Ancient Fossils Reveal Egg-Laying Secrets of Mammal Ancestors and Dinosaurs

A fossil discovery announced in April 2026 has provided the first direct evidence that the ancestors of mammals laid eggs, resolving a long-standing question in paleontology about the reproductive biology of early synapsids. The finding centers on a 250-million-year-old egg containing a preserved embryo of Lystrosaurus, a hardy, plant-eating therapsid that thrived in the aftermath of the End-Permian mass extinction.

An international team led by Professor Julien Benoit of the University of the Witwatersrand in Johannesburg, Professor Jennifer Botha of the Evolutionary Studies Institute at the same institution, and Dr. Vincent Fernandez of the European Synchrotron Radiation Facility (ESRF) in France analyzed three fossil specimens using x-ray microcomputed tomography (CT) and synchrotron radiation computed tomography (SRCT). These non-invasive imaging techniques allowed researchers to examine the bones inside the rock nodules without damaging the fossils.