{"status":"ok","message-type":"work","message-version":"1.0.0","message":{"indexed":{"date-parts":[[2026,7,30]],"date-time":"2026-07-30T09:16:41Z","timestamp":1785403001387,"version":"3.56.0"},"reference-count":39,"publisher":"PeerJ","license":[{"start":{"date-parts":[[2026,7,30]],"date-time":"2026-07-30T00:00:00Z","timestamp":1785369600000},"content-version":"unspecified","delay-in-days":0,"URL":"https:\/\/creativecommons.org\/licenses\/by\/4.0\/"}],"funder":[{"DOI":"10.13039\/501100001809","name":"National Natural Science Foundation of China","doi-asserted-by":"crossref","award":["42572021"],"award-info":[{"award-number":["42572021"]}],"id":[{"id":"10.13039\/501100001809","id-type":"DOI","asserted-by":"crossref"}]},{"DOI":"10.13039\/501100001809","name":"National Natural Science Foundation of China","doi-asserted-by":"crossref","award":["U2544205"],"award-info":[{"award-number":["U2544205"]}],"id":[{"id":"10.13039\/501100001809","id-type":"DOI","asserted-by":"crossref"}]},{"name":"Zhejiang Province Cultural Relics Protection Technology Program","award":["2025010"],"award-info":[{"award-number":["2025010"]}]}],"content-domain":{"domain":[],"crossmark-restriction":false},"short-container-title":[],"abstract":"<jats:p>\n                    The mode of reproduction is a key aspect for an organism to adapt to its environment. However, understanding the reproductive strategies in extinct animals from deep time remains challenging, due to the scarcity of fossils providing such information. Although viviparity is well-documented across various ichthyosaur lineages, our understanding of the reproductive characteristics of early-diverging Triassic taxa is still limited, particularly regarding their litter sizes. Here, we describe an exceptionally preserved gravid specimen of the Middle Triassic ichthyosaur\n                    <jats:italic>Mixosaurus<\/jats:italic>\n                    from southwestern China, containing at least eleven fetuses. This represents the largest litter sizes ever recorded in a Triassic ichthyosaur, and even one of the highest numbers of fetuses known in all ichthyosaurs. The well-articulated fetal skeletons exhibit consistent developmental stages and advanced ossification, indicating a late prenatal or perinatal state. Their variable body orientations further suggest the absence of a fixed head-first or tail-first birth posture. The presence of over ten fetuses in\n                    <jats:italic>Mixosaurus<\/jats:italic>\n                    demonstrates that high fecundity, potentially indicative of a more r- than K-selection reproductive strategy, had evolved early in ichthyosaur evolutionary history. This reproductive trait may have been a significant factor contributing to the success of this ichthyosaur group with enormous populations and widespread distribution across Northern Hemisphere during the Triassic.\n                  <\/jats:p>","DOI":"10.7717\/peerj.21620","type":"journal-article","created":{"date-parts":[[2026,7,30]],"date-time":"2026-07-30T08:52:51Z","timestamp":1785401571000},"page":"e21620","source":"Crossref","is-referenced-by-count":0,"title":["High fecundity in an early-diverging ichthyosaur with over ten fetuses"],"prefix":"10.7717","volume":"14","author":[{"given":"Shengxiao","family":"Gu","sequence":"first","affiliation":[{"name":"Zhejiang Museum of Natural History, Hangzhou, China"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Wei","family":"Wang","sequence":"additional","affiliation":[{"name":"Institute of Vertebrate Paleontology and Paleoanthropology, Chinese Academy of Sciences, Beijing, China"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Xiuti","family":"Li","sequence":"additional","affiliation":[{"name":"Zhejiang Museum of Natural History, Hangzhou, China"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Chun","family":"Li","sequence":"additional","affiliation":[{"name":"Institute of Vertebrate Paleontology and Paleoanthropology, Chinese Academy of Sciences, Beijing, China"}],"role":[{"vocabulary":"crossref","role":"author"}]}],"member":"4443","published-online":{"date-parts":[[2026,7,30]]},"reference":[{"key":"10.7717\/peerj.21620\/ref-1","doi-asserted-by":"publisher","first-page":"e11179","DOI":"10.7717\/peerj.11179","article-title":"Cranial anatomy of Besanosaurus leptorhynchus Dal Sasso & Pinna, 1996 (Reptilia: Ichthyosauria) from the Middle Triassic Besano Formation of Monte San Giorgio, Italy\/Switzerland: taxonomic and palaeobiological implications","volume":"9","author":"Bindellini","year":"2021","journal-title":"PeerJ"},{"key":"10.7717\/peerj.21620\/ref-2","first-page":"1","article-title":"Neue Erkenntnisse \u00fcber die Fortpfanzungsbiologie der Ichthyosaurier (Reptilia)","volume":"164","author":"B\u00f6ttcher","year":"1990","journal-title":"Stuttgarter Beitr\u00e4ge zur Naturkunde Ser. 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Italy): Societ\u00e0 Italiana di scienze naturali","volume":"4","author":"Dal Sasso","year":"1996","journal-title":"Museo Civico di storia naturale di Milano"},{"key":"10.7717\/peerj.21620\/ref-9","first-page":"423","article-title":"An ichthyosaur embryo from the Lower Lias (Jurassic-Hettangian) of Somerset England with comments on the reproductive biology of ichthyosaurs","volume":"18","author":"Deeming","year":"1993","journal-title":"Modern Geology"},{"key":"10.7717\/peerj.21620\/ref-10","doi-asserted-by":"publisher","first-page":"1460","DOI":"10.1080\/02724634.2014.866573","article-title":"An Upper Triassic (Norian) ichthyosaur (Reptilia, Ichthyopterygia) from northern Alaska and dietary insight based on gut contents","volume":"34","author":"Druckenmiller","year":"2014","journal-title":"Journal of Vertebrate Paleontology"},{"key":"10.7717\/peerj.21620\/ref-11","doi-asserted-by":"publisher","first-page":"e0285939","DOI":"10.1371\/journal.pone.0285939","article-title":"Exceptional X-ray contrast: radiography imaging of a Middle Triassic mixosaurid from Svalbard","volume":"18","author":"Engelschi\u00f8n","year":"2023","journal-title":"PLOS ONE"},{"key":"10.7717\/peerj.21620\/ref-12","doi-asserted-by":"publisher","first-page":"263","DOI":"10.4202\/app.01133.2024","article-title":"A new species of mixosaurid ichthyosaur from the Middle Triassic of Luxi County, Yunnan Province, South China","volume":"69","author":"Fang","year":"2024","journal-title":"Acta Palaeontologica Polonica"},{"issue":"2","key":"10.7717\/peerj.21620\/ref-13","doi-asserted-by":"publisher","first-page":"e4366","DOI":"10.1371\/journal.pone.0004366","article-title":"New protocetid whale from the middle Eocene of Pakistan: birth on land, precocial development, and sexual dimorphism","volume":"4","author":"Gingerich","year":"2009","journal-title":"PLOS ONE"},{"key":"10.7717\/peerj.21620\/ref-14","doi-asserted-by":"publisher","first-page":"60","DOI":"10.1671\/0272-4634(2006)26[60:Anmift]2.0.Co;2","article-title":"A new mixosaurid ichthyosaur from the Middle Triassic of China","volume":"26","author":"Jiang","year":"2006","journal-title":"Journal of Vertebrate Paleontology"},{"key":"10.7717\/peerj.21620\/ref-15","doi-asserted-by":"publisher","first-page":"602","DOI":"10.1666\/05092.1","article-title":"The mixosaurid ichthyosaur Phalarodon cf. 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