Detailed imaging of a 250-million-year-old fossil has revealed the first proof that the ancestors of mammals laid eggs. The discovery answers a long-standing question about the reproductive biology of our ancient forerunners and hints at how they managed to flourish in the aftermath of the biggest mass extinction in Earth’s history.

Scientists have long assumed that the ancestors of mammals—a group known as the therapsids—laid eggs like today’s platypuses and echidnas do. But they lacked any direct evidence of synapsid eggs in the fossil record.

In the new study, Julien Benoit of the University of the Witwatersrand, Johannesburg, in South Africa and his colleagues analyzed three specimens of rocks containing fossils of a therapsid known as Lystrosaurus. The team used x-ray microcomputed tomography (CT) and synchrotron radiation computed tomography (SRCT) to examine the bones inside the rocks noninvasively.

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The three specimens represent slightly different stages of development shortly before or after birth. Multiple lines of evidence point to the biologically youngest Lystrosaurus having still been inside its egg when it died, according to the authors. For instance, the dimensions of the rock nodule in which it is preserved are consistent with those of an egg. And the curled posture of the embryo follows an ovoid shape like that of an egg. The disjointed nature of the pelvis, the vertebrae at the base of the spine and the ribs indicate that the pelvis and cartilage, meanwhile, could not yet support the animal’s weight, as would be expected of an individual that had yet to hatch.

Most importantly, the new images reveal that the two halves of the lower jaw had yet to fuse in the youngest Lystrosaurus specimen. In turtles and birds, the lower jaw fuses before birth, allowing the baby to feed itself after hatching. The unfused lower jaw of this Lystrosaurus is therefore another indication that the animaldied while still in its egg. The other two specimens exhibit signs of having been somewhat more mature; the largest one was preserved in a splayed-out posture that shows it was not in an egg and had traveled some distance before dying.

Lystrosaurus, a pig-sized plant-eater with two tusks and a beak,was one of the few tetrapods—backboned animals with four limbs—to survive the Permian mass extinction event that occurred around 252 million years ago and wiped out about 90 percent of the planet’s species. In the wake of the extinction, on a planet beset by extreme swings in climate, with long periods of searing heat and brutal drought, Lystrosaurus flourished, becoming the most abundant terrestrial vertebrate around.

Reproducing by laying eggs may have been a secret of its success. Reconstruction of the Lystrosaurus egg indicates that it was relatively large. The shell was probably soft and leathery and didn’t fossilize readily, which would explain why scientists have not found signs of therapsid eggs until now. Big eggs, with their smaller surface-to-volume ratio, are more resistant to drying out—a boon in drought conditions. Moreover, the babies of modern-day tetrapods that lay large eggs tend to be more fully developed and capable of fending for themselves when they hatch compared with babies that develop in smaller eggs. In contrast, the babies of mammals—even egg-laying ones—need to be fed milk for a time after birth.

The new findings have implications for understanding the fate of species under pressure in today’s changing world. “Understanding how past organisms survived global upheaval helps scientists better predict how species today might respond to ongoing environmental stress,” Benoit said in a statement.