From “Oldest-Ever Reptile Embryos Unearthed” (ScienceDaily, Apr. 11, 2012), we learn,
Dating back 280 million years or so, the oldest known fossil reptile embryos have been unearthed in Uruguay and Brazil. They belong to the ancient aquatic reptiles, mesosaurs. The study of these exceptionally well-preserved fossils suggests that mesosaurs were either viviparous (pushing back this mode of reproduction by 60 million years) or that they laid eggs in advanced stages of development.
So much for the long, slow development of live birth. Question:
This research therefore reveals the oldest known fossil amniote embryos from the Paleozoic (543 to 250 million years BP) and the first examples of embryo retention (and perhaps viviparity), pushing back this reproductive mechanism by some 60 million years. But do the reproductive characteristics of mesosaurs highlighted in this study reflect their aquatic way of life (since viviparity is frequent in aquatic reptiles), or was it rather a fairly widespread condition among early reptiles?