A fossil in South Africa has revealed that our mammalian ancestors were producing hard-shelled embryos as early as 250 million years ago, according to a study in the journal Plos One. The discovery, led by paleontologist Jennifer Botha of the University of the Witwatersrand, revealed that the species in question was a therapsid, a group of animals that was the forefather to modern-day mammals, including humans. The study also revealed that a fossilized egg and embryo belonged to a species called Lystrosaurus, a pig-like herbivore with a turtle-like beak, naked skin two tusks. The unique reproductive habits of these creatures may have helped them survive a mass extinction event around 252 million years old, which occurred during the Great Dying period.