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The Jurassic period extended from about 200 to 146 million years ago. The Jurassic period was right smack in the middle of the "Age of Reptiles," also known as the "Age of Dinosaurs." In the late Triassic, immediately before the Jurassic, almost all the important dinosaur groups evolved — theropods (which includes all carnivorous dinosaurs), sauropods (long-necked herbivores which grew to absolutely gargantuan sizes during the Jurassic), and assorted herbivores such as Stegosaurus. Other non-dinosaur reptiles also had evolved in the late Triassic, including pliosaurs (marine reptiles) and pterosaurs (large winged reptiles).
The Jurassic was immediately before the age of flowering plants, so the landscape was dominated by conifers, cycads, and ferns. Cycads, tropical trees with a stout trunk, experienced their greatest success during the Jurassic period, and even today, many intuitively recognize these plants as "common in the time of the dinosaurs." Unlike the arid Triassic before it, or the cold Cenozoic of today, the Jurassic was lush and warm, including at higher altitudes, where forests extended to the poles. This climate was extremely encouraging towards life at the time.
By the start of the Jurassic, dinosaurs had already ruled the Earth for about 30 million years. There were two main groups: saurischians (lizard-hipped dinosaurs), and ornithischians (bird-hipped dinosaurs). The saurischians included the theropods (all carnivores) and the sauropods (the largest herbivores). The ornithischians included a variety of other medium-to-large sized herbivores, though all smaller than most sauropods. Confusingly, birds actually evolved in the Late Jurassic from the lizard-hipped dinosaurs. They acquired a distinctive bird hip, in an example of parallel evolution.
Various large theropods dominated the ecosystems of the Jurassic, such as the carnivore Allosaurus, which had the same body plan as Tyrannosaurus rex, which did not evolve until 90 million years later. The largest of this species reached 12 m (39 ft) in length, and were among the most terrifying terrestrial carnivores of all time. No terrestrial carnivore today comes even close.
The Jurassic is considered the golden age of sauropods, huge long-necked dinosaurs which used their height to eat leaves off the tallest trees. Many sauropods had such bulk and size that they had little to worry about even from the largest predators such as Allosaurus. One late Jurassic sauropod, Supersaurus, had a length up to 34 m (112 ft) in length, and a weight of up to 40 tons.