Viviparity (giving birth to live young) in fossil reptiles has been known only in a few marine groups: ichthyosaurs, pachypleurosaurs, and mosasaurs. Here, we report a pregnant specimen of the Early Cretaceous Hyphalosaurus baitaigouensis, a species of Choristodera, a diapsid group known from unequivocal fossil remains from the Middle Jurassic to the early Miocene (about 165 to 20 million years ago). This specimen provides the first evidence of viviparity in choristoderan reptiles and is also the sole record of viviparity in fossil reptiles which lived in freshwater ecosystems. This exquisitely preserved specimen contains up to 18 embryos arranged in pairs. Size comparison with small free-living individuals and the straight posture of the posterior-most pair suggest that those embryos were at term and had probably reached parturition. The posterior-most embryo on the left side has the head positioned toward the rear, contrary to normal position, suggesting a complication that may have contributed to the mother’s death. Viviparity would certainly have freed species of Hyphalosaurus from the need to return to land to deposit eggs; taking this advantage, they would have avoided intense competition with contemporaneous terrestrial carnivores such as dinosaurs.
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We thank the Ministry of Science and Technology of China (973-Project-2006CB701405) and the China Geological Survey (110914218) for supporting this project and also Mr. Zhang Yu-qing of the CAGS for the preparation of the specimen. We particularly wish to thank Dr. Steve Cumbaa and Richard Day of the Canadian Museum of Nature for reading and editing the final version of the manuscript. X.-c. Wu was supported by a research grant (RS34) from the CMN and is grateful to the Institute of Geology of CAGS for the support during his visit in 2007. We wish to acknowledge Drs. Tamaki Sato of Tokyo Gakugei University for her comments and suggestions on the study and Wen-shan Wang of the National Museum of Natural Science for the stimulating discussion of the reproductive patterns of extant squamates. Dr. Michael Caldwell of University of Alberta and an anonymous reviewer reviewed the manuscript, offering critical comments which led to its great improvement.
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Ji, Q., Wu, Xc. & Cheng, Yn. Cretaceous choristoderan reptiles gave birth to live young. Naturwissenschaften 97, 423–428 (2010). https://doi.org/10.1007/s00114-010-0654-2