Mammal ancestor Lystrosaurus laid large leathery eggs and may have aided survival after the mass extinction

Synchrotron X-ray CT scans revealed a 250-million-year-old fossilized egg containing a curled Lystrosaurus embryo, offering direct evidence that mammal ancestors laid eggs. The specimen appears to have held a large, leathery egg with a relatively undeveloped embryo, which researchers link to yolk-rich development and more mature hatchlings.

Researchers report that a fossilized, curled embryo found in South Africa is an intact egg from Lystrosaurus, a therapsid lineage ancestral to mammals. The specimen was scanned using synchrotron X-ray CT at the European Synchrotron Radiation Facility to study internal structures without damage. Scans revealed developmental features indicating the embryo had not yet hatched and a shell that the team interprets as leathery and now dissolved. The find addresses a long-standing question about whether mammal ancestors laid eggs and provides context for how some species persisted after the Permian–Triassic extinction. Key findings: - The fossil was discovered in 2008 near the Xhariep district in South Africa and contains a tightly curled embryo identified as Lystrosaurus. - High-resolution CT scans revealed small internal details, including an unfused lower jaw, indicating the embryo was not developed enough to feed independently. - The researchers interpret the outer shell as leathery and likely dissolved during fossilization, rather than preserved as a hard shell. - The egg is unusually large for the animal's body size, which the authors say implies a yolk-rich egg and more complete development before hatching. - The team links large, leathery eggs and relatively mature hatchlings to traits that may have helped Lystrosaurus survive and become abundant after the Permian–Triassic event. Summary: The study provides direct evidence that Lystrosaurus, a therapsid ancestor of mammals, laid large, leathery eggs and that its young were relatively mature at hatching. Undetermined at this time.