The theropods were all the predatory dinosaurs except the herrerasaurians. From the smallest dinosaurs to the largest meat-eaters, the theropods had the most different kinds of saurischian dinosaurs of all suborders. These two-legged meat-eaters had clawed feet with no more than three functional toes.
The wings and feet of birds are similar to the arms and feet of theropod skeletons. Also like birds, all theropods to some extent had hollow hones. The best ancestral bird is the small, feathered, theropodlike Archaeopteryx from the Late Jurassic.
The theropods evolved into two major groups: the Ceratosauria, with flexible tails; and the Tetanurae, with stiff tails. All the earliest theropods were ceratosaurians. Their fossil record is from the Late Triassic through the Late Jurassic. The tetanurans appeared in the Middle Jurassic, diversified in the Late Jurassic, and were the main northern-hemisphere predators until the Late Cretaceous.
Family: Podokesauridae: The earliest ceratosaurians include Coelophysis from the Late Triassic of western North America. It was small and nimble and had a long, slender skull with many teeth.
Families: Halticosauridae and Ceratosauridae: Dilophosaurus, which lived during the Early Jurassic, had a double crest on its head. Ceratosaurus was from the Late Jurassic and had a horn on its head. Both were from North America and are examples of later members of the ceratosaurians. After the Late Jurassic, ceratosaurians apparently vanished in the northern hemisphere but survived in South America.
Family: Abelisauridae: The abelisaurids are a group of medium to large African and South American theropods characterized by short, tall skulls. Carnotaurus from Argentina and Majungatholus from Madagascar are similar with the exception that Carnotaurus has two large horns on the skull.
The tetanurans, the most advanced theropods, included several groups where the relationships are not well understood. Crests and other decorations on the head were usually not present. Their hands had three or fewer fingers, and the "thumb" usually had the largest claw.
Family: Compsognathidae: The most primitive tetanuran was Compsognathus from the Late Jurassic of Europe. It was the smallest theropod, about three feet long and lightly built.
Family: Coeluridae: Ornitholestes and Coelurus, which lived during the Late Jurassic in western North America, were fast-running, lightly built theropods that were two to three feet tall at the hips and from six to ten feet long.
Family: Carcharodontosauridae: This group of giant theropods from Gondwana includes enormous predatory dinosaurs, Giganotosaurus from Argentina and Car-charodontosaurus from North Africa.
Family: Therizinosauridae: The therizinosaurids were apparently herbivorous or omnivorous theropods known from the Late Cretaceous of Asia and North America. The unusual, birdlike pelves and almost prosauropod-like skulls of therizinosaurids have resulted in uncertainty about their evolutionary position, but they have recently been shown to be theropods closely related to the Ornithomimidae.
Family: Spinosauridae: Spinosaurids are a distinctive group of theropods with long, crocodile-like snouts and elongated vertebral spines that may have formed sail-like structures on their backs. Spinosaurids are restricted to the Cretaceous but are known from Africa, South America, and Europe.
Family: Oviraptorisauridae: Another curious theropod from the Cretaceous of Asia is Oviraptor, which has a tall, highly pneumatic skull with a turtle-like beak. Oviraptor got its name ("egg predator") because specimens were found in Mongolia with what were originally thought to be nests of ceratopsian eggs.
Family: Allosauridae: This family is typical of the larger Jurassic and Early Cretaceous theropods that were from 15 to 35 feet long or longer. The biggest allosaurid may have been more than 40 feet long. Allosaurids were slender but dangerous predators.
Family: Tyrannosauridae: Most dramatic of all the theropods were the tyrannosaurids, which probably came from allosauridlike ancestors at the beginning of the Late Cretaceous. Unlike other tetanurans, they had massive bodies, unusually shaped heads, and small two-fingered hands.
Some tyrannosaurids were nearly 50 feet long and became the largest meat-eating land animals known. The smallest, such as Nanotyrannus, were about 18 feet long. The medium-size tyrannosaurids, such as Albertosaurus and Daspletosaurus, were about 30 feet long
Family: Ornithomimidae: Many kinds of small theropods also arose in the Cretaceous. Their skeletons were birdlike. Most had changes in their front legs and hands for a powerful striking action. The "quick-strike" motion of the front limbs may have been the beginning of the power stroke of birds' wings.
The "ostrich dinosaurs" or Ornithomimidae are known best from the Late Cretaceous of eastern Asia and western North America. They had small heads and they usually had no teeth. They had long necks and short, stiff backs. Their front limbs were long, and their powerful rear legs were built for speed. They are thought to have been the fastest dinosaurs.
Family: Dromaeosauridae: Deinonychus and other "sickle claw" theropods are among the best-studied dinosaurs. The discovery of Deinonychus supported the idea of a bird-dinosaur relationship and started the debate about dinosaurs being warm-blooded. Each foot had a large, sickle-shaped claw on the second toe. The end of the tail had vertebrae that locked together to make it stiff.
Family: Troodontidae: This group of "sickle claw" theropods had large brains and large eyes that faced forward. Some, such as Troodon and Saurornithoides, may have been almost as smart as some mammals.