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

[erronis]

https://phys.org/news/2026-04-mammal-ancestors-laid-eggs-million.html
Wits University

Egg photographed in the control room of the ESRF in France. Credit: Professor Julien Benoit

A remarkable new discovery is shedding light on one of the greatest survival stories in Earth's history, and answering a decades-old scientific mystery. Lystrosaurus, a hardy, plant-eating mammal ancestor, rose to prominence in the wake of the End-Permian Mass Extinction some 252 million years ago, the most devastating extinction event our planet has ever experienced.

While countless species vanished, Lystrosaurus not only survived but thrived in a world marked by extreme environmental instability, intense heat, and prolonged droughts.

Now, groundbreaking research published in PLOS ONE reveals a discovery that transforms our understanding of this iconic survivor. An international team led by Professor Julien Benoit, Professor Jennifer Botha (Evolutionary Studies Institute, University of the Witwatersrand, South Africa), and Dr. Vincent Fernandez (ESRF--The European Synchrotron, France) has identified, for the first time, an egg containing an embryo of Lystrosaurus, dating back approximately 250 million years.

This extraordinary fossil represents the first-ever egg discovered from a mammal ancestor, finally answering a long-standing question: Did the ancestors of mammals lay eggs?

The answer is yes.

. . .

[no_hypocrisy]

1. Pre-MAGA heads exploded during the Scopes Monkey Trial.

Now, (fast forward), Eve's laid an egg.

[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.

[erronis]

3. Thanks for enlightening comments. It seems there might even be overlap between

individuals within a species that laid eggs and those that gave live birth - all during the same long time period. Perhaps depending on environmental conditions.

Or would that automatically classify them as being different species?