This is the oldest embryo ever discovered on Earth, found in Uruguay and Brazil. They belong to a baby mesosaur, a group of small aquatic reptiles from the early Permian. According to the researchers, they're the earliest known case of viviparity.
The study — published in the journal Historic Biology by Graciela Piñeiroa, Jorge Ferigolob, Melitta Meneghelc and Michel Laurind, from France's National Center for Research — is very important. Until now, scientists didn't have such an early record of viviparity, which is key to understand the evolution of vertebrates.
The unborn baby fossil — partially articulated and well-preserved — was discovered inside their mother and it had no recognisable eggshell. This discovery demonstrates that, instead of laying eggs in which the animals develop from the embryo stage, the embryo actually grew up inside the body of the mesosaur mother, eventually leading to live birth.
Mesosaurus were small alligator-like creatures that could go as long as 2m in length and probably fed on crustaceans. [CNRS (French)]