Triassic marine reptiles gave birth to live young

[Tim Williams <twilliams_alpha@hotmail.com>]

Yen-Nien Cheng, Xiao-Chun Wu and Qiang Ji (2004).  Triassic marine reptiles 
gave birth to live young.  Nature 432: 383 - 386


Abstract: 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). 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) did. Viviparity has been 
proposed for plesiosaur, pachypleurosaur and nothosaur sauropterygians, 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.