Ancient egg fossil rewrites the story of mammal origins

A fossil from Oviston, South Africa, shows the therapsid Lystrosaurus laid eggs about 252–250 million years ago. X-ray analysis revealed an in-egg embryonic beak not yet fused, indicating hatchlings that could break out of a leathery shell, a trait shared with turtles and birds and providing the first strong evidence that mammal ancestors laid eggs. This egg-based reproduction may have helped these creatures survive the Great Dying and reshapes our understanding of mammalian origins, alongside today’s egg-laying mammals like the platypus and echidna.