World Paleogeography: Western Pangea had already assembled, and Eastern Pangea began to assemble from terranes in the Paleo-Tethys Ocean. Cimmeria and the Cathaysian terranes collided with Siberia in the Middle Triassic. This collision closed the Paleo-Tethys Ocean and opened the Tethys Ocean. By the latest Triassic, just as the formation of Pangea was complete, the supercontinent began to break up, and rifts developed between North America, Africa, and South America. The large emergent landmass and rain shadows from several high mountain belts created an arid climate and resulted in large seasonal temperature variations over most of the supercontinent throughout the Triassic. North American Paleogeography: As Pangea began to break apart, rift basins developed along the modern East Coast and filled with sediment and lava flows. During the rifting, regions that once were part of Africa remained attached to North America, including parts of the Atlantic Coastal Plain and most of Florida. Highlands in eastern North America shed copious amounts of sediment into the lower areas that dominated the central part of the continent. Shallow seas covered most of the continent west of the modern Rocky Mountains, but subduction along the continental margin generated small volcanic landmasses that began to form the core of the ancestral Sierra Nevada. Paleontology: The Triassic Period was a time of transition as life on Earth was recovering from the great mass extinction that ended the Paleozoic Era. The dinosaurs made their first appearance in the Triassic, and diversified to dominate the terrestrial faunas for the next 180 million years. The Triassic seas were home to other large reptiles, such as ichthyosaurs and plesiosaurs, as well as invertebrate survivors of the Permian extinction, such as a few ammonites, brachiopods, and molluscs. The earliest flying vertebrates, the pterosaurs, evolved during the late Triassic. One other vertebrate group evolved in the Triassic at about the same time as the dinosaurs: the mammals. The first mammals were tiny, about the size of the modern shrew or mouse. Mammals diversified slightly during the Mesozoic, but would not become major players until the Cenozoic.