About 250 million years ago, the worst mass extinction Earth has ever seen left a gaping hole in ecosystems the world over. The organisms that filled that hole would come to define the world of the Mesozoic and leave fundamental changes in their ecology that still shape the living world today. On land, there was an explosive diversification of reptiles, including among its array of weird wonders the ancestors of modern lizards, turtles, and crocodiles. In the seas, an invasion of new predators and shell-crushers set off an arms race that produced the diverse community of fast-moving fish and heavily-armored invertebrates we see today. And under the feet of the new ruling reptiles, a last gasp of the Permian world would make one of the most significant morphological innovations in mammalian history (Benton et al, Dal Corso et al). Then, after only 50 million years, the world would be wiped clean again, ending a range of Triassic experiments and giving way to the more recognizable world of the Jurassic and Cretaceous. Among the beneficiaries of this new world were the dinosaurs, who started as a small part of an already crowded fauna, but, by the beginning of the Jurassic, had come to dominate a newly-emptied ecosystem, and, with very little serious competition, radiated out into the variety of forms that characterized the later Mesozoic (Dunne et al). But what caused this change, from thriving ecosystems full of evolutionary novelties to barren landscapes monopolized by a small handful of survivors, and what advantage did they have that allowed them to make it through and recolonize this barren world?