigraphic deposits and by preserving specimens, allowing species identification. However, the biodiversity in terms of genera is also the highest for the entire province (183 genera; see Fig. 1 ), which supports the idea that the NEPP-Ypresian can be considered the maximum marine biodiversity (Fig. 1 ), regardless of the hyperthermal events from the Early Eocene. The NEPP is on an active continental margin (Ubilla et al., 2009 ). Geological processes have dramatically impacted the western edge of North America since the Mesozoic era. The West Coast of the United States and northwestern Mexico coexisted on a convergent margin where the now-extinct Farallon tectonic plate sank beneath the North American plate, causing sudden changes in coastal topography (Ubilla et al., 2009 ; Garrison, 2009 ; Santiesteban-Mendívil, 2019). This coastal topographic variation of the NEPP caused by geological processes (subduction and orogeny) could have enabled the survival of molluscs. In this context, the coastline of an active margin is characterised by steep, narrow, and rocky coasts, which is one of the most biologically rich benthic habitats of the intertidal zone and accounts for high in situ primary production (Satheesh & El-Sherbiny, 2022 ). The environments recorded in our study for the NEPP during the Ypresian and Lutetian range from the coast (rocky coast, lagoon, estuary, bay, and delta) to deep environments (deep subtidal, submarine canyon, submarine fans, bathyal, and abyssal), typical of an active margin (Fig. 7 ). Molluscs are abundant in the intertidal zone, and their physiological adaptations allow them to survive extreme temperature, salinity, humidity and wave action changes. Thus, molluscs adapted to these conditions were able to resist the increase in sea surface temperatures and deoxygenation during the epoch. Furthermore, it is also possible that these organisms migrated latitudinally (high latitudes) and vertically (depth). In this sense, it has been noted that during the Eocene, the fauna thrived at the latitudinal range of 30° to 50° in both hemispheres, with which it is inferred that the tropical and subtropical belts were much broader than the ones we know at present. On the other hand, the vertical distribution of molluscs depends on their physiological tolerance to abiotic ( e.g ., temperature, desiccation, and light) and biotic ( e.g. , predation and competition) factors. Most molluscs can move vertically, which may have enabled them to survive under less favourable conditions (Camacho, 2008 ; Das & Halder, 2018 ; Satheesh & El-Sherbiny, 2022 ). Another factor supporting the idea that the Early Eocene hyperthermal events did not produce significant changes in mollusc diversification is body size. An increase in temperature within the system can influence the reduction in body size, promote parasitism and/or alter habitat preferences (Foster et al., 2020 ). However, body size could not have been affected since we recorded large specimens, such as the gastropods Campanile sp. (Morales-Ortega & González-Barba, 2018 ) and Gisortia sp. (Perrillat, 1996); the bivalves Crassostrea contracta (Morales-Ortega et al., 2016 ), Acutostrea gaviota (Squires, 2018 ), and Phygraea stewarti (Squires, 2018 ); and the cephalopod Nautilus sudcalifornianus (Morales-Ortega et al., 2023 ). The sizes of these species resemble those recorded with Tethyan affinity (Squires & Demetrion, 1992 ; Squires, 1993 ; Morales-Ortega et al., 2016 ). During the Early Eocene, the NEPP-Ypresian and NEPP-Lutetian mollusc assemblages were unaffected by the hyperthermal events (Fig. 7 ). The West Coast of the USA and the Baja California peninsula in Mexico were warm-temperate regions during this time (Squires, 2003 ), where the sea levels rose significantly (Squires, 2018 ). This combination of warm temperatures and rising sea levels enabled warm-water mollusc species from the Tethys province to spread to the aforementioned areas. This led to the highest point of Eocene faunal exchange among Eurasia, North Africa, and GP through the "Central American seaway" (Squires, 1987 ; Squires & Demetrion, 1992 ; Lozouet, 2014 ; Morales-Ortega et al., 2015 , 2016 ; Das & Halder, 2018 ; Squires, 2018 ). On the other hand, the NEPP-Bartonian exhibits a smaller number of genera (120) and species (182) than the GP-Bartonian (see Fig. 1 ). Three factors can explain this decrease in NEPP-Bartonian records: (1) There could be a difference in sampling effort among sedimentary deposits. (2) There could be fewer sedimentary deposits within this age range than others. For example, in the Adelholzen section, Germany, the Lutetian and Priabonian stages are widely recorded, while the Bartonian is absent (Gebhardt et al., 2013 ). (3) Environmental changes caused by the MECO could have affected the mollusc community, as its effects were not globally uniform (Bohaty et al., 2009 ). For example, low primary productivity has been reported at Site 1263 (Walvis Ridge, southeast Atlantic) due to increased surface water temperatures (≈ 4°C), which possibly led to the starvation of heterotrophs (mainly planktonic foraminifera) (Boscolo-Galazzo et al., 2014 ). A contrasting case was observed in Monte Cagnero (Umbria-Marche Basin), Italy, where an increased primary productivity was detected, probably stimulated by an increased aeolian supply to surface waters (Savian et al., 2014 ). It is worth noting that an increase in nutrients (particularly nitrogen or phosphorus) can generate 'dead zones' (Dybas, 2005 ; Foster et al., 2020 ), which are significant drivers of ecological change in modern shallow-water marine ecosystems and perhaps one of the most critical drivers of benthic invertebrate loss. Changes in water circulation patterns, wind, high temperatures, an increase in runoff, and an excess of nutrients are the main determinant factors of 'dead zones' expansion and, concomitantly, the level of marine biodiversity. Currently, 'dead zones' are developing in regions of the Gulf Coastal Plain, surrounding the mouth of the Mississippi River, and coastal areas of the Northwestern Pacific (Dybas, 2005 ; Díaz & Rosenberg, 2008; Gebhardt et al., 2013 ; Sluijs et al., 2014 ; Foster et al., 2020 ; NOAA, 2023). Thus, the development of ‘dead zones' could explain the low presence of molluscs during the Bartonian of the NEPP (Fig. 1 ) (although our work does not account with data on the determinant factors mentioned earlier –winds, runoff, etc.). 'Dead zones' could have been temporary, and benthic communities could have been able to rapidly recolonise habitats without significant renewal, which could explain the increase in genera and species during the Priabonian (Fig. 1 ) (Foster et al., 2020 ). To a certain extent, the NEPP during the Priabonian “does not record” biodiversity variations, mainly within cooling events, compared to the GP-Priabonian, which shows a dramatic decline in genera and species (Fig. 1 ). NEPP-Priabonian records a slight increase in genera (139) and species (212) compared to NEPP-Bartonian (Fig. 1 ). As mentioned earlier, the Tethys Sea provided a migratory route; however, at the end of the Eocene, this sea became narrower and, concomitantly, reduced gene flow. For instance, molluscs began to diverge from the Middle Eocene (Zhao et al., 2022 ). This is depicted by the presence of specific genera such as the bivalves Adula , Bathymodiolus , Calyptogena , † Elongatolucina , Maorithyas , and Tellidorella ; the gastropods Desbruyeresia , and Thalassonerita ; and the scaphopod Coccodentalium . These genera appear from the Lutetian in the NEPP (Paleontology Database, 2024 ). In the same sense, it has been mentioned that the fauna of the North American Pacific could be differentiated into two biogeographic components. The first component could be defined by the presence of genera with Tethyan and Indo-Pacific Tethyan affinity (Tethys Realm), which can be characterised by the reported Early Eocene (NEPP-Ypresian) and Middle Eocene (NEPP-Lutetian) faunas (Figs. 1 , 7 ). The second component would be represented by the fauna reported from the northeastern Pacific during the Late Eocene (NEPP-Priabonian) (Figs. 1 , 7 ; Oleinik, 2001 ; Hickman, 2015 ). The climatic conditions during the Late Eocene were already sufficiently different from the previous Eocene stages to cause differences in the composition of mollusc faunas (Piccolli & Savazzi, 1983). The oceanic circulation of the North Pacific had a pattern similar to the modern oceanic circulation, with a subtropical anticyclonic gyre and a subarctic cyclonic gyre as its main characteristics. However, the latitudinal extent, circulation intensity, and local shallow water patterns are still unknown (Oleinick, 2001). We record genera from the high and middle latitudes during the Late Eocene, including the Australasian, northwestern and northeastern Pacific regions (Piccolli & Savazzi, 1983; Olenick, 2001; Hickman, 2015 ). Some genera include the bivalves, Conchocele (Japan, Russia), Lucinoma (Russia), † Nitidavenus (Russia), Nuttallia (Russia), † Pseudocardium (Russia), Tivela (Canada, Russia), and † Vertipecten (Canada); and the gastropods, Argobuccinum (Antarctica), Bathybembix (Tonga), Cellana (Antarctica), Colus (Japan, Russia), Homalopoma (Australia, Russia), † Molopopophorus (Russia, Canada), Nekewis (Japan, Russia), Neptunea (Antarctica, Japan, Russia), † Parasyrinx (Russia), and † Turrinosyrinx (Russia). Some of these taxa are extant genera in the North Pacific and, hence, are potentially relevant to the biogeographic reconstruction of the Pacific Basin (Oleinik, 2001 ; Hickman, 2015 ). During the Middle to Late Eocene, there were shifts in the distribution of molluscs in the northeastern Pacific (including the CP-Priabonian fauna). These changes showed a combination of faunas with North Pacific and Tethys affinities, evidenced by tropical, subtropical, and temperate taxa (Fig. 7 ; Oleinik, 2001 ; Schweitzer et al., 2006 ). It has been suggested that the reduction in the latitudinal range of genera with Tethys affinity was restricted to southern California during the Late Eocene. In contrast, genera related to the North Pacific (bivalves: Cardiomya , † Cryptolucina , Cyclinella , † Elongatolucina , Mactromeris , Modiolus , Mytilus , Thracia , and Yoldia ; and gastropods: † Paraseraphs , † Perse , and Rimella ) and cosmopolitan genera (bivalves: Costacallista , Fimbria , Lucina , Macrocallista , Miltha , Myrtea , Phacoides , Pitar , Tellina , † Venericardia , and † Venericor ; and gastropods: Acrilla , Acteon , Bonellitia , Calyptraea , Conus , Crepidula , Exilia , Ficus , Fusinus , Lyria , Margarites , Odostomia , Olivella , Polynices , Retusa , Scaphander , Siphonalia , Solariella , Turricula , and Turritella ) did not undergo significant changes in the latitudinal ranges. This pattern suggests that high-latitude faunas were “unaffected” by global climate changes (cooling), at least during the Late Eocene. Indeed, the ocean circulation patterns and climate changes limited the latitudinal migration of temperature-sensitive equatorial genera to high latitudes (Piccoli & Savazzi, 1983 ; Oleinik, 2001 ). Moreover, at the end of the Eocene, there was an increase in heteroconch bivalve fauna, mainly in Oregon and Washington (Fig. 7 ; Nesbitt, 2003 ; Kiel, 2006 ; Hickman, 2015 ). The heteroconchs have successfully developed morphological and physiological adaptations to extreme environments (chemosynthetic, for example, 'cold-seep'). Hickman ( 2015 ) noted that the heteroconch bivalve fauna from Oregon records the climatic transition of the Late Eocene and Early Oligocene and a replacement of tropical taxa by the cryophilic taxa that currently dominate the high-latitude fauna of the North Pacific. Undoubtedly, the Priabonian in the NEPP is of great interest for studying changes in the North Pacific regions. The mixture of fauna, the ability to tolerate extreme conditions, the closure of Tethys, the change in deep and surface ocean circulation, and the diversity of marine environments caused by a tectonically active margin could favour mollusc communities within the transition to the 'coolhouse' (Fig. 7 ), compared to the decrease in fauna of the GP-Priabonian (Fig. 1 ) (Oleinik, 2001 ; Schweitzer et al., 2006 ). Gulf Province, Lutetian and Priabonian (cluster one) After the hyperthermal events of the Early Eocene, the beginning of the Middle Eocene was marked by a “stable” climate (Zachos et al., 2008 ). Indeed, it has been pointed out as an interval of relative cooling, known as the “Lutetian cooling”, as indicated by the presence of temporary ice sheets in both hemispheres (Lear et al., 2000 ; Zachos et al., 2008 ; Payros et al., 2009 ; Huyghe et al., 2012 ). This “cold stage” could have influenced the mean annual sea surface temperatures, causing the formation of “temperate zones” (or less hot zones) that enabled the increase in marine biodiversity (Huyghe et al., 2012 ). Similarly, a regional transgression (presence of glauconite, Fig. 8 ) could also favour the rise in the mollusc fauna during the GP-Lutetian. Transgressions can increase the benthic zone and concomitantly enhance the benthos' light, food and oxygen conditions (Huyghe et al., 2012 ; Das & Halder, 2018 ). The presence of genera similar to those found in tropical, subtropical, and temperate environments (bivalve: Pitar ; gastropods: Agaronia , Ancilla , Architectonica , † Ficopsis , Hexaplex , Raphitoma , Siphonalia , † Stellaxis , and † Terebrifusus ; and scaphopod: Dentalium ) suggests a faunal mix that was able to survive under changing conditions (Fig. 8 ). This could have allowed an increase in the GP-Lutetian genera and species (Fig. 1 ). For example, the Paris Basin was characterised by warm summers and relatively “cold” winters, where the “cooling” was not a negative factor in establishing the maximum marine biodiversity of the Parisian Lutetian (Huyghe et al., 2012 , p. 588). The dendrogram analysis in our study showed a connection between the Lutetian and Priabonian of the GP (see Fig. 3 ), which could be attributed to the presence of genera and species related to cooler environmental conditions (Fig. 8 ). However, unlike the Lutetian, where conditions were favourable for the increase in fauna, there was a significant reduction in genera and species in the Priabonian. It has been estimated that the turnover of molluscs was over 90% (Hansen, 1987 ; Ivany et al., 2000 ; Hansen et al., 2004 ). However, using the data obtained in our work, we estimated a 52% decrease in genera and a 74% decrease in species (see Fig. 1 ). These results indicate that the environmental disturbances with a cooling trend from the late Middle Eocene to the Eocene-Oligocene transition (EOT) significantly impacted mollusc assemblages (Ivany et al., 2003 ). Some authors consider the Late Eocene a time of regional "extinction" waves (Hansen, 1992 ; Ivany et al., 2000 ; Hansen et al., 2004 ). The "loss" of mollusc faunas (mainly species) is deemed to have occurred in a prolonged and transitional manner, with warm-water taxa being more affected than cold-water taxa, including the loss of endemic taxa adapted to high temperatures (Ivany et al., 2003 ; Foster et al., 2020 ). In addition, the 'coolhouse' transition may have influenced the migration of taxa, caused by the reduction of paleobiogeographic belts parallel to the Equator, which moved southwards in response to low temperatures. This could have caused warm-water species to migrate in an equatorward direction (Hansen et al., 2004 ; Westerhold et al., 2020 ). Consistently, the Late Eocene to EOT is not considered a global extinction event but rather a faunal "rotation" event (Dockery, 1986 ; Dockery & Lozouet, 2003 ; Hansen et al., 2004 ). A decrease in sea level can affect habitat diversity, and circulation patterns can force a biogeographic change in taxa. While both factors could cause a decrease in taxa, the decrease in temperature is the most notable factor in faunal change (Hansen, 1987 ; Ivany et al., 2000 ; Hansen et al., 2004 ; Das & Halder, 2018 ). The different responses of molluscs to cooling and warming events highlight the relationships between evolutionary history, ecological structure and environmental change. Thus, a significant change in environmental state (‘cold or hot’) will “affect” biodiversity; however, its effects (‘positive or negative’) will depend on the initial state of the system (Ivany et al., 2003 ; Foster et al., 2020 ). Conclusions Using the dendrogram, we could distinguish the level of similarity and shared features of the taxa based on province and age range, resulting in five distinct groups. However, this does not imply significant differences between the provinces. The dendrogram enabled us to elucidate how environmental changes specific to each region and the global climate from the Epoch impacted the molluscan assemblages in each province. Undoubtedly, all the paleobiogeographic provinces proposed in our work showed evidence that the distribution of molluscs was strongly influenced by Tethys (Tethys Realm). However, the extensive faunal exchange was reflected mainly in the provinces of the northern hemisphere (NEPP, GP and CP), certainly due to their geographical proximity. At the same time, the 'Central American seaway' allowed this exchange. The extensive record of taxa with Tethyan affinity in the American continent can be comparable with the Eurasian, North African and South American faunas. Thus, our work highlights the significant faunal exchange between the NEPP and the GP during the Ypresian and Lutetian. Moreover, our results show that the maximum marine biodiversity stages coincide with hyperthermal events (EECO and MECO). The GP-Bartonian is considered the stage with the maximum biodiversity, which increases in species turnover rates could explain. For the NEPP, the maximum biodiversity was reached in the Ypresian, possibly due to the extensive faunal exchange from the Tethys towards the Pacific coast of North America. The Priabonian in the GP and NEPP provinces showed significant differences. The cooling (the most conspicuous factor) in the GP wreaked havoc on the mollusc fauna, and a considerable decrease was observed in the record of genera and species. The NEPP reflects a discrete increase in the Bartonian, which could be explained by the flow of the North Pacific currents, causing a greater faunal exchange with the North Western Pacific. However, the dynamics of the currents in the Pacific basin are still unknown. In contrast, the SEPP reflects a geographical “remoteness”, at least during the Ypresian and Lutetian stages, and few records indicate a closer relationship with faunas from the Early Paleogene of Peru during these stages. To a certain extent, this province was related to the northern hemisphere regions during the Bartonian and Priabonian. This could be explained by the displacement of the South American plate towards the north and the persistent Tethyan faunal affinity. Finally, we recognise that Tethys strongly influences the provincial relationships of molluscs. However, the regional environmental changes produced by the progressive and abrupt events in the global geological and climatic conditions during the epoch had a different impact on each province. Likewise, we reinforce several ideas proposed by other authors and include new regionalised and provincial information. It is essential to mention that we need to record more faunal data (adding records from different countries) and include regional environmental data (geochemistry, specify age, strengthen lithostratigraphic data, regional tectonic data and changes in regional sea level) to improve and reinforce the conclusions achieved in our work. Abbreviations PETM Paleocene–Eocene Thermal Maximum ETM2 Eocene Thermal Maximum 2 ETM3 Eocene Thermal Maximum 3 MECO Middle Eocene Climatic Optimum EOT Eocene–Oligocene Transition Ma Million years ago USA United States of America NGMDB National Geologic Map Database Project USGS United States Geological Survey AASG Association of American State Geologists UPGMA Unweighted pair–group method using arithmetic averages NEPP Northeastern Pacific Ocean Province SEPP Southeastern Pacific Ocean Province CP Caribbean Province GP Gulf Province ODP Ocean Drilling Program CCP Calcite Compensation Depth CONAHCYT Consejo Nacional de Humanidades, Ciencias y Tecnologías Declarations Ethics declarations Ethics approval and consent to participate The authors declare that they have no competing interests. Competing interests The authors declare no competing interests. Availability of data and materials All fossil genera and species included in this study have been previously described and published. 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Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {"props":{"pageProps":{"initialData":{"identity":"rs-5278171","acceptedTermsAndConditions":true,"allowDirectSubmit":true,"archivedVersions":[],"articleType":"Research Article","associatedPublications":[],"authors":[{"id":378308786,"identity":"982a3da5-b242-4bef-9b34-479023b7ef2f","order_by":0,"name":"Priscila Morales-Ortega","email":"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAAA4UlEQVRIiWNgGAWjYFACxgYog/kAkJCQIVaLARCzJYC08BBrFUgLD4hgIKzFXPpw84ePe/7Iybuf+fzqRo0FDwP74aMb8Gmx7Etsk5zxzMDY8EzuNuucY0CH8aSl3cDrojOMbcw8BwwSNzbkbjPOYQNqkeAxI6Sl+TNQS/3G/jfPjHP+EaelQRqoJUFeIof5cW4bEVosexiBfjlgbLhB4pkZc26fBA8bIb+Y87A//vDhgJy8fH/y48853+rk+NkPH8PvMDjjAAObBIjBhk85ihb5BgbmD4RUj4JRMApGwcgEAGLdRp6kXck7AAAAAElFTkSuQmCC","orcid":"","institution":"Universidad Autónoma de Baja California Sur (UABCS), Baja California Sur","correspondingAuthor":true,"prefix":"","firstName":"Priscila","middleName":"","lastName":"Morales-Ortega","suffix":""},{"id":378308787,"identity":"39cacbdc-8ba0-4a5b-90f5-57828333c059","order_by":1,"name":"Gerardo Gonzalez-Barba","email":"","orcid":"","institution":"Universidad Autónoma de Baja California Sur (UABCS), Baja California Sur","correspondingAuthor":false,"prefix":"","firstName":"Gerardo","middleName":"","lastName":"Gonzalez-Barba","suffix":""}],"badges":[],"createdAt":"2024-10-16 19:53:10","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-5278171/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-5278171/v1","draftVersion":[],"editorialEvents":[],"editorialNote":"","failedWorkflow":false,"files":[{"id":70104689,"identity":"d429c5b5-18e4-44e4-b93a-e8386efcdd92","added_by":"auto","created_at":"2024-11-28 11:09:28","extension":"jpeg","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":421346,"visible":true,"origin":"","legend":"\u003cp\u003eThe total number of genera (sharp bars) and species (dim bars) was recorded per stage. The four paleoclimatic events (PETM, EECO, MECO, and EOT) are depicted (grey bars). Black arrows indicate the stage with the maximum marine biodiversity (based on Huyghe et al., 2012 and Westerhold et al., 2020).\u003c/p\u003e","description":"","filename":"floatimage1.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-5278171/v1/5834b871ce49468fef7c3c75.jpeg"},{"id":70104690,"identity":"38dde816-3b51-4738-b7e0-fdf6b564d8d7","added_by":"auto","created_at":"2024-11-28 11:09:28","extension":"jpeg","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":99300,"visible":true,"origin":"","legend":"\u003cp\u003eRelationship between localities and occurrences by stage for all provinces. Stages: Ypr, Ypresian; Lut, Lutetian; Bar, Bartonian; and Pri, Priabonian.\u003c/p\u003e","description":"","filename":"floatimage2.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-5278171/v1/45231d4ad1889896ba6a7d3a.jpeg"},{"id":70104692,"identity":"2d12f840-8cea-40e3-ab7c-03ebb2ac858c","added_by":"auto","created_at":"2024-11-28 11:09:28","extension":"jpeg","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":480211,"visible":true,"origin":"","legend":"\u003cp\u003eSimilarity analysis depicts five defined clusters (similarity level of 0.5). Provinces: northeastern Pacific Ocean Province (NEPP), southeastern Pacific Ocean Province (SEPP), Gulf Province (Gulf of GP), and Caribbean Province (Caribbean or CP). Stages: Ypr, Ypresian; Lut, Lutetian; Bar, Bartonian; and Pri, Priabonian.\u003c/p\u003e","description":"","filename":"floatimage3.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-5278171/v1/2684c1ddec69237723cd6fec.jpeg"},{"id":70104694,"identity":"03f107cc-8e60-4259-aae3-cb4b4944dfeb","added_by":"auto","created_at":"2024-11-28 11:09:28","extension":"jpeg","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":247562,"visible":true,"origin":"","legend":"\u003cp\u003eCluster five, Southeastern Pacific Ocean Province (SEPP) during the Ypresian and Lutetian. The colour bar shows the percentages of species by order and stage. The lithostratigraphic formations, depositional environments, and the map of localities by stage are depicted (illustrations: map and colour bar, PaleoTax; information on lithostratigraphic formations obtained from six research publications).\u003c/p\u003e","description":"","filename":"floatimage4.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-5278171/v1/47425f35f92f4e166be4b85a.jpeg"},{"id":70104693,"identity":"1fe640fa-cd32-4fe6-9b9b-317982bba5d2","added_by":"auto","created_at":"2024-11-28 11:09:28","extension":"jpeg","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":305067,"visible":true,"origin":"","legend":"\u003cp\u003eCluster four, Southeastern Pacific Ocean Province (SEPP) during the Bartonian and Priabonian. The colour bar shows the percentages of species by order and stage. The lithostratigraphic formations, depositional environments, and the map of localities by stage are depicted (illustrations: map and colour bar, PaleoTax; information on lithostratigraphic formations obtained from six research publications).\u003c/p\u003e","description":"","filename":"floatimage5.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-5278171/v1/ca25b4692acc57c64c6f6e42.jpeg"},{"id":70104697,"identity":"7b3c8d38-6528-41f0-8166-01e67c905eed","added_by":"auto","created_at":"2024-11-28 11:09:29","extension":"jpeg","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":559052,"visible":true,"origin":"","legend":"\u003cp\u003eCluster three, Gulf Province (Gulf or GP) during the Ypresian and Bartonian, and Caribbean Province (Caribbean or CP) during the Lutetian. The colour bar shows the percentages of species by order and stage. The lithostratigraphic formations, depositional environments, and the map of localities by stage are depicted (illustrations: map and colour bar, PaleoTax; information on lithostratigraphic formations obtained from 17 research publications).\u003c/p\u003e","description":"","filename":"floatimage6.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-5278171/v1/153f91ec84ea51a3b5b26c34.jpeg"},{"id":70104696,"identity":"611b010a-9ca5-47e1-ade4-5c07f2d05ea1","added_by":"auto","created_at":"2024-11-28 11:09:29","extension":"jpeg","order_by":7,"title":"Figure 7","display":"","copyAsset":false,"role":"figure","size":924188,"visible":true,"origin":"","legend":"\u003cp\u003eCluster two, Northeastern Pacific Ocean Province (NEPP). The colour bar shows the percentages of species by order and stage. The lithostratigraphic formations, depositional environments, and the map of localities by stage are depicted (illustrations: map and colour bar, PaleoTax; information on lithostratigraphic formations obtained from 50 research publications).\u003c/p\u003e","description":"","filename":"floatimage7.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-5278171/v1/e63f7d6f5830eedc5848dd14.jpeg"},{"id":70104973,"identity":"a16209b6-0c4a-4765-ac6a-85e491ce6028","added_by":"auto","created_at":"2024-11-28 11:17:28","extension":"jpeg","order_by":8,"title":"Figure 8","display":"","copyAsset":false,"role":"figure","size":991724,"visible":true,"origin":"","legend":"\u003cp\u003eCluster one, Gulf Province (Gulf or GP) during the Lutetian and Priabonian. The colour bar shows the percentages of species by order and stage. The lithostratigraphic formations, depositional environments, and the map of localities by stage are depicted (illustrations: map and colour bar, PaleoTax; information on lithostratigraphic formations obtained from the 13 research publications).\u003c/p\u003e","description":"","filename":"floatimage8.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-5278171/v1/3f3985babc05e391706f25a8.jpeg"},{"id":71449919,"identity":"beb60bc3-61aa-4064-af46-8a62993a23cd","added_by":"auto","created_at":"2024-12-15 16:16:38","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":4810928,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-5278171/v1/0007902c-3362-45d0-adec-4bd6fc77d8ad.pdf"}],"financialInterests":"No competing interests reported.","formattedTitle":"Effects of the global and regional climate events of the Eocene on the Pan-American provinces of marine molluscs","fulltext":[{"header":"Introduction","content":"\u003cp\u003eThe Eocene was one of the most active epochs in Earth's geological history. Climatic events during this epoch are known to be associated with the movement of tectonic plates, shifting ocean currents, significant changes in atmospheric circulation, alterations in biogeochemical cycles, and even variation in orbital cycles; all of these variations resulted in a constantly changing and highly variable biosphere (Coxall \u0026amp; Pearson, \u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e2007\u003c/span\u003e; Collins, \u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e2011\u003c/span\u003e; Westerhold \u0026amp; R\u0026ouml;hl, 2012; Gebhardt et al., \u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e2013\u003c/span\u003e).\u003c/p\u003e \u003cp\u003eThe phylum Mollusca is one of the most studied groups of the Eocene, as there was radiation and a wide distribution of families globally. However, this was \"interrupted\" by progressive and abrupt events in geological, environmental and climatic conditions during the Early Eocene (hyperthermal), Middle Eocene (cooling trend and sudden warming), and Late Eocene (\u0026lsquo;cooling\u0026rsquo;) (Piccoli \u0026amp; Savazzi, \u003cspan citationid=\"CR55\" class=\"CitationRef\"\u003e1983\u003c/span\u003e; McGowran et al., \u003cspan citationid=\"CR35\" class=\"CitationRef\"\u003e2000\u003c/span\u003e; Oleinik, \u003cspan citationid=\"CR47\" class=\"CitationRef\"\u003e2001\u003c/span\u003e; Squires, \u003cspan citationid=\"CR70\" class=\"CitationRef\"\u003e2003\u003c/span\u003e; Lozouet, \u003cspan citationid=\"CR33\" class=\"CitationRef\"\u003e2014\u003c/span\u003e; Das \u0026amp; Halder, \u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e2018\u003c/span\u003e; Morales-Ortega \u0026amp; Gonz\u0026aacute;lez-Barba, \u003cspan citationid=\"CR40\" class=\"CitationRef\"\u003e2018\u003c/span\u003e; Hutchinson et al., \u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e2021\u003c/span\u003e). These events affected the marine biota; hence, in recent years, attempts have been made to correlate the faunal assemblages of molluscs from different parts of the world to understand regional palaeoenvironmental changes and shifts in the geographical redistribution of species (Coxall \u0026amp; Pearson, \u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e2007\u003c/span\u003e; Das \u0026amp; Halder, \u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e2018\u003c/span\u003e; Foster et al., \u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e2020\u003c/span\u003e). Thus, the main aim of our study was to evaluate the relationship among faunal assemblages of molluscs reported in different formations of the American continent in order to identify the interprovincial relationship and the effects caused by regional and global climatic-environmental events during the Eocene.\u003c/p\u003e\n\u003ch3\u003eClimate events\u003c/h3\u003e\n\u003cp\u003eThe Early Eocene was characterised by reaching the highest mean annual temperatures of the entire Cenozoic Era, with relatively low pole-to-pole temperature gradients and high precipitation in an ice-free world (Zachos et al., \u003cspan citationid=\"CR80\" class=\"CitationRef\"\u003e2001\u003c/span\u003e). From \u0026asymp;\u0026thinsp;55 to \u0026asymp;\u0026thinsp;52 Ma, there was a series of short-term changes in the carbon isotope composition of the ocean due to the release of carbon from the ocean to the atmosphere, which caused an increase in ocean surface temperature of \u0026asymp;\u0026thinsp;4 to 8\u0026deg;C (Galeotti et al., \u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e2010\u003c/span\u003e). These short-term hyperthermal events are known as the Paleocene-Eocene Thermal Maximum (PETM) and the Eocene Thermal Maximum 2 and 3 (ETM2 and ETM3) (Lunt et al., \u003cspan citationid=\"CR34\" class=\"CitationRef\"\u003e2011\u003c/span\u003e; Thomas et al., \u003cspan citationid=\"CR75\" class=\"CitationRef\"\u003e2018\u003c/span\u003e). The most recently accepted hypothesis is that orbital parameters such as eccentricity and obliquity may have triggered ETM2 and ETM3 (Galeotti et al., \u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e2010\u003c/span\u003e; Lunt et al., \u003cspan citationid=\"CR34\" class=\"CitationRef\"\u003e2011\u003c/span\u003e). These thermal events significantly perturbed planktonic and benthic foraminifera; however, the regional and global impact on other marine fauna remains unknown (Galeotti et al., \u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e2010\u003c/span\u003e; Lunt et al., \u003cspan citationid=\"CR34\" class=\"CitationRef\"\u003e2011\u003c/span\u003e; Sexton et al., \u003cspan citationid=\"CR62\" class=\"CitationRef\"\u003e2011\u003c/span\u003e).\u003c/p\u003e \u003cp\u003eMoreover, the Middle Eocene marked the transition from a warm to a cold climate (\u0026lsquo;warmhouse to coolhouse\u0026rsquo;; \u0026asymp;49 Ma) (Bohaty et al., \u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2009\u003c/span\u003e; Westerhold et al., \u003cspan citationid=\"CR78\" class=\"CitationRef\"\u003e2020\u003c/span\u003e). Carbon and oxygen isotopes indicate a shift towards global cooling caused by a significant decrease in greenhouse gases (mainly CO2), a change in ocean circulation patterns, and global heat transport, which resulted in a cold climate by the end of the Eocene (Speelman et al., \u003cspan citationid=\"CR65\" class=\"CitationRef\"\u003e2009\u003c/span\u003e; Straume et al., \u003cspan citationid=\"CR73\" class=\"CitationRef\"\u003e2022\u003c/span\u003e). However, during this global cooling trend, there was a significant reversal of warming in the Bartonian, known as the Middle Eocene Climatic Optimum (MECO) (Bohaty \u0026amp; Zachos, \u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e2003\u003c/span\u003e).\u003c/p\u003e \u003cp\u003eThe MECO is considered a warming period due to the increased atmospheric CO2 with a maximum of 4,000 ppm, the highest amount of CO2 detected during the epoch (Bohaty \u0026amp; Zachos, \u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e2003\u003c/span\u003e; Pearson, \u003cspan citationid=\"CR52\" class=\"CitationRef\"\u003e2010\u003c/span\u003e). The increase in this gas was driven by the rate of expansion of the ocean floor, the metamorphic decarbonisation reactions between Australia and Antarctica, and the increase in volcanism in the region, in addition to continental drift and the collision of India with Asia (formation of the Himalayas). However, the exact timing of the release of atmospheric CO2 is still unknown (Bohaty \u0026amp; Zachos, \u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e2003\u003c/span\u003e). Additionally, an increase in the surface temperature of the Tethys Sea, ranging from 32 to 36\u0026deg;C, was reported (Cramwinckel et al., \u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e2023\u003c/span\u003e), as well as acidification of the deep sea. The MECO event is estimated to have lasted\u0026thinsp;\u0026asymp;\u0026thinsp;500 kyr, with a maximum warming period of \u0026lt;\u0026thinsp;100 kyr (Bohaty et al., \u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2009\u003c/span\u003e; Spofforth et al., \u003cspan citationid=\"CR67\" class=\"CitationRef\"\u003e2010\u003c/span\u003e).\u003c/p\u003e \u003cp\u003eAfter the MECO, oxygen isotopic records indicate a return to cooling at \u0026asymp;\u0026thinsp;40 Ma (Bohaty \u0026amp; Zachos, \u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e2003\u003c/span\u003e). Undoubtedly, the climatological transition from the late Eocene to the early Oligocene is the most prominent within the Cenozoic (Hutchinson et al., \u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e2021\u003c/span\u003e). This change was primarily marked by (1) the cooling of Antarctica due to tectonic plate reorganisation and related changes in ocean circulation controlling poleward heat transport and (2) a threshold response to decreasing atmospheric CO2, which determined the Earth's modern glacial climate (Coxall \u0026amp; Pearson, \u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e2007\u003c/span\u003e; Gebhardt, 2013; Hutchinson et al., \u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e2021\u003c/span\u003e; Straume et al., \u003cspan citationid=\"CR73\" class=\"CitationRef\"\u003e2022\u003c/span\u003e). This cooling brought significant climatic and geographical changes, while the flora and fauna recorded a global shift towards species more adapted to the cold climate (Fenero et al., \u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e2010\u003c/span\u003e; Hutchinson et al., \u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e2021\u003c/span\u003e). However, the fossil record documents a gradual turnover pattern indicating an adjustment to food and nutrient availability changes, habitat, and climatic regime (Coxall \u0026amp; Pearson, \u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e2007\u003c/span\u003e).\u003c/p\u003e"},{"header":"Material and methods","content":"\u003cp\u003eOne of our main objectives was to record the mollusc species of the classes Bivalvia, Gastropoda, Cephalopoda, and Scaphopoda reported in various Pan-American lithostratigraphic formations. Thus, we reviewed faunal lists of molluscs from the United States of America (USA) (West Coast and Gulf Coast), Mexico (Baja California Sur, Chiapas, Tamaulipas, and Nuevo Leon), Central America (Jamaica), and Peru (north and south-central).\u003c/p\u003e\n\u003ch3\u003eRecord of taxa and localities\u003c/h3\u003e\n\u003cp\u003eIn this study, we formulated a database to organise and analyse the data obtained. We designed the database entirely without any previously recorded data or elements. Each item recorded in the database was analysed and classified under certain conditions.\u003c/p\u003e \u003cp\u003e \u003cb\u003eFor taxa\u003c/b\u003e: we reviewed each faunal list to obtain the most significant quantity and quality of records for genera and species. We compared each taxon with other faunal lists or databases to recognise whether or not that genera or species is catalogued as a valid taxon. We excluded genera and/or species with a question mark (\u003cem\u003e?\u003c/em\u003e); genera or species \u003cem\u003eaffinis\u003c/em\u003e (\u003cem\u003eaff\u003c/em\u003e.) or \u003cem\u003ecofer\u003c/em\u003e (\u003cem\u003ecf\u003c/em\u003e./\u003cem\u003ecfr\u003c/em\u003e.); genera or subgenera currently in doubt or not accepted (WoRMS, \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://www.marinespecies.org\u003c/span\u003e\u003cspan address=\"https://www.marinespecies.org\" targettype=\"URL\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e); genera or species with doubt in the age range; and genera or species with erroneous locality data.\u003c/p\u003e \u003cp\u003e \u003cb\u003eFor localities\u003c/b\u003e: to corroborate locality data such as coordinates, depositional environments, and age range, we used databases such as Paleobiology DataBase (\u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://paleobiodb.org/#/\u003c/span\u003e\u003cspan address=\"https://paleobiodb.org/#/\" targettype=\"URL\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e), Mindat \u0026ndash; The Hudson Institute of Mineralogy (\u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://www.mindat.org\u003c/span\u003e\u003cspan address=\"https://www.mindat.org\" targettype=\"URL\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e), and NGMDB-USGS \u0026ndash; National Geologic Map Database Project (NGMDB: \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://ngmdb.usgs.gov/Geolex\u003c/span\u003e\u003cspan address=\"https://ngmdb.usgs.gov/Geolex\" targettype=\"URL\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e) in collaboration with the United States Geological Survey (USGS) and the Association of American State Geologists (AASG).\u003c/p\u003e \u003cdiv id=\"Sec5\" class=\"Section2\"\u003e \u003ch2\u003eData analysis\u003c/h2\u003e \u003cp\u003eWe used 'PaleoTax' (\u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttp://paleotax.de\u003c/span\u003e\u003cspan address=\"http://paleotax.de\" targettype=\"URL\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e) and PAST (\u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://past.en.lo4d.com\u003c/span\u003e\u003cspan address=\"https://past.en.lo4d.com\" targettype=\"URL\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e) software for data analysis. PaleoTax is a universal information system and a valuable tool for taxonomic work. The system facilitates the management of a large number of taxa so that each species, genus, family or order can be correlated with locality, lithostratigraphic formation, depositional environments, provincial data and more, depending on the quantity and quality of the information recorded in this database. In addition, PaleoTax has other tools such as PaleoTax/Map (\u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://www.paleotax.de/map\u003c/span\u003e\u003cspan address=\"https://www.paleotax.de/map\" targettype=\"URL\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e), with which we were able to generate maps of the localities (we also used CorelDRAW, 2021 to detail figures) and PaleoTax/Graph (\u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://www.paleotax.de/pgraph\u003c/span\u003e\u003cspan address=\"https://www.paleotax.de/pgraph\" targettype=\"URL\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e), with which we generated the correlation matrices, which we exported to the PAST software for analysis.\u003c/p\u003e \u003cp\u003eIn PAST, we used the Jaccard similarity analysis, a qualitative or incidence index that gauges the similarity between the provinces evaluated. This analysis is based on incidence data and does not include shared absences, as it considers that comparing the absence of a species in two different areas does not provide relevant information (Flores-Contreras \u0026amp; Luna-Reyes, \u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e2017\u003c/span\u003e). The index values range from 0, when no shared species exist between provinces and age intervals, to 1, when stations have the same species composition. Similarity values were grouped by the UPGMA (unweighted pair-group method using arithmetic averages) (Reyes \u0026amp; Torres-Florez, \u003cspan citationid=\"CR57\" class=\"CitationRef\"\u003e2009\u003c/span\u003e; Flores-Contreras \u0026amp; Luna-Reyes, \u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e2017\u003c/span\u003e).\u003c/p\u003e \u003c/div\u003e"},{"header":"Results","content":"\u003cp\u003eWe reviewed 73 faunal lists for the study region, with records from 1915 to 2023. From these lists, 1,479 species, 557 genera, 188 families, 35 orders, 1,625 localities, and 12,626 occurrences were obtained (Figs.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e, \u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e). We recorded taxonomic data, locality(s), depositional environments, and age intervals (Ypresian, Lutetian, Bartonian, and Priabonian) for each genus and/or species. Likewise, we separated the taxa into four paleobiogeographic provinces based on the studies of Piccolli \u0026amp; Savazzi, 1983; Oleinik, \u003cspan citationid=\"CR47\" class=\"CitationRef\"\u003e2001\u003c/span\u003e; Squires, \u003cspan citationid=\"CR70\" class=\"CitationRef\"\u003e2003\u003c/span\u003e; and Das \u0026amp; Halder, \u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e2018\u003c/span\u003e as follows:\u003c/p\u003e \u003cp\u003e \u003cul\u003e \u003cli\u003e \u003cp\u003eNortheastern Pacific Ocean Province (NEPP): Washington, Oregon, California, USA; and Baja California Sur, Mexico.\u003c/p\u003e \u003c/li\u003e \u003cli\u003e \u003cp\u003eSoutheastern Pacific Ocean Province (SEPP): Peru.\u003c/p\u003e \u003c/li\u003e \u003cli\u003e \u003cp\u003eCaribbean Province (CP or Caribbean): Chiapas, Mexico, and Jamaica.\u003c/p\u003e \u003c/li\u003e \u003cli\u003e \u003cp\u003eGulf Province (GP or Gulf): Texas, Louisiana, Mississippi, Alabama and Florida, USA; and Nuevo Leon and Tamaulipas, Mexico.\u003c/p\u003e \u003c/li\u003e \u003c/ul\u003e \u003c/p\u003e \u003cp\u003eIt is essential to mention that the four provinces were limited by age interval; for example, the Gulf Province was divided into four: GP-Ypresian, GP-Lutetian, GP-Bartonian, and GP-Priabonian. Therefore, we correlated 22 provinces for the similarity analysis, of which 15 (except CP-Bartonian, no data) presented a well-defined age interval (Ypresian, Lutetian, Bartonian, and Priabonian) (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e, bold and underlined letters). In contrast, seven provinces showed an extended age interval; for example, GP-Ypresian-Lutetian, GP-Lutetian-Priabonian, etc. (see Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e, single letters). We used the 22 provinces for the analysis to ensure greater data representativeness. In the similarity analysis, we obtained a dendrogram from which five groups could be defined (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e). Also, we described in detail the percentages of species per class (Tab. I; Figs.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e, \u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e7\u003c/span\u003e) and the number of formations and localities per province and age range (Tab. I).\u003c/p\u003e \u003cp\u003e\u003cstrong\u003eTable I.\u0026nbsp;\u003c/strong\u003eOverview of the five defined clusters. Each cluster indicates the number of formations per province and stage. Furthermore, the percentage of species per class, province, and stage. Provinces: northeastern Pacific Ocean Province (NEPP), southeastern Pacific Ocean Province (SEPP), Gulf Province (Gulf of GP), and Caribbean Province (Caribbean or CP). Stages: Ypr, Ypresian; Lut, Lutetian; Bar, Bartonian; and Pri, Priabonian. Fms: Formations. Class: Bi, Bivalvia; Ce: Cephalopoda; Ga: Gastropoda; and Sca: Scaphopoda.\u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"No\" id=\"Taba\" border=\"1\"\u003e \u003ccolgroup cols=\"4\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\"\u003e \u003cp\u003eCluster\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003eProvinces\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003eNo. of Fms.\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c4\"\u003e \u003cp\u003e% of species per class\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\" morerows=\"1\" rowspan=\"2\"\u003e \u003cp\u003e1\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eGP-Pri\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e6\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003eBi: 23.7; Ce: 1.2; Ga: 73; \u0026amp; Sca: 2.4\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eGP-Lut\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e9\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003eBi: 27.8; Ce: 1.6; Ga: 69.3; \u0026amp; Sca: 1.3\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\" morerows=\"3\" rowspan=\"4\"\u003e \u003cp\u003e2\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eNEPP-Pri\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e10\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003eBi: 39.9; Ce: \u0026lt;1; Ga: 57.8; \u0026amp; Sca: 1.4\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eNEPP-Bar\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e5\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003eBi: 37.5; Ce: 1.6; Ga: 59.3; \u0026amp; Sca: 1.6\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eNEPP-Lut\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e12\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003eBi: 38.7; Ce: 1.8; Ga: 57.7; \u0026amp; Sca: 1.8\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eNEPP-Ypr\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e19\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003eBi: 34.8; Ce: \u0026lt;1; Ga: 63.1; \u0026amp; Sca: 1.6\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\" morerows=\"2\" rowspan=\"3\"\u003e \u003cp\u003e3\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eGP-Bar\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e9\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003eBi: 22.9; Ce: 1.1; Ga: 74.9; \u0026amp; Sca: 1.1\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eCP-Lut\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e2\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003eGa: 100\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eGP-Ypr\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e7\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003eBi: 26.5; Ce: \u0026lt;2; Ga: 69.8; \u0026amp; Sca: 1.7\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\" morerows=\"1\" rowspan=\"2\"\u003e \u003cp\u003e4\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eSEPP-Pri\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e3\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003eBi: 30.2; Ce: 2.9; \u0026amp; Ga: 67.1\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eSEPP-Bar\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e4\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003eBi: 30.9; Ce: 2.8; Ga: 64.8; \u0026amp; Sca: 1.4\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\" morerows=\"1\" rowspan=\"2\"\u003e \u003cp\u003e5\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eSEPP-Lut\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e3\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003eBi: 42.9; Ce: 14.3; \u0026amp; Ga: 42.9\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eSEPP-Ypr\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e2\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003eBi: 37.5; Ce: 12.5; \u0026amp; Ga: 50\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003e \u003c/p\u003e"},{"header":"Discussion","content":"\u003cp\u003eThe dendrogram (read from right to left, with a similarity level of 0.5) allowed us to identify and infer what the faunal exchange was like during the epoch and how global and regional geological, environmental and climatic events caused changes in the distribution of the species (Morales-Ortega et al., \u003cspan citationid=\"CR41\" class=\"CitationRef\"\u003e2015\u003c/span\u003e, \u003cspan citationid=\"CR42\" class=\"CitationRef\"\u003e2016\u003c/span\u003e; Das \u0026amp; Halder, \u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e2018\u003c/span\u003e). According to the reading, we discuss the clusters from right to left, from group number five to one (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e).\u003c/p\u003e \u003cp\u003e \u003cb\u003eSoutheastern Pacific Ocean Province, Ypresian and Lutetian (cluster five).\u003c/b\u003e \u003c/p\u003e \u003cp\u003eGroup five is composed of the SEPP-Ypresian and SEPP-Lutetian-Bartonian. Despite the level of similarity excludes SEPP-Lutetian and SEPP-Ypresian-Lutetian, the latter age intervals are related between them. Thus, we propose that SEPP-Ypresian and SEPP-Lutetian were correlated (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e). In Peru (the only country included in the SEPP), the Paleogene was widely distributed, although in isolated basins controlled by megaregional geological features ('Andean magmatic arc') and regional faults and suture zones (Monges, \u003cspan citationid=\"CR39\" class=\"CitationRef\"\u003e1995\u003c/span\u003e). It is recognised that during the Peruvian Cenozoic, prolonged floods (transgressions) resulted in bioclastic and silty sandstone deposits (DeVries, \u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e2019\u003c/span\u003e).\u003c/p\u003e \u003cp\u003eSpecifically, we report the Caballas and Negritos Formations with an age range of the Ypresian; and the Pale Greda, Paracas and Caballas Formations for the Lutetian (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e). In addition, we reported the genera \u003cem\u003eOstrea\u003c/em\u003e, \u003cem\u003ePotamides\u003c/em\u003e, \u003cem\u003eTurritella\u003c/em\u003e and \u003cem\u003eClavilithes\u003c/em\u003e, at least for the Caballas Formation, which marks a transition from fluvial and alluvial plain environments to marine paleoenvironments. In this sense, the presence of oysters and potamids is consistent with the interpretation of a brackish water environment and the deposition environments reported in our study, which are summarised in coastal habitats such as plains, estuaries and lagoons (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e) (DeVries, \u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e2017\u003c/span\u003e; \u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e2019\u003c/span\u003e).\u003c/p\u003e \u003cp\u003eHowever, DeVries (\u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e2019\u003c/span\u003e) mentions that the majority of mollusc species from the Caballas Formation exhibit a pattern of mixed affinities with faunas from the Early Eocene and Late Cretaceous in northern Peru, which is the reason why this fauna is considered to be of an Early Paleogene age. It is not easy for us to determine whether the faunal assemblages reported in our study are from the appointed time, given that we did not obtain sufficient data, as only eight genera were recorded, including eight species for the SEPP-Ypresian and seven genera with seven species from the SEPP-Lutetian (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e), and the age was not specified either (this should be done in future research). Therefore, we consider that these faunas are from the early Cenozoic era, as proposed by DeVries (\u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e2019\u003c/span\u003e).\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cdiv id=\"Sec8\" class=\"Section2\"\u003e \u003ch2\u003eSoutheastern Pacific Ocean Province, Bartonian and Priabonian (cluster four)\u003c/h2\u003e \u003cp\u003eMost of the continental masses detected at present were identifiable at the beginning of the Cenozoic (66 Ma). However, their relative positions and shapes changed until they reached their present geographical configuration (Ubilla et al., \u003cspan citationid=\"CR76\" class=\"CitationRef\"\u003e2009\u003c/span\u003e). The South American plate shifted both longitudinally, which allowed the expansion of the Atlantic and at the same time stimulated the 'Andean orogeny', and latitudinally, which led to the formation of the Caribbean Island arc and the opening of the Drake Passage (\u0026asymp;\u0026thinsp;45 Ma) (Ramos, \u003cspan citationid=\"CR56\" class=\"CitationRef\"\u003e2009\u003c/span\u003e; Ghiglione, \u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e2016\u003c/span\u003e).\u003c/p\u003e \u003cp\u003eIn northwest Peru, the Talara Basin contains one of South America's most extensive Paleogene sequences (\u0026asymp;\u0026thinsp;4,500 m). Towards the boundary of the Middle Eocene to Late Eocene, several sequences of marine origin have been recognized within the basin, such as the Talara Formation (Middle Eocene), recorded as a deep marine sequence, and the Sandstone-Talara (Middle Eocene), Verdun and Chira (Upper Eocene) Formations, with characteristics of deltaic systems and shallow marine sequences. Similarly, the Paracas Formation (Middle Eocene), south Peru (Ica), is recognized as a marine sequence (Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e5\u003c/span\u003e) (Monges, \u003cspan citationid=\"CR39\" class=\"CitationRef\"\u003e1995\u003c/span\u003e).\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eThe marine sedimentary sequences may be a consequence of the sea level variations that occurred during the Middle and Late Eocene, which are related to tectonic development, and also of a\u0026thinsp;\u0026asymp;\u0026thinsp;40 m rise in sea level under near ice-free conditions during the MECO (\u0026asymp;\u0026thinsp;40.1 Ma) and a fall of the sea level of \u0026asymp;\u0026thinsp;20 m (39.5 Ma) (Monges, \u003cspan citationid=\"CR39\" class=\"CitationRef\"\u003e1995\u003c/span\u003e; Bohaty \u0026amp; Zachos, \u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e2003\u003c/span\u003e; Miller et al., \u003cspan citationid=\"CR36\" class=\"CitationRef\"\u003e2020\u003c/span\u003e). In addition, a drop in sea level during the Late Eocene (\u0026asymp;\u0026thinsp;40 m) has also been recorded, rising again in almost ice-free conditions (\u0026asymp;\u0026thinsp;35 Ma) (sea-level data from Ocean Drilling Program -ODP- sites 1209 -Western Pacific- and 1218 -Eastern Pacific-) (Miller et al., \u003cspan citationid=\"CR36\" class=\"CitationRef\"\u003e2020\u003c/span\u003e, p. 6). All of these variations may have favoured transition and coastal plains, the inner and outer shelves, and deep environments (deep subtidal and submarine fans), allowing the presence of molluscs (Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e5\u003c/span\u003e).\u003c/p\u003e \u003cp\u003eMoreover, it is known that the Tethys currents allowed faunal exchange between the Atlantic and Pacific oceans. The movement of the South American plate towards the north may have favoured the \u0026ldquo;vicinity\u0026rdquo; of species from the Caribbean and Gulf provinces (see Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e). With the dendrogram obtained in our study, we confirm the hypothesis that the surface currents of the Tethys Ocean allowed a faunal exchange across the Atlantic, as this ocean was narrower in the past, and faunas represent a constant process in the equatorial area (Piccoli \u0026amp; Savazzi, \u003cspan citationid=\"CR55\" class=\"CitationRef\"\u003e1983\u003c/span\u003e; Morales-Ortega et al., \u003cspan citationid=\"CR41\" class=\"CitationRef\"\u003e2015\u003c/span\u003e). Das \u0026amp; Halder (\u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e2018\u003c/span\u003e) stated that the western Atlantic provinces, \u003cem\u003ee.g.\u003c/em\u003e the Caribbean Northern South American Province (Colombia, Peru, Venezuela, Jamaica, Trinidad \u0026amp; Tobago, Panama, and Haiti) and the Gulf Coastal Province (USA: Texas, Arkansas, Alabama, Tennessee, Louisiana, Mississippi, Georgia, Florida, North Carolina, South Carolina, Virginia, Maryland, New Jersey) exhibit a remarkable similarity in generic composition throughout the Eocene. Coincidingly, our work proposes that SEPP and GP depict a similar relationship. However, it is worth noting that this affinity was stronger during the Bartonian and Priabonian periods in Peru. Likewise, the Peruvian faunas reported for the Middle Eocene (SEPP-Bartonian) and Late Eocene (SEPP-Priabonian) are related to the tropical and subtropical belts of Tethys (Tethys Realm) (Piccoli \u0026amp; Savazzi, \u003cspan citationid=\"CR55\" class=\"CitationRef\"\u003e1983\u003c/span\u003e; Lozouet, \u003cspan citationid=\"CR33\" class=\"CitationRef\"\u003e2014\u003c/span\u003e; Das \u0026amp; Halder, \u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e2018\u003c/span\u003e; DeVries, \u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e2019\u003c/span\u003e). This is confirmed by the number of genera and species of molluscs reported in our study (Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e5\u003c/span\u003e), which mainly represent shallow water environments and warm to subtropical seas typical of Tethys (Das \u0026amp; Halder, \u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e2018\u003c/span\u003e).\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003e \u003cb\u003eGulf Province, Ypresian and Bartonian; and its proximity to the Caribbean Province during the Lutetian (cluster three)\u003c/b\u003e \u003c/p\u003e \u003cp\u003eThe Gulf Province (GP or Gulf) and the Caribbean Province (CP or Caribbean) are known for their high interprovincial similarity (Das \u0026amp; Halder, \u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e2018\u003c/span\u003e). This similarity reflects their geographic proximity, and the genera and species reported in our work are known to exhibit faunal similarities with those of the Mediterranean and Central Europe. These results indicate and reinforce the idea that the transatlantic Tethys current drove the planktonic larval transport (Das \u0026amp; Halder, \u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e2018\u003c/span\u003e). At present, major surface currents are known to transport marine fauna from the eastern Atlantic westward across the mid-Atlantic, developing some resemblance between the faunas of Western Europe and Eastern America (Silva et al., \u003cspan citationid=\"CR63\" class=\"CitationRef\"\u003e2014\u003c/span\u003e; Cox, 2020). Some genera reflecting the wide dispersal during the Eocene are the bivalve \u003cem\u003eVenericor\u003c/em\u003e; the gastropods \u003cem\u003eCornulina\u003c/em\u003e, \u003cem\u003eRaphitoma\u003c/em\u003e, \u003cem\u003eHastula\u003c/em\u003e, \u003cem\u003eTurricula\u003c/em\u003e, \u003cem\u003eCoronia\u003c/em\u003e, \u003cem\u003eEopleurotoma\u003c/em\u003e and \u003cem\u003eSurculites\u003c/em\u003e; and the cuttlefish \u003cem\u003eBelosaepia\u003c/em\u003e of the family Belosaepiidae (Paleontology Database, \u003cspan citationid=\"CR48\" class=\"CitationRef\"\u003e2024\u003c/span\u003e).\u003c/p\u003e \u003cp\u003eThe environments recorded in our study are mostly shallow; the lithostratigraphic formations contain calcareous sands, marls, silts, and clays, both lithified and non-lithified (Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e6\u003c/span\u003e). These fine-grained deposits that accumulate considerably slowly contain organic materials such as phosphate and glauconite (minerals deposited in a shallow marine sedimentary environment), which are related to transgressions (maximum flooding surfaces) (Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e6\u003c/span\u003e) (Garrison, \u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e2009\u003c/span\u003e). In this sense, global sea-level changes estimated during the Early and Middle Eocene could have favoured this type of shallow environment. Additionally, the GP is located in a passive margin, allowing a slow sedimentary deposition (Garrison, \u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e2009\u003c/span\u003e; Miller et al., \u003cspan citationid=\"CR36\" class=\"CitationRef\"\u003e2020\u003c/span\u003e; Zhao et al., \u003cspan citationid=\"CR82\" class=\"CitationRef\"\u003e2022\u003c/span\u003e).\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eThe assemblages of molluscs and sedimentary sequences reported in our work are some of the consequences of changes in sea level, which allowed the dispersal of species and occupation of new niches during the Ypresian and Bartonian in the GP and the Lutetian in the CP (Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e6\u003c/span\u003e). However, it is interesting to note that the fauna is related to the hyperthermal events of the Ypresian (EECO) and the Bartonian (MECO), especially in the GP (Lunt et al., \u003cspan citationid=\"CR34\" class=\"CitationRef\"\u003e2011\u003c/span\u003e; Thomas et al., \u003cspan citationid=\"CR75\" class=\"CitationRef\"\u003e2018\u003c/span\u003e; Foster et al., \u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e2020\u003c/span\u003e).\u003c/p\u003e \u003cp\u003eSuperimposed on the long-term warming events (estimated increase of \u0026asymp;\u0026thinsp;6\u0026deg;C on the Earth's surface), EECO and MECO are short-duration intervals recognized as hyperthermal. One hyperthermal event is the PETM (marking the beginning of the Eocene), and the other two hyperthermal events are known as ETM2 (\u0026asymp;\u0026thinsp;54 Ma) and ETM3 (\u0026asymp;\u0026thinsp;53 Ma) during the Early Eocene. These hyperthermals are characterised by injections of carbon into the atmosphere, ocean acidification and variable increases in sea surface temperature (Zachos et al., \u003cspan citationid=\"CR81\" class=\"CitationRef\"\u003e2008\u003c/span\u003e; Lunt et al., \u003cspan citationid=\"CR34\" class=\"CitationRef\"\u003e2011\u003c/span\u003e; Foster et al., \u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e2020\u003c/span\u003e). Foster and collaborators (2020) mentioned that the environmental changes associated with hyperthermal events were not harmful to shallow-water mollusc faunas despite increased temperatures, acidification, and deoxygenation. Instead, the authors inferred that these hyperthermal events produced slight changes, which could have been reversible and short-lived.\u003c/p\u003e \u003cp\u003eIn our study, we reported 117 genera and 171 species for the PG-Ypresian, and 179 genera and 390 species for the GP-Bartonian (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e). The highest mean annual temperatures for the entire Cenozoic Era have been recorded in both age ranges (Pearson \u0026amp; Palmer, \u003cspan citationid=\"CR53\" class=\"CitationRef\"\u003e2000\u003c/span\u003e). However, the taxonomic richness was not \"affected\" by the hyperthermal events, especially during the Bartonian, which could be considered as the maximum marine biodiversity stage of the Gulf Coastal Plain, in contrast with the maximum marine biodiversity reported for the Paris Basin, which occurred in the Lutetian (Huyghe et al., \u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e2012\u003c/span\u003e). Despite the CP-Lutetian (proposed in our work) could be considered the maximum marine biodiversity of molluscs (19 genera and 22 species, see Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e), more taxonomic data and the inclusion of other Caribbean countries are needed in future research as we only recorded data from the sedimentary sequences of Chiapas, Mexico and Saint James, Jamaica (Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e6\u003c/span\u003e).\u003c/p\u003e \u003cp\u003eSimilarly, regarding GP, we attribute the GP-Bartonian to be the maximum marine biodiversity of molluscs, although it is worth noting that the Early Eocene hyperthermal events (Ypresian) did not observe \u0026ldquo;negative effects\u0026rdquo; on the reported fauna. Instead, the molluscs resisted the hostile conditions (mainly of the PETM), which allowed unoccupied areas to be recolonised once the environmental conditions improved (Foster et al., \u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e2020\u003c/span\u003e). Geochemical studies have suggested that during the PETM (similar to ETM2 and ETM3 but at different intensities), there was a reduction in calcite compensation depth (CCD) and ocean surface acidification (Foster et al., \u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e2020\u003c/span\u003e). In this sense, the lysocline reduction could explain the extinction of deep-sea benthic foraminifera (Molina, \u003cspan citationid=\"CR38\" class=\"CitationRef\"\u003e2007\u003c/span\u003e; Speijer et al., \u003cspan citationid=\"CR66\" class=\"CitationRef\"\u003e2012\u003c/span\u003e), while acidification would have harmed the metamorphosis, growth and survival of planktonic mollusc larvae (Talmage \u0026amp; Gobler, \u003cspan citationid=\"CR74\" class=\"CitationRef\"\u003e2009\u003c/span\u003e). However, the molluscs reported in our study for the GP-Ypresian mostly suggest a shallow water environment (Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e6\u003c/span\u003e). Thus, it is possible that these environments were not affected by CCD and acidification and that molluscs were able to cope with the acidification of the oceans by regulating the pH and the chemistry of carbonates in their habitats (Foster et al., \u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e2020\u003c/span\u003e).\u003c/p\u003e \u003cp\u003eAs mentioned earlier, after the hyperthermal events of the Early Eocene, the development of the epoch was led by a global cooling trend until the sudden warming of the Bartonian (MECO) (Boscolo-Galazzo et al., \u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e2014\u003c/span\u003e; Savian et al., \u003cspan citationid=\"CR60\" class=\"CitationRef\"\u003e2014\u003c/span\u003e). The MECO resembled its predecessor events from the Early Eocene, with increased atmospheric carbon dioxide (Bohaty et al., \u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2009\u003c/span\u003e). However, this hyperthermal did not impact the species richness of the GP-Bartonian; in fact, this stage is considered the maximum marine biodiversity of the province (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e). As with the Early Eocene, in the Bartonian (late Middle Eocene), the temperature rise, ocean acidification and deoxygenation were not detrimental, with which we persist in the idea that shallow marine environments \"did not undergo detrimental changes\" for the fauna (Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e6\u003c/span\u003e). Furthermore, a sea level rise of \u0026asymp;\u0026thinsp;40 m (\u0026asymp;\u0026thinsp;40.1 Ma) (Miller et al., \u003cspan citationid=\"CR36\" class=\"CitationRef\"\u003e2020\u003c/span\u003e) has been recorded during this period. This rise triggered transgressions (presence of glauconite) (Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e6\u003c/span\u003e) and may have favoured the development of shallow shelves, allowing remarkable species diversification without leading to long-term changes in faunal diversity or composition at the genus level (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e) (Huyghe et al., \u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e2012\u003c/span\u003e; Das \u0026amp; Halder, \u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e2018\u003c/span\u003e; Foster et al., \u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e2020\u003c/span\u003e). It is important to note the noticeable increase in GP-Bartonian species, which could be explained by species stratigraphic ranges being short and concomitantly exhibiting higher turnover rates across the age range, suggesting that Eocene hyperthermal events impacted benthic mollusc turnover rates at the species level, but not at the genus level (see Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e) (Foster et al., \u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e2020\u003c/span\u003e).\u003c/p\u003e \u003cp\u003eThe hyperthermal events during the Early and Middle Eocene did not have a \u0026ldquo;negative effect\u0026rdquo; on molluscan assemblages; instead, these benthic communities (genera) were warm-water tolerant and thus tolerated the high temperatures (Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e6\u003c/span\u003e). Since the Mesozoic greenhouse periods, the mollusc faunal assemblages of the Gulf Coastal Plain have been dominated by taxa that tolerate high temperatures. Indeed, most of these molluscs are associated with warm-temperate climates (Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e6\u003c/span\u003e) and tend to inhabit broader niches than tropical species, which explains the low impact of hyperthermal events (Foster et al., \u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e2020\u003c/span\u003e).\u003c/p\u003e \u003cp\u003eFinally, in the Middle Eocene, the biodiversity of marine molluscs significantly increased, particularly in the Paris Basin (middle Lutetian) and Gulf Province (Bartonian). The temperate conditions of the long-term relatively stable cooling trend during this period enabled the increase in marine mollusc biodiversity. In addition, the hyperthermal events allowed for greater thermal tolerance and plasticity among the genera (Huyghe et al., \u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e2012\u003c/span\u003e; Foster et al., \u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e2020\u003c/span\u003e). Future studies could address dated diversification and extinction rates of representative taxa of each biogeographic province under a phylogenetic context to further evaluate the biodiversity patterns of marine molluscs.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e\n\u003ch3\u003eNortheastern Pacific Ocean Province (cluster two)\u003c/h3\u003e\n\u003cp\u003eBased on the dendrogram obtained in our study, the NEEP showed the highest similarity (0.675). The clustering of the Caribbean-Priabonian in group two is further evidence of faunal exchange (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e). The Tethys Current provided a faunal migratory route between the Atlantic and the Indo-West Pacific through the 'Central American seaway'. The presence of taxa with Tethyan affinity reflects this connectivity, although this current was almost closed by the end of the Eocene (Oleinik, \u003cspan citationid=\"CR47\" class=\"CitationRef\"\u003e2001\u003c/span\u003e; Morales-Ortega et al., \u003cspan citationid=\"CR41\" class=\"CitationRef\"\u003e2015\u003c/span\u003e; Zhao et al., \u003cspan citationid=\"CR82\" class=\"CitationRef\"\u003e2022\u003c/span\u003e).\u003c/p\u003e \u003cp\u003eWith our findings, we can infer that the marine molluscs from this area exhibit a certain resemblance to the faunas reported from Western Europe and Eastern America, at least concerning the taxa reported from the Ypresian (mainly) and Lutetian of the NEPP (Silva et al., \u003cspan citationid=\"CR63\" class=\"CitationRef\"\u003e2014\u003c/span\u003e). This high interconnectivity is reflected with some bivalve genera such as \u003cem\u003eAnodontia\u003c/em\u003e, \u003cem\u003eChama\u003c/em\u003e, \u0026dagger;\u003cem\u003eCubitostrea\u003c/em\u003e, \u0026dagger;\u003cem\u003eGryphaeostrea\u003c/em\u003e, \u003cem\u003ePlicatula\u003c/em\u003e and \u003cem\u003eVaricorbula\u003c/em\u003e; the gastropod genera \u003cem\u003eAthleta\u003c/em\u003e, \u003cem\u003eBathytoma\u003c/em\u003e, \u003cem\u003eCampanile\u003c/em\u003e, \u0026dagger;\u003cem\u003eClavilithes\u003c/em\u003e, \u003cem\u003eConomitra\u003c/em\u003e, \u0026dagger;\u003cem\u003eCrommium\u003c/em\u003e, \u003cem\u003eCypraea\u003c/em\u003e, \u0026dagger;\u003cem\u003eCypraedia\u003c/em\u003e, \u0026dagger;\u003cem\u003eDomenginella\u003c/em\u003e, \u0026dagger;\u003cem\u003eEocithara\u003c/em\u003e, \u0026dagger;\u003cem\u003eEocypraea\u003c/em\u003e, \u0026dagger;\u003cem\u003eEosurcula\u003c/em\u003e, \u003cem\u003eEuspira\u003c/em\u003e, \u003cem\u003eGaleodea\u003c/em\u003e, \u003cem\u003eGemmula\u003c/em\u003e, \u003cem\u003eHarpa\u003c/em\u003e, \u0026dagger;\u003cem\u003eKeepingia\u003c/em\u003e, \u003cem\u003eLatirus\u003c/em\u003e, \u003cem\u003eMesalia\u003c/em\u003e, \u003cem\u003eMetula\u003c/em\u003e, \u003cem\u003eOliva\u003c/em\u003e, \u0026dagger;\u003cem\u003ePlatyoptera\u003c/em\u003e, \u0026dagger;\u003cem\u003ePleurofusia\u003c/em\u003e, \u003cem\u003ePuncturella\u003c/em\u003e, \u003cem\u003eRingicula\u003c/em\u003e, \u0026dagger;\u003cem\u003eStrepsidura\u003c/em\u003e, \u0026dagger;\u003cem\u003eSurculites\u003c/em\u003e, \u0026dagger;\u003cem\u003eTrypanotoma\u003c/em\u003e, \u003cem\u003eTurbonilla\u003c/em\u003e, \u0026dagger;\u003cem\u003eVelates\u003c/em\u003e and \u003cem\u003eXenophora\u003c/em\u003e; and the cephalopod genera \u0026dagger;\u003cem\u003eAturia\u003c/em\u003e, \u0026dagger;\u003cem\u003eEutrephoceras\u003c/em\u003e and \u003cem\u003eNautilus\u003c/em\u003e (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e7\u003c/span\u003e).\u003c/p\u003e \u003cp\u003eIn group two, the NEPP-Ypresian represents the maximum marine biodiversity of molluscs (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e7\u003c/span\u003e), contrary to what is reflected in the Paris Basin (middle Lutetian) (Huyghe et al., \u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e2012\u003c/span\u003e) and the Gulf Province (Bartonian) (our study). This maximum biodiversity (275 species, see Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e) could be explained by the more significant number of lithostratigraphic deposits and by preserving specimens, allowing species identification. However, the biodiversity in terms of genera is also the highest for the entire province (183 genera; see Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e), which supports the idea that the NEPP-Ypresian can be considered the maximum marine biodiversity (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e), regardless of the hyperthermal events from the Early Eocene.\u003c/p\u003e \u003cp\u003eThe NEPP is on an active continental margin (Ubilla et al., \u003cspan citationid=\"CR76\" class=\"CitationRef\"\u003e2009\u003c/span\u003e). Geological processes have dramatically impacted the western edge of North America since the Mesozoic era. The West Coast of the United States and northwestern Mexico coexisted on a convergent margin where the now-extinct Farallon tectonic plate sank beneath the North American plate, causing sudden changes in coastal topography (Ubilla et al., \u003cspan citationid=\"CR76\" class=\"CitationRef\"\u003e2009\u003c/span\u003e; Garrison, \u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e2009\u003c/span\u003e; Santiesteban-Mend\u0026iacute;vil, 2019). This coastal topographic variation of the NEPP caused by geological processes (subduction and orogeny) could have enabled the survival of molluscs. In this context, the coastline of an active margin is characterised by steep, narrow, and rocky coasts, which is one of the most biologically rich benthic habitats of the intertidal zone and accounts for high \u003cem\u003ein situ\u003c/em\u003e primary production (Satheesh \u0026amp; El-Sherbiny, \u003cspan citationid=\"CR59\" class=\"CitationRef\"\u003e2022\u003c/span\u003e). The environments recorded in our study for the NEPP during the Ypresian and Lutetian range from the coast (rocky coast, lagoon, estuary, bay, and delta) to deep environments (deep subtidal, submarine canyon, submarine fans, bathyal, and abyssal), typical of an active margin (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e7\u003c/span\u003e).\u003c/p\u003e \u003cp\u003eMolluscs are abundant in the intertidal zone, and their physiological adaptations allow them to survive extreme temperature, salinity, humidity and wave action changes. Thus, molluscs adapted to these conditions were able to resist the increase in sea surface temperatures and deoxygenation during the epoch. Furthermore, it is also possible that these organisms migrated latitudinally (high latitudes) and vertically (depth). In this sense, it has been noted that during the Eocene, the fauna thrived at the latitudinal range of 30\u0026deg; to 50\u0026deg; in both hemispheres, with which it is inferred that the tropical and subtropical belts were much broader than the ones we know at present. On the other hand, the vertical distribution of molluscs depends on their physiological tolerance to abiotic (\u003cem\u003ee.g\u003c/em\u003e., temperature, desiccation, and light) and biotic (\u003cem\u003ee.g.\u003c/em\u003e, predation and competition) factors. Most molluscs can move vertically, which may have enabled them to survive under less favourable conditions (Camacho, \u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e2008\u003c/span\u003e; Das \u0026amp; Halder, \u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e2018\u003c/span\u003e; Satheesh \u0026amp; El-Sherbiny, \u003cspan citationid=\"CR59\" class=\"CitationRef\"\u003e2022\u003c/span\u003e).\u003c/p\u003e \u003cp\u003eAnother factor supporting the idea that the Early Eocene hyperthermal events did not produce significant changes in mollusc diversification is body size. An increase in temperature within the system can influence the reduction in body size, promote parasitism and/or alter habitat preferences (Foster et al., \u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e2020\u003c/span\u003e). However, body size could not have been affected since we recorded large specimens, such as the gastropods \u003cem\u003eCampanile\u003c/em\u003e sp. (Morales-Ortega \u0026amp; Gonz\u0026aacute;lez-Barba, \u003cspan citationid=\"CR40\" class=\"CitationRef\"\u003e2018\u003c/span\u003e) and \u003cem\u003eGisortia\u003c/em\u003e sp. (Perrillat, 1996); the bivalves \u003cem\u003eCrassostrea contracta\u003c/em\u003e (Morales-Ortega et al., \u003cspan citationid=\"CR42\" class=\"CitationRef\"\u003e2016\u003c/span\u003e), \u003cem\u003eAcutostrea gaviota\u003c/em\u003e (Squires, \u003cspan citationid=\"CR71\" class=\"CitationRef\"\u003e2018\u003c/span\u003e), and \u003cem\u003ePhygraea stewarti\u003c/em\u003e (Squires, \u003cspan citationid=\"CR71\" class=\"CitationRef\"\u003e2018\u003c/span\u003e); and the cephalopod \u003cem\u003eNautilus sudcalifornianus\u003c/em\u003e (Morales-Ortega et al., \u003cspan citationid=\"CR43\" class=\"CitationRef\"\u003e2023\u003c/span\u003e). The sizes of these species resemble those recorded with Tethyan affinity (Squires \u0026amp; Demetrion, \u003cspan citationid=\"CR72\" class=\"CitationRef\"\u003e1992\u003c/span\u003e; Squires, \u003cspan citationid=\"CR69\" class=\"CitationRef\"\u003e1993\u003c/span\u003e; Morales-Ortega et al., \u003cspan citationid=\"CR42\" class=\"CitationRef\"\u003e2016\u003c/span\u003e).\u003c/p\u003e \u003cp\u003eDuring the Early Eocene, the NEPP-Ypresian and NEPP-Lutetian mollusc assemblages were unaffected by the hyperthermal events (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e7\u003c/span\u003e). The West Coast of the USA and the Baja California peninsula in Mexico were warm-temperate regions during this time (Squires, \u003cspan citationid=\"CR70\" class=\"CitationRef\"\u003e2003\u003c/span\u003e), where the sea levels rose significantly (Squires, \u003cspan citationid=\"CR71\" class=\"CitationRef\"\u003e2018\u003c/span\u003e). This combination of warm temperatures and rising sea levels enabled warm-water mollusc species from the Tethys province to spread to the aforementioned areas. This led to the highest point of Eocene faunal exchange among Eurasia, North Africa, and GP through the \"Central American seaway\" (Squires, \u003cspan citationid=\"CR68\" class=\"CitationRef\"\u003e1987\u003c/span\u003e; Squires \u0026amp; Demetrion, \u003cspan citationid=\"CR72\" class=\"CitationRef\"\u003e1992\u003c/span\u003e; Lozouet, \u003cspan citationid=\"CR33\" class=\"CitationRef\"\u003e2014\u003c/span\u003e; Morales-Ortega et al., \u003cspan citationid=\"CR41\" class=\"CitationRef\"\u003e2015\u003c/span\u003e, \u003cspan citationid=\"CR42\" class=\"CitationRef\"\u003e2016\u003c/span\u003e; Das \u0026amp; Halder, \u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e2018\u003c/span\u003e; Squires, \u003cspan citationid=\"CR71\" class=\"CitationRef\"\u003e2018\u003c/span\u003e).\u003c/p\u003e \u003cp\u003eOn the other hand, the NEPP-Bartonian exhibits a smaller number of genera (120) and species (182) than the GP-Bartonian (see Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e). Three factors can explain this decrease in NEPP-Bartonian records: (1) There could be a difference in sampling effort among sedimentary deposits. (2) There could be fewer sedimentary deposits within this age range than others. For example, in the Adelholzen section, Germany, the Lutetian and Priabonian stages are widely recorded, while the Bartonian is absent (Gebhardt et al., \u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e2013\u003c/span\u003e). (3) Environmental changes caused by the MECO could have affected the mollusc community, as its effects were not globally uniform (Bohaty et al., \u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2009\u003c/span\u003e). For example, low primary productivity has been reported at Site 1263 (Walvis Ridge, southeast Atlantic) due to increased surface water temperatures (\u0026asymp;\u0026thinsp;4\u0026deg;C), which possibly led to the starvation of heterotrophs (mainly planktonic foraminifera) (Boscolo-Galazzo et al., \u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e2014\u003c/span\u003e). A contrasting case was observed in Monte Cagnero (Umbria-Marche Basin), Italy, where an increased primary productivity was detected, probably stimulated by an increased aeolian supply to surface waters (Savian et al., \u003cspan citationid=\"CR60\" class=\"CitationRef\"\u003e2014\u003c/span\u003e). It is worth noting that an increase in nutrients (particularly nitrogen or phosphorus) can generate 'dead zones' (Dybas, \u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e2005\u003c/span\u003e; Foster et al., \u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e2020\u003c/span\u003e), which are significant drivers of ecological change in modern shallow-water marine ecosystems and perhaps one of the most critical drivers of benthic invertebrate loss. Changes in water circulation patterns, wind, high temperatures, an increase in runoff, and an excess of nutrients are the main determinant factors of 'dead zones' expansion and, concomitantly, the level of marine biodiversity. Currently, 'dead zones' are developing in regions of the Gulf Coastal Plain, surrounding the mouth of the Mississippi River, and coastal areas of the Northwestern Pacific (Dybas, \u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e2005\u003c/span\u003e; D\u0026iacute;az \u0026amp; Rosenberg, 2008; Gebhardt et al., \u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e2013\u003c/span\u003e; Sluijs et al., \u003cspan citationid=\"CR64\" class=\"CitationRef\"\u003e2014\u003c/span\u003e; Foster et al., \u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e2020\u003c/span\u003e; NOAA, 2023). Thus, the development of \u0026lsquo;dead zones' could explain the low presence of molluscs during the Bartonian of the NEPP (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e) (although our work does not account with data on the determinant factors mentioned earlier \u0026ndash;winds, runoff, etc.). 'Dead zones' could have been temporary, and benthic communities could have been able to rapidly recolonise habitats without significant renewal, which could explain the increase in genera and species during the Priabonian (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e) (Foster et al., \u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e2020\u003c/span\u003e).\u003c/p\u003e \u003cp\u003eTo a certain extent, the NEPP during the Priabonian \u0026ldquo;does not record\u0026rdquo; biodiversity variations, mainly within cooling events, compared to the GP-Priabonian, which shows a dramatic decline in genera and species (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e). NEPP-Priabonian records a slight increase in genera (139) and species (212) compared to NEPP-Bartonian (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e). As mentioned earlier, the Tethys Sea provided a migratory route; however, at the end of the Eocene, this sea became narrower and, concomitantly, reduced gene flow. For instance, molluscs began to diverge from the Middle Eocene (Zhao et al., \u003cspan citationid=\"CR82\" class=\"CitationRef\"\u003e2022\u003c/span\u003e). This is depicted by the presence of specific genera such as the bivalves \u003cem\u003eAdula\u003c/em\u003e, \u003cem\u003eBathymodiolus\u003c/em\u003e, \u003cem\u003eCalyptogena\u003c/em\u003e, \u0026dagger;\u003cem\u003eElongatolucina\u003c/em\u003e, \u003cem\u003eMaorithyas\u003c/em\u003e, and \u003cem\u003eTellidorella\u003c/em\u003e; the gastropods \u003cem\u003eDesbruyeresia\u003c/em\u003e, and \u003cem\u003eThalassonerita\u003c/em\u003e; and the scaphopod \u003cem\u003eCoccodentalium\u003c/em\u003e. These genera appear from the Lutetian in the NEPP (Paleontology Database, \u003cspan citationid=\"CR48\" class=\"CitationRef\"\u003e2024\u003c/span\u003e).\u003c/p\u003e \u003cp\u003eIn the same sense, it has been mentioned that the fauna of the North American Pacific could be differentiated into two biogeographic components. The first component could be defined by the presence of genera with Tethyan and Indo-Pacific Tethyan affinity (Tethys Realm), which can be characterised by the reported Early Eocene (NEPP-Ypresian) and Middle Eocene (NEPP-Lutetian) faunas (Figs.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e, \u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e7\u003c/span\u003e). The second component would be represented by the fauna reported from the northeastern Pacific during the Late Eocene (NEPP-Priabonian) (Figs.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e, \u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e7\u003c/span\u003e; Oleinik, \u003cspan citationid=\"CR47\" class=\"CitationRef\"\u003e2001\u003c/span\u003e; Hickman, \u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e2015\u003c/span\u003e).\u003c/p\u003e \u003cp\u003eThe climatic conditions during the Late Eocene were already sufficiently different from the previous Eocene stages to cause differences in the composition of mollusc faunas (Piccolli \u0026amp; Savazzi, 1983). The oceanic circulation of the North Pacific had a pattern similar to the modern oceanic circulation, with a subtropical anticyclonic gyre and a subarctic cyclonic gyre as its main characteristics. However, the latitudinal extent, circulation intensity, and local shallow water patterns are still unknown (Oleinick, 2001). We record genera from the high and middle latitudes during the Late Eocene, including the Australasian, northwestern and northeastern Pacific regions (Piccolli \u0026amp; Savazzi, 1983; Olenick, 2001; Hickman, \u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e2015\u003c/span\u003e). Some genera include the bivalves, \u003cem\u003eConchocele\u003c/em\u003e (Japan, Russia), \u003cem\u003eLucinoma\u003c/em\u003e (Russia), \u0026dagger;\u003cem\u003eNitidavenus\u003c/em\u003e (Russia), \u003cem\u003eNuttallia\u003c/em\u003e (Russia), \u0026dagger;\u003cem\u003ePseudocardium\u003c/em\u003e (Russia), \u003cem\u003eTivela\u003c/em\u003e (Canada, Russia), and \u0026dagger;\u003cem\u003eVertipecten\u003c/em\u003e (Canada); and the gastropods, \u003cem\u003eArgobuccinum\u003c/em\u003e (Antarctica), \u003cem\u003eBathybembix\u003c/em\u003e (Tonga), \u003cem\u003eCellana\u003c/em\u003e (Antarctica), \u003cem\u003eColus\u003c/em\u003e (Japan, Russia), \u003cem\u003eHomalopoma\u003c/em\u003e (Australia, Russia), \u0026dagger;\u003cem\u003eMolopopophorus\u003c/em\u003e (Russia, Canada), \u003cem\u003eNekewis\u003c/em\u003e (Japan, Russia), \u003cem\u003eNeptunea\u003c/em\u003e (Antarctica, Japan, Russia), \u0026dagger;\u003cem\u003eParasyrinx\u003c/em\u003e (Russia), and \u0026dagger;\u003cem\u003eTurrinosyrinx\u003c/em\u003e (Russia). Some of these taxa are extant genera in the North Pacific and, hence, are potentially relevant to the biogeographic reconstruction of the Pacific Basin (Oleinik, \u003cspan citationid=\"CR47\" class=\"CitationRef\"\u003e2001\u003c/span\u003e; Hickman, \u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e2015\u003c/span\u003e).\u003c/p\u003e \u003cp\u003eDuring the Middle to Late Eocene, there were shifts in the distribution of molluscs in the northeastern Pacific (including the CP-Priabonian fauna). These changes showed a combination of faunas with North Pacific and Tethys affinities, evidenced by tropical, subtropical, and temperate taxa (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e7\u003c/span\u003e; Oleinik, \u003cspan citationid=\"CR47\" class=\"CitationRef\"\u003e2001\u003c/span\u003e; Schweitzer et al., \u003cspan citationid=\"CR61\" class=\"CitationRef\"\u003e2006\u003c/span\u003e). It has been suggested that the reduction in the latitudinal range of genera with Tethys affinity was restricted to southern California during the Late Eocene. In contrast, genera related to the North Pacific (bivalves: \u003cem\u003eCardiomya\u003c/em\u003e, \u0026dagger;\u003cem\u003eCryptolucina\u003c/em\u003e, \u003cem\u003eCyclinella\u003c/em\u003e, \u0026dagger;\u003cem\u003eElongatolucina\u003c/em\u003e, \u003cem\u003eMactromeris\u003c/em\u003e, \u003cem\u003eModiolus\u003c/em\u003e, \u003cem\u003eMytilus\u003c/em\u003e, \u003cem\u003eThracia\u003c/em\u003e, and \u003cem\u003eYoldia\u003c/em\u003e; and gastropods: \u0026dagger;\u003cem\u003eParaseraphs\u003c/em\u003e, \u0026dagger;\u003cem\u003ePerse\u003c/em\u003e, and \u003cem\u003eRimella\u003c/em\u003e) and cosmopolitan genera (bivalves: \u003cem\u003eCostacallista\u003c/em\u003e, \u003cem\u003eFimbria\u003c/em\u003e, \u003cem\u003eLucina\u003c/em\u003e, \u003cem\u003eMacrocallista\u003c/em\u003e, \u003cem\u003eMiltha\u003c/em\u003e, \u003cem\u003eMyrtea\u003c/em\u003e, \u003cem\u003ePhacoides\u003c/em\u003e, \u003cem\u003ePitar\u003c/em\u003e, \u003cem\u003eTellina\u003c/em\u003e, \u0026dagger;\u003cem\u003eVenericardia\u003c/em\u003e, and \u0026dagger;\u003cem\u003eVenericor\u003c/em\u003e; and gastropods: \u003cem\u003eAcrilla\u003c/em\u003e, \u003cem\u003eActeon\u003c/em\u003e, \u003cem\u003eBonellitia\u003c/em\u003e, \u003cem\u003eCalyptraea\u003c/em\u003e, \u003cem\u003eConus\u003c/em\u003e, \u003cem\u003eCrepidula\u003c/em\u003e, \u003cem\u003eExilia\u003c/em\u003e, \u003cem\u003eFicus\u003c/em\u003e, \u003cem\u003eFusinus\u003c/em\u003e, \u003cem\u003eLyria\u003c/em\u003e, \u003cem\u003eMargarites\u003c/em\u003e, \u003cem\u003eOdostomia\u003c/em\u003e, \u003cem\u003eOlivella\u003c/em\u003e, \u003cem\u003ePolynices\u003c/em\u003e, \u003cem\u003eRetusa\u003c/em\u003e, \u003cem\u003eScaphander\u003c/em\u003e, \u003cem\u003eSiphonalia\u003c/em\u003e, \u003cem\u003eSolariella\u003c/em\u003e, \u003cem\u003eTurricula\u003c/em\u003e, and \u003cem\u003eTurritella\u003c/em\u003e) did not undergo significant changes in the latitudinal ranges. This pattern suggests that high-latitude faunas were \u0026ldquo;unaffected\u0026rdquo; by global climate changes (cooling), at least during the Late Eocene. Indeed, the ocean circulation patterns and climate changes limited the latitudinal migration of temperature-sensitive equatorial genera to high latitudes (Piccoli \u0026amp; Savazzi, \u003cspan citationid=\"CR55\" class=\"CitationRef\"\u003e1983\u003c/span\u003e; Oleinik, \u003cspan citationid=\"CR47\" class=\"CitationRef\"\u003e2001\u003c/span\u003e).\u003c/p\u003e \u003cp\u003eMoreover, at the end of the Eocene, there was an increase in heteroconch bivalve fauna, mainly in Oregon and Washington (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e7\u003c/span\u003e; Nesbitt, \u003cspan citationid=\"CR44\" class=\"CitationRef\"\u003e2003\u003c/span\u003e; Kiel, \u003cspan citationid=\"CR31\" class=\"CitationRef\"\u003e2006\u003c/span\u003e; Hickman, \u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e2015\u003c/span\u003e). The heteroconchs have successfully developed morphological and physiological adaptations to extreme environments (chemosynthetic, for example, 'cold-seep'). Hickman (\u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e2015\u003c/span\u003e) noted that the heteroconch bivalve fauna from Oregon records the climatic transition of the Late Eocene and Early Oligocene and a replacement of tropical taxa by the cryophilic taxa that currently dominate the high-latitude fauna of the North Pacific.\u003c/p\u003e \u003cp\u003eUndoubtedly, the Priabonian in the NEPP is of great interest for studying changes in the North Pacific regions. The mixture of fauna, the ability to tolerate extreme conditions, the closure of Tethys, the change in deep and surface ocean circulation, and the diversity of marine environments caused by a tectonically active margin could favour mollusc communities within the transition to the 'coolhouse' (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e7\u003c/span\u003e), compared to the decrease in fauna of the GP-Priabonian (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e) (Oleinik, \u003cspan citationid=\"CR47\" class=\"CitationRef\"\u003e2001\u003c/span\u003e; Schweitzer et al., \u003cspan citationid=\"CR61\" class=\"CitationRef\"\u003e2006\u003c/span\u003e).\u003c/p\u003e \u003cp\u003e \u003c/p\u003e\n\u003ch3\u003eGulf Province, Lutetian and Priabonian (cluster one)\u003c/h3\u003e\n\u003cp\u003eAfter the hyperthermal events of the Early Eocene, the beginning of the Middle Eocene was marked by a \u0026ldquo;stable\u0026rdquo; climate (Zachos et al., \u003cspan citationid=\"CR81\" class=\"CitationRef\"\u003e2008\u003c/span\u003e). Indeed, it has been pointed out as an interval of relative cooling, known as the \u0026ldquo;Lutetian cooling\u0026rdquo;, as indicated by the presence of temporary ice sheets in both hemispheres (Lear et al., \u003cspan citationid=\"CR32\" class=\"CitationRef\"\u003e2000\u003c/span\u003e; Zachos et al., \u003cspan citationid=\"CR81\" class=\"CitationRef\"\u003e2008\u003c/span\u003e; Payros et al., \u003cspan citationid=\"CR51\" class=\"CitationRef\"\u003e2009\u003c/span\u003e; Huyghe et al., \u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e2012\u003c/span\u003e). This \u0026ldquo;cold stage\u0026rdquo; could have influenced the mean annual sea surface temperatures, causing the formation of \u0026ldquo;temperate zones\u0026rdquo; (or less hot zones) that enabled the increase in marine biodiversity (Huyghe et al., \u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e2012\u003c/span\u003e). Similarly, a regional transgression (presence of glauconite, Fig.\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003e) could also favour the rise in the mollusc fauna during the GP-Lutetian. Transgressions can increase the benthic zone and concomitantly enhance the benthos' light, food and oxygen conditions (Huyghe et al., \u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e2012\u003c/span\u003e; Das \u0026amp; Halder, \u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e2018\u003c/span\u003e).\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eThe presence of genera similar to those found in tropical, subtropical, and temperate environments (bivalve: \u003cem\u003ePitar\u003c/em\u003e; gastropods: \u003cem\u003eAgaronia\u003c/em\u003e, \u003cem\u003eAncilla\u003c/em\u003e, \u003cem\u003eArchitectonica\u003c/em\u003e, \u0026dagger;\u003cem\u003eFicopsis\u003c/em\u003e, \u003cem\u003eHexaplex\u003c/em\u003e, \u003cem\u003eRaphitoma\u003c/em\u003e, \u003cem\u003eSiphonalia\u003c/em\u003e, \u0026dagger;\u003cem\u003eStellaxis\u003c/em\u003e, and \u0026dagger;\u003cem\u003eTerebrifusus\u003c/em\u003e; and scaphopod: \u003cem\u003eDentalium\u003c/em\u003e) suggests a faunal mix that was able to survive under changing conditions (Fig.\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003e). This could have allowed an increase in the GP-Lutetian genera and species (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e). For example, the Paris Basin was characterised by warm summers and relatively \u0026ldquo;cold\u0026rdquo; winters, where the \u0026ldquo;cooling\u0026rdquo; was not a negative factor in establishing the maximum marine biodiversity of the Parisian Lutetian (Huyghe et al., \u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e2012\u003c/span\u003e, p. 588).\u003c/p\u003e \u003cp\u003eThe dendrogram analysis in our study showed a connection between the Lutetian and Priabonian of the GP (see Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e), which could be attributed to the presence of genera and species related to cooler environmental conditions (Fig.\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003e). However, unlike the Lutetian, where conditions were favourable for the increase in fauna, there was a significant reduction in genera and species in the Priabonian. It has been estimated that the turnover of molluscs was over 90% (Hansen, \u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e1987\u003c/span\u003e; Ivany et al., \u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e2000\u003c/span\u003e; Hansen et al., \u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e2004\u003c/span\u003e). However, using the data obtained in our work, we estimated a 52% decrease in genera and a 74% decrease in species (see Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e). These results indicate that the environmental disturbances with a cooling trend from the late Middle Eocene to the Eocene-Oligocene transition (EOT) significantly impacted mollusc assemblages (Ivany et al., \u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e2003\u003c/span\u003e).\u003c/p\u003e \u003cp\u003eSome authors consider the Late Eocene a time of regional \"extinction\" waves (Hansen, \u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e1992\u003c/span\u003e; Ivany et al., \u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e2000\u003c/span\u003e; Hansen et al., \u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e2004\u003c/span\u003e). The \"loss\" of mollusc faunas (mainly species) is deemed to have occurred in a prolonged and transitional manner, with warm-water taxa being more affected than cold-water taxa, including the loss of endemic taxa adapted to high temperatures (Ivany et al., \u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e2003\u003c/span\u003e; Foster et al., \u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e2020\u003c/span\u003e). In addition, the 'coolhouse' transition may have influenced the migration of taxa, caused by the reduction of paleobiogeographic belts parallel to the Equator, which moved southwards in response to low temperatures. This could have caused warm-water species to migrate in an equatorward direction (Hansen et al., \u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e2004\u003c/span\u003e; Westerhold et al., \u003cspan citationid=\"CR78\" class=\"CitationRef\"\u003e2020\u003c/span\u003e). Consistently, the Late Eocene to EOT is not considered a global extinction event but rather a faunal \"rotation\" event (Dockery, \u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e1986\u003c/span\u003e; Dockery \u0026amp; Lozouet, \u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e2003\u003c/span\u003e; Hansen et al., \u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e2004\u003c/span\u003e).\u003c/p\u003e \u003cp\u003eA decrease in sea level can affect habitat diversity, and circulation patterns can force a biogeographic change in taxa. While both factors could cause a decrease in taxa, the decrease in temperature is the most notable factor in faunal change (Hansen, \u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e1987\u003c/span\u003e; Ivany et al., \u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e2000\u003c/span\u003e; Hansen et al., \u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e2004\u003c/span\u003e; Das \u0026amp; Halder, \u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e2018\u003c/span\u003e). The different responses of molluscs to cooling and warming events highlight the relationships between evolutionary history, ecological structure and environmental change. Thus, a significant change in environmental state (\u0026lsquo;cold or hot\u0026rsquo;) will \u0026ldquo;affect\u0026rdquo; biodiversity; however, its effects (\u0026lsquo;positive or negative\u0026rsquo;) will depend on the initial state of the system (Ivany et al., \u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e2003\u003c/span\u003e; Foster et al., \u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e2020\u003c/span\u003e).\u003c/p\u003e \u003cp\u003e \u003c/p\u003e"},{"header":"Conclusions","content":"\u003cp\u003eUsing the dendrogram, we could distinguish the level of similarity and shared features of the taxa based on province and age range, resulting in five distinct groups. However, this does not imply significant differences between the provinces. The dendrogram enabled us to elucidate how environmental changes specific to each region and the global climate from the Epoch impacted the molluscan assemblages in each province.\u003c/p\u003e \u003cp\u003eUndoubtedly, all the paleobiogeographic provinces proposed in our work showed evidence that the distribution of molluscs was strongly influenced by Tethys (Tethys Realm). However, the extensive faunal exchange was reflected mainly in the provinces of the northern hemisphere (NEPP, GP and CP), certainly due to their geographical proximity. At the same time, the 'Central American seaway' allowed this exchange. The extensive record of taxa with Tethyan affinity in the American continent can be comparable with the Eurasian, North African and South American faunas. Thus, our work highlights the significant faunal exchange between the NEPP and the GP during the Ypresian and Lutetian.\u003c/p\u003e \u003cp\u003eMoreover, our results show that the maximum marine biodiversity stages coincide with hyperthermal events (EECO and MECO). The GP-Bartonian is considered the stage with the maximum biodiversity, which increases in species turnover rates could explain. For the NEPP, the maximum biodiversity was reached in the Ypresian, possibly due to the extensive faunal exchange from the Tethys towards the Pacific coast of North America.\u003c/p\u003e \u003cp\u003eThe Priabonian in the GP and NEPP provinces showed significant differences. The cooling (the most conspicuous factor) in the GP wreaked havoc on the mollusc fauna, and a considerable decrease was observed in the record of genera and species. The NEPP reflects a discrete increase in the Bartonian, which could be explained by the flow of the North Pacific currents, causing a greater faunal exchange with the North Western Pacific. However, the dynamics of the currents in the Pacific basin are still unknown.\u003c/p\u003e \u003cp\u003eIn contrast, the SEPP reflects a geographical \u0026ldquo;remoteness\u0026rdquo;, at least during the Ypresian and Lutetian stages, and few records indicate a closer relationship with faunas from the Early Paleogene of Peru during these stages. To a certain extent, this province was related to the northern hemisphere regions during the Bartonian and Priabonian. This could be explained by the displacement of the South American plate towards the north and the persistent Tethyan faunal affinity.\u003c/p\u003e \u003cp\u003eFinally, we recognise that Tethys strongly influences the provincial relationships of molluscs. However, the regional environmental changes produced by the progressive and abrupt events in the global geological and climatic conditions during the epoch had a different impact on each province. Likewise, we reinforce several ideas proposed by other authors and include new regionalised and provincial information. It is essential to mention that we need to record more faunal data (adding records from different countries) and include regional environmental data (geochemistry, specify age, strengthen lithostratigraphic data, regional tectonic data and changes in regional sea level) to improve and reinforce the conclusions achieved in our work.\u003c/p\u003e"},{"header":"Abbreviations","content":"\u003cdiv class=\"DefinitionList\"\u003e \u003cdiv class=\"DefinitionListEntry\"\u003e \u003cdiv class=\"Term\"\u003ePETM\u003c/div\u003e \u003cdiv class=\"Description\"\u003e \u003cp\u003ePaleocene\u0026ndash;Eocene Thermal Maximum\u003c/p\u003e \u003c/div\u003e \u003c/div\u003e \u003cdiv class=\"DefinitionListEntry\"\u003e \u003cdiv class=\"Term\"\u003eETM2\u003c/div\u003e \u003cdiv class=\"Description\"\u003e \u003cp\u003eEocene Thermal Maximum 2\u003c/p\u003e \u003c/div\u003e \u003c/div\u003e \u003cdiv class=\"DefinitionListEntry\"\u003e \u003cdiv class=\"Term\"\u003eETM3\u003c/div\u003e \u003cdiv class=\"Description\"\u003e \u003cp\u003eEocene Thermal Maximum 3\u003c/p\u003e \u003c/div\u003e \u003c/div\u003e \u003cdiv class=\"DefinitionListEntry\"\u003e \u003cdiv class=\"Term\"\u003eMECO\u003c/div\u003e \u003cdiv class=\"Description\"\u003e \u003cp\u003eMiddle Eocene Climatic Optimum\u003c/p\u003e \u003c/div\u003e \u003c/div\u003e \u003cdiv class=\"DefinitionListEntry\"\u003e \u003cdiv class=\"Term\"\u003eEOT\u003c/div\u003e \u003cdiv class=\"Description\"\u003e \u003cp\u003eEocene\u0026ndash;Oligocene Transition\u003c/p\u003e \u003c/div\u003e \u003c/div\u003e \u003cdiv class=\"DefinitionListEntry\"\u003e \u003cdiv class=\"Term\"\u003eMa\u003c/div\u003e \u003cdiv class=\"Description\"\u003e \u003cp\u003eMillion years ago\u003c/p\u003e \u003c/div\u003e \u003c/div\u003e \u003cdiv class=\"DefinitionListEntry\"\u003e \u003cdiv class=\"Term\"\u003eUSA\u003c/div\u003e \u003cdiv class=\"Description\"\u003e \u003cp\u003eUnited States of America\u003c/p\u003e \u003c/div\u003e \u003c/div\u003e \u003cdiv class=\"DefinitionListEntry\"\u003e \u003cdiv class=\"Term\"\u003eNGMDB\u003c/div\u003e \u003cdiv class=\"Description\"\u003e \u003cp\u003eNational Geologic Map Database Project\u003c/p\u003e \u003c/div\u003e \u003c/div\u003e \u003cdiv class=\"DefinitionListEntry\"\u003e \u003cdiv class=\"Term\"\u003eUSGS\u003c/div\u003e \u003cdiv class=\"Description\"\u003e \u003cp\u003eUnited States Geological Survey\u003c/p\u003e \u003c/div\u003e \u003c/div\u003e \u003cdiv class=\"DefinitionListEntry\"\u003e \u003cdiv class=\"Term\"\u003eAASG\u003c/div\u003e \u003cdiv class=\"Description\"\u003e \u003cp\u003eAssociation of American State Geologists\u003c/p\u003e \u003c/div\u003e \u003c/div\u003e \u003cdiv class=\"DefinitionListEntry\"\u003e \u003cdiv class=\"Term\"\u003eUPGMA\u003c/div\u003e \u003cdiv class=\"Description\"\u003e \u003cp\u003eUnweighted pair\u0026ndash;group method using arithmetic averages\u003c/p\u003e \u003c/div\u003e \u003c/div\u003e \u003cdiv class=\"DefinitionListEntry\"\u003e \u003cdiv class=\"Term\"\u003eNEPP\u003c/div\u003e \u003cdiv class=\"Description\"\u003e \u003cp\u003eNortheastern Pacific Ocean Province\u003c/p\u003e \u003c/div\u003e \u003c/div\u003e \u003cdiv class=\"DefinitionListEntry\"\u003e \u003cdiv class=\"Term\"\u003eSEPP\u003c/div\u003e \u003cdiv class=\"Description\"\u003e \u003cp\u003eSoutheastern Pacific Ocean Province\u003c/p\u003e \u003c/div\u003e \u003c/div\u003e \u003cdiv class=\"DefinitionListEntry\"\u003e \u003cdiv class=\"Term\"\u003eCP\u003c/div\u003e \u003cdiv class=\"Description\"\u003e \u003cp\u003eCaribbean Province\u003c/p\u003e \u003c/div\u003e \u003c/div\u003e \u003cdiv class=\"DefinitionListEntry\"\u003e \u003cdiv class=\"Term\"\u003eGP\u003c/div\u003e \u003cdiv class=\"Description\"\u003e \u003cp\u003eGulf Province\u003c/p\u003e \u003c/div\u003e \u003c/div\u003e \u003cdiv class=\"DefinitionListEntry\"\u003e \u003cdiv class=\"Term\"\u003eODP\u003c/div\u003e \u003cdiv class=\"Description\"\u003e \u003cp\u003eOcean Drilling Program\u003c/p\u003e \u003c/div\u003e \u003c/div\u003e \u003cdiv class=\"DefinitionListEntry\"\u003e \u003cdiv class=\"Term\"\u003eCCP\u003c/div\u003e \u003cdiv class=\"Description\"\u003e \u003cp\u003eCalcite Compensation Depth\u003c/p\u003e \u003c/div\u003e \u003c/div\u003e \u003cdiv class=\"DefinitionListEntry\"\u003e \u003cdiv class=\"Term\"\u003eCONAHCYT\u003c/div\u003e \u003cdiv class=\"Description\"\u003e \u003cp\u003eConsejo Nacional de Humanidades, Ciencias y Tecnolog\u0026iacute;as\u003c/p\u003e \u003c/div\u003e \u003c/div\u003e \u003c/div\u003e"},{"header":"Declarations","content":"\u003cp\u003e\u003cstrong\u003eEthics declarations\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eEthics approval and consent to participate\u003c/p\u003e\n\u003cp\u003eThe authors declare that they have no competing interests.\u003c/p\u003e\n\u003cp\u003eCompeting interests\u003c/p\u003e\n\u003cp\u003eThe authors declare no competing interests.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAvailability of data and materials\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eAll fossil genera and species included in this study have been previously described and published. They are accessible in the referenced papers and databases, including localities (see the material and methods section for details).\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eFunding\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eNot applicable.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAuthors' contributions\u0026nbsp;\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003ePMO was responsible for analysing, writing the manuscript, and preparing the figures. GGB acted as the PhD thesis director.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAcknowledgements\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe authors are grateful for the valuable comments and suggestions made by the reviewers. Priscila Morales-Ortega acknowledges the grant number CVU 389635 from the Consejo Nacional de Humanidades, Ciencias y Tecnologías (CONAHCYT). We thank Dr. Hannes Löser for his valuable help with the PaleoTax software.\u0026nbsp;\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\n\u003cli\u003eBohaty, S.M., \u0026amp; Zachos, J.C. (2003). Significant Southern Ocean warming event in the late middle Eocene. \u003cem\u003eGeology, 31\u003c/em\u003e, 1017‒1020.\u003c/li\u003e\n\u003cli\u003eBohaty, S.M., Zachos, J.C., Florindo, F., \u0026amp; Delaney, M.L. (2009). Coupled greenhouse warming and deep-sea acidification in the middle Eocene. \u003cem\u003ePaleoceanography, 24\u003c/em\u003e, PA2207. http://dx.doi.org/10.1029/2008PA001676.\u003c/li\u003e\n\u003cli\u003eBoscolo-Galazzo, F., Thomas, E., \u0026amp; Giusberti, L. (2014). 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However, these phenomena were \u0026ldquo;interrupted\u0026rdquo; by progressive and abrupt events in the geological, environmental, and climatic global conditions during the Early Eocene (hyperthermal), Middle Eocene (cooling trend and sudden warming), and Late Eocene (\u0026lsquo;cooling\u0026rsquo;). In recent years, attempts have been made to correlate faunal associations of molluscs to understand regional paleoenvironmental changes and changes in the geographical redistribution of species. In this study, we correlate the faunistic assemblages of marine molluscs among four provinces: Northeastern Pacific Ocean Province (NEPP), Southeastern Pacific Ocean Province (SEPP), Caribbean Province (CP or Caribbean), and Gulf Province (GP or Gulf). The provinces of the northern hemisphere (NEPP, GP, and CP) reflected a closer interprovincial relationship. The Tethys current, at least during the Ypresian and Lutetian stages, allowed this faunal exchange, where the 'Central American seaway' was crucial. Moreover, our study shows that the maximum marine biodiversity of molluscs coincides with hyperthermal events. For the NEPP during the Ypresian (Early Eocene Climatic Optimum), while for the GP during the Bartonian (Middle Eocene Climatic Optimum). The Priabonian of the NEPP was a period of redistribution, possibly due to the dynamics of the surface currents of the northwestern Pacific. Meanwhile, cooling caused a significant reduction of taxa in the GP. Moreover, the SEPP during the Bartonian and Priabonian is correlated to the provinces of the northern hemisphere, which could be explained by the displacement of the South American plate towards the north and by the persistence of affinity of the Tethys fauna (Tethys Realm).\u003c/p\u003e","manuscriptTitle":"Effects of the global and regional climate events of the Eocene on the Pan-American provinces of marine molluscs","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2024-11-28 11:09:23","doi":"10.21203/rs.3.rs-5278171/v1","editorialEvents":[{"type":"communityComments","content":0}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true}}],"origin":"","ownerIdentity":"6407dd72-c703-4ab0-8556-3d781fd48ac8","owner":[],"postedDate":"November 28th, 2024","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"posted","subjectAreas":[],"tags":[],"updatedAt":"2024-12-15T16:08:29+00:00","versionOfRecord":[],"versionCreatedAt":"2024-11-28 11:09:23","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-5278171","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-5278171","identity":"rs-5278171","version":["v1"]},"buildId":"qtupq5eGEP_6zYnWcrvyt","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}