Dinosaurs, as a group of specialized descendants of archosaurs (which are descendants of the diapsids), arose in triassic time, and radiated into almost all the niches now inhabited by today"s mammals. They were clearly very successful, because their fossil record extends from triassic to cretaceous time, all together about 160 million years" worth of success! Today we will look at some features of this diversity: remember, the archosaurs include the dinosaurs, pterosaurs, and birds, as well as crocodiles, we also want to remember that the dinosaurs are classified into two general groups: The saurischian dinosaurs are most commonly known in popular literature. They include: carnivores, such as allosaurus, tyrannosaurus, and deinonychus (all. Theropods): top predators with skeletal features including bipedalism (ran on two legs), hollow bones, recurved teeth, powerful jaws. Most of these walked on all four legs. To explore dinosaur diversity, we will look at three groups (it was a very tough choice, but i had to make it).