Charles Choi, LiveScience Contributor
Newly found fossils of embryos from the first aquatic reptiles called mesosaurs — along with a pregnant female — may be the oldest known example of birth given to live young instead of eggs, scientists report.
Both mammals and reptiles wrap their developing embryos in protective layers, something that helped the little ones survive and ultimately helped their ancestors conquer the land. Mammals often keep these membrane-swaddled offspring within them, giving birth to live young, while reptiles typically lay their membrane-bundled progeny in eggs.
However, there are some oddballs: Some mammals, such as the platypus, lay eggs, while some reptiles, such as most vipers, are viviparous, giving birth to live young.
The fact that mammals and reptiles surround their embryos with these protective layers makes them known as amniotes. The fossil record of amniotic eggs and embryos is very sparse, and as such, scientists have little information about when, how and why they evolved.
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