Paleontologists have found the first evidence that the ancestors of mammals laid eggs, a claim that a pre-mammal called Lystrosaurus survived during the Great Dying, a period when it rose to prominence during Earth's most severe extinction event. The discovery was made through advanced X-ray tomography at the European Synchrotron Radiation Facility in France. The study suggests that Lystrosauruses survived this period, possibly due to their egg-laying. The findings support existing ideas about the origins of lactation, but also suggest that the eggs may not have been used as a way to feed babies, or for hormonal signaling.