For more than 180 years, scientists have hypothesized that the ancestors of mammals laid eggs , but haven't found any fossils to prove it. Now, paleontologists finally have some tantalizing evidence to back up those long-held suspicions. Around 250 million years ago, a tusked, pig-like pre-mammal called Lystrosaurus rose to prominence among the ashy molten landscapes, acid rain , and poisoned seas of Earth's most severe extinction event, known as the Great Dying . Lystrosaurus survived that