Ancient mammal kin laid eggs, fossil rewrites mammal origins

A 250-million-year-old Lystrosaurus embryo fossil provides the first direct evidence that mammal ancestors laid eggs. High‑resolution imaging shows the embryo’s jaws were not fully fused, indicating an egg inside the egg with a soft leathery shell, suggesting water‑conserving eggs helped this dry‑habitat survivor endure the Great Dying. The discovery offers clues about the evolution of lactation and live birth, and implies that hatchlings were relatively mature at hatching, aiding early mammal lineages before milk secretion became widespread.