Cretaceous Burmese Amber First Reveals the Nearly 100-Million-Year-Old Gregarious Molting Behavior in Scorpions

Cretaceous Burmese Amber First Reveals the Nearly 100-Million-Year-Old Gregarious Molting Behavior in Scorpions | 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 Article Cretaceous Burmese Amber First Reveals the Nearly 100-Million-Year-Old Gregarious Molting Behavior in Scorpions Weijia Huang This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-8604487/v1 This work is licensed under a CC BY 4.0 License Status: Posted Version 1 posted You are reading this latest preprint version Abstract This study reports the first evidence of social aggregation and suspected group molting behavior in Cretaceous scorpions from Burmese amber (approximately 98.79 million years ago). Morphological observations of three scorpion exuvial fossil samples revealed that Cretaceous scorpions underwent synchronized ecdysis on resin-coated arboreal substrates—a striking contrast to the cryptic or cave-dwelling molting habits exhibited by extant scorpions. Taphonomic evidence has revealed rapid resin entombment during active exuviation, preserving exoskeletal separation patterns and postmolt aggregation. Critically, this finding confirms that scorpion gregarious molting dates back nearly 100 million years, and highlights scenarios in which scorpion molting behavior occurred outside caves during the Cretaceous period. redefining our understanding of arthropod behavioral complexity during the Cenomanian. Biological sciences/Evolution/Palaeontology Biological sciences/Ecology/Behavioural ecology Scorpiones Ecdysis Burmese amber Cretaceous (Cenomanian) Palaeoethology Gregarious behaviour Figures Figure 1 Figure 2 Figure 3 Figure 4 1. Introduction Understanding the processes and behaviors of molting in the fossil record is crucial, as this life-history strategy imposes constraints on morphology throughout the evolution of modern animal groups (Daley & Drage, 2016 ). Scorpions, as a significant biological group within the class Arachnida, have been a focal subject in ecological, evolutionary biological, and biodiversity studies because of their unique biological characteristics and ecological functions. Currently, the order Scorpiones comprises 24 families with approximately 2900 extant species and subspecies, alongside 111 extinct species (The Scorpion Files website < https://www.ntnu.no/ub/scorpion-files/%3E ). This numerical estimate fluctuates according to geographical variations and new taxonomic discoveries. Up to now, the comprehensive statistics of scorpion taxa (Scorpiones) in Burmese amber are available for reference in the attachment.The modern parvorders ​ ​ Buthida​​ and ​​Iurida​​ likely existed during the Triassic period. Families, including Buthidae, Chaerilidae, Chactidae, and Hormuridae, all existed in the Cretaceous. ​​Euscorpiidae​​ is known from the Palaeogene, while ​​Scorpionidae​​ may (although unconfirmed) date back to the Neogene period (Dunlop&Garwood, 2024). Burma Amber Fauna Shows Connections to Gondwana and Transported Gondwanan Lineages to the Northern Hemisphere (Wood& Wunderlich, 2023 ). These specimens not only fill the Jurassic-Cretaceous gap in scorpion fossil records but also provide critical insights into early scorpion diversification through behavioral evidence (e.g., predation remnants, maternal care) and phylogenetic signals (e.g., cheliceral articulation, pectine structure). Notably, several ancient representatives of extant scorpion families have been discovered in Burmese amber. For example, Electrochaerilus buckleyi represents the earliest record of the family Chaerilidae, while Archaeoananteroides and Cretaceousbuthus are assigned to the family Buthidae. These findings indicate that some modern scorpion families had already emerged by the mid-Cretaceous and have undergone stable evolution for at least 100 million years. Scorpions preserved in Burmese amber exhibit remarkable variations in body size and morphology. Most of these amber-entombed scorpions are small-sized (usually 1–2 cm in body length), a trait that may not only reflect their actual size distribution but also result from sampling bias—smaller individuals are more readily entrapped by tree resin. In terms of morphological characteristics, these fossil scorpions display a mosaic of plesiomorphic and apomorphic traits, providing critical evidence for reconstructing the evolutionary history of scorpions. The rigid exoskeletons of arthropods primarily serve a protective function and are shed between developmental stages through a uniquely evolutionarily conserved pattern. Molting is triggered by steroid hormones (Cheong et al., 2015 ). During the molting process, all exoskeletal structures are shed simultaneously. (Francke, O. F., & Sissom, W. D. 1984 ). When the humidity and environmental safety requirements are met, a new epidermis forms under the old skin, and the old exoskeleton splits along the midline of the cephalothorax and abdomen. The scorpion body then releases its claw, leg, and posterior abdomen through muscle twisting and gravity, and the arthropod initiates ecdysis through coordinated cheliceral and pedipalpal movements, coupled with internal physiological processes that rupture the pleural membrane ventral to the anterior and lateral margins of the carapace. The carapace is then elevated from beneath, creating an opening through which the animal emerges—typically in a prone position. Whether emerging in a prone or supine position, the resulting exuvium remains relatively intact, encompassing the cuticle of booklungs, bristles, and sensilla, with only a wedge-shaped aperture at the anterior end marking the site of escape (Gaban, R. D&Farley, R. D, 2002 ). During the molting process, arthropods are unable to escape threats and are prone to desiccation. As a result, they have evolved new behavioral adaptations, such as group molting behavior and seeking shelter during molting (Daley & Drage, 2016 ). Solitary molting behavior is widely adopted by most modern arthropods, whereas gregarious molting has also been observed in various taxa, including spiders (Kim et al.,2001), shrimp (Webster et al.,1982), prawns (Howe et al.,1981), crabs, and springtails (Leinaas et al.,1983) (Stone et al.,1999). First-instar scorpions climb onto their mothers after birth to continue developing and undergo their first molting (Polis, G.A.1991) (Brownell et al.,2011). The young second-instar scorpions then disperse and exist independently. Thereafter, they typically undergo several additional molts over an extended period ranging from 6–83 months before reaching maturity (Polis, G.A.1991) (Brownell et al.,2011). Most scorpion species undergo ecdysis within subterranean retreats, rock crevices, bark shelters, or other protected microhabitats rather than exposed environments. However, contemporary observations document occasional surface molting events under specific ecological conditions, although such behavior remains atypical. While these exceptional cases demonstrate environmental plasticity in molting site selection, such behavior remains nonsystematic and context dependent. The predominant evolutionary strategy maintains concealed and secure locations for ecdysis. Fossilized exuviae are relatively rare in the fossil record because of the fragility of the material and specific preservation conditions. However, in certain depositional environments, such as lakes or wetlands, exuviae may be rapidly buried, increasing their chances of preservation. Some Palaeozoic fossils and research on modern arthropods have shown that exuviae are closely related to the growth stages of arthropods because they grow in larger increments with each molt or achieve the same size with fewer molts (Grunert LW et al.,2015; Liu & Zhang, 2015 ). Molting not only is a marker of growth but also may be related to ecological adaptation (Liu & Zhang,2015). The preservation and characteristics of exuviae provide valuable information about past ecological conditions. For example, some arthropod exuviae found in specific sedimentary layers are linked to certain environmental factors, such as water depth, temperature, and salinity. These studies help reconstruct ancient ecosystems. The synchronization of molting can be triggered by external abiotic factors, and Johnson et al. ( 1957 ) reported synchronized molting in aphids, which they attributed to circadian rhythms. In terrestrial arthropods, synchronized molting is often induced through pheromone communication.This phenomenon has been confirmed in colonial spiders (Fowler HG&Gobbi N,1998) and gregarious springtails (Leinaas HP, 1983 ). Large-scale molting to reduce cannibalism also applies to other cases of synchronized molting in predatory species (Huang et al., 2013). Although Burmese amber preserves abundant scorpion fossils, no unequivocal evidence of gregarious molting behavior in scorpions has been documented to date. Studying fossilized arthropod remains and exuviae is crucial for a more comprehensive and precise assessment of their roles in ecosystems (McCoy, e.g., McCoy and Brandt,2009). 2. Materials and methods The newly described samples in this study are preserved in Burmese amber sourced from the Hukawng Valley of Kachin State, northern Myanmar (Grimaldi et al., 2002 ; Cruickshank and Ko, 2003 ; for geological images, refer to Guo et al. 2024 , Cretaceous Research 164, 105968). Radiometric dating of zircons conducted by Shi et al. ( 2012 ) established the amber depositional age as the earliest Cenomanian stage of the Late Cretaceous, with precise U–Pb dating yielding an absolute age of 98.79 ± 0.62 million years ago. The samples are deposited in the Blue Miracle Museum, Guangzhou, China, and are available for examination upon request. These amber samples were legally acquired prior to the closure of the Kachin amber mines by the Myanmar military in November 2017 and therefore bear no association with any ongoing armed conflicts or ethnic tensions in Myanmar. 3. Systematic palaeontology Order Scorpiones Koch, 1837 Suborder Neoscorpionina Thorell & Lindström, 1885 Infraorder Orthosterni Pocock, 1911 Superfamily Buthoidea C. L. Koch, 1837 Family Buthidae C. L. Koch, 1837 Genus and species undetermined Material: BMM 4772 (FIGURE 2 : A.B.D.). BMM4773 (FIGURE 3 ). BMM4773 (FIGURE 2 :C) Depository: Blue Miracle Museum, Guangzhou, China. Given the obscuration of diagnostic characteristics (e.g., pectinal tooth count, cheliceral dentition) by carbonization, the samples should be tentatively assigned to the Incertae familiae within Scorpiones ​ ​ rather than a specific genus. Notably, the high carbonization of arthrodial regions and the deformation of exuvial remains caused by resin flow render the measurement data meaningless, and prone to induce errors in species identification. The primary focus of this paper remains the ecological characteristics provided by the samples themselves. The sample (BMM 4772) exhibited moderate preservation with notable carbonization of the principal structure, rendering diagnostic morphological details noninformative and susceptible to erroneous interpretation. Three exuviae are observable on the right lateral aspect in a dorsoventral arrangement, wherein exuvium A and exuvium B retain well-defined ecdysial cleavage lines and exuvial fractures. The cuticle ruptures along the lateral sides and anterior margin of the carapace, whereas the third exuvium preserves a nearly complete telson (FIGURE. 2: A.D). The left ventrolateral region contains three carbonized scorpion samples in tight sequential alignment: the first retains the anterior body portion, whereas the second and third preserve the major mesosoma. Their spatial separation from the exuviae, combined with angular displacement, suggests that postmolt displacement is potentially mediated by the active avoidance of resin flow or passive transport via resin dynamics. A solitary insect inclusion is preserved within the central amber matrix. For sample BMM 4773 (FIGURE 3 ), a transparent and impurity-free amber resin rapidly and completely encapsulates a scorpion during molting. Most of the scorpion's body had already detached from the exuviated shell, and traces of struggling and disturbance were generated because it was rapidly encapsulated by the resin. In the amber of specimen BMM4774 (FIGURE 2 :C), three aggregated scorpion bodies are enclosed. Scorpion 5 preserves the complete telson, and scorpion 6 preserves the anterior body, including two pedipalps and partial body segments. Scorpion 7 preserves a part of the anterior body and a part of the leg. Supplementary reference (FIGURES 8–9) 4. Ecology ​​Molting represents an evolutionarily conserved adaptive strategy.​​ In modern social species, such as Opisthacanthus cayaporum , most, if not all, juveniles molt within a short period of time, usually on the same night. This behavior indicates the presence of a group effect (Lourenço, W. R. 2018 ).The co-occurrence of multiple molting individuals suggests that this behavior stabilized by the mid-Cretaceous (~ 98.79 million years ago).(FIGURE 1 ). The correlation between molting frequency and body size increases (0.5–0.7 cm per molt) (Polis, 1990 ) ​ ​. demonstrates precise resource allocation. This periodicity likely originated as an adaptation to terrestrial aridity, where molting repaired the osmoregulatory function of the exoskeleton. Notably, Lebanese amber from similar palaeoclimates lacks such behavior, potentially due to differences in resin viscosity or polymerization rates (Grimaldi et al., 2002 ). Palaeoclimatic reconstructions indicate a substantially elevated global thermal regime during the amber deposition period (early Cenomanian), which is supported by substantial data that warm conditions at high latitudes were the norm, with a distinct long-term climatic stasis persisting until the Late Cretaceous. (Voigt et al., 2003 ; Price et al., 2013 ). This multiproxy evidence collectively supports the interpretation of a stable warm-tropical palaeoenvironment The discovery of gregarious molting in Burmese amber must be contextualized within global Cretaceous arachnid fossil records. The main known Cretaceous amber deposits are located between 34°N and 74°N, and the palaeolatitude (10–15°N) of the amber deposits in Myanmar and Lebanon is the southernmost of all the major Cretaceous amber deposits. Compared with contemporaneous amber deposits (e.g., Lebanese amber [~ 130 Ma], Spanish Álava amber [~ 105 Ma]) and sedimentary strata (e.g., Jehol Biota [~ 125 Ma], China), Burmese amber deposits present greater scorpion fossil abundance, completeness, and behavioral preservation (Supplementary). Lebanese amber yields only one solitary species ( Archaeobuthus estephani ) (Lourenço WR. 2001 ); Spanish Álava amber lacks any scorpion fossils (Delcl`os X et al., 2007). This disparity highlights the controlling role of palaeoclimate on behaviour: Myanmar's tropical rainforest (palaeolatitude 5°N) (Shi et al., 2012 ) provides optimal conditions for synchronous moulting, and the Jehol Biota experiences a temperate humid climate with seasonal variability, which contrasts with the stable tropics of Myanmar (Zhou et al., 2003; T. Yu et al., 2019 ) and appears more suitable for a burrowing moulting strategy, resulting in a stark contrast between the two. Moreover, Palaeoeus corpius gallicus from French amber (45.94°N) inhabited warm and humid estuarine environments during the same period (Lourenço, Wilson R. 2003). During the Early Cretaceous (~ 145–100 Ma), atmospheric oxygen levels began rising from approximately 15%. They peaked at approximately 23% during the Aptian–Albian stages (~ 125–100 Ma). During the Late Cretaceous (~ 100–66 Ma), the oxygen content gradually decreased from this peak to approximately 18%. (Clapham & Karr 2012). During the Cenomanian (94–100 Ma), atmospheric CO 2 peaked at 1,700 ppm (Wang et al., 2014). Gregarious molting likely represents an ​​adaptive innovation under Cretaceous greenhouse crises​​. The mid-Cretaceous (∼124–90 MA) was a transitional period in the nature of the marine climate system (Leckie,2002). The Burmese amber fauna, which is situated in a coastal environment, was inevitably affected. The reorganization of the global biosphere during the mid-Cretaceous paralleled significant changes in the marine climate system, i ndicating a causal relationship between biological evolution and environmental stimuli. From a stratigraphic perspective (Fischer and Arthur, 1977 ; Rich et al., 1986; Leckie, 1989; Thurow et al., 1992 ; Vermeij, 1995 ), the communal molting horizon (98.79 ± 0.62 MA) provides a precise temporal anchor for studying biological responses to greenhouse environments. However, owing to the insufficient quantity of collected samples, a small number of Burmese amber samples cannot cover the complete time span. Therefore, the relationship between this behavioral pattern and Cretaceous climate change requires cautious discussion. At present, this remains merely a plausible conjecture—there is, in fact, no evidence to support the claim that synchronous molting behavior is exclusive to tropical climates or that it emerged only in the Cretaceous period and not earlier. 5. Discussion According to Buskirk ( 1981 ), social behavior is exhibited by only a small number of arachnid species. In scorpions, "social phases" typically involve only interactions during mating and the maternal care of offspring from birth to shortly after their first molt. However, multiple field reports on scorpion aggregations have shown that such groups consist of both unrelated and related individuals. These observations, summarized by Polis & Lourenço ( 1986 ), suggest that living in groups may be a significant trait of certain scorpion species. Synchronized molting may have been facilitated by chemical communication, analogous to modern crustaceans and spiders (Howe et al.,1981; Kim et al.,2001), although direct fossil evidence is lacking. In specimen BMM 4772, the phenomenon of three exuviae being spatially separated from three scorpion bodies can alternatively be interpreted as passive aggregation caused by resin flow rather than active clustering behavior. In modern times, clustered molting is predominantly observed in viviparous and ovoviviparous juveniles (e.g., Pandinus imperator , Hadrurus arizonensis ; Polis, G. A. (ed.), 1990), reflecting maternal care or shared microhabitat needs. Most neonate scorpions molt and survive on their mother’s back, with an approximate survival rate of 100%. In the absence of maternal care, these juveniles survive through aggregative molting behavior on sandy substrates (89.83% ± 1.91%). In modern scorpions, first-instar juveniles aggregating on the mother scorpion have unsclerotized exoskeletons and remain tightly connected via leg entanglement and maternal secretions, which provide fixed anchorage for molting. Once molting is complete and they enter the second instar, their distal claws mature, enabling them to detach from the mother and acquire molting ability through their own anchoring strength. As distal claws are unnecessary during molting, the molting behavior of older scorpions transitions from aggregating to solitary. Currently, the survival rate of first-instar juveniles living solitarily in the wild—even when simulating maternal environments through aggregative molting—remains at only ~ 50% (Guo et al.,2024). This modern scenario clearly cannot explain the Cretaceous observations, rendering the social molting behavior of ancient scorpions more perplexing. However, we can confirm that social behavior in Cretaceous scorpions has occurred and that they do not always act independently (FIGURE 2 ). On the basis of the current size of the samples, these scorpions do not appear to be in their first instar. That is, their aggregation is most likely a form of social gathering behavior rather than a result of detachment from their mother. We welcome further discussions regarding this hypothesis. Another possibility merits consideration: In many extant species, individuals remove their exuviae from the shelter postmolting. These shed exoskeletons may accumulate and become encased by flowing amber resin. While this scenario is plausible, observations reveal that the exuviae preserved in the amber are remarkably clean and intact—devoid of soil and debris that would have adhered to them if they were transported from a burrow, and showed no significant distortion. They thus do not appear to have been intentionally carried from burrows to resin-flowing areas (which are inherently hazardous) for deposition. The simultaneous occurrence of numerous such exuviae within a single amber inclusion further confirms that molting occurred synchronously among a gathering of individuals. However, regardless of the interpretation, scorpion molting clearly did not occur inside caves with abundant soil. On the basis of the fossil samples, we reconstructed an artistic interpretation of scorpions engaged in postmolting aggregation, partially engulfed by resin, to visualize this palaeobiological behavior (FIGURE 4 ). Notably, the samples maintained a degree of aggregation postmolting rather than being immediately dispersed. While current evidence cannot confirm whether this clustered behavior potentially facilitates group mating after ecdysis, such evolutionary adaptation should not be entirely dismissed. It cannot be ruled out that taphonomic processes (such as resin flow) may have caused spatial aggregation of organisms. New material shows evidence for gregarious behaviour, and perhaps synchronized moulting, in subadults, possibly belongs to the buthid family. These hypotheses warrant further investigation. We anticipate that future discoveries of analogous palaeontological materials will yield additional insights into these arthropod social behaviors and life history strategies. 6. Conclusion This specimen provides a novel observational perspective. This completely transparent and impurity-free amber demonstrates that the molting event occurred not within caves or on muddy ground but rather on tree trunks, showing marked divergence from modern species' behavioral patterns. Moreover, this species may be semi-gregarious with a large population.This fossil represents the earliest known record of gregarious postmolting behavior in ancient scorpions. Declarations Ethics approval and consent to participate Not applicable Permission Statement The specimen is deposited in the Blue Miracle Museum, identified by the authors of this paper, and permission for its use has been obtained. Author contributions All the authors contributed to manuscript preparation and critical revision. All the authors have modified, finalized and approved the manuscript. Institutional review board statement Not applicable. Informed consent statement Not applicable. Funding declaration not applicable Data availability statement All the data are reported in this paper. Conflicts of interest The authors declare that they have no conflicts of interest. Acknowledgements We express our gratitude to Maria Rose Petrizzo, PhD. We would like to thank Dr. Andy Ross from National Museums Scotland for his assistance with the statistics of scorpion species. We would like to thank the anonymous reviewers for their valuable comments and suggestions which have helped improve the manuscript. References Alonso, A., et al., 2000. A new fossil resin with biological inclusions in Lower Cretaceous deposits from Álava (northern Spain, Basque-Cantabrian Basin). J. Palaeontol. 74, 158–178. https://doi.org/10.1017/S002233600003434X. Brownell, P., Polis, G., 2001. Scorpion Biology and Research. Oxford Univ. Press, Oxford. Buskirk, R. E., 1981. Sociality in the arachnida. Soc. Insects 2, 281–367. Cruickshank, R. D., Ko, K., 2003. 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Earth Sci. 92, 285–299. https://doi.org/10.1007/s00531-003-0371-1. Additional Declarations There is NO Competing Interest. Supplementary Files SupplementaryCompletestatistics.docx Supplementary Complete statistics Supplementaryreference.docx Supplementary reference Cite Share Download PDF Status: Posted Version 1 posted You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. As a division of Research Square Company, we’re committed to making research communication faster, fairer, and more useful. We do this by developing innovative software and high quality services for the global research community. Our growing team is made up of researchers and industry professionals working together to solve the most critical problems facing scientific publishing. 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-8604487","acceptedTermsAndConditions":true,"allowDirectSubmit":true,"archivedVersions":[],"articleType":"Article","associatedPublications":[],"authors":[{"id":575223380,"identity":"d66a2d56-edba-4102-87f6-9a4ead5f2592","order_by":0,"name":"Weijia Huang","email":"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAAA3UlEQVRIiWNgGAWjYLCCDwYScvzszQcOfCBWB+OMAgtjyZ5jiQdnEKuFmedDReKGGznGh3mIUc53I/nYBx4DCWOGM8cSDtuUWTPwt3cn4NUieSMteYYE0C+M7c0HDuecS2eQOHN2A14tBrdzjBkMgLYw8wBtyW07zGAgkUtIS/5nhgQDicQ2iRyDw5bEaclhZjgA1NID0sJIjBbJ+8+MGRuADpMAOuxgz7l0HoJ+4Ttz+DHznz91cvbHmw9/+FFmLcff3otfC8MBFB4bMxFRg66FsI5RMApGwSgYcQAAGbhN+B+jwHkAAAAASUVORK5CYII=","orcid":"https://orcid.org/0000-0002-5112-8931","institution":"Blue Miracle Museum","correspondingAuthor":true,"prefix":"","firstName":"Weijia","middleName":"","lastName":"Huang","suffix":""}],"badges":[],"createdAt":"2026-01-14 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13:20:15","extension":"jpg","order_by":11,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":891477,"visible":true,"origin":"","legend":"","description":"","filename":"fig1.jpg","url":"https://assets-eu.researchsquare.com/files/rs-8604487/v1/ee9475888ee6d2ee3a6aaa27.jpg"},{"id":100415048,"identity":"04c252ec-6efb-4807-b532-89e242566ddc","added_by":"auto","created_at":"2026-01-16 13:20:26","extension":"png","order_by":12,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":15579236,"visible":true,"origin":"","legend":"","description":"","filename":"fig2.png","url":"https://assets-eu.researchsquare.com/files/rs-8604487/v1/854853ad341de17f9508c6f8.png"},{"id":100414976,"identity":"28c5d8b7-2445-4d90-ad64-6b264c1a9820","added_by":"auto","created_at":"2026-01-16 13:20:23","extension":"xml","order_by":18,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":76106,"visible":true,"origin":"","legend":"","description":"","filename":"COMMSBIO2605150structuring.xml","url":"https://assets-eu.researchsquare.com/files/rs-8604487/v1/7cd88a82cd465ed2f7efe0fd.xml"},{"id":100414883,"identity":"a644b1fb-6c50-4c98-aa17-11a4c1f6a1b7","added_by":"auto","created_at":"2026-01-16 13:20:13","extension":"html","order_by":19,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":86765,"visible":true,"origin":"","legend":"","description":"","filename":"earlyproof.html","url":"https://assets-eu.researchsquare.com/files/rs-8604487/v1/6710afc03eb07c6b6ecc2102.html"},{"id":100414897,"identity":"a18b5136-f3c7-46c6-a800-9217a19e340a","added_by":"auto","created_at":"2026-01-16 13:20:15","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":905073,"visible":true,"origin":"","legend":"\u003cp\u003eCretaceous paleogeographic maps and modern maps showing the amber-producing area of the Hukawng Valley in Myanmar, which is located near Tanaing Township, Myitkyina District, Kachin State, Myanmar. The modern map is sourced from Google Maps.\u003c/p\u003e","description":"","filename":"1.png","url":"https://assets-eu.researchsquare.com/files/rs-8604487/v1/9f89bfd36677d92c24a33626.png"},{"id":100414850,"identity":"e5fee408-a2ad-42c5-97f9-e40f03740548","added_by":"auto","created_at":"2026-01-16 13:20:10","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":626834,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eA\u003c/strong\u003e: BMM 4772;The right lateral portion of the amber fossil presents three exuviae in its morphological structure. \u003cstrong\u003eB\u003c/strong\u003e: BMM 4772;The left ventrolateral morphology of the amber fossil displays three scenes. \u003cstrong\u003eC\u003c/strong\u003e: BMM4773;three scorpions are aggregated in the same piece of amber, each preserving a part of their bodies (BMM4774).\u003cstrong\u003eD\u003c/strong\u003e: BMM 4772;The complete morphology of the amber fossil reveals 3 scorpions and 3 exuviae.\u003c/p\u003e","description":"","filename":"2.png","url":"https://assets-eu.researchsquare.com/files/rs-8604487/v1/4be3346ba8af8bb0851a264a.png"},{"id":100414895,"identity":"cff1b9a1-13ad-4e5d-ac41-ffc9bdf19acb","added_by":"auto","created_at":"2026-01-16 13:20:14","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":442684,"visible":true,"origin":"","legend":"\u003cp\u003eDuring molting, a scorpion is enclosed in a piece of transparent amber (BMM 4773).\u003c/p\u003e","description":"","filename":"3.png","url":"https://assets-eu.researchsquare.com/files/rs-8604487/v1/224990c78626fbcdcbb134cd.png"},{"id":100414851,"identity":"e438e55c-4eb5-4070-b15c-c2da5fc6b035","added_by":"auto","created_at":"2026-01-16 13:20:10","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":721913,"visible":true,"origin":"","legend":"\u003cp\u003eArtistic reconstruction. This restored illustration simulates a scene where a group of scorpions gather on a fallen tree trunk to molt, with resin flowing around them. Modern scorpions were used as props, and AI-simulated flowing resin was added during postproduction to enhance realism.\u003c/p\u003e","description":"","filename":"4.png","url":"https://assets-eu.researchsquare.com/files/rs-8604487/v1/2909f605f2aeb7a8f4fe26ca.png"},{"id":100547193,"identity":"79a4a7b1-f7d5-4dad-9929-d75efeece922","added_by":"auto","created_at":"2026-01-19 08:14:50","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":3588247,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-8604487/v1/bd3c144e-168c-42dd-b41c-8015fec96fd4.pdf"},{"id":100415142,"identity":"9a243ebd-167e-413b-8de4-6788c73c6c81","added_by":"auto","created_at":"2026-01-16 13:20:35","extension":"docx","order_by":1,"title":"","display":"","copyAsset":false,"role":"supplement","size":29000,"visible":true,"origin":"","legend":"Supplementary Complete statistics","description":"","filename":"SupplementaryCompletestatistics.docx","url":"https://assets-eu.researchsquare.com/files/rs-8604487/v1/563019078aac7e9b2aa4e6ca.docx"},{"id":100414908,"identity":"0d873577-44c2-432e-950a-55e5ed677ac5","added_by":"auto","created_at":"2026-01-16 13:20:17","extension":"docx","order_by":3,"title":"","display":"","copyAsset":false,"role":"supplement","size":685312,"visible":true,"origin":"","legend":"Supplementary reference","description":"","filename":"Supplementaryreference.docx","url":"https://assets-eu.researchsquare.com/files/rs-8604487/v1/f4b9f18e689275f38122bfd7.docx"}],"financialInterests":"There is \u003cb\u003eNO\u003c/b\u003e Competing Interest.","formattedTitle":"Cretaceous Burmese Amber First Reveals the Nearly 100-Million-Year-Old Gregarious Molting Behavior in Scorpions","fulltext":[{"header":"1. Introduction","content":"\u003cp\u003eUnderstanding the processes and behaviors of molting in the fossil record is crucial, as this life-history strategy imposes constraints on morphology throughout the evolution of modern animal groups (Daley \u0026amp; Drage, \u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e2016\u003c/span\u003e). Scorpions, as a significant biological group within the class Arachnida, have been a focal subject in ecological, evolutionary biological, and biodiversity studies because of their unique biological characteristics and ecological functions. Currently, the order Scorpiones comprises 24 families with approximately 2900 extant species and subspecies, alongside 111 extinct species (The Scorpion Files website\u0026thinsp;\u0026lt;\u0026thinsp;\u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://www.ntnu.no/ub/scorpion-files/%3E\u003c/span\u003e\u003cspan address=\"https://www.ntnu.no/ub/scorpion-files/%3E\" targettype=\"URL\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e). This numerical estimate fluctuates according to geographical variations and new taxonomic discoveries. Up to now, the comprehensive statistics of scorpion taxa (Scorpiones) in Burmese amber are available for reference in the attachment.The modern parvorders ​\u003cb\u003e​\u003c/b\u003eButhida​​ and ​​Iurida​​ likely existed during the Triassic period. Families, including Buthidae, Chaerilidae, Chactidae, and Hormuridae, all existed in the Cretaceous. ​​Euscorpiidae​​ is known from the Palaeogene, while ​​Scorpionidae​​ may (although unconfirmed) date back to the Neogene period (Dunlop\u0026amp;Garwood, 2024). Burma Amber Fauna Shows Connections to Gondwana and Transported Gondwanan Lineages to the Northern Hemisphere (Wood\u0026amp; Wunderlich, \u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e2023\u003c/span\u003e). These specimens not only fill the Jurassic-Cretaceous gap in scorpion fossil records but also provide critical insights into early scorpion diversification through behavioral evidence (e.g., predation remnants, maternal care) and phylogenetic signals (e.g., cheliceral articulation, pectine structure). Notably, several ancient representatives of extant scorpion families have been discovered in Burmese amber. For example, \u003cem\u003eElectrochaerilus buckleyi\u003c/em\u003e represents the earliest record of the family Chaerilidae, while \u003cem\u003eArchaeoananteroides\u003c/em\u003e and \u003cem\u003eCretaceousbuthus\u003c/em\u003e are assigned to the family Buthidae. These findings indicate that some modern scorpion families had already emerged by the mid-Cretaceous and have undergone stable evolution for at least 100\u0026nbsp;million years. Scorpions preserved in Burmese amber exhibit remarkable variations in body size and morphology. Most of these amber-entombed scorpions are small-sized (usually 1\u0026ndash;2 cm in body length), a trait that may not only reflect their actual size distribution but also result from sampling bias\u0026mdash;smaller individuals are more readily entrapped by tree resin. In terms of morphological characteristics, these fossil scorpions display a mosaic of plesiomorphic and apomorphic traits, providing critical evidence for reconstructing the evolutionary history of scorpions.\u003c/p\u003e \u003cp\u003eThe rigid exoskeletons of arthropods primarily serve a protective function and are shed between developmental stages through a uniquely evolutionarily conserved pattern. Molting is triggered by steroid hormones (Cheong et al., \u003cspan citationid=\"CR32\" class=\"CitationRef\"\u003e2015\u003c/span\u003e). During the molting process, all exoskeletal structures are shed simultaneously. (Francke, O. F., \u0026amp; Sissom, W. D. \u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e1984\u003c/span\u003e). When the humidity and environmental safety requirements are met, a new epidermis forms under the old skin, and the old exoskeleton splits along the midline of the cephalothorax and abdomen. The scorpion body then releases its claw, leg, and posterior abdomen through muscle twisting and gravity, and the arthropod initiates ecdysis through coordinated cheliceral and pedipalpal movements, coupled with internal physiological processes that rupture the pleural membrane ventral to the anterior and lateral margins of the carapace. The carapace is then elevated from beneath, creating an opening through which the animal emerges\u0026mdash;typically in a prone position. Whether emerging in a prone or supine position, the resulting exuvium remains relatively intact, encompassing the cuticle of booklungs, bristles, and sensilla, with only a wedge-shaped aperture at the anterior end marking the site of escape (Gaban, R. D\u0026amp;Farley, R. D, \u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e2002\u003c/span\u003e).\u003c/p\u003e \u003cp\u003eDuring the molting process, arthropods are unable to escape threats and are prone to desiccation. As a result, they have evolved new behavioral adaptations, such as group molting behavior and seeking shelter during molting (Daley \u0026amp; Drage, \u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e2016\u003c/span\u003e). Solitary molting behavior is widely adopted by most modern arthropods, whereas gregarious molting has also been observed in various taxa, including spiders (Kim et al.,2001), shrimp (Webster et al.,1982), prawns (Howe et al.,1981), crabs, and springtails (Leinaas et al.,1983) (Stone et al.,1999). First-instar scorpions climb onto their mothers after birth to continue developing and undergo their first molting (Polis, G.A.1991) (Brownell et al.,2011). The young second-instar scorpions then disperse and exist independently. Thereafter, they typically undergo several additional molts over an extended period ranging from 6\u0026ndash;83 months before reaching maturity (Polis, G.A.1991) (Brownell et al.,2011).\u003c/p\u003e \u003cp\u003eMost scorpion species undergo ecdysis within subterranean retreats, rock crevices, bark shelters, or other protected microhabitats rather than exposed environments. However, contemporary observations document occasional surface molting events under specific ecological conditions, although such behavior remains atypical. While these exceptional cases demonstrate environmental plasticity in molting site selection, such behavior remains nonsystematic and context dependent. The predominant evolutionary strategy maintains concealed and secure locations for ecdysis.\u003c/p\u003e \u003cp\u003eFossilized exuviae are relatively rare in the fossil record because of the fragility of the material and specific preservation conditions. However, in certain depositional environments, such as lakes or wetlands, exuviae may be rapidly buried, increasing their chances of preservation. Some Palaeozoic fossils and research on modern arthropods have shown that exuviae are closely related to the growth stages of arthropods because they grow in larger increments with each molt or achieve the same size with fewer molts (Grunert LW et al.,2015; Liu \u0026amp; Zhang, \u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e2015\u003c/span\u003e). Molting not only is a marker of growth but also may be related to ecological adaptation (Liu \u0026amp; Zhang,2015). The preservation and characteristics of exuviae provide valuable information about past ecological conditions. For example, some arthropod exuviae found in specific sedimentary layers are linked to certain environmental factors, such as water depth, temperature, and salinity. These studies help reconstruct ancient ecosystems. The synchronization of molting can be triggered by external abiotic factors, and Johnson et al. (\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e1957\u003c/span\u003e) reported synchronized molting in aphids, which they attributed to circadian rhythms. In terrestrial arthropods, synchronized molting is often induced through pheromone communication.This phenomenon has been confirmed in colonial spiders (Fowler HG\u0026amp;Gobbi N,1998) and gregarious springtails (Leinaas HP, \u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e1983\u003c/span\u003e). Large-scale molting to reduce cannibalism also applies to other cases of synchronized molting in predatory species (Huang et al., 2013). Although Burmese amber preserves abundant scorpion fossils, no unequivocal evidence of gregarious molting behavior in scorpions has been documented to date. Studying fossilized arthropod remains and exuviae is crucial for a more comprehensive and precise assessment of their roles in ecosystems (McCoy, e.g., McCoy and Brandt,2009).\u003c/p\u003e"},{"header":"2. Materials and methods","content":"\u003cp\u003eThe newly described samples in this study are preserved in Burmese amber sourced from the Hukawng Valley of Kachin State, northern Myanmar (Grimaldi et al., \u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e2002\u003c/span\u003e; Cruickshank and Ko, \u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e2003\u003c/span\u003e; for geological images, refer to Guo et al. \u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e2024\u003c/span\u003e, Cretaceous Research 164, 105968). Radiometric dating of zircons conducted by Shi et al. (\u003cspan citationid=\"CR33\" class=\"CitationRef\"\u003e2012\u003c/span\u003e) established the amber depositional age as the earliest Cenomanian stage of the Late Cretaceous, with precise U\u0026ndash;Pb dating yielding an absolute age of 98.79\u0026thinsp;\u0026plusmn;\u0026thinsp;0.62\u0026nbsp;million years ago. The samples are deposited in the Blue Miracle Museum, Guangzhou, China, and are available for examination upon request. These amber samples were legally acquired prior to the closure of the Kachin amber mines by the Myanmar military in November 2017 and therefore bear no association with any ongoing armed conflicts or ethnic tensions in Myanmar.\u003c/p\u003e"},{"header":"3. Systematic palaeontology","content":"\u003cp\u003eOrder Scorpiones Koch, 1837\u003c/p\u003e \u003cp\u003eSuborder Neoscorpionina Thorell \u0026amp; Lindstr\u0026ouml;m, 1885\u003c/p\u003e \u003cp\u003eInfraorder Orthosterni Pocock, 1911\u003c/p\u003e \u003cp\u003eSuperfamily Buthoidea C. L. Koch, 1837\u003c/p\u003e \u003cp\u003eFamily Buthidae C. L. Koch, 1837\u003c/p\u003e \u003cp\u003eGenus and species undetermined\u003c/p\u003e \u003cp\u003eMaterial: BMM 4772 (FIGURE \u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e2\u003c/span\u003e: A.B.D.). BMM4773 (FIGURE \u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e3\u003c/span\u003e). BMM4773 (FIGURE \u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e2\u003c/span\u003e:C)\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eDepository: Blue Miracle Museum, Guangzhou, China.\u003c/p\u003e \u003cp\u003eGiven the obscuration of diagnostic characteristics (e.g., pectinal tooth count, cheliceral dentition) by carbonization, the samples should be tentatively assigned to the Incertae familiae within Scorpiones\u003cem\u003e​\u003c/em\u003e\u003cb\u003e​\u003c/b\u003e rather than a specific genus. Notably, the high carbonization of arthrodial regions and the deformation of exuvial remains caused by resin flow render the measurement data meaningless, and prone to induce errors in species identification. The primary focus of this paper remains the ecological characteristics provided by the samples themselves.\u003c/p\u003e \u003cp\u003eThe sample (BMM 4772) exhibited moderate preservation with notable carbonization of the principal structure, rendering diagnostic morphological details noninformative and susceptible to erroneous interpretation. Three exuviae are observable on the right lateral aspect in a dorsoventral arrangement, wherein exuvium A and exuvium B retain well-defined ecdysial cleavage lines and exuvial fractures. The cuticle ruptures along the lateral sides and anterior margin of the carapace, whereas the third exuvium preserves a nearly complete telson (FIGURE. 2: A.D). The left ventrolateral region contains three carbonized scorpion samples in tight sequential alignment: the first retains the anterior body portion, whereas the second and third preserve the major mesosoma. Their spatial separation from the exuviae, combined with angular displacement, suggests that postmolt displacement is potentially mediated by the active avoidance of resin flow or passive transport via resin dynamics. A solitary insect inclusion is preserved within the central amber matrix.\u003c/p\u003e \u003cp\u003eFor sample BMM 4773 (FIGURE \u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e3\u003c/span\u003e), a transparent and impurity-free amber resin rapidly and completely encapsulates a scorpion during molting. Most of the scorpion's body had already detached from the exuviated shell, and traces of struggling and disturbance were generated because it was rapidly encapsulated by the resin.\u003c/p\u003e \u003cp\u003eIn the amber of specimen BMM4774 (FIGURE \u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e2\u003c/span\u003e:C), three aggregated scorpion bodies are enclosed. Scorpion 5 preserves the complete telson, and scorpion 6 preserves the anterior body, including two pedipalps and partial body segments. Scorpion 7 preserves a part of the anterior body and a part of the leg.\u003c/p\u003e \u003cp\u003eSupplementary reference (FIGURES 8\u0026ndash;9)\u003c/p\u003e"},{"header":"4. Ecology","content":"\u003cp\u003e​​Molting represents an evolutionarily conserved adaptive strategy.​​ In modern social species, such as \u003cem\u003eOpisthacanthus cayaporum\u003c/em\u003e, most, if not all, juveniles molt within a short period of time, usually on the same night. This behavior indicates the presence of a group effect (Louren\u0026ccedil;o, W. R. \u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e2018\u003c/span\u003e).The co-occurrence of multiple molting individuals suggests that this behavior stabilized by the mid-Cretaceous (~\u0026thinsp;98.79\u0026nbsp;million years ago).(FIGURE \u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e1\u003c/span\u003e). The correlation between molting frequency and body size increases (0.5\u0026ndash;0.7 cm per molt) (Polis, \u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e1990\u003c/span\u003e) \u003cb\u003e​\u003c/b\u003e​. demonstrates precise resource allocation. This periodicity likely originated as an adaptation to terrestrial aridity, where molting repaired the osmoregulatory function of the exoskeleton. Notably, Lebanese amber from similar palaeoclimates lacks such behavior, potentially due to differences in resin viscosity or polymerization rates (Grimaldi et al., \u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e2002\u003c/span\u003e). Palaeoclimatic reconstructions indicate a substantially elevated global thermal regime during the amber deposition period (early Cenomanian), which is supported by substantial data that warm conditions at high latitudes were the norm, with a distinct long-term climatic stasis persisting until the Late Cretaceous. (Voigt et al., \u003cspan citationid=\"CR38\" class=\"CitationRef\"\u003e2003\u003c/span\u003e; Price et al., \u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e2013\u003c/span\u003e). This multiproxy evidence collectively supports the interpretation of a stable warm-tropical palaeoenvironment\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eThe discovery of gregarious molting in Burmese amber must be contextualized within global Cretaceous arachnid fossil records. The main known Cretaceous amber deposits are located between 34\u0026deg;N and 74\u0026deg;N, and the palaeolatitude (10\u0026ndash;15\u0026deg;N) of the amber deposits in Myanmar and Lebanon is the southernmost of all the major Cretaceous amber deposits. Compared with contemporaneous amber deposits (e.g., Lebanese amber [~\u0026thinsp;130 Ma], Spanish \u0026Aacute;lava amber [~\u0026thinsp;105 Ma]) and sedimentary strata (e.g., Jehol Biota [~\u0026thinsp;125 Ma], China), Burmese amber deposits present greater scorpion fossil abundance, completeness, and behavioral preservation (Supplementary). Lebanese amber yields only one solitary species (\u003cem\u003eArchaeobuthus estephani\u003c/em\u003e) (Louren\u0026ccedil;o WR. \u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e2001\u003c/span\u003e); Spanish \u0026Aacute;lava amber lacks any scorpion fossils (Delcl`os X et al., 2007). This disparity highlights the controlling role of palaeoclimate on behaviour: Myanmar's tropical rainforest (palaeolatitude 5\u0026deg;N) (Shi et al., \u003cspan citationid=\"CR33\" class=\"CitationRef\"\u003e2012\u003c/span\u003e) provides optimal conditions for synchronous moulting, and\u003c/p\u003e \u003cp\u003ethe Jehol Biota experiences a temperate humid climate with seasonal variability, which contrasts with the stable tropics of Myanmar (Zhou et al., 2003; T. Yu et al., \u003cspan citationid=\"CR36\" class=\"CitationRef\"\u003e2019\u003c/span\u003e) and appears more suitable for a burrowing moulting strategy, resulting in a stark contrast between the two. \u003cem\u003eMoreover, Palaeoeus corpius gallicus\u003c/em\u003e from French amber (45.94\u0026deg;N) inhabited warm and humid estuarine environments during the same period (Louren\u0026ccedil;o, Wilson R. 2003).\u003c/p\u003e \u003cp\u003eDuring the Early Cretaceous (~\u0026thinsp;145\u0026ndash;100 Ma), atmospheric oxygen levels began rising from approximately 15%. They peaked at approximately 23% during the Aptian\u0026ndash;Albian stages (~\u0026thinsp;125\u0026ndash;100 Ma). During the Late Cretaceous (~\u0026thinsp;100\u0026ndash;66 Ma), the oxygen content gradually decreased from this peak to approximately 18%. (Clapham \u0026amp; Karr 2012). During the Cenomanian (94\u0026ndash;100 Ma), atmospheric CO\u003csub\u003e2\u003c/sub\u003e peaked at 1,700 ppm (Wang et al., 2014). Gregarious molting likely represents an ​​adaptive innovation under Cretaceous greenhouse crises​​.\u003c/p\u003e \u003cp\u003eThe mid-Cretaceous (\u0026sim;124\u0026ndash;90 MA) was a transitional period in the nature of the marine climate system (Leckie,2002). The Burmese amber fauna, which is situated in a coastal environment, was inevitably affected. The reorganization of the global biosphere during the mid-Cretaceous paralleled significant changes in the marine climate system, \u003cspan type=\"Underline\" class=\"Underline\" name=\"Emphasis\"\u003ei\u003c/span\u003endicating a causal relationship between biological evolution and environmental stimuli. From a stratigraphic perspective (Fischer and Arthur, \u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e1977\u003c/span\u003e; Rich et al., 1986; Leckie, 1989; Thurow et al., \u003cspan citationid=\"CR35\" class=\"CitationRef\"\u003e1992\u003c/span\u003e; Vermeij, \u003cspan citationid=\"CR37\" class=\"CitationRef\"\u003e1995\u003c/span\u003e), the communal molting horizon (98.79\u0026thinsp;\u0026plusmn;\u0026thinsp;0.62 MA) provides a precise temporal anchor for studying biological responses to greenhouse environments. However, owing to the insufficient quantity of collected samples, a small number of Burmese amber samples cannot cover the complete time span. Therefore, the relationship between this behavioral pattern and Cretaceous climate change requires cautious discussion. At present, this remains merely a plausible conjecture\u0026mdash;there is, in fact, no evidence to support the claim that synchronous molting behavior is exclusive to tropical climates or that it emerged only in the Cretaceous period and not earlier.\u003c/p\u003e"},{"header":"5. Discussion","content":"\u003cp\u003eAccording to Buskirk (\u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e1981\u003c/span\u003e), social behavior is exhibited by only a small number of arachnid species. In scorpions, \"social phases\" typically involve only interactions during mating and the maternal care of offspring from birth to shortly after their first molt. However, multiple field reports on scorpion aggregations have shown that such groups consist of both unrelated and related individuals. These observations, summarized by Polis \u0026amp; Louren\u0026ccedil;o (\u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e1986\u003c/span\u003e), suggest that living in groups may be a significant trait of certain scorpion species. Synchronized molting may have been facilitated by chemical communication, analogous to modern crustaceans and spiders (Howe et al.,1981; Kim et al.,2001), although direct fossil evidence is lacking.\u003c/p\u003e \u003cp\u003eIn specimen BMM 4772, the phenomenon of three exuviae being spatially separated from three scorpion bodies can alternatively be interpreted as passive aggregation caused by resin flow rather than active clustering behavior. In modern times, clustered molting is predominantly observed in viviparous and ovoviviparous juveniles (e.g., \u003cem\u003ePandinus imperator\u003c/em\u003e, \u003cem\u003eHadrurus arizonensis\u003c/em\u003e; Polis, G. A. (ed.), 1990), reflecting maternal care or shared microhabitat needs. Most neonate scorpions molt and survive on their mother\u0026rsquo;s back, with an approximate survival rate of 100%. In the absence of maternal care, these juveniles survive through aggregative molting behavior on sandy substrates (89.83% \u0026plusmn; 1.91%). In modern scorpions, first-instar juveniles aggregating on the mother scorpion have unsclerotized exoskeletons and remain tightly connected via leg entanglement and maternal secretions, which provide fixed anchorage for molting. Once molting is complete and they enter the second instar, their distal claws mature, enabling them to detach from the mother and acquire molting ability through their own anchoring strength. As distal claws are unnecessary during molting, the molting behavior of older scorpions transitions from aggregating to solitary. Currently, the survival rate of first-instar juveniles living solitarily in the wild\u0026mdash;even when simulating maternal environments through aggregative molting\u0026mdash;remains at only\u0026thinsp;~\u0026thinsp;50% (Guo et al.,2024). This modern scenario clearly cannot explain the Cretaceous observations, rendering the social molting behavior of ancient scorpions more perplexing. However, we can confirm that social behavior in Cretaceous scorpions has occurred and that they do not always act independently (FIGURE \u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e2\u003c/span\u003e). On the basis of the current size of the samples, these scorpions do not appear to be in their first instar. That is, their aggregation is most likely a form of social gathering behavior rather than a result of detachment from their mother. We welcome further discussions regarding this hypothesis.\u003c/p\u003e \u003cp\u003eAnother possibility merits consideration: In many extant species, individuals remove their exuviae from the shelter postmolting. These shed exoskeletons may accumulate and become encased by flowing amber resin. While this scenario is plausible, observations reveal that the exuviae preserved in the amber are remarkably clean and intact\u0026mdash;devoid of soil and debris that would have adhered to them if they were transported from a burrow, and showed no significant distortion. They thus do not appear to have been intentionally carried from burrows to resin-flowing areas (which are inherently hazardous) for deposition. The simultaneous occurrence of numerous such exuviae within a single amber inclusion further confirms that molting occurred synchronously among a gathering of individuals.\u003c/p\u003e \u003cp\u003eHowever, regardless of the interpretation, scorpion molting clearly did not occur inside caves with abundant soil. On the basis of the fossil samples, we reconstructed an artistic interpretation of scorpions engaged in postmolting aggregation, partially engulfed by resin, to visualize this palaeobiological behavior (FIGURE \u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e).\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eNotably, the samples maintained a degree of aggregation postmolting rather than being immediately dispersed. While current evidence cannot confirm whether this clustered behavior potentially facilitates group mating after ecdysis, such evolutionary adaptation should not be entirely dismissed. It cannot be ruled out that taphonomic processes (such as resin flow) may have caused spatial aggregation of organisms. New material shows evidence for gregarious behaviour, and perhaps synchronized moulting, in subadults, possibly belongs to the buthid family. These hypotheses warrant further investigation. We anticipate that future discoveries of analogous palaeontological materials will yield additional insights into these arthropod social behaviors and life history strategies.\u003c/p\u003e"},{"header":"6. Conclusion","content":"\u003cp\u003eThis specimen provides a novel observational perspective. This completely transparent and impurity-free amber demonstrates that the molting event occurred not within caves or on muddy ground but rather on tree trunks, showing marked divergence from modern species' behavioral patterns. Moreover, this species may be semi-gregarious with a large population.This fossil represents the earliest known record of gregarious postmolting behavior in ancient scorpions.\u003c/p\u003e"},{"header":"Declarations","content":"\u003cp\u003e\u003cstrong\u003eEthics approval and consent to participate\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eNot applicable\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003ePermission Statement\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe specimen is deposited in the Blue Miracle Museum, identified by the authors of this paper, and permission for its use has been obtained.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAuthor contributions\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eAll the authors contributed to manuscript preparation and critical revision. All the authors have modified, finalized and approved the manuscript.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eInstitutional review board statement\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eNot applicable.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eInformed consent statement\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eNot applicable.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eFunding declaration\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003enot applicable\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eData availability statement\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eAll the data are reported in this paper.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eConflicts of interest\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe authors declare that they have no conflicts of interest.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAcknowledgements\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eWe express our gratitude to Maria Rose Petrizzo, PhD. We would like to thank Dr. Andy Ross from National Museums Scotland for his assistance with the statistics of scorpion species. We would like to thank the anonymous reviewers for their valuable comments and suggestions which have helped improve the manuscript.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\n\u003cli\u003eAlonso, A., et al., 2000. A new fossil resin with biological inclusions in Lower Cretaceous deposits from \u0026Aacute;lava (northern Spain, Basque-Cantabrian Basin). J. Palaeontol. 74, 158\u0026ndash;178. https://doi.org/10.1017/S002233600003434X.\u003c/li\u003e\n\u003cli\u003eBrownell, P., Polis, G., 2001. Scorpion Biology and Research. Oxford Univ. Press, Oxford.\u003c/li\u003e\n\u003cli\u003eBuskirk, R. E., 1981. Sociality in the arachnida. Soc. Insects 2, 281\u0026ndash;367.\u003c/li\u003e\n\u003cli\u003eCruickshank, R. D., Ko, K., 2003. Geology of an amber locality in the Hukawng Valley, northern Myanmar. J. 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Earth Sci. 92, 285\u0026ndash;299. https://doi.org/10.1007/s00531-003-0371-1.\u003c/li\u003e\n\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":true,"hideJournal":true,"highlight":"","institution":"","isAcceptedByJournal":false,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true},"keywords":"Scorpiones, Ecdysis, Burmese amber, Cretaceous (Cenomanian), Palaeoethology, Gregarious behaviour","lastPublishedDoi":"10.21203/rs.3.rs-8604487/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-8604487/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eThis study reports the first evidence of social aggregation and suspected group molting behavior in Cretaceous scorpions from Burmese amber (approximately 98.79\u0026nbsp;million years ago). Morphological observations of three scorpion exuvial fossil samples revealed that Cretaceous scorpions underwent synchronized ecdysis on resin-coated arboreal substrates\u0026mdash;a striking contrast to the cryptic or cave-dwelling molting habits exhibited by extant scorpions. Taphonomic evidence has revealed rapid resin entombment during active exuviation, preserving exoskeletal separation patterns and postmolt aggregation. Critically, this finding confirms that scorpion gregarious molting dates back nearly 100\u0026nbsp;million years, and highlights scenarios in which scorpion molting behavior occurred outside caves during the Cretaceous period. redefining our understanding of arthropod behavioral complexity during the Cenomanian.\u003c/p\u003e","manuscriptTitle":"Cretaceous Burmese Amber First Reveals the Nearly 100-Million-Year-Old Gregarious Molting Behavior in Scorpions","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2026-01-16 11:51:59","doi":"10.21203/rs.3.rs-8604487/v1","editorialEvents":[{"type":"communityComments","content":0}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true}}],"origin":"","ownerIdentity":"a5a7fa78-693e-4b6c-8e95-e0b45d1763c2","owner":[],"postedDate":"January 16th, 2026","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"posted","subjectAreas":[{"id":61206993,"name":"Biological sciences/Evolution/Palaeontology"},{"id":61206994,"name":"Biological sciences/Ecology/Behavioural ecology"}],"tags":[],"updatedAt":"2026-01-16T19:25:48+00:00","versionOfRecord":[],"versionCreatedAt":"2026-01-16 11:51:59","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-8604487","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-8604487","identity":"rs-8604487","version":["v1"]},"buildId":"XKTyCvWXoU3ODBz1xrDgd","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}