A gravid lizard from the Cretaceous of China and the early history of squamate viviparity
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Although viviparity is most often associated with mammals, roughly one fifth of extant squamate reptiles give birth to live young. Phylogenetic analyses indicate that the trait evolved more than 100 times within Squamata, a frequency greater than that of all other vertebrate clades combined. However, there is debate as to the antiquity of the trait and, until now, the only direct fossil evidence of squamate viviparity was in Late Cretaceous mosasauroids, specialised marine lizards without modern equivalents. Here, we document viviparity in a specimen of a more generalised lizard, Yabeinosaurus, from the Early Cretaceous of China. The gravid female contains more than 15 young at a level of skeletal development corresponding to that of late embryos of living viviparous lizards. This specimen documents the first occurrence of viviparity in a fossil reptile that was largely terrestrial in life, and extends the temporal distribution of the trait in squamates by at least 30 Ma. As Yabeinosaurus occupies a relatively basal position within crown-group squamates, it suggests that the anatomical and physiological preconditions for viviparity arose early within Squamata.
KeywordsViviparity Squamata Reptilia Cretaceous China
This work formed part of a joint Anglo-Chinese project funded by the Royal Society, London and the National Natural Science Foundation of China. Field work was also supported by grant 41072015 from the NNSF, China. Our thanks go to Jie Zhang (IVPP) for photographing the fossil and to Jack Conrad, Marcelo Sánchez-Villagra and an anonymous reviewer for comments on an earlier draft.
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