Dinosaur

The carnosaurs were a group of saurischian predators, or theropods, that grew large and had enormous hind limbs but tiny fore limbs. Tyrannosaurus rex was the largest carnivore that has ever stalked Earth; its scientific name is derived from Greek words for "tyrant reptile king." This fearsome, bipedal predator of the Late Cretaceous could grow to a length of 45 ft (14 m) and may have weighed as much as 9 tons (8.2 metric tons). Tyrannosaurus rex (T. rex) had a massive head and a mouth full of about 60 dagger-shaped, 6-in-long (15-cm-long), very sharp, serrated teeth, which were renewed throughout the life of the animal. This predator probably ran in a lumbering fashion using its powerful hind legs, which may also have been wielded as sharp-clawed, kicking weapons. It is thought that T. rex may have initially attacked its prey with powerful head-butts and then torn the animal apart with its enormous, 3 ft long (1 m long) jaws. Alternatively, T. rex may have been a scavenger of dead dinosaurs. The relatively tiny fore legs of T. rex probably only had little use. The long and heavy tail of T. rex was used as a counter-balance for the animal while it was running and as a stabilizing prop while it was standing.

Albertosaurus was also a large theropod of the Late Cretaceous. Albertosaurus was similar to Tyrannosaurus, but it was a less massively built animal at about 25 ft (8 m) long and 2 tons (1.8 metric tons) in weight. Albertosaurus probably moved considerably faster than Tyrannosaurus.

Allosaurus was a gigantic, bipedal predator of the Late Jurassic. Allosaurus could grow to a length of 36 ft (12 m) and a weight of 2 tons (1.8 metric tons). The jaws of Allosaurus were loosely hinged, and they could detach to swallow large chunks of prey.

Spinosaurus was a "fin-back" (or "sail-back") dinosaur of the Late Cretaceous period that was distantly related to Allosaurus. Spinosaurus had long, erect, skin-covered, bony projections from its vertebrae that may have been used to regulate body temperature or perhaps for behavioral displays to impress others or attract a mate. Spinosaurus could have achieved a length of 40 ft (13 m) and a weight of 7 tons (6.3 metric tons). These animals had small, sharp teeth and were probably carnivores. Dimetrodon and Edaphosaurus, early Permian pelycosaurs (mammal-like reptiles, not dinosaurs), are sometimes confused with Spinosaurus as they also had sail-like back spines.

Not all of the fearsome dinosaurian predators or theropods were enormous. Deinonychus, for example, was an Early Cretaceous dinosaur that grew to about 10 ft (3 m) and weighed around 220 lb (100 kg). Deinonychus was one of the so-called running lizards, which were fast, agile predators that likely hunted in packs. As a result, Deinonychus was probably a fearsome predator of animals much larger than itself. Deinonychus had one of its hind claws enlarged into a sharp, sickle-like, slashing weapon, which was wielded by jumping on its prey and then kicking, slashing, and disemboweling the victim. The scientific name of Deinonychus is derived from the Greek words for terrible claw.

The most infamous small theropod is Velociraptor, or "swift plunderer," a 6-ft-long (2-m-long) animal of the Late Cretaceous. Restorations of this fearsome, highly intelligent, pack-hunting, killing machine were used in the movie Jurassic Park.

Oviraptosaurs (egg-stealing reptiles) were relatively small, probably highly intelligent theropods that were fast-running hunters of small animals, and some are believed to have also been specialized predators of the nests of other dinosaurs. The best known of these animals is Late Cretaceous Oviraptor. Ingenia, a somewhat smaller oviraptorsaur, was about 6 ft (2 m) long, weighed about 55 lb (25 kg), and also lived during the Late Cretaceous. Microvenator of the early Cretaceous was less than 3 ft (1 m) long and weighed about 12 lb (6 kg).