easier to extract from the atmosphere. The two-thirds drop in atmospheric oxygen certainly lit the fuse to an evolutionary bomb, which exploded in the Triassic.
Thus, the diversity of Triassic animal plans is analogous to the diversity of marine body plans that resulted from the Cambrian Explosion. It also occurred for nearly the same reasons and, as will be shown, was as important for animal life on land as the Cambrian Explosion was for marine animal life. As we saw in Chapter 3, the Cambrian Explosion followed a mass extinction (of the Ediacaran fauna), and it was a time of lower oxygen than today. The latter stimulated much new design. Finally, the Cambrian Period itself ended in a mass extinction—mainly of trilobites that we know of but also among many of the more exotic arthropods known from the Burgess Shale, such as Anomalocaris. In similar fashion, on land the Triassic Explosion followed a mass extinction, was a time of lower oxygen, and ended in a mass extinction. Before this extinction, mammals had evolved, as had true dinosaurs, but many of the other kinds of body plans disappeared and dinosaurs, were the dominant land animals. In this end-Triassic mass extinction the dinosaurs suffered least of all. Why dinosaurs? This chapter will look at those questions.
Let’s begin by looking back to the middle part of the Triassic period. In this middle-late Triassic world, 215 million years ago, on land at least we seem to have arrived among a veritable smorgasbord of animal body plans. Many quite different kinds of vertebrates inhabit this world. Dog-like creatures walk beneath the conifer- and tree-fern dominated vegetation. They are cynodonts, carnivorous varieties, but there are massive herbivores belonging to the same group as well. They are all very mammalian in appearance and behavior, except in one aspect. They move little and seem to tire easily. The carnivores mostly lay in wait, and the herbivores browse stolidly. The cynodonts are not the only mammal-reptiles here, for rhino-sized dicynodonts also browse the low brushy vegetation. Their odd, name-giving tusks extending from a parrot-like beaked mouth make them look like nothing of our