Theropods are part of the saurischian class of dinosaurs. Dating from the middle of the Triassic period, approximately 225 million years ago, saurischians are characterized by flexible hips, hollow bones, grasping hands and long mobile necks. Theropods were bipedal (walking on two legs) and carniverous (meat eating).
Ceratosaurs, the "horned reptile" theropods were birdlike with hollow bones and slender s-shaped necks. Most ceratosaurs have by a large lower tooth that fit into a notch between the upper and lower jaw. Ceratosaurs include: Ceratosaurus nasicornis (25 feet long with a prominent nasal horn), Segisaurus halli (3 feet long), Coelophysis bauri (6 feet long). Most famous is Dilophosaurus, with its large painted crest on both sides of its head that it flared when angry or threatened.
Greek for "meat-eating lizards," carnisaurus were ferocious carnivores with narrow jaws, muscled forelimbs and large claws. Allosaurus fragilis (40 feet long) also had talons on its hind feet that scientists speculate were used to kill prey. Spinosaurus aegypticus (40 feet long) had spines, up to six feet long, on its back. Finally, the first dinosaur, identified in 1842, Megalosaurus, is unlikely to have existed. Many scientists, including those at the University of California Museum of Paleontology use Megalosaur as a generic term for miscellaneous and undistinguished carosaurs.
Coleurosaurus are generally described as predators that were very fast and strong for their weight with larger than average brains. The Tyrannosaurus Rex (up to 40 feet long) was the largest coleurosaur. Originally classed as a carnosaur, Tyrannosaurus Rex did not have the heavily muscled forearms and claws typical of carnosaurs. Another coelurosaur is the Velociraptor ("speedy thief"). Velociraptors were 6 feet long and weighed 30 pounds with a large sickle-shaped, retractible claw on its second toe. One of the fastest dinosaurs, scientists believe Velociraptors could run up to 40 miles-per-hour in short bursts. Other colelurosaurs include Gallimimus bullatus and Struthionimus altus and Ornithomimus velox. All three were very fast with delicate birdlike builds.
As the University of California Museum of Paleontology warns, the study of dinosaurs is a rapidly evolving and changing field. A theory put forth one day can be disproved by a fossil find the next. Questions like "Is T. Rex a carnosaur or a coelursaur?" and "How fast was the Velociraptor?" are works in progress. T. Rex and raptors are two dinosaurs commonly known. However, everyday birds are living dinosaurs.