Lystrosaurus

One of the most well-known non-mammal synapsids was Lystrosaurus. Lystrosaurus was a dicynodont therapsid, and as such, it had a horny, toothless beak used for grinding up vegetation, and a pair of tusks - its only teeth. Its legs were set at a semi-erect position, and judging from the size of its forelimbs, it was probably a powerful burrower. Of course, its most striking aspect is that it was one of the few animals to make it past the Permian-Triassic boundary, which was denoted by a catastrophic mass extinction. Fossils of it were found in both strata, an example of this is that Lystrosaurus fossils can be found in both Permian and Triassic strata in the Karoo Basin in South Africa. Many animals have done the same, but none were as prosperous and bountiful as Lystrosaurus, which was the most plentiful terrestrial vertebrate in the Early Triassic - so much in fact, that its herds accounted for a staggering 95% of the Earth's tetrapod population for a time. How such an animal could have survived is a mystery. Some scientists say that the since atmospheric conditions during the Great Dying lacked sufficient oxygen, its burrowing lifestyle and unique respiratory adaptations are what preserved it. Others claim that a semiaquatic lifestyle enabled it to preserve, or sheer generalization, lack of predators, and some say that it survived just out of chance. Nevertheless, never again would a single species prosper so well, let alone after a catastrophic mass extinction. As a side note, the location of Lystrosaurus fossils were used as evidence to prove the theory of continental drift. In Walking With Monsters, a herd of Lystrosaurus was pressured to migrate in search of more food; along the way, they had to cross a wide river, where one of their few predators, Proterosuchus, lurked. Lystrosaurus was also shown to have evolved from Diictodon, another dicynodont. It isn't logical for Lystrosaurus to have descended from Diictodon, as the two actually coexisted with one another.