Mammal ancestors laid eggs, and this 250-million-year-old fossil finally proves it

In the aftermath of Earth’s most catastrophic extinction event, one unlikely survivor rose to dominate a shattered world: Lystrosaurus. Now, a stunning fossil discovery—an ancient egg containing a curled-up embryo—has finally answered a decades-old mystery about whether mammal ancestors laid eggs. Using advanced imaging technology, scientists confirmed that these resilient creatures did reproduce this way, likely producing large, soft-shelled eggs packed with nutrients.