Mammal ancestors laid eggs--and this 250-million-year-old fossil proves it

[eppur_se_muova]

2. Well, we pretty well knew that, just not where the dividing line was.

Lystrosaurus was a "mammal-like reptile", which might have laid eggs like reptiles, or given live birth like mammals, until we had this evidence. Since its ancestors were more reptile-like we'd expect them to lay eggs. Lystrosaurus was more of an open question before now.

A surprising number of modern reptiles give birth to live young w/out eggs, and some can even switch between the two. But just when, and in what genera, live birth became the norm we don't know -- and remember that monotreme mammals still lay eggs! I would have thought that live birth in Lystrosaurus would have been more of a revelation than ovipary.

Still, an interesting discovery, assisted by developments in engineering new radiation sources and detectors.