First-Ever Egg of a Mammal Ancestor Discovered

Therapsids laid eggs!

Research by Julien Benoit, Jennifer Botha (University of the Witwatersrand) and Vincent Fernandez (European Synchrotron Radiation Facility) just pulished in PLoS ONE on an egg containing a 250-million-year-old embryo of the therapsid Lystrosaurus presents the first-ever egg discovered from a mammal ancestor, finally answering a long-standing question: Did the ancestors of mammals lay eggs? Yes, they did! Therapsid eggs were likely soft-shelled, explaining why they have remained elusive for so long.

Lystrosaurus laid relatively large eggs for its size. Larger eggs typically contain more yolk, providing all the nutrients an embryo needs to develop independently, without parental feeding after hatching, suggesting that Lystrosaurus did not produce milk for its young. Large eggs are also more resistant to drying out. In the harsh, drought-prone environment following the Permo-Triassic Mass Extinction, this would have been a critical survival trait. The Lystrosaurus hatchlings were likely precocial, born at an advanced stage of development. These young animals would have been capable of feeding themselves, escaping predators, and reaching reproductive maturity quickly, all factors important for their survival during the Early Triassic post-extinction phase.