Gorgosaurus (meaning “dreadful lizard”) is a genus of tyrannosaurid theropod dinosaur that lived in western North America during the Late Cretaceous Period, between about 76.6 and 75.1 million years ago. Fossil remains have been found in the Canadian province of Alberta and possibly the U.S. state of Montana. Paleontologists recognize only the type species, G. libratus, although other species have been erroneously referred to the genus.
Like most known tyrannosaurids, Gorgosaurus was a bipedal predator weighing more than two metric tons as an adult; dozens of large, sharp teeth lined its jaws, while its two-fingered forelimbs were comparatively small. Gorgosaurus was most closely related to Albertosaurus, and more distantly related to the larger Tyrannosaurus. Gorgosaurus and Albertosaurus are extremely similar, distinguished mainly by subtle differences in the teeth and skull bones. Some experts consider G. libratusto be a species of Albertosaurus; this would make Gorgosaurus a junior synonym of that genus.
Gorgosaurus lived in a lush floodplain environment along the edge of an inland sea. It was an apex predator, preying upon abundant ceratopsids and hadrosaurs. In some areas, Gorgosaurus coexisted with another tyrannosaurid, Daspletosaurus. Although these animals were roughly the same size, there is some evidence of niche differentiation between the two. Gorgosaurus is the best-represented tyrannosaurid in the fossil record, known from dozens of specimens. These plentiful remains have allowed scientists to investigate its ontogeny, life history and other aspects of its biology.
Gorgosaurus was smaller than Tyrannosaurus or Tarbosaurus, closer in size to Albertosaurus and Daspletosaurus. Adults reached 8 to 9 m (26 to 30 ft) from snout to tail. Paleontologists have estimated full-grown adults to weigh about 2.5 tonnes (2.8 short tons), perhaps approaching 2.8 tonnes (3.1 short tons). The largest known skull measures 99 cm (39 in) long, just slightly smaller than that of Daspletosaurus. As in other tyrannosaurids, the skull was large compared to its body size, although chambers within the skull bones and large openings (fenestrae) between bones reduced its weight. Albertosaurusand Gorgosaurus share proportionally longer and lower skulls than Daspletosaurus and other tyrannosaurids. The end of the snout was blunt, and the nasal and parietal bones were fused along the midline of the skull, as in all other members of the family. The eye socket was circular rather than oval or keyhole-shaped as in other tyrannosaurid genera. A tall crest rose from the lacrimal bone in front of each eye, similar to Albertosaurus and Daspletosaurus. Differences in the shape of bones surrounding the brain set Gorgosaurus apart from Albertosaurus.
Most specimens of Gorgosaurus libratus have been recovered from the Dinosaur Park Formation in Alberta. This formation dates to the middle of the Campanian, between 76.5 and 74.8 million years ago, and Gorgosaurus libratus fossils are known specifically from the lower to middle section of the formation, between 76.6 and 75.1 million years ago.