The Jurassic Period, 190-135 million years ago, was the middle segment of the Mesozoic Era, the Age of Dinosaurs. While small, herbivorous dinosaurs had appeared in the Triassic, the Jurassic witnessed the rise of their larger cousins, such as allosaurus, stegosaurus and brontosaurus. Earth's continents, now split west to east by the Tethys Sea, still occupied the warmer latitudes, fostering dense vegetation that sustained these giants. Small mammals already scurried beneath the copious plant life but would be dominated by dinosaurs throughout the Mesozoic.
Flowering plants appeared during the Jurassic, some 200 million years after ferns and early conifers graced the scene. Other newcomers included pterosaurs and archaeopteryx; while the former were superb gliders, the latter was perhaps the most primitive relative of modern birds. The Atlantic began to open in the middle of the Period and the Sundance Sea covered most of what is now the High Plains and Rocky Mountain region of North America; this shallow sea deposited the Morrison Formation, now famous for its cargo of Jurassic fossils.
In concert with the opening of the Atlantic, the Andes began to rise (as the Farallon Plate subducted beneath future South America) and the Smartville Block, a large exotic terrain, collided with western North America, adding much of Nevada and California to the Continent; it was within this terrain that the massive Sierra Batholith began to form, not to rise until the end of the Tertiary (some four million years ago). On the other side of the young Atlantic, Madagascar rifted from Africa, drifting southward to rendezvous with the interconnected mass of Antarctica, Australia and India. All in all, the Jurassic was certainly a momentous Period in the history of our planet.