World Paleogeography: During the Tertiary, the last phase of the breakup of Pangea was accompanied by several continental collisions. North America and Greenland split from each other and from Europe, while Arabia was rifting away from Africa. These continental movements also formed the Gulf of Mexico, the African Rift Valley, and the Red Sea, while rifting in Japan opened the Sea of Japan. Elsewhere, multiple small terranes and continents collided, resulting in several mountain chains that we know today, such as the Pyrenees, Alps, and Zagros Mountains. The largest and fastest collision brought India crashing into Southeast Asia, forming the Himalayas and Tibetan Plateau. The multitude of continental collisions raised high mountains and resulted in lower sea levels around the world, causing shifting climates throughout the Tertiary. North American Paleogeography: As sea level dropped, the inland sea that covered much of North America during the Cretaceous withdrew. Grasslands, large lakes, and small mountain ranges covered the central and western states until tectonic changes occurred on the western margin. Rapid uplift in the west created the modern Rocky Mountains, eroded much of the ancestral Sierra Nevada, and generated volcanism that helped form its modern equivalent. Fossils indicate a warm-temperate to subtropical climate in the Early Tertiary of North America. Later in the period, the climate cooled significantly, and many of the warm-weather organisms disappeared from the fossil record in North America. This pattern was repeated with another warming trend followed by a much colder climate in the Late Tertiary. Paleontology: The extinction at the end of the Cretaceous opened numerous ecological niches. These were filled mostly by mammals, which underwent a dramatic evolutionary radiation. By the Late Tertiary, North America was home to mastodons, ground sloths, armadillos, camels, horses, saber tooth cats, giant wolves, giant beavers, and giant bears. Tertiary seas would have looked fairly familiar to us: gastropods and bivalves were very similar to modern forms. Squid replaced the ammonites, which died out at the end of the Cretaceous. Sea urchins and single-celled foraminifera continued to be abundant, but new forms appeared. Sharks and bony fishes were common.