Fossil fish assemblage of the Laguna Formation, Philippines: Unveiling the uniqueness of Pleistocene freshwater ecosystems in Southeast Asia

Fossil fish assemblage of the Laguna Formation, Philippines: Unveiling the uniqueness of Pleistocene freshwater ecosystems in Southeast Asia | 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 Fossil fish assemblage of the Laguna Formation, Philippines: Unveiling the uniqueness of Pleistocene freshwater ecosystems in Southeast Asia Tomáš Přikryl, Abigael Castro, Allan Gil Fernando, Jaan Ruy Conrad Nogot, and 4 more This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-5055249/v1 This work is licensed under a CC BY 4.0 License Status: Published Journal Publication published 16 Jan, 2025 Read the published version in Swiss Journal of Palaeontology → Version 1 posted 9 You are reading this latest preprint version Abstract This study offers a comprehensive analysis and detailed description of the fossil fish assemblage from the Pleistocene Laguna Formation in Luzon Island, Philippines. The fish fossils were collected from the deeper lacustrine facies of the formation, and a total of three fish families were identified. The identification is based on the recognizable synapomorphies in 10 moderately preserved semi-articulated individuals of ray-finned fish specimens, some of which include counterparts. The assemblage is predominantly composed of small clupeiforms of the family Dorosomatidae, accompanied by a gobioid fish (Gobiidae or Oxudercidae) and a synbranchid specimen (Synbranchidae), each represented by a single specimen. This taphocoenosis preserves free-swimming dorosomatids and demersal gobioid and synbranchid, suggesting an autochthonous assemblage with minimal postmortem sorting. Despite all recognized taxa being tolerant to changes in salinity, the environment is inferred to have been freshwater, analogous to modern Taal Lake, where a freshwater sardine thrives. This study represents the first systematic description of freshwater fish fossils in the Philippines and marks the first discovery of these taxa in the tropical West Pacific region. Paleoichthyology Dorosomatidae Gobioidei Synbranchidae West Pacific Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Figure 7 Introduction The Philippines is recognized for its rich freshwater biodiversity, particularly its number of freshwater fish species (Capuli & Froese, 1997 ; Papa & Briones, 2017 ). According to Froese & Pauly ( 2024 ), approximately 362 freshwater fish species have been recorded in the Philippines, with ongoing efforts to describe newly documented species (Bestre et al., 2018 ; Maeda et al., 2021 ). The most recent comprehensive overview of the freshwater fishes of the Philippines by Jamandre ( 2023 ) revealed a total of 374 species. It offered a provisional glimpse into the diversity within this area, highlighting the remarkable progress made in studying various living groups. However, the evolutionary history and distribution of freshwater fish in the Philippines remain unexplored, representing a vast terra incognita in ichthyological research. The fossil record in the Philippines, particularly concerning fish, is notably sparse. Only a handful of fossil specimens have been described or mentioned in previous studies, with the most recent review by Mediodia et al. ( 2024 ) emphasizing the scarcity of these records. This limited evidence has provided only fragmented insights into the past biodiversity and evolutionary history of the region's freshwater ecosystems. The absence of freshwater fossil records leaves a significant gap in our understanding of how this ecosystem has changed over time, especially in response to climatic and geological events. The primary objective of this study is to document a collection of freshwater fish fossils from the Pleistocene Laguna Formation in Antipolo, Rizal Province. It represents the first systematic study of freshwater fish fossils in the Philippines and provides valuable data that can be used to compare with modern ecosystems. Finally, our results improve our understanding of the biodiversity and evolutionary history of Southeast Asia's freshwater ecosystems. Geology of central Luzon Basin The study area is underlain by the rock units of the Central Luzon Basin (CLB, Fig. 1 ). It is part of the 8,000 m-thick sedimentary sequence of the N-S trending Central Luzon Basin, which flanks western Luzon Island (Mines and Geosciences Bureau [MGB], 2010). The CLB is structurally controlled by the major branches of the northern segment of the Philippine Fault, specifically the Vigan-Aggao Fault (Pinet & Stephan, 1990 ), the Tuba Fault, the Pugo Fault, and the Coastal Thrust Fault (Rimando & Rimando, 2020 ). The CLB is subdivided into western and eastern sections. The western CLB is composed of Eocene to Pleistocene sediments, which directly overlie the Eocene Zambales Ophiolite Complex. The western CLB is dominated by marine-derived sediments with some tuffaceous clastic units (Tumanda, 1984 Lorentz, 1984 ; De Leon et al., 1998 ; MGB, 2010; Guballa & Fernando, 2015 ). The eastern CLB is composed of Late Cretaceous basalts and volcanic flows with metasedimentary units, Late Eocene to Early Oligocene andesitic and pyroclastic rocks with intercalations of deep marine sediments, and Lower Miocene to Pliocene marine sedimentary units with few tuffaceous clastic strata. The Pleistocene section is characterized by the thick conglomerate-sandstone-mudstone-tuff sequence of the Guadalupe Formation. Guadalupe Formation is equivalent to the Laguna Formation, further described as interbeds of tephra and pyroclastic flow units, lahars, lake deposits, and basaltic flows (Schoell et al., 1985 ). Geology and stratigraphy of the fossil locality The fossil site (N 14.617102, E 121.174509) is located in Antipolo City, Rizal Province (Fig. 1 ). The collection of the fossils was done in 2014 and 2015 during the construction of a housing project that exposed several facies of the Laguna Formation. No systematic collection of fossils was conducted due to time constraints, but a detailed stratigraphic log of the outcrops is available (Fig. 2 ). According to MGB (2010), the Laguna Formation is underlain by the Late Pliocene to Pleistocene Laguna Formation, a sequence of epiclastic sediments laid down in a terrestrial environment, consisting of tuffs deposited in alluvial setting, stream and lake deposits, and lahars (Schoell et al., 1985 ; Foronda et al., 1987 ). Stegodon teeth, bones, and tusks were embedded in tuffs (Schoell et al., 1987 ), while fish fossils were reported in the lacustrine facies (Foronda et al., 1987 ). These fish fossils were assigned to the family Clupeidae, closely resembling Sardinella tawilis (Herre, 1927 ) by Foronda et al. ( 1987 , as Harengula tawilis ). Radiometric age dating of the basalt flow and ignimbrite deposits yielded ages of 1.0 Ma and 1.7 Ma, respectively (Wolfe, 1981 ). In the study area, only the lacustrine facies were recognized. The lacustrine facies consist of thinly laminated bedded mudstones and sandstones, and the laminated mudstones appear tuffaceous and are referred to as “white shale” due to their fissile nature. In between the mudstones are fine- to medium-grained sandstone beds, which Foronda et al. ( 1987 ) interpreted to be lake turbidites, as evidenced by the presence of erosional/undulating base, normal grading, load casts, and flame structures. The sequence in the study area probably corresponds to the basal (deeper) portion of the lacustrine facies (Foronda et al., 1987 ) based on the dominance of the laminated mudstones. Fossils in the “white shale” and tuffaceous sandstone layers include semi-articulated fish, mollusks, leaf imprints, and plant remains. Investigation of the fish fossils also resulted in recognizing disarticulated frog fossils. Material and methods The fish fossils described in this study were recovered from the surface exposures of the site (see above). A total of 14 articulated and semi-articulated specimens (representing 10 individuals) were examined in this study. The specimens are currently deposited at the National Institute of Geological Sciences-University of the Philippines (under the code COG) and at the National Museum of the Philippines (under the code NMP). The following living specimens were used for comparison: (a) eight semi-articulated skeletons of Herklotsichthys quadrimaculatus (Rüppell, 1837) from Wanli locality, NE Taiwan, GLU-2023-0247 to GLU-2023-0254 (SL from 91 to 100 mm); (b) three semi-articulated skeletons of Monopetrus albus (Zuiew, 1793 ) from Kaohsiung, S Taiwan, GLU-2023-0093 to GLU-2023-0095 (TL from 489 to 569 mm); and (c) two clear and stained specimens of the same species from the same site, GLU-2023-0143 and GLU-2023-0144 (TL from 444 to 486 mm). In addition, X-rays of Opisternon bengalense McClelland, 1844 (ZRC62758, 250 mm TL; Singapore: Pandan mangroves; March 2022) and Herklotsichthys quadrimaculatus (Rüppell, 1837) published by Shao ( 2024 ) were consulted. Other comparative information was extracted from the references cited in the relevant sections of this publication. Morphometric characters were extracted using ImageJ (Schneider et al., 2012 ) and standardized relative to their relationship with standard length (SL) or head length (HL) in the case of orbit diameter. The clear and stained method by Taylor & van Dyke ( 1985 ) was followed. Systematic paleontology Order Clupeiformes Goodrich, 1909 Family Dorosomatidae Gill, 1861 Genus Herklotsichthys Whitley, 1951 Herklotsichthys sp. Figures 3 & 4 Material : The available material comprises remains of eight individuals preserved as semi-articulated fossils, partially as part and counterpart: COG-005 (Fig. 3A) & COG-003 (Fig. 3B) – part and counterpart; NMP-1415a & NMP-1416b (Fig. S1) – part and counterpart; NMP-1415d & NMP1415e (Fig. S1) – part and counterpart; NMP-1415c & COG-002 (Fig. S1) – part and counterpart; COG-004 (Fig. 3C); COG-006 (Fig. S1); COG-007 (Fig. S1); NMP-1416c (Fig. S1). Description : The fossils preserve the remains of small fish with a maximal SL of about 45 mm. The dorsal profile of the body is nearly straight or very slightly convex, while the ventral body margin is convex and almost semicircular. The head is large and triangular, with its length and depth slightly more than 30% of SL and about 27% of SL, respectively. The eye is large, comprising about 25% of HL. The maximal body depth is approximately at the level of the dorsal fin. Other proportions and available measurements are presented in Table 1. The bones are strongly crushed, making many details difficult or impossible to recognize. The neurocranium is relatively deep and represents about half of the skull depth. The frontal bone bears approximately five to seven striations, which extend to the parietals. The pterotic bears two recognizable bullae. The parasphenoid is straight, horizontally set, and well-recognizable in the orbit. The jaw is toothless and subterminal, oriented slightly dorsally. The jaw joint is located at the level of the anterior half of the orbit. There is a recognizable second supramaxilla with an asymmetrical shape, a more or less drop-shaped posterior part, and an anteriorly oriented stick-like process (Fig. 4; compare with fig. 2A in Whitehead, 1963). The opercle is smooth. The preopercle appears to have a vertical limb about twice as long as the horizontal one. The vertebral column consists of 32 to 34 preural centra. The pleural ribs are long, and the epineurals and epipleurals are recognizable. The caudal skeleton shows a typical clupeoid configuration, with the second hypural plate fused with the first ural centrum (Grande, 1985; Schultze & Arratia, 2013). The caudal fin is deeply forked and appears to consist of 19 principal rays (I+9+8+I), plus eight or nine dorsal and six or seven ventral procurrent rays. The caudal peduncle is short and relatively deep, measuring slightly more than 10% of SL. The dorsal fin inserts just in front of the midpoint of the body (predorsal length is slightly less than 50% of SL) and consists of about 14 rays, with the last rays not elongated. There are approximately six predorsal bones. The anal fin inserts in the posterior part of the body (preanal length is slightly more than 70% of SL). The fin is long-based and consists of about 20 rays, but the precise number of fin rays is not preserved in any specimen. The last ray of the anal fin is not elongated. The pectoral fin sets close to the ventral profile of the body and consists of about 14 or 15 rays. The pelvic fins insert under the first half of the dorsal fin, approximately at the midpoint of the SL, with the exact number of fin rays being unknown. Both pre-pelvic and post-pelvic scutes are developed. The scales are rounded and cycloid, with a relatively low number of continuous radii. Notes : Although the specimens under consideration are moderately preserved, a unique combination of several features supports their assignment within the genus Herklotsichthys (Whitehead, 1963, 1985). These features include the presence of scutes along the belly anteriorly and posteriorly to pelvic fins, the location of the lower jaw joint under the orbit, a moderately developed anal fin with less than 30 fin rays, a mouth not in the inferior position, non-elongated last dorsal and anal fin rays, a moderately striated skull roof, and an asymmetrically shaped second supramaxilla. The genus Herklotsichthys was traditionally considered a member of the dorosomatid clupeid (see Whitehead, 1985). Although Nelson et al. (2016) placed it within the family Clupeidae, subfamily Clupeinae, we follow the classification according to Whitehead (1985). According to Froese & Pauly (2024), the genus Herklotsichthys consists of 12 primarily marine species restricted to the Indian and Pacific Oceans. Among these, H. dispilonotus (Bleeker, 1852) also occurs in brackish environments in the Western Central Pacific and four species are found in freshwater and brackish environments: H. koningsbergeri (Weber & de Beaufort, 1912) from the Eastern Indian Ocean, H. gotoi Wongratana, 1983 from the Western Central Pacific, H. castelnaui (Ogilby, 1897) from the Southwest Pacific, and H. quadrimaculatus (Rüppell, 1837) from the Indo-Pacific. According to Jamandre (2023), the freshwater regions of the Philippines are inhabited by Herklotsichthys quadrimaculatus, along with three other freshwater clupeiforms: Sardinella tawilis, Dussumieria acuta , and Pellona ditchela . However, due to significant morphological differences, the studied fossils cannot be associated with any of these species. This includes differences from H. quadrimaculatus , which possesses a significantly different number of preural vertebrae (32 to 34 vs. 40 to 43, personal observation). Data for other potential species candidates are practically non-existent. Consequently, the fossils are left in open nomenclature, although the possibility of them representing separate species is not excluded. Gaudant (1991) stated that clupeomorph remains are relatively numerous in the fossil record, but they often lack features usable for precise systematic classification. Several fossil clupeiforms have been described from the Pacific region, such as those from the Miocene deposits of California (David, 1943), Neogene deposits of Chile (Oyanadel-Urbina et al., 2021), Miocene and Pleistocene deposits of Japan (Yabumoto & Uyeno, 1994; Yabumoto et al., 2005), and the Miocene deposits of Sakhalin (Nazarkin, 2021). However, most (if not all) of these records are restricted to marine deposits. Series Percomorpha Hay, 1903 Order Gobiiformes Bleeker, 1859 sensu Günther, 1880 Suborder Gobioidei Günther, 1880 Family Gobiidae Cuvier, 1816 vel Oxudercidae Günther, 1861 Genus and species indet. Figures 5 & 6 Material : Single specimen NMP-2235 (Fig. 5) preserving semi-articulated fish remains mainly in the ventral view. Description : The small fish reaches a SL of 69 mm, with a large head representing slightly more than 30% of SL. Due to the preservation of the fossil exposing the ventral view and its partial disarticulation, other body proportions are not discernible. The remains of the skull (Fig. 6) allow recognition of a post-orbitally enlarged neurocranium with partially preserved frontal, mesethmoid, parasphenoid, pterotic, and basioccipital bones. The left frontal (exposing its internal side) is about two and a half times wider in the postorbital section than in its interorbital section. The mesethmoid is a sub-squarish bone with no distinct details. The parasphenoid is thin, narrow anteriorly, and enlarged posteriorly, with well-developed ascending processes. The pterotic is manifested as a well-discernable postero-laterally oriented spike at the posterior margin of the neurocranium. The basioccipital is partially preserved, exposing the articulation part for the first abdominal vertebra. The remains of the oral jaw allow the discernment of the maxillae, partially preserved premaxilla, and dentary. Teeth in the premaxilla are arranged in at least two rows (similarly as in the dentary), with the outer row bearing about five enlarged teeth just aside from the symphyses. The lower jaw joint was located at the level of the anterior orbital margin. The palatine is “T”-shaped, with the ethmoid process larger than the maxillary one. The quadrate is poorly preserved, with only the relatively robust preopercular process being well recognizable. The symplectic and hyomandibula are mostly preserved as natural imprints in the sediment, but the “symplectic foramen” is determinable. The preopercle and opercle are preserved mainly as imprints on the surface of the sediment, suggesting a slightly longer horizontal ramus of the preopercle and a triangular opercle with a convex dorsal margin. The vertebral column is too poorly preserved to provide details about its morphology and the number of vertebrae. The caudal skeleton is partially preserved and allows the recognition of two large hypural plates, with each large plate representing fused hypurals: the lower one hyp 1+2 and the upper one hyp 3+4. The upper large plate is fused with the ural vertebra. The caudal fin is preserved with about 18 elongated fin rays, with the maximal length slightly more than 20% of the SL. Other unpaired fins and their supportive skeletons are not sufficiently preserved. The pectoral fin is composed of approximately 13 elongated fin rays, with a maximal length of about 15% of the SL. The pectoral girdle, the pelvic fin, and the pelvic girdle are not recognizable. The body is covered by large, most probably cycloid, scales. No ctenii are observed on any scale. The scales are ornamented with almost parallelly arranged radii in high numbers; a scale in the caudal section of the body shows 15 of them. Notes : Wiley & Johnson (2010) listed 14 synapomorphies diagnosing the order Gobiiformes ( sensu Günther, 1880), and although only a single character is recognizable (fused hypurals 1 & 2 and 3 & 4, with the latter also fused with the urostyle), this character unambiguously places the fossil within the order Gobiiformes. The specimen also shows the overall physiognomy of the skull and the presence of a symplectic foramen (i.e., suspensorial interspace), supporting its placement within the suborder Gobioidei. The Gobioidei suborder is highly diverse, with approximately 2,200 extant species classified into more than 270 genera and eight families (Nelson et al., 2016). Although the fossil does not preserve usable morphological characters for precise classification, the “T”-shaped palatine is typically restricted to gobioid families with five branchiostegal rays, specifically Gobiidae and Oxudercidae (Regan, 1911; Hoese, 1984; Reichenbacher et al., 2020). Both families are highly diversified, with many species inhabiting muddy and silty environments (Nelson et al., 2016). Due to the missing number of osteological characters (especially otoliths, the hyoid arch with associated branchiostegal rays, and the endoskeleton of the unpaired fins), and thus a high degree of uncertainty, we refrain from attempting a classification to the lower level of the specimen. However, the fossil record in the Pacific Ocean does indeed preserve several semi-articulated gobioid species from the Plio-Pleistocene and Pleistocene deposits of Japan, namely species of the genera Tridentiger Gill, 1859, Chaenogobius Gill, 1859, Amblychaeturichthys Bleeker, 1874, and Rhinogobius Gill, 1859 (Uyeno & Iwato, 1975; Yabumoto, 1987; Yabumoto & Uyeno, 1994). The latter genus, with scales presenting numerous parallel radii (e.g., Yabumoto, 1987: fig. 5), especially resembles our Antipolo specimen. Until better-preserved specimens become available, the true systematic position remains unclear. Order Synbranchiformes Berg, 1940 Suborder Synbranchoidei Boulenger, 1904 Family Synbranchidae Bonaparte, 1835 Genus Ophisternon McClelland, 1844 Ophisternon sp. Figure 7 Material : Single specimen COG-001 (Fig. 7) preserving the semi-articulated remains of the anterior part of the body, including the skull. Description : The remains of this small fish have a measurable length of about 48 mm, with a large head measuring ca 20 mm. The skull is preserved in a dorsolateral view, exposing its left side and part of the dorsi-cranium. However, the margins among individual bones are not discernable due to insufficient preservation. The neurocranium is antero-posteriorly elongated. Two protrusions are recognizable in the anterior part of the skull, interpreted to be remains of the palatine and lateral ethmoid. Both frontals are recognizable, and although their precise shape is indeterminable, they were strongly elongated. Two additional protrusions are preserved in the otic part of the skull and interpreted as remains of the sphenotic and pterotic. In the midline of the skull, a short occipital bone is recognizable with a short and low occipital crest. The remains of the lower jaw preserve the anguloarticular, a small and rounded retroarticular, and partially the dentary. The remains of the dentary preserve an elongated lower limb reaching the level of articulation with quadrate bone (see arrow in Fig. 7B). Anguloarticular seems to have a more or less developed coronoid process. The articulation of the lower jaw is at the midpoint between the lateral ethmoid and sphenotic protrusions. Five short branchiostegal rays are recognizable, with the third one being the longest. The posterior-most tips of the branchiostegal rays reach the level of the ventral tip of the cleithrum. The cleithrum is crescent-shaped, anteriorly elongated, and relatively close to the skull. The posttemporal is developed, connecting the skull with the dorsal section of the pectoral girdle, and preserves a more robust and longer dorsal limb than the ventral one, which is about half as long and significantly thinner. Other remains of the skull and the attached pectoral girdle are not recognizable. A fragment of the vertebral column preserves the remains of 16 vertebral centra. While the first six vertebrae are preserved more or less in a dorso-ventral view, the more posterior ones are visible in lateral aspects. Skull fragments partly cover the anterior-most vertebra, but the socket-like articulation for the occipital part of the skull is determinable. The vertebrae bear well-developed lateral apophyses, particularly well discernable in the 9 th and 10 th vertebrae. These lateral apophyses served for articulation with pleural ribs. From the fifth vertebra onwards, the dorsal prezygapophyses and ventral postzygapophyses are recognizable—these structures would likely be present in the more anteriorly located vertebrae, but the preservation does not allow clear determination. The pleural ribs are thin, set sub-horizontally, and recognizable in association with several vertebrae, including the second one (the condition of the first vertebra is not clear). Notes : Swamp eels (Synbranchiformes) are highly specialized percomorph fish classified within three families: Synbranchidae, Chaudhuriidae, and Mastacembelidae (Nelson et al., 2016). Wiley & Johnson (2010) listed six synapomorphies diagnosing this order. Although the specimen is relatively poorly preserved, the extension of the dentary postero-ventrally along the ventral margin of the anguloarticular supports its classification within this order. The order is divided into two suborders: Mastacembeloidei and Synbranchoidei. Based on the partially recognizable articular plug of the first vertebra and the absence of the pectoral fin and scapulocoracoid, it is reasonable to assume that the specimen belongs to the suborder Synbranchoidei, which includes a single family, Synbranchidae (Wiley & Johnson, 2010). The entire family was revised by Rosen & Greenwood (1976), who recognized four separate genera: Macrotrema Regan, 1912, Synbranchus Bloch, 1795, Ophisternon McClelland, 1844, and Monopterus Lacepède, 1800, but research of this peculiar group has progressed significantly in recent years, leading to a deeper understanding to its diversity and anatomy (see Kottelat, 2013; Britz et al., 2016, 2020a, b, c, 2023). Due to the presence of the posttemporal, which connects the pectoral girdle with the skull, the probable presence of the triangular coronoid process at the anguloarticular, and the distal tips of the branchiostegal rays reaching the level of the ventral tip of the cleithrum, it is possible to classify the specimen as a member of the genus Ophisternon (see Rosen & Greenwood, 1976). Although Rosen & Greenwood (1976) listed six branchiostegal rays for the genus, only five are recognizable in the fossil. This condition is interpreted as an artifact of preservation rather than a natural state. According to Jamandre (2023), the only synbranchiform family native to the Philippines is Synbranchidae, which includes two species of the genus Monopterus ( M. albus (Zuiew, 1793) and M. javanensis (Lacepède, 1800)) and a single species of the genus Ophisternon ( O. bengalense McClelland, 1844). The genus Ophisternon is remarkably distributed from Middle and South America, Africa, India, Southeast Asia, and Australia (Rosen, 1975) and includes six species (Nelson et al., 2016). Due to insufficient and incomplete preservation, species determination is not possible at this moment. On the other hand, the specimen represents the first fossil record of swamp eels worldwide—until now, only Holocene remains of synbranchids ( Synbranchus ) were reported from southwestern Amazonia (Prestes-Carneiro et al., 2018, 2020). Discussion Preservation and taphonomy The preservation of the dorosomatid and gobioid fossils shows well-discernable traces of disarticulation, which were likely caused by natural decomposition processes. The disarticulation of skulls, body parts, and fins suggests that these individuals, after death, remained in the free water column for some time but with minimal or no disturbances from the scavenger activities, as evidenced by the lack of obvious bio-erosion markers (Foronda et al., 1987). This condition could be explained by a hypoxic water layer just above the bottom of the basin, which would have inhibited scavenger activity. Subsequently, these partially disarticulated bodies were covered by additional fine sediment layers. This preservation mode is in strong contrast with that of the swamp eel specimen. The swamp eel is fully articulated, showing no signs of decomposition or disarticulation in the water column. Instead, it appears that the individual died and was immediately buried in the sediment. The presence of invertebrates, plant remains, and several fish specimens in various stages of decomposition and size suggests that there were very few, if any, selective agents, such as bottom currents, in the basin. This further indicates a relatively calm depositional environment, which likely contributed to the good preservation of the fossils. This assemblage, therefore, indicates an autochthonous assemblage with minimal postmortem sorting. Biogeographical implication The freshwater fossils in this study were identified under three families: Dorosomatidae, Gobiidae (or Oxudercidae), and Synbranchidae, with Dorosomatidae being the most abundant in the collection. Dorosomatidae has 30 valid genera and 116 species and is considered the second-largest family of Clupeiformes (Fricke et al., 2024). These species are globally distributed and primarily found in the tropics (Froese & Pauly, 2024). In the Philippines, two species have been reported: Herklotsichthys quadrimaculatus , considered native and distributed in wetlands and river basins, and Sardinella tawilis , which is endemic to Lake Taal, Luzon Island (Jamandre, 2023). The family Gobiidae (167 genera and 1417 species) and Oxudercidae (110 genera and 726 species) are also widely distributed globally, inhabiting both tropical and temperate regions (Fricke et al., 2024). There are 89 species of Gobiidae recorded in the Philippines, making it the most diverse fish family in the country, with 20 species being endemic. These species are primarily found in the river basins of the three major islands: Luzon, Visayas, and Mindanao (Jamandre, 2023). In Taal Lake, genera such as Glossogobius , Acentrogobius , Odontamblyopus , Oligolepis , Psammagobius , and Redigobius have been reported (Corpuz et al., 2016; Mutia et al., 2017). Four native species of Oxudercidae have been reported in the river basins and wetlands across the Philippines: Boleophthalmus boddarti, Periophthalmus argentilineatus , Periophthalmus kalolo , and Periophthalmus malaccensis (Jamandre, 2023). Synbranchidae has seven valid genera and 27 valid species worldwide (Fricke et al., 2024). It is widely distributed across Southeast Asia, northern Australia, West Africa, and Middle America (Berra, 2001). In the Philippines, four species of swamp eels have been reported: the Asian swamp eels ( Monopterus albus , Monopterus javanensis ), freshwater mud eel ( Monopterus cuchia ), and Bengal eel ( Ophisternon bengalense ) (Bucol et al., 2010; Guerrero, 2014; Roy et al., 2016). The Asian swamp eel was introduced to the Philippines in 1905 for aquaculture purposes in freshwater streams and rice paddies (Guerrero, 2014). However, there are no reports of synbranchids in Taal Lake. Paleoenvironmental interpretation The presence of demersal swamp eels and gobioid fishes suggests an aquatic ecosystem with a silty and muddy bottom environment. The close association between species from both the pelagic (dorosomatids) and benthic zones indicates a shallow-water environment where these habitats overlapped. Although some, if not all, of the identified taxa may tolerate brackish and/or marine environments, the deposits are best interpreted as freshwater. This interpretation is further supported by the presence of other freshwater elements, such as frog remains and leaf imprints, although these are not the primary focus of this study. The euryhaline fish assemblage in lacustrine freshwater deposits is reminiscent of the situation in Taal Lake (Herre, 1927), which transitioned from a brackish to a freshwater environment. A channel once connected Taal Lake to Balayan Bay in the South China Sea, which ultimately became isolated and freshwater due to the volcanic eruption of Taal Volcano in 1754 (Ramos, 2002). This environment is ideal for marine species to adapt to freshwater environments, as seen in Sardinella tawilis , the only extant species of Sardinella that inhabit freshwater (Herre, 1927). Comparison with other Pleistocene fish assemblages in the West Pacific In the West Pacific, the most extensively documented freshwater fish fossils from the Pleistocene are found in the deposits of Japan. Miyata (2019) provided a comprehensive and updated list of taxa and strata from the Quaternary of Japan. The fossil localities are predominantly centered in the western part of Japan, and the taxonomic composition is mainly confined to the most diverse orders of freshwater fishes: Cypriniformes and Siluriformes (e.g., Watanabe & Uyeno, 1999; Miyata et al., 2018), with occasional occurrences of the family Gobiidae (Yabumoto, 1987). In addition, Pleistocene deposits in southern Taiwan have yielded cypriniform pharyngeal teeth (Tao & Hu 2001). Uyeno (1978) also reported the presence of Cyprinus carpio (Cyprinidae) pharyngeal teeth and a bagrid spine (Bagridae, Siluriformes) from southern Taiwan, although the age of these fossils was not well-constrained, ranging from the Miocene to the Pleistocene. These represent the only known freshwater fish fossils from Taiwan and have been assigned to three species of cyprinids (Lin et al., 2021). Similarly, Yudha et al. (2020) compiled Pleistocene fish fossils from Java, highlighting fragmentary bones predominantly from Siluriformes, along with less common Cypriniformes and Anabantidae. Fossil Cypriniformes are also abundantly represented in continental China, though most specimens are from the Paleogene and Neogene strata, with Quaternary fossils being relatively rarely described (Chang & Zhou, 1993; Chang & Chen, 2008). The Chinese records provide a broader temporal context for understanding the earlier evolutionary history of freshwater fishes in East Asia (Chang & Chen, 2000); however, since the Philippines is an archipelago with island systems that differ from the continental environments of China, we do not address further into this comparison. The taxonomic composition of these assemblages generally reflects the dominant role these major groups—Cypriniformes and Siluriformes—play in present-day freshwater fish communities. Although detailed documentation exists for articulate skeletons, particularly for the Japanese specimens, the fossils are often fragmentary, with pharyngeal teeth (Cypriniformes) and fin spines (Siluriformes) being the most frequently recorded elements. In contrast, the Philippine assemblage we describe here stands out in terms of both taxonomic composition and preservation status. The discovery of this well-preserved freshwater fish fossil assemblage in the Philippines not only suggests a different paleoenvironmental condition, as discussed earlier, but, more importantly, it uncovers an extraordinary paleodiversity that has not been previously reported from the region. Conclusion The fish fossil assemblage in the Pleistocene Laguna Formation is predominantly composed of the families Dorosomatidae, a Gobiidae vel Oxudercidae, and a Synbranchidae. These fossils represent the first records of freshwater fish in the Philippines and the first discovery of these taxa in the tropical West Pacific region. Notably, this study also provides the first-ever record of a synbranchid fossil globally. We infer that the environment of the Pleistocene Laguna Formation was freshwater, analogous to the modern Taal Lake. Our findings pave the way for promising scientific exploration and further understanding of the region's paleodiversity. Abbreviations Anatomical abbreviations: BD body depth CPD caudal peduncle depth CPL caudal peduncle length HD head depth HL head length pre A–preanal length pre D–predorsal length SL standard length TL total length. Institutional abbreviations: GLU–Institute of Geology of the Czech Academy of Sciences, Prague ZRC Zoological Reference Collection, formerly the Raffles Natural History Collection, Singapore. Declarations Acknowledgments Dr. Heok Hui Tan (National University of Singapore) is acknowledged for providing a comparative X-ray of the Ophisternon , and Dr. Ralf Britz (Senckenberg Naturhistorische Sammlungen, Dresden) is acknowledged for providing PDF copies of his papers regarding the synbranchid fish. Authors’ contributions Conceptualization: TP and C-HL. Methodology: TP, AC, AGF. Investigation and formal analyses: TP and AC. Visualization: TP, AC, AGF. Field work-sampling: AC, AGF, CM, KG, JRN. Supervision: TP, AGF, C-HL. Funding: TP, AGF, C-HL. Writing-original draft: TP, AGF, DM, C-HL. All authors contributed and approved the final manuscript. Funding This study was mainly supported by the Mobility Plus Project between Academia Sinica, Taipei, Taiwan and the Czech Academy of Sciences, Czech Republic: Cenozoic fossil fishes from Taiwan and the Czech Republic – the once thrived ichthyofaunas to Chien-Hsiang Lin and Tomáš Přikryl. Tomáš Přikryl is supported by the Institute of Geology of the Czech Academy of Sciences (RVO67985831). Chien-Hsiang Lin is also supported by the National Science and Technology Council, Taiwan (Grant No. 111-2116-M-001-033, 112-2116-M-001- 017-MY3) and Academia Sinica, Taipei, Taiwan. Allan Gil Fernando is supported by Emerging Interdisciplinary Research (EIDR) Program “Discovering the world of first hominins in the Philippines – geology, palaeoenvironment, and palaeoecology of archaic hominins in the Philippines – Project 3: Geological Environments” under the Office of the Vice President for Academic Affairs of the University of the Philippines. Funding and a travel grant were also given by the Taiwan International Internship Program (Academia Sinica) in 2023 to Abigael Castro’s summer internship at the Biodiversity Research Center, Academia Sinica. 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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-5055249","acceptedTermsAndConditions":true,"allowDirectSubmit":false,"archivedVersions":[],"articleType":"Research Article","associatedPublications":[],"authors":[{"id":388294412,"identity":"96429f2e-67d8-4bc2-8378-0d0ddd47112c","order_by":0,"name":"Tomáš Přikryl","email":"","orcid":"","institution":"Institute of Geology of the Czech Academy of Sciences","correspondingAuthor":false,"prefix":"","firstName":"Tomáš","middleName":"","lastName":"Přikryl","suffix":""},{"id":388294413,"identity":"d66f72be-983d-458d-9ca6-006c7b4e1d78","order_by":1,"name":"Abigael Castro","email":"","orcid":"","institution":"National Museum of the Philippines","correspondingAuthor":false,"prefix":"","firstName":"Abigael","middleName":"","lastName":"Castro","suffix":""},{"id":388294414,"identity":"62966894-ad80-47d2-aa2c-8b535e081a2c","order_by":2,"name":"Allan Gil Fernando","email":"","orcid":"","institution":"University of the Philippines Diliman","correspondingAuthor":false,"prefix":"","firstName":"Allan","middleName":"Gil","lastName":"Fernando","suffix":""},{"id":388294415,"identity":"3920ad21-c7c7-4be5-b7cb-ab66ef056143","order_by":3,"name":"Jaan Ruy Conrad Nogot","email":"","orcid":"","institution":"National Museum of the Philippines","correspondingAuthor":false,"prefix":"","firstName":"Jaan","middleName":"Ruy Conrad","lastName":"Nogot","suffix":""},{"id":388294416,"identity":"ae969fca-8dfa-4b28-8aee-5bbc33989260","order_by":4,"name":"Clarence Magtoto","email":"","orcid":"","institution":"University of the Philippines","correspondingAuthor":false,"prefix":"","firstName":"Clarence","middleName":"","lastName":"Magtoto","suffix":""},{"id":388294417,"identity":"5585e16b-40f2-49e8-b397-4234f114f021","order_by":5,"name":"Kevin Garas","email":"","orcid":"","institution":"Mines and Geoscience Bureau","correspondingAuthor":false,"prefix":"","firstName":"Kevin","middleName":"","lastName":"Garas","suffix":""},{"id":388294418,"identity":"d77ee3fd-063d-4585-a55c-268fbaa29492","order_by":6,"name":"Dominique Mediodia","email":"","orcid":"","institution":"Academia Sinica","correspondingAuthor":false,"prefix":"","firstName":"Dominique","middleName":"","lastName":"Mediodia","suffix":""},{"id":388294419,"identity":"e37d6074-f745-4bee-87ad-a58b662a9d8c","order_by":7,"name":"Chien-Hsiang Lin","email":"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAAA5UlEQVRIie3PMQrCMBSA4ffI4FLJGm/RA3gTJxHcArp10JJSsIsHqCj1Crp0bnlQF8FV0EHwCC6OplUEhxrdBPMP4Q3vIwmAzfaLCaayamigYuDpgTFlIFgRAQyDELYlQTOBOwEMcVLOBsLnQUADaPucseg8TMYdHmly9dL6S465ohj6ohXqh83SjYwJFU63h1riiq4iB0i4pEkzLaTShOHkY7Io5PJLokZyZSJiXxK335rpv8ydIpNrTfJ3f+Fxjy6O1+a8EZ0uzsiXyY7y09WrJ4/nPSeqzsyw/5L/zbLNZrP9STfvAlkOdrJoZwAAAABJRU5ErkJggg==","orcid":"","institution":"Academia Sinica","correspondingAuthor":true,"prefix":"","firstName":"Chien-Hsiang","middleName":"","lastName":"Lin","suffix":""}],"badges":[],"createdAt":"2024-09-09 04:50:59","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-5055249/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-5055249/v1","draftVersion":[],"editorialEvents":[{"content":"https://doi.org/10.1186/s13358-024-00347-0","type":"published","date":"2025-01-16T15:56:50+00:00"}],"editorialNote":"","failedWorkflow":false,"files":[{"id":72330097,"identity":"3cb4a58e-ae4d-43c4-b083-2684c30eb20e","added_by":"auto","created_at":"2024-12-25 14:02:53","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":1154665,"visible":true,"origin":"","legend":"\u003cp\u003eMap showing the sampling site. \u003cstrong\u003ea\u003c/strong\u003eMap of northern Philippines showing the tectonic setting of the Central Luzon Basin. \u003cstrong\u003eb\u003c/strong\u003e Facies distribution map of the Laguna Formation (Modified from Foronda et al., 1987) with the location of the fossil site.\u003c/p\u003e","description":"","filename":"Fig1.png","url":"https://assets-eu.researchsquare.com/files/rs-5055249/v1/a9227ec9bdcab5dc5e1376db.png"},{"id":72330817,"identity":"b6784057-137c-49c5-90fd-5ba08bdadff4","added_by":"auto","created_at":"2024-12-25 14:10:53","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":3355569,"visible":true,"origin":"","legend":"\u003cp\u003eOutcrop and measured section of the fossil site. \u003cstrong\u003ea\u003c/strong\u003e Field photograph showing the Laguna Formation in Antipolo, Rizal. \u003cstrong\u003eb\u003c/strong\u003e Stratigraphic log of the Laguna Formation observed at the fossil site. The white arrow in (\u003cstrong\u003ea\u003c/strong\u003e) shows the layer where the fish fossils were collected.\u003c/p\u003e","description":"","filename":"Fig2.png","url":"https://assets-eu.researchsquare.com/files/rs-5055249/v1/ca60759bc282c2d69638da59.png"},{"id":72330100,"identity":"7db64f36-0f36-4a67-8a03-72d7debd2490","added_by":"auto","created_at":"2024-12-25 14:02:53","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":1818282,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cem\u003eHerklotsichthys\u003c/em\u003e sp. from the Laguna Formation, Philippines.\u003cstrong\u003e a\u003c/strong\u003e Overview of the specimen COG-005.\u003cstrong\u003e b\u003c/strong\u003e Specimen COG-003.\u003cstrong\u003e c\u003c/strong\u003e Specimen COG-004.\u003c/p\u003e","description":"","filename":"Figure3herklosichthys.png","url":"https://assets-eu.researchsquare.com/files/rs-5055249/v1/68b3f818f5059d7118c0cbc9.png"},{"id":72330095,"identity":"6ebc1172-34d4-42ab-8ab5-6bc78ead1c6a","added_by":"auto","created_at":"2024-12-25 14:02:53","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":83081,"visible":true,"origin":"","legend":"\u003cp\u003eInterpretative drawing of the supramaxilla of \u003cem\u003eHerklotsichthys\u003c/em\u003e sp. (specimen NMP-1415C). Anterior to the left.\u003c/p\u003e","description":"","filename":"Figure4clupeidsupramaxilla.png","url":"https://assets-eu.researchsquare.com/files/rs-5055249/v1/b92cc0c579618587c765c4b7.png"},{"id":72330101,"identity":"88859e0e-6d1d-4965-bb6c-4378b9b5a10f","added_by":"auto","created_at":"2024-12-25 14:02:53","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":676901,"visible":true,"origin":"","legend":"\u003cp\u003eGeneral view of specimen NMP-2235 (Gobiidae \u003cem\u003evel \u003c/em\u003eOxudercidae, \u003cem\u003eGenus et species indetermined\u003c/em\u003e) from the Laguna Formation, Philippines.\u003c/p\u003e","description":"","filename":"Figure5GOBYcomplete.png","url":"https://assets-eu.researchsquare.com/files/rs-5055249/v1/e48fb7d7c3786518bd18a2ea.png"},{"id":72330098,"identity":"22f9bdcf-4627-4f16-aa51-db57fe9ec68f","added_by":"auto","created_at":"2024-12-25 14:02:53","extension":"png","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":1162757,"visible":true,"origin":"","legend":"\u003cp\u003eInterpretative drawing of the skull of the specimen NMP-2235 (Gobiidae \u003cem\u003evel \u003c/em\u003eOxudercidae, \u003cem\u003eGenus et species indetermined\u003c/em\u003e). Anterior to the right.\u003c/p\u003e","description":"","filename":"Figure6GOBYskull.png","url":"https://assets-eu.researchsquare.com/files/rs-5055249/v1/5664bcc50a64db41749c3b25.png"},{"id":72330102,"identity":"3ce50a8d-d5bb-43cc-871c-ca4e78e13122","added_by":"auto","created_at":"2024-12-25 14:02:53","extension":"png","order_by":7,"title":"Figure 7","display":"","copyAsset":false,"role":"figure","size":1011354,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cem\u003eOphisternon\u003c/em\u003e sp. (specimen COG-001) from the Laguna Formation, Philippines. \u003cstrong\u003ea\u003c/strong\u003e Overview of the specimen. \u003cstrong\u003eb\u003c/strong\u003e Interpretative drawing of the specimen. The arrow in (\u003cstrong\u003eb\u003c/strong\u003e) marks the posterior-most margin of the lower dentary limb.\u003c/p\u003e","description":"","filename":"Figure7synbranchiform.png","url":"https://assets-eu.researchsquare.com/files/rs-5055249/v1/f396c2fb6cd47219f41ac3c7.png"},{"id":74284372,"identity":"4eaa02a4-93ee-4974-8393-f24b763b0e93","added_by":"auto","created_at":"2025-01-20 16:02:42","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":9882247,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-5055249/v1/4217b436-38ad-4e40-a568-d8c4b18ed721.pdf"},{"id":72330816,"identity":"5c160fb7-c8fc-4690-9bd6-7e8578d0fc11","added_by":"auto","created_at":"2024-12-25 14:10:53","extension":"xlsx","order_by":0,"title":"","display":"","copyAsset":false,"role":"supplement","size":13462,"visible":true,"origin":"","legend":"","description":"","filename":"Table1.xlsx","url":"https://assets-eu.researchsquare.com/files/rs-5055249/v1/498880c8207d8cc8d682e34c.xlsx"},{"id":72330103,"identity":"072d7340-e774-4993-8e60-35e7d21353d4","added_by":"auto","created_at":"2024-12-25 14:02:54","extension":"png","order_by":1,"title":"","display":"","copyAsset":false,"role":"supplement","size":15842010,"visible":true,"origin":"","legend":"","description":"","filename":"SupplementaryFig.S1.png","url":"https://assets-eu.researchsquare.com/files/rs-5055249/v1/12771c1a6e2876059676c91c.png"}],"financialInterests":"No competing interests reported.","formattedTitle":"Fossil fish assemblage of the Laguna Formation, Philippines: Unveiling the uniqueness of Pleistocene freshwater ecosystems in Southeast Asia","fulltext":[{"header":"Introduction","content":"\u003cp\u003eThe Philippines is recognized for its rich freshwater biodiversity, particularly its number of freshwater fish species (Capuli \u0026amp; Froese, \u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e1997\u003c/span\u003e; Papa \u0026amp; Briones, \u003cspan citationid=\"CR59\" class=\"CitationRef\"\u003e2017\u003c/span\u003e). According to Froese \u0026amp; Pauly (\u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e2024\u003c/span\u003e), approximately 362 freshwater fish species have been recorded in the Philippines, with ongoing efforts to describe newly documented species (Bestre et al., \u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e2018\u003c/span\u003e; Maeda et al., \u003cspan citationid=\"CR47\" class=\"CitationRef\"\u003e2021\u003c/span\u003e). The most recent comprehensive overview of the freshwater fishes of the Philippines by Jamandre (\u003cspan citationid=\"CR41\" class=\"CitationRef\"\u003e2023\u003c/span\u003e) revealed a total of 374 species. It offered a provisional glimpse into the diversity within this area, highlighting the remarkable progress made in studying various living groups. However, the evolutionary history and distribution of freshwater fish in the Philippines remain unexplored, representing a vast \u003cem\u003eterra incognita\u003c/em\u003e in ichthyological research.\u003c/p\u003e \u003cp\u003eThe fossil record in the Philippines, particularly concerning fish, is notably sparse. Only a handful of fossil specimens have been described or mentioned in previous studies, with the most recent review by Mediodia et al. (\u003cspan citationid=\"CR49\" class=\"CitationRef\"\u003e2024\u003c/span\u003e) emphasizing the scarcity of these records. This limited evidence has provided only fragmented insights into the past biodiversity and evolutionary history of the region's freshwater ecosystems. The absence of freshwater fossil records leaves a significant gap in our understanding of how this ecosystem has changed over time, especially in response to climatic and geological events.\u003c/p\u003e \u003cp\u003eThe primary objective of this study is to document a collection of freshwater fish fossils from the Pleistocene Laguna Formation in Antipolo, Rizal Province. It represents the first systematic study of freshwater fish fossils in the Philippines and provides valuable data that can be used to compare with modern ecosystems. Finally, our results improve our understanding of the biodiversity and evolutionary history of Southeast Asia's freshwater ecosystems.\u003c/p\u003e"},{"header":"Geology of central Luzon Basin","content":"\u003cp\u003eThe study area is underlain by the rock units of the Central Luzon Basin (CLB, Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e1\u003c/span\u003e). It is part of the 8,000 m-thick sedimentary sequence of the N-S trending Central Luzon Basin, which flanks western Luzon Island (Mines and Geosciences Bureau [MGB], 2010). The CLB is structurally controlled by the major branches of the northern segment of the Philippine Fault, specifically the Vigan-Aggao Fault (Pinet \u0026amp; Stephan, \u003cspan citationid=\"CR60\" class=\"CitationRef\"\u003e1990\u003c/span\u003e), the Tuba Fault, the Pugo Fault, and the Coastal Thrust Fault (Rimando \u0026amp; Rimando, \u003cspan citationid=\"CR68\" class=\"CitationRef\"\u003e2020\u003c/span\u003e). The CLB is subdivided into western and eastern sections. The western CLB is composed of Eocene to Pleistocene sediments, which directly overlie the Eocene Zambales Ophiolite Complex. The western CLB is dominated by marine-derived sediments with some tuffaceous clastic units (Tumanda, \u003cspan citationid=\"CR82\" class=\"CitationRef\"\u003e1984\u003c/span\u003e Lorentz, \u003cspan citationid=\"CR46\" class=\"CitationRef\"\u003e1984\u003c/span\u003e; De Leon et al., \u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e1998\u003c/span\u003e; MGB, 2010; Guballa \u0026amp; Fernando, \u003cspan citationid=\"CR34\" class=\"CitationRef\"\u003e2015\u003c/span\u003e). The eastern CLB is composed of Late Cretaceous basalts and volcanic flows with metasedimentary units, Late Eocene to Early Oligocene andesitic and pyroclastic rocks with intercalations of deep marine sediments, and Lower Miocene to Pliocene marine sedimentary units with few tuffaceous clastic strata. The Pleistocene section is characterized by the thick conglomerate-sandstone-mudstone-tuff sequence of the Guadalupe Formation. Guadalupe Formation is equivalent to the Laguna Formation, further described as interbeds of tephra and pyroclastic flow units, lahars, lake deposits, and basaltic flows (Schoell et al., \u003cspan citationid=\"CR75\" class=\"CitationRef\"\u003e1985\u003c/span\u003e).\u003c/p\u003e \u003cdiv id=\"Sec3\" class=\"Section2\"\u003e \u003ch2\u003eGeology and stratigraphy of the fossil locality\u003c/h2\u003e \u003cp\u003eThe fossil site (N 14.617102, E 121.174509) is located in Antipolo City, Rizal Province (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e1\u003c/span\u003e). The collection of the fossils was done in 2014 and 2015 during the construction of a housing project that exposed several facies of the Laguna Formation. No systematic collection of fossils was conducted due to time constraints, but a detailed stratigraphic log of the outcrops is available (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e2\u003c/span\u003e). According to MGB (2010), the Laguna Formation is underlain by the Late Pliocene to Pleistocene Laguna Formation, a sequence of epiclastic sediments laid down in a terrestrial environment, consisting of tuffs deposited in alluvial setting, stream and lake deposits, and lahars (Schoell et al., \u003cspan citationid=\"CR75\" class=\"CitationRef\"\u003e1985\u003c/span\u003e; Foronda et al., \u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e1987\u003c/span\u003e). \u003cem\u003eStegodon\u003c/em\u003e teeth, bones, and tusks were embedded in tuffs (Schoell et al., \u003cspan citationid=\"CR76\" class=\"CitationRef\"\u003e1987\u003c/span\u003e), while fish fossils were reported in the lacustrine facies (Foronda et al., \u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e1987\u003c/span\u003e). These fish fossils were assigned to the family Clupeidae, closely resembling \u003cem\u003eSardinella tawilis\u003c/em\u003e (Herre, \u003cspan citationid=\"CR39\" class=\"CitationRef\"\u003e1927\u003c/span\u003e) by Foronda et al. (\u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e1987\u003c/span\u003e, as \u003cem\u003eHarengula tawilis\u003c/em\u003e). Radiometric age dating of the basalt flow and ignimbrite deposits yielded ages of 1.0 Ma and 1.7 Ma, respectively (Wolfe, \u003cspan citationid=\"CR91\" class=\"CitationRef\"\u003e1981\u003c/span\u003e). In the study area, only the lacustrine facies were recognized.\u003c/p\u003e \u003cp\u003eThe lacustrine facies consist of thinly laminated bedded mudstones and sandstones, and the laminated mudstones appear tuffaceous and are referred to as \u0026ldquo;white shale\u0026rdquo; due to their fissile nature. In between the mudstones are fine- to medium-grained sandstone beds, which Foronda et al. (\u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e1987\u003c/span\u003e) interpreted to be lake turbidites, as evidenced by the presence of erosional/undulating base, normal grading, load casts, and flame structures. The sequence in the study area probably corresponds to the basal (deeper) portion of the lacustrine facies (Foronda et al., \u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e1987\u003c/span\u003e) based on the dominance of the laminated mudstones. Fossils in the \u0026ldquo;white shale\u0026rdquo; and tuffaceous sandstone layers include semi-articulated fish, mollusks, leaf imprints, and plant remains. Investigation of the fish fossils also resulted in recognizing disarticulated frog fossils.\u003c/p\u003e \u003c/div\u003e"},{"header":"Material and methods","content":"\u003cp\u003eThe fish fossils described in this study were recovered from the surface exposures of the site (see above). A total of 14 articulated and semi-articulated specimens (representing 10 individuals) were examined in this study. The specimens are currently deposited at the National Institute of Geological Sciences-University of the Philippines (under the code COG) and at the National Museum of the Philippines (under the code NMP).\u003c/p\u003e \u003cp\u003eThe following living specimens were used for comparison: (a) eight semi-articulated skeletons of \u003cem\u003eHerklotsichthys quadrimaculatus\u003c/em\u003e (R\u0026uuml;ppell, 1837) from Wanli locality, NE Taiwan, GLU-2023-0247 to GLU-2023-0254 (SL from 91 to 100 mm); (b) three semi-articulated skeletons of \u003cem\u003eMonopetrus albus\u003c/em\u003e (Zuiew, \u003cspan citationid=\"CR97\" class=\"CitationRef\"\u003e1793\u003c/span\u003e) from Kaohsiung, S Taiwan, GLU-2023-0093 to GLU-2023-0095 (TL from 489 to 569 mm); and (c) two clear and stained specimens of the same species from the same site, GLU-2023-0143 and GLU-2023-0144 (TL from 444 to 486 mm). In addition, X-rays of \u003cem\u003eOpisternon bengalense\u003c/em\u003e McClelland, \u003cspan citationid=\"CR48\" class=\"CitationRef\"\u003e1844\u003c/span\u003e (ZRC62758, 250 mm TL; Singapore: Pandan mangroves; March 2022) and \u003cem\u003eHerklotsichthys quadrimaculatus\u003c/em\u003e (R\u0026uuml;ppell, 1837) published by Shao (\u003cspan citationid=\"CR77\" class=\"CitationRef\"\u003e2024\u003c/span\u003e) were consulted. Other comparative information was extracted from the references cited in the relevant sections of this publication. Morphometric characters were extracted using ImageJ (Schneider et al., \u003cspan citationid=\"CR73\" class=\"CitationRef\"\u003e2012\u003c/span\u003e) and standardized relative to their relationship with standard length (SL) or head length (HL) in the case of orbit diameter. The clear and stained method by Taylor \u0026amp; van Dyke (\u003cspan citationid=\"CR80\" class=\"CitationRef\"\u003e1985\u003c/span\u003e) was followed.\u003c/p\u003e\u003cp\u003e\u003cstrong\u003eSystematic paleontology\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eOrder Clupeiformes Goodrich, 1909\u003c/p\u003e\n\u003cp\u003eFamily Dorosomatidae Gill, 1861\u003c/p\u003e\n\u003cp\u003eGenus \u003cem\u003eHerklotsichthys\u003c/em\u003e Whitley, 1951\u003c/p\u003e\n\u003cp\u003e\u003cem\u003eHerklotsichthys\u003c/em\u003e sp.\u003c/p\u003e\n\u003cp\u003eFigures 3 \u0026amp; 4\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eMaterial\u003c/strong\u003e: The available material comprises remains of eight individuals preserved as semi-articulated fossils, partially as part and counterpart: COG-005 (Fig. 3A) \u0026amp; COG-003 (Fig. 3B) \u0026ndash; part and counterpart; NMP-1415a \u0026amp; NMP-1416b (Fig. S1) \u0026ndash; part and counterpart; NMP-1415d \u0026amp; NMP1415e (Fig. S1) \u0026ndash; part and counterpart; NMP-1415c \u0026amp; COG-002 (Fig. S1) \u0026ndash; part and counterpart; COG-004 (Fig. 3C); COG-006 (Fig. S1); COG-007 (Fig. S1); NMP-1416c (Fig. S1).\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eDescription\u003c/strong\u003e: The fossils preserve the remains of small fish with a maximal SL of about 45 mm. The dorsal profile of the body is nearly straight or very slightly convex, while the ventral body margin is convex and almost semicircular. The head is large and triangular, with its length and depth slightly more than 30% of SL and about 27% of SL, respectively. The eye is large, comprising about 25% of HL. The maximal body depth is approximately at the level of the dorsal fin. Other proportions and available measurements are presented in Table 1. The bones are strongly crushed, making many details difficult or impossible to recognize. The neurocranium is relatively deep and represents about half of the skull depth. The frontal bone bears approximately five to seven striations, which extend to the parietals. The pterotic bears two recognizable bullae. The parasphenoid is straight, horizontally set, and well-recognizable in the orbit. The jaw is toothless and subterminal, oriented slightly dorsally. The jaw joint is located at the level of the anterior half of the orbit. There is a recognizable second supramaxilla with an asymmetrical shape, a more or less drop-shaped posterior part, and an anteriorly oriented stick-like process (Fig. 4; compare with fig. 2A in Whitehead, 1963). The opercle is smooth. The preopercle appears to have a vertical limb about twice as long as the horizontal one. The vertebral column consists of 32 to 34 preural centra. The pleural ribs are long, and the epineurals and epipleurals are recognizable. The caudal skeleton shows a typical clupeoid configuration, with the second hypural plate fused with the first ural centrum (Grande, 1985; Schultze \u0026amp; Arratia, 2013).\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eThe caudal fin is deeply forked and appears to consist of 19 principal rays (I+9+8+I), plus eight or nine dorsal and six or seven ventral procurrent rays. The caudal peduncle is short and relatively deep, measuring slightly more than 10% of SL. The dorsal fin inserts just in front of the midpoint of the body (predorsal length is slightly less than 50% of SL) and consists of about 14 rays, with the last rays not elongated. There are approximately six predorsal bones.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eThe anal fin inserts in the posterior part of the body (preanal length is slightly more than 70% of SL). The fin is long-based and consists of about 20 rays, but the precise number of fin rays is not preserved in any specimen. The last ray of the anal fin is not elongated. The pectoral fin sets close to the ventral profile of the body and consists of about 14 or 15 rays. The pelvic fins insert under the first half of the dorsal fin, approximately at the midpoint of the SL, with the exact number of fin rays being unknown. Both pre-pelvic and post-pelvic scutes are developed. The scales are rounded and cycloid, with a relatively low number of continuous radii.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eNotes\u003c/strong\u003e: Although the specimens under consideration are moderately preserved, a unique combination of several features supports their assignment within the genus \u003cem\u003eHerklotsichthys\u003c/em\u003e (Whitehead, 1963, 1985). These features include the presence of scutes along the belly anteriorly and posteriorly to pelvic fins, the location of the lower jaw joint under the orbit, a moderately developed anal fin with less than 30 fin rays, a mouth not in the inferior position, non-elongated last dorsal and anal fin rays, a moderately striated skull roof, and an asymmetrically shaped second supramaxilla.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eThe genus \u003cem\u003eHerklotsichthys\u003c/em\u003e was traditionally considered a member of the dorosomatid clupeid (see Whitehead, 1985). Although Nelson et al. (2016) placed it within the family Clupeidae, subfamily Clupeinae, we follow the classification according to Whitehead (1985). According to Froese \u0026amp; Pauly (2024), the genus \u003cem\u003eHerklotsichthys\u003c/em\u003e consists of 12 primarily marine species restricted to the Indian and Pacific Oceans. Among these, \u003cem\u003eH. dispilonotus\u0026nbsp;\u003c/em\u003e(Bleeker, 1852) also occurs in brackish environments in the Western Central Pacific and four species are found in freshwater and brackish environments: \u003cem\u003eH.\u0026nbsp;koningsbergeri\u003c/em\u003e (Weber \u0026amp; de Beaufort, 1912) from the Eastern Indian Ocean, \u003cem\u003eH. gotoi\u0026nbsp;\u003c/em\u003eWongratana, 1983 from the Western Central Pacific, \u003cem\u003eH.\u0026nbsp;castelnaui\u003c/em\u003e (Ogilby, 1897) from the Southwest Pacific, and \u003cem\u003eH. quadrimaculatus\u003c/em\u003e (R\u0026uuml;ppell, 1837) from the Indo-Pacific.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eAccording to Jamandre (2023), the freshwater regions of the Philippines are inhabited by \u003cem\u003eHerklotsichthys quadrimaculatus,\u003c/em\u003e along with three other freshwater clupeiforms: \u003cem\u003eSardinella tawilis,\u003c/em\u003e \u003cem\u003eDussumieria acuta\u003c/em\u003e, and\u003cem\u003e\u0026nbsp;Pellona ditchela\u003c/em\u003e. However, due to significant morphological differences, the studied fossils cannot be associated with any of these species. This includes differences from \u003cem\u003eH.\u003c/em\u003e \u003cem\u003equadrimaculatus\u003c/em\u003e, which possesses a significantly different number of preural vertebrae (32 to 34 vs. 40 to 43, personal observation). Data for other potential species candidates are practically non-existent. Consequently, the fossils are left in open nomenclature, although the possibility of them representing separate species is not excluded.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eGaudant (1991) stated that clupeomorph remains are relatively numerous in the fossil record, but they often lack features usable for precise systematic classification. Several fossil clupeiforms have been described from the Pacific region, such as those from the Miocene deposits of California (David, 1943), Neogene deposits of Chile (Oyanadel-Urbina et al., 2021), Miocene and Pleistocene deposits of Japan (Yabumoto \u0026amp; Uyeno, 1994; Yabumoto et al., 2005), and the Miocene deposits of Sakhalin (Nazarkin, 2021). However, most (if not all) of these records are restricted to marine deposits.\u003c/p\u003e\n\u003cp\u003eSeries Percomorpha Hay, 1903\u003c/p\u003e\n\u003cp\u003eOrder Gobiiformes Bleeker, 1859 \u003cem\u003esensu\u003c/em\u003e G\u0026uuml;nther, 1880\u003c/p\u003e\n\u003cp\u003eSuborder Gobioidei G\u0026uuml;nther, 1880\u003c/p\u003e\n\u003cp\u003eFamily Gobiidae Cuvier, 1816 \u003cem\u003evel\u0026nbsp;\u003c/em\u003eOxudercidae G\u0026uuml;nther, 1861\u003c/p\u003e\n\u003cp\u003e\u003cem\u003eGenus and species indet.\u003c/em\u003e\u003c/p\u003e\n\u003cp\u003eFigures 5 \u0026amp; 6\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eMaterial\u003c/strong\u003e: Single specimen NMP-2235 (Fig. 5) preserving semi-articulated fish remains mainly in the ventral view.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eDescription\u003c/strong\u003e: The small fish reaches a SL of 69 mm, with a large head representing slightly more than 30% of SL. Due to the preservation of the fossil exposing the ventral view and its partial disarticulation, other body proportions are not discernible. The remains of the skull (Fig. 6) allow recognition of a post-orbitally enlarged neurocranium with partially preserved frontal, mesethmoid, parasphenoid, pterotic, and basioccipital bones. The left frontal (exposing its internal side) is about two and a half times wider in the postorbital section than in its interorbital section. The mesethmoid is a sub-squarish bone with no distinct details. The parasphenoid is thin, narrow anteriorly, and enlarged posteriorly, with well-developed ascending processes. The pterotic is manifested as a well-discernable postero-laterally oriented spike at the posterior margin of the neurocranium. The basioccipital is partially preserved, exposing the articulation part for the first abdominal vertebra.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eThe remains of the oral jaw allow the discernment of the maxillae, partially preserved premaxilla, and dentary. Teeth in the premaxilla are arranged in at least two rows (similarly as in the dentary), with the outer row bearing about five enlarged teeth just aside from the symphyses. The lower jaw joint was located at the level of the anterior orbital margin. The palatine is \u0026ldquo;T\u0026rdquo;-shaped, with the ethmoid process larger than the maxillary one. The quadrate is poorly preserved, with only the relatively robust preopercular process being well recognizable. The symplectic and hyomandibula are mostly preserved as natural imprints in the sediment, but the \u0026ldquo;symplectic foramen\u0026rdquo; is determinable. The preopercle and opercle are preserved mainly as imprints on the surface of the sediment, suggesting a slightly longer horizontal ramus of the preopercle and a triangular opercle with a convex dorsal margin.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eThe vertebral column is too poorly preserved to provide details about its morphology and the number of vertebrae. The caudal skeleton is partially preserved and allows the recognition of two large hypural plates, with each large plate representing fused hypurals: the lower one hyp 1+2 and the upper one hyp 3+4. The upper large plate is fused with the ural vertebra. The caudal fin is preserved with about 18 elongated fin rays, with the maximal length slightly more than 20% of the SL. Other unpaired fins and their supportive skeletons are not sufficiently preserved.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eThe pectoral fin is composed of approximately 13 elongated fin rays, with a maximal length of about 15% of the SL. The pectoral girdle, the pelvic fin, and the pelvic girdle are not recognizable. The body is covered by large, most probably cycloid, scales. No ctenii are observed on any scale. The scales are ornamented with almost parallelly arranged radii in high numbers; a scale in the caudal section of the body shows 15 of them.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eNotes\u003c/strong\u003e: Wiley \u0026amp; Johnson (2010) listed 14 synapomorphies diagnosing the order Gobiiformes (\u003cem\u003esensu\u003c/em\u003e G\u0026uuml;nther, 1880), and although only a single character is recognizable (fused hypurals 1 \u0026amp; 2 and 3 \u0026amp; 4, with the latter also fused with the urostyle), this character unambiguously places the fossil within the order Gobiiformes. The specimen also shows the overall physiognomy of the skull and the presence of a symplectic foramen (i.e., suspensorial interspace), supporting its placement within the suborder Gobioidei. The Gobioidei suborder is highly diverse, with approximately 2,200 extant species classified into more than 270 genera and eight families (Nelson et al., 2016).\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eAlthough the fossil does not preserve usable morphological characters for precise classification, the \u0026ldquo;T\u0026rdquo;-shaped palatine is typically restricted to gobioid families with five branchiostegal rays, specifically Gobiidae and Oxudercidae (Regan, 1911; Hoese, 1984; Reichenbacher et al., 2020). Both families are highly diversified, with many species inhabiting muddy and silty environments (Nelson et al., 2016). Due to the missing number of osteological characters (especially otoliths, the hyoid arch with associated branchiostegal rays, and the endoskeleton of the unpaired fins), and thus a high degree of uncertainty, we refrain from attempting a classification to the lower level of the specimen.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eHowever, the fossil record in the Pacific Ocean does indeed preserve several semi-articulated gobioid species from the Plio-Pleistocene and Pleistocene deposits of Japan, namely species of the genera\u003cem\u003e\u0026nbsp;Tridentiger\u003c/em\u003e Gill, 1859,\u003cem\u003e\u0026nbsp;Chaenogobius\u003c/em\u003e Gill, 1859,\u003cem\u003e\u0026nbsp;Amblychaeturichthys\u0026nbsp;\u003c/em\u003eBleeker, 1874, and\u003cem\u003e\u0026nbsp;Rhinogobius\u003c/em\u003e Gill, 1859 (Uyeno \u0026amp; Iwato, 1975; Yabumoto, 1987; Yabumoto \u0026amp; Uyeno, 1994). The latter genus, with scales presenting numerous parallel radii (e.g., Yabumoto, 1987: fig. 5), especially resembles our Antipolo specimen. Until better-preserved specimens become available, the true systematic position remains unclear.\u003c/p\u003e\n\u003cp\u003eOrder Synbranchiformes Berg, 1940\u003c/p\u003e\n\u003cp\u003eSuborder Synbranchoidei Boulenger, 1904\u003c/p\u003e\n\u003cp\u003eFamily Synbranchidae Bonaparte, 1835\u003c/p\u003e\n\u003cp\u003eGenus \u003cem\u003eOphisternon\u003c/em\u003e McClelland, 1844\u003c/p\u003e\n\u003cp\u003e\u003cem\u003eOphisternon\u003c/em\u003e sp.\u003c/p\u003e\n\u003cp\u003eFigure 7\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eMaterial\u003c/strong\u003e: Single specimen COG-001 (Fig. 7) preserving the semi-articulated remains of the anterior part of the body, including the skull.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eDescription\u003c/strong\u003e: The remains of this small fish have a measurable length of about 48 mm, with a large head measuring ca 20 mm. The skull is preserved in a dorsolateral view, exposing its left side and part of the dorsi-cranium. However, the margins among individual bones are not discernable due to insufficient preservation. The neurocranium is antero-posteriorly elongated. Two protrusions are recognizable in the anterior part of the skull, interpreted to be remains of the palatine and lateral ethmoid. Both frontals are recognizable, and although their precise shape is indeterminable, they were strongly elongated. Two additional protrusions are preserved in the otic part of the skull and interpreted as remains of the sphenotic and pterotic. In the midline of the skull, a short occipital bone is recognizable with a short and low occipital crest.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eThe remains of the lower jaw preserve the anguloarticular, a small and rounded retroarticular, and partially the dentary. The remains of the dentary preserve an elongated lower limb reaching the level of articulation with quadrate bone (see arrow in Fig. 7B). Anguloarticular seems to have a more or less developed coronoid process. The articulation of the lower jaw is at the midpoint between the lateral ethmoid and sphenotic protrusions. Five short branchiostegal rays are recognizable, with the third one being the longest. The posterior-most tips of the branchiostegal rays reach the level of the ventral tip of the cleithrum. The cleithrum is crescent-shaped, anteriorly elongated, and relatively close to the skull. The posttemporal is developed, connecting the skull with the dorsal section of the pectoral girdle, and preserves a more robust and longer dorsal limb than the ventral one, which is about half as long and significantly thinner.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eOther remains of the skull and the attached pectoral girdle are not recognizable. A fragment of the vertebral column preserves the remains of 16 vertebral centra. While the first six vertebrae are preserved more or less in a dorso-ventral view, the more posterior ones are visible in lateral aspects. Skull fragments partly cover the anterior-most vertebra, but the socket-like articulation for the occipital part of the skull is determinable. The vertebrae bear well-developed lateral apophyses, particularly well discernable in the 9\u003csup\u003eth\u003c/sup\u003e and 10\u003csup\u003eth\u003c/sup\u003e vertebrae. These lateral apophyses served for articulation with pleural ribs. From the fifth vertebra onwards, the dorsal prezygapophyses and ventral postzygapophyses are recognizable\u0026mdash;these structures would likely be present in the more anteriorly located vertebrae, but the preservation does not allow clear determination. The pleural ribs are thin, set sub-horizontally, and recognizable in association with several vertebrae, including the second one (the condition of the first vertebra is not clear).\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eNotes\u003c/strong\u003e: Swamp eels (Synbranchiformes) are highly specialized percomorph fish classified within three families: Synbranchidae, Chaudhuriidae, and Mastacembelidae (Nelson et al., 2016). Wiley \u0026amp; Johnson (2010) listed six synapomorphies diagnosing this order. Although the specimen is relatively poorly preserved, the extension of the dentary postero-ventrally along the ventral margin of the anguloarticular supports its classification within this order.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eThe order is divided into two suborders: Mastacembeloidei and Synbranchoidei. Based on the partially recognizable articular plug of the first vertebra and the absence of the pectoral fin and scapulocoracoid, it is reasonable to assume that the specimen belongs to the suborder Synbranchoidei, which includes a single family, Synbranchidae (Wiley \u0026amp; Johnson, 2010). The entire family was revised by Rosen \u0026amp; Greenwood (1976), who recognized four separate genera: \u003cem\u003eMacrotrema\u003c/em\u003e Regan, 1912, \u003cem\u003eSynbranchus\u003c/em\u003e Bloch, 1795, \u003cem\u003eOphisternon\u003c/em\u003e McClelland, 1844, and \u003cem\u003eMonopterus\u003c/em\u003e Lacep\u0026egrave;de, 1800, but research of this peculiar group has progressed significantly in recent years, leading to a deeper understanding to its diversity and anatomy (see Kottelat, 2013; Britz et al., 2016, 2020a, b, c, 2023). Due to the presence of the posttemporal, which connects the pectoral girdle with the skull, the probable presence of the triangular coronoid process at the anguloarticular, and the distal tips of the branchiostegal rays reaching the level of the ventral tip of the cleithrum, it is possible to classify the specimen as a member of the genus \u003cem\u003eOphisternon\u003c/em\u003e (see Rosen \u0026amp; Greenwood, 1976). Although Rosen \u0026amp; Greenwood (1976) listed six branchiostegal rays for the genus, only five are recognizable in the fossil. This condition is interpreted as an artifact of preservation rather than a natural state.\u003c/p\u003e\n\u003cp\u003eAccording to Jamandre (2023), the only synbranchiform family native to the Philippines is Synbranchidae, which includes two species of the genus \u003cem\u003eMonopterus\u003c/em\u003e (\u003cem\u003eM. albus\u003c/em\u003e (Zuiew, 1793) and \u003cem\u003eM. javanensis\u003c/em\u003e (Lacep\u0026egrave;de, 1800)) and a single species of the genus \u003cem\u003eOphisternon\u003c/em\u003e (\u003cem\u003eO. bengalense\u003c/em\u003e McClelland, 1844). The genus \u003cem\u003eOphisternon\u003c/em\u003e is remarkably distributed from Middle and South America, Africa, India, Southeast Asia, and Australia (Rosen, 1975) and includes six species (Nelson et al., 2016). Due to insufficient and incomplete preservation, species determination is not possible at this moment. On the other hand, the specimen represents the first fossil record of swamp eels worldwide\u0026mdash;until now, only Holocene remains of synbranchids (\u003cem\u003eSynbranchus\u003c/em\u003e) were reported from southwestern Amazonia (Prestes-Carneiro et al., 2018, 2020).\u003c/p\u003e"},{"header":"Discussion","content":"\u003cp\u003e\u003cstrong\u003ePreservation and taphonomy\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe preservation of the dorosomatid and gobioid fossils shows well-discernable traces of disarticulation, which were likely caused by natural decomposition processes. The disarticulation of skulls, body parts, and fins suggests that these individuals, after death, remained in the free water column for some time but with minimal or no disturbances from the scavenger activities, as evidenced by the lack of obvious bio-erosion markers (Foronda et al., 1987). This condition could be explained by a hypoxic water layer just above the bottom of the basin, which would have inhibited scavenger activity. Subsequently, these partially disarticulated bodies were covered by additional fine sediment layers.\u003c/p\u003e\n\u003cp\u003eThis preservation mode is in strong contrast with that of the swamp eel specimen. The swamp eel is fully articulated, showing no signs of decomposition or disarticulation in the water column. Instead, it appears that the individual died and was immediately buried in the sediment.\u003c/p\u003e\n\u003cp\u003eThe presence of invertebrates, plant remains, and several fish specimens in various stages of decomposition and size suggests that there were very few, if any, selective agents, such as bottom currents, in the basin. This further indicates a relatively calm depositional environment, which likely contributed to the good preservation of the fossils. This assemblage, therefore, indicates an autochthonous assemblage with minimal postmortem sorting.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eBiogeographical implication\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe freshwater fossils in this study were identified under three families: Dorosomatidae, Gobiidae (or Oxudercidae), and Synbranchidae, with Dorosomatidae being the most abundant in the collection. Dorosomatidae has 30 valid genera and 116 species and is considered the second-largest family of Clupeiformes (Fricke et al., 2024). These species are globally distributed and primarily found in the tropics (Froese \u0026amp; Pauly, 2024). In the Philippines, two species have been reported: \u003cem\u003eHerklotsichthys quadrimaculatus\u003c/em\u003e, considered native and distributed in wetlands and river basins, and \u003cem\u003eSardinella tawilis\u003c/em\u003e, which is endemic to Lake Taal, Luzon Island (Jamandre, 2023).\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eThe family Gobiidae (167 genera and 1417 species) and Oxudercidae (110 genera and 726 species) are also widely distributed globally, inhabiting both tropical and temperate regions (Fricke et al., 2024). There are 89 species of Gobiidae recorded in the Philippines, making it the most diverse fish family in the country, with 20 species being endemic. These species are primarily found in the river basins of the three major islands: Luzon, Visayas, and Mindanao (Jamandre, 2023). In Taal Lake, genera such as \u003cem\u003eGlossogobius\u003c/em\u003e, \u003cem\u003eAcentrogobius\u003c/em\u003e, \u003cem\u003eOdontamblyopus\u003c/em\u003e, \u003cem\u003eOligolepis\u003c/em\u003e, \u003cem\u003ePsammagobius\u003c/em\u003e, and \u003cem\u003eRedigobius\u0026nbsp;\u003c/em\u003ehave been reported (Corpuz et al., 2016; Mutia et al., 2017). Four native species of Oxudercidae have been reported in the river basins and wetlands across the Philippines:\u003cem\u003e\u0026nbsp;Boleophthalmus boddarti, Periophthalmus argentilineatus\u003c/em\u003e, \u003cem\u003ePeriophthalmus kalolo\u003c/em\u003e, and \u003cem\u003ePeriophthalmus malaccensis\u003c/em\u003e (Jamandre, 2023).\u003c/p\u003e\n\u003cp\u003eSynbranchidae has seven valid genera and 27 valid species worldwide (Fricke et al., 2024). It is widely distributed across Southeast Asia, northern Australia, West Africa, and Middle America (Berra, 2001). In the Philippines, four species of swamp eels have been reported: the Asian swamp eels (\u003cem\u003eMonopterus albus\u003c/em\u003e, \u003cem\u003eMonopterus javanensis\u003c/em\u003e), freshwater mud eel (\u003cem\u003eMonopterus cuchia\u003c/em\u003e), and Bengal eel (\u003cem\u003eOphisternon bengalense\u003c/em\u003e) (Bucol et al., 2010; Guerrero, 2014; Roy et al., 2016). The Asian swamp eel was introduced to the Philippines in 1905 for aquaculture purposes in freshwater streams and rice paddies (Guerrero, 2014). However, there are no reports of synbranchids in Taal Lake.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003ePaleoenvironmental interpretation\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe presence of demersal swamp eels and gobioid fishes suggests an aquatic ecosystem with a silty and muddy bottom environment. The close association between species from both the pelagic (dorosomatids) and benthic zones indicates a shallow-water environment where these habitats overlapped.\u003c/p\u003e\n\u003cp\u003eAlthough some, if not all, of the identified taxa may tolerate brackish and/or marine environments, the deposits are best interpreted as freshwater. This interpretation is further supported by the presence of other freshwater elements, such as frog remains and leaf imprints, although these are not the primary focus of this study. The euryhaline fish assemblage in lacustrine freshwater deposits is reminiscent of the situation in Taal Lake (Herre, 1927), which transitioned from a brackish to a freshwater environment. A channel once connected Taal Lake to Balayan Bay in the South China Sea, which ultimately became isolated and freshwater due to the volcanic eruption of Taal Volcano in 1754 (Ramos, 2002). This environment is ideal for marine species to adapt to freshwater environments, as seen in \u003cem\u003eSardinella tawilis\u003c/em\u003e, the only extant species of \u003cem\u003eSardinella\u003c/em\u003e that inhabit freshwater (Herre, 1927).\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eComparison with other Pleistocene fish assemblages in the West Pacific\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eIn the West Pacific, the most extensively documented freshwater fish fossils from the Pleistocene are found in the deposits of Japan. Miyata (2019) provided a comprehensive and updated list of taxa and strata from the Quaternary of Japan. The fossil localities are predominantly centered in the western part of Japan, and the taxonomic composition is mainly confined to the most diverse orders of freshwater fishes: Cypriniformes and Siluriformes (e.g., Watanabe \u0026amp; Uyeno, 1999; Miyata et al.,\u0026nbsp;2018), with occasional occurrences of the family Gobiidae (Yabumoto,\u0026nbsp;1987).\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp;\u0026nbsp; In addition, Pleistocene deposits in southern Taiwan have yielded cypriniform pharyngeal teeth (Tao \u0026amp; Hu 2001). Uyeno (1978) also reported the presence of \u003cem\u003eCyprinus carpio\u003c/em\u003e (Cyprinidae) pharyngeal teeth and a bagrid spine (Bagridae, Siluriformes) from southern Taiwan, although the age of these fossils was not well-constrained, ranging from the Miocene to the Pleistocene. These represent the only known freshwater fish fossils from Taiwan and have been assigned to three species of cyprinids (Lin et al., 2021). Similarly, Yudha et al. (2020) compiled Pleistocene fish fossils from Java, highlighting fragmentary bones predominantly from Siluriformes, along with less common Cypriniformes and Anabantidae.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eFossil Cypriniformes are also abundantly represented in continental China, though most specimens are from the Paleogene and Neogene strata, with Quaternary fossils being relatively rarely described (Chang \u0026amp; Zhou, 1993; Chang \u0026amp; Chen, 2008). The Chinese records provide a broader temporal context for understanding the earlier evolutionary history of freshwater fishes in East Asia (Chang \u0026amp; Chen, 2000); however, since the Philippines is an archipelago with island systems that differ from the continental environments of China, we do not address further into this comparison.\u003c/p\u003e\n\u003cp\u003eThe taxonomic composition of these assemblages generally reflects the dominant role these major groups\u0026mdash;Cypriniformes and Siluriformes\u0026mdash;play in present-day freshwater fish communities. Although detailed documentation exists for articulate skeletons, particularly for the Japanese specimens, the fossils are often fragmentary, with pharyngeal teeth (Cypriniformes) and fin spines (Siluriformes) being the most frequently recorded elements.\u003c/p\u003e\n\u003cp\u003e\u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp;\u0026nbsp;In contrast, the Philippine assemblage we describe here stands out in terms of both taxonomic composition and preservation status. The discovery of this well-preserved freshwater fish fossil assemblage in the Philippines not only suggests a different paleoenvironmental condition, as discussed earlier, but, more importantly, it uncovers an extraordinary paleodiversity that has not been previously reported from the region.\u003c/p\u003e"},{"header":"Conclusion","content":"\u003cp\u003eThe fish fossil assemblage in the Pleistocene Laguna Formation is predominantly composed of the families Dorosomatidae, a Gobiidae \u003cem\u003evel\u003c/em\u003e Oxudercidae, and a Synbranchidae. These fossils represent the first records of freshwater fish in the Philippines and the first discovery of these taxa in the tropical West Pacific region. Notably, this study also provides the first-ever record of a synbranchid fossil globally. We infer that the environment of the Pleistocene Laguna Formation was freshwater, analogous to the modern Taal Lake. Our findings pave the way for promising scientific exploration and further understanding of the region\u0026apos;s paleodiversity.\u003c/p\u003e"},{"header":"Abbreviations","content":"\u003cdiv class=\"DefinitionList\"\u003e \u003cdiv class=\"DefinitionListEntry\"\u003e \u003cdiv class=\"Term\"\u003eAnatomical abbreviations: BD\u003c/div\u003e \u003cdiv class=\"Description\"\u003e \u003cp\u003ebody depth\u003c/p\u003e \u003c/div\u003e \u003c/div\u003e \u003cdiv class=\"DefinitionListEntry\"\u003e \u003cdiv class=\"Term\"\u003eCPD\u003c/div\u003e \u003cdiv class=\"Description\"\u003e \u003cp\u003ecaudal peduncle depth\u003c/p\u003e \u003c/div\u003e \u003c/div\u003e \u003cdiv class=\"DefinitionListEntry\"\u003e \u003cdiv class=\"Term\"\u003eCPL\u003c/div\u003e \u003cdiv class=\"Description\"\u003e \u003cp\u003ecaudal peduncle length\u003c/p\u003e \u003c/div\u003e \u003c/div\u003e \u003cdiv class=\"DefinitionListEntry\"\u003e \u003cdiv class=\"Term\"\u003eHD\u003c/div\u003e \u003cdiv class=\"Description\"\u003e \u003cp\u003ehead depth\u003c/p\u003e \u003c/div\u003e \u003c/div\u003e \u003cdiv class=\"DefinitionListEntry\"\u003e \u003cdiv class=\"Term\"\u003eHL\u003c/div\u003e \u003cdiv class=\"Description\"\u003e \u003cp\u003ehead length\u003c/p\u003e \u003c/div\u003e \u003c/div\u003e \u003cdiv class=\"DefinitionListEntry\"\u003e \u003cdiv class=\"Term\"\u003epre\u003c/div\u003e \u003cdiv class=\"Description\"\u003e \u003cp\u003eA\u0026ndash;preanal length\u003c/p\u003e \u003c/div\u003e \u003c/div\u003e \u003cdiv class=\"DefinitionListEntry\"\u003e \u003cdiv class=\"Term\"\u003epre\u003c/div\u003e \u003cdiv class=\"Description\"\u003e \u003cp\u003eD\u0026ndash;predorsal length\u003c/p\u003e \u003c/div\u003e \u003c/div\u003e \u003cdiv class=\"DefinitionListEntry\"\u003e \u003cdiv class=\"Term\"\u003eSL\u003c/div\u003e \u003cdiv class=\"Description\"\u003e \u003cp\u003estandard length\u003c/p\u003e \u003c/div\u003e \u003c/div\u003e \u003cdiv class=\"DefinitionListEntry\"\u003e \u003cdiv class=\"Term\"\u003eTL\u003c/div\u003e \u003cdiv class=\"Description\"\u003e \u003cp\u003etotal length. Institutional abbreviations: GLU\u0026ndash;Institute of Geology of the Czech Academy of Sciences, Prague\u003c/p\u003e \u003c/div\u003e \u003c/div\u003e \u003cdiv class=\"DefinitionListEntry\"\u003e \u003cdiv class=\"Term\"\u003eZRC\u003c/div\u003e \u003cdiv class=\"Description\"\u003e \u003cp\u003eZoological Reference Collection, formerly the Raffles Natural History Collection, Singapore.\u003c/p\u003e \u003c/div\u003e \u003c/div\u003e \u003c/div\u003e"},{"header":"Declarations","content":"\u003cp\u003e\u003cstrong\u003eAcknowledgments\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eDr. Heok Hui Tan (National University of Singapore) is acknowledged for providing a comparative X-ray of the \u003cem\u003eOphisternon\u003c/em\u003e, and Dr. Ralf Britz (Senckenberg Naturhistorische Sammlungen, Dresden) is acknowledged for providing PDF copies of his papers regarding the synbranchid fish.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAuthors\u0026rsquo; contributions\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eConceptualization: TP and C-HL. Methodology: TP, AC, AGF. Investigation and formal analyses: TP and AC. Visualization: TP, AC, AGF. Field work-sampling: AC, AGF, CM, KG, JRN. Supervision: TP, AGF, C-HL. Funding: TP, AGF, C-HL. Writing-original draft: TP, AGF, DM, C-HL. All authors contributed and approved the final manuscript.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eFunding\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThis study was mainly supported by the Mobility Plus Project between Academia Sinica, Taipei, Taiwan and the Czech Academy of Sciences, Czech Republic: Cenozoic fossil fishes from Taiwan and the Czech Republic \u0026ndash; the once thrived ichthyofaunas to Chien-Hsiang Lin and Tom\u0026aacute;\u0026scaron; Přikryl. Tom\u0026aacute;\u0026scaron; Přikryl is supported by the Institute of Geology of the Czech Academy of Sciences (RVO67985831). Chien-Hsiang Lin is also supported by the National Science and Technology Council, Taiwan (Grant No. 111-2116-M-001-033, 112-2116-M-001- 017-MY3) and Academia Sinica, Taipei, Taiwan. Allan Gil Fernando is supported by Emerging Interdisciplinary Research (EIDR) Program \u0026ldquo;Discovering the world of first hominins in the Philippines \u0026ndash; geology, palaeoenvironment, and palaeoecology of archaic hominins in the Philippines \u0026ndash; Project 3: Geological Environments\u0026rdquo; under the Office of the Vice President for Academic Affairs of the University of the Philippines. Funding and a travel grant were also given by the Taiwan International Internship Program (Academia Sinica) in 2023 to Abigael Castro\u0026rsquo;s summer internship at the Biodiversity Research Center, Academia Sinica.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAvailability of data and materials\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eAll studied/figured specimens were deposited at the National Institute of Geological Sciences-University of the Philippines (under the code COG) and at the National Museum of the Philippines (under the code NMP). All data generated or analyzed during this study are included in this published article or available from the corresponding author at reasonable request.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eEthics approval and consent to participate\u0026nbsp;\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eNot applicable\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eConsent for publication\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eAll authors declare the consent to publications.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eCompeting interests\u0026nbsp;\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe authors declare no competing interests.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\u003cli\u003e\u003cspan\u003eBerra, T.M. (2001). \u003cem\u003eFreshwater fish distribution\u003c/em\u003e. Academic Press, San Diego.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eBerg, L.S. (1940). Classification of fishes, both recent and fossil. 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(1793). \u003cem\u003eBiga Muraenarum\u003c/em\u003e, novae species. \u003cem\u003eNova Acta Academiae Scientiarum Imperialis Petropolitanae, 7\u003c/em\u003e(for 1789), 296\u0026ndash;301.\u003c/span\u003e\u003c/li\u003e\u003c/ol\u003e"},{"header":"Tables","content":"\u003cp\u003eTable 1 is available in the Supplementary Files section.\u003c/p\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":false,"highlight":"","institution":"","isAcceptedByJournal":true,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"swiss-journal-of-palaeontology","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"sjpa","sideBox":"Learn more about [Swiss Journal of Palaeontology](https://sjpp.springeropen.com/)","snPcode":"13358","submissionUrl":"https://submission.nature.com/new-submission/13358/3","title":"Swiss Journal of Palaeontology","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"stoa","reportingPortfolio":"Springer Open","inReviewEnabled":true,"inReviewRevisionsEnabled":true},"keywords":"Paleoichthyology, Dorosomatidae, Gobioidei, Synbranchidae, West Pacific","lastPublishedDoi":"10.21203/rs.3.rs-5055249/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-5055249/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eThis study offers a comprehensive analysis and detailed description of the fossil fish assemblage from the Pleistocene Laguna Formation in Luzon Island, Philippines. The fish fossils were collected from the deeper lacustrine facies of the formation, and a total of three fish families were identified. The identification is based on the recognizable synapomorphies in 10 moderately preserved semi-articulated individuals of ray-finned fish specimens, some of which include counterparts. The assemblage is predominantly composed of small clupeiforms of the family Dorosomatidae, accompanied by a gobioid fish (Gobiidae or Oxudercidae) and a synbranchid specimen (Synbranchidae), each represented by a single specimen. This taphocoenosis preserves free-swimming dorosomatids and demersal gobioid and synbranchid, suggesting an autochthonous assemblage with minimal postmortem sorting. Despite all recognized taxa being tolerant to changes in salinity, the environment is inferred to have been freshwater, analogous to modern Taal Lake, where a freshwater sardine thrives. This study represents the first systematic description of freshwater fish fossils in the Philippines and marks the first discovery of these taxa in the tropical West Pacific region.\u003c/p\u003e","manuscriptTitle":"Fossil fish assemblage of the Laguna Formation, Philippines: Unveiling the uniqueness of Pleistocene freshwater ecosystems in Southeast Asia","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2024-12-25 14:02:48","doi":"10.21203/rs.3.rs-5055249/v1","editorialEvents":[{"type":"communityComments","content":0},{"type":"decision","content":"Revision requested","date":"2024-12-10T01:53:49+00:00","index":"","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2024-12-09T19:17:30+00:00","index":"hide","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2024-11-19T18:48:36+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"264164899018609431659836492385307004843","date":"2024-11-11T15:13:11+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"286494342306795549068472418258766333252","date":"2024-11-08T18:20:58+00:00","index":"hide","fulltext":""},{"type":"reviewersInvited","content":"","date":"2024-11-01T13:16:44+00:00","index":"","fulltext":""},{"type":"editorAssigned","content":"","date":"2024-09-10T10:37:03+00:00","index":"","fulltext":""},{"type":"checksComplete","content":"","date":"2024-09-10T10:35:24+00:00","index":"","fulltext":""},{"type":"submitted","content":"Swiss Journal of Palaeontology","date":"2024-09-09T04:47:42+00:00","index":"","fulltext":""}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"swiss-journal-of-palaeontology","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"sjpa","sideBox":"Learn more about [Swiss Journal of Palaeontology](https://sjpp.springeropen.com/)","snPcode":"13358","submissionUrl":"https://submission.nature.com/new-submission/13358/3","title":"Swiss Journal of Palaeontology","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"stoa","reportingPortfolio":"Springer Open","inReviewEnabled":true,"inReviewRevisionsEnabled":true}}],"origin":"","ownerIdentity":"99b2d28a-6a39-42ae-a5b8-9b3b504092e0","owner":[],"postedDate":"December 25th, 2024","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"published-in-journal","subjectAreas":[],"tags":[],"updatedAt":"2025-01-20T15:58:15+00:00","versionOfRecord":{"articleIdentity":"rs-5055249","link":"https://doi.org/10.1186/s13358-024-00347-0","journal":{"identity":"swiss-journal-of-palaeontology","isVorOnly":false,"title":"Swiss Journal of Palaeontology"},"publishedOn":"2025-01-16 15:56:50","publishedOnDateReadable":"January 16th, 2025"},"versionCreatedAt":"2024-12-25 14:02:48","video":"","vorDoi":"10.1186/s13358-024-00347-0","vorDoiUrl":"https://doi.org/10.1186/s13358-024-00347-0","workflowStages":[]},"version":"v1","identity":"rs-5055249","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-5055249","identity":"rs-5055249","version":["v1"]},"buildId":"qtupq5eGEP_6zYnWcrvyt","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}