Their ancestors have survived two mass extinctions over 230 million years and scientists have uncovered a new secret to their evolutionary success and longevity. Mammals have typically been regarded as the best group to use to study mass extinction survival, since we have a generalist diet that allows us to thrive in different habitats. Crocodylomorphs survived both the end-Triassic mass extinction (roughly 201.4 million years ago) and the end-Cretaceous mass extinction (about 66 million years ago), which killed all non-avian dinosaur species. By the end-Cretaceous mass extinction event, most of the survivors were the semiaquatic generalists and one group of aquatic carnivores. The teeth of this fossil Borealosuchus skull typify the toothy grin of semi-aquatic generalist predators that survived the end-Cretaceous mass extinction.