A subcategory of the lizard-hipped dinosaur order Saurischia, the Theropoda includes all carnivorous, or meat-eating, dinosaurs. The theropods were a highly diverse group that ranged in size from the relatively small 100-pound (45-kilogram) Velociraptor, to the massive 6-ton Tyrannosaurus rex. Fossil evidence indicates that modern birds share a common ancestor with a branch of the theropods.
Two of the oldest known dinosaurs are theropods. Both of these animals were relatively small when compared with most of the known dinosaurs. Herrerasaurus measured approximately 10 to 20 feet (3 to 6 meters) in length. Eodromaeus was only about 4 feet (1.2 meters) long. Both animals lived during the late Triassic period (approximately 208–230 million years ago). Eodromaeus is the older of the two dinosaurs; scientific tests revealed that it lived about 230 million years ago.
All theropods were bipedal, meaning that they stood upright and ran or walked on their two hind limbs. Their legs were long and muscular; in many species, the tibia, or shin bone, was longer than the femur, or thigh bone. Theropods had fairly short arms that ended in grasping hands armed with two or three clawed fingers, depending upon the species. In all theropods, however, the fourth and fifth fingers were greatly reduced in size, if present at all. Although the name Theropoda comes from the Latin words for “beast foot,” the feet of theropods were actually birdlike. Theropods had five toes—three clawed toes pointed forward, the first, or big, toe was shortened and pointed backward, and the fifth toe was reduced. Like modern birds, most species of theropods had hollow bones.
Theropods are most noted for the rows of razor-sharp teeth that lined their powerful jaws, enabling them to tear the flesh of their prey easily. Unlike most vertebrates, theropods also had a set of abdominal ribs. This unusual feature helped to protect the vulnerable and highly critical abdominal organs from serious injury.
The classification of dinosaurs changes frequently as scientists find different ways to organize the relationships among different taxa, or groups. Theropoda includes several subcategories, the largest of which are the Ceratosauria and Tetanurae.
The Ceratosauria, which contains most of the earliest theropods, were generally small to medium in size; this group includes Ceratosaurus and Dilophosaurus.
The Tetanurae is a widely diverse group, containing the more advanced theropods. There are many theories regarding how to subdivide this category, none of which is universally accepted. Among the best-known tetanurans are Compsognathus, the smallest of the theropods; this dinosaur grew only as large as a modern chicken. Another well-known tetanuran is Ornithomimus, a beaked, ostrich-sized theropod whose long legs adapted it for running. The agile, sickle-clawed raptor dinosaurs, most notably Velociraptor and Deinonychus, also belong to the Tetanurae, as does Oviraptor. The largest and most advanced of the Tetanurae thrived until the end of the Cretaceous period (roughly 66 million years ago). Among the best-known members of this group are Allosaurus and Tyrannosaurus rex, which could reach a weight of 8 tons. All of the modern birds are classified in the group Tetanurae.