World Paleogeography: In the Early Jurassic, Pangea was breaking up between modern North America, Africa, and South America. By the Middle Jurassic, these rifts were opening up the Central Atlantic Ocean and the Gulf of Mexico formed. Rifting between North America and Eurasia began forming the North Atlantic Ocean while closing the Tethys Ocean. At the same time, volcanic activity began along adjacent margins of East Africa, Antarctica, and Madagascar, where the South Atlantic Ocean would later form. Global climate during the Jurassic was relatively warm and moist over much of northern North America, Eurasia, and Indo-Australian Gondwana. Desert and seasonally wet conditions were present throughout much of Africa, South America, and southern North America. North American Paleogeography: As Pangea rifted apart, North America moved northwest during the Jurassic. Much of the interior of North America was covered with shallow seas. The highlands that dominated eastern North America eroded and large dune deposits formed along the coast, which stood east of the modern Rocky Mountains. Subduction on the western edge of the continent caused frequent volcanic eruptions, forming the igneous rocks in the core of the ancestral Sierra Nevada. Paleontology: Dinosaurs diversified to fill most of the major ecological niches for land animals. Jurassic dinosaurs reached tremendous sizes: Supersaurus reached over 40 meters in length and Brachiosaurus tipped the scales at about 55 tons. The stegosaurs became abundant, and carnivores such as Allosaurus and Megalosaurus preyed on the abundant herbivores, large and small. Ammonites, coiled and straight-shelled relatives of the modern chambered Nautilus, were major predators among the invertebrates. Ammonite fossils were so abundant and so distinctive in many Jurassic and Cretaceous rocks that they can be used to very finely subdivide these periods.