Sauropterygians form the largest and most diverse group of ancient marine reptiles that lived throughout nearly the entire Mesozoic era (from 250 to 65 million years ago)1,2. Although thousands of specimens of this group have been collected around the world since the description of the first plesiosaur in 1821 (ref. 3), no direct evidence has been found to determine whether any sauropterygians came on shore to lay eggs (oviparity) like sea turtles, or gave birth in the water to live young (viviparity) as ichthyosaurs and mosasauroids (marine lizards) did4,5,6. Viviparity has been proposed for plesiosaur, pachypleurosaur and nothosaur sauropterygians7,8,9,10, but until now no concrete evidence has been advanced. Here we report two gravid specimens of Keichousaurus hui Young from the Middle Triassic of China. These exquisitely preserved specimens not only provide the first unequivocal evidence of reproductive mode and sexual dimorphism in sauropterygians, but also indicate that viviparity could have been expedited by the evolution of a movable pelvis in pachypleurosaurs. By extension, this has implications for the reproductive pattern of other sauropterygians and Mesozoic marine reptiles that possessed a movable pelvis.
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We thank R. Holmes and T. Sato of the Canadian Museum of Nature (CMN), Ottawa, for advice, suggestions and access to their reference collections, and R. Holmes for reading earlier drafts and for editorial assistance. X.-c. W. was supported by research grants from the CMN.
The authors declare that they have no competing financial interests.
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Cheng, Yn., Wu, Xc. & Ji, Q. Triassic marine reptiles gave birth to live young. Nature 432, 383–386 (2004). https://doi.org/10.1038/nature03050
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