'dreadful lizard') is a genus of tyrannosaurid theropod dinosaur that lived in western North America during the Late Cretaceous Period (Campanian), between about 76.5 and 75 million years ago.

[5] Prospectors from the American Museum of Natural History in New York City were active along the Red Deer River in Alberta at the same time, collecting hundreds of spectacular dinosaur specimens, including four complete G. libratus skulls, three of which were associated with skeletons.

[5] One specimen from Montana (TCMI 2001.89.1), housed in the Children's Museum of Indianapolis, shows evidence of severe pathologies, including healed leg, rib, and vertebral fractures, osteomyelitis (infection) at the tip of the lower jaw resulting in permanent tooth loss, and possibly a brain tumor.

A complete skull of a small tyrannosaurid (CMNH 7541), found in the younger, late Maastrichtian-age Hell Creek Formation of Montana, was named Gorgosaurus lancensis by Charles Whitney Gilmore in 1946.

The eight premaxillary teeth at the front of the snout were smaller than the rest, closely packed and D-shaped in cross section.

In several smaller specimens of Gorgosaurus, the tibia was longer than the femur, a proportion typical of fast-running animals.

[26] In the Encyclopedia of Dinosaurs Kenneth Carpenter pointed out that traces of skin impressions from the tail of Gorgosaurus showed similar small rounded or hexagonal scales.

[5] Appalachiosaurus was described as a basal tyrannosauroid just outside Tyrannosauridae,[28] although American paleontologist Thomas Holtz published a phylogenetic analysis in 2004 which indicated it was an albertosaurine.

[24][30] The close similarities between Gorgosaurus libratus and Albertosaurus sarcophagus have led many experts to combine them into one genus over the years.

[7] Canadian paleontologist Phil Currie claims there are as many anatomical differences between Albertosaurus and Gorgosaurus as there are between Daspletosaurus and Tyrannosaurus, which are almost always kept separate.

[37][38] Thomas R. Holtz Jr., a paleontologist who also previously theorized that tyrannosaurs underwent a big dietary shift with maturation, said that the fossil "looks like it was Thanksgiving," as the juvenile Gorgosaurus was mostly eating the legs of Citipes.

[39] Gregory Erickson and colleagues have studied the growth and life history of tyrannosaurids using bone histology, which can determine the age of a specimen when it died.

This pattern is seen in modern Komodo dragons, whose hatchlings start off as tree-dwelling insectivores and slowly mature into massive apex predators capable of taking down large vertebrates.

[7] Other tyrannosaurids, including Albertosaurus, have been found in aggregations that some have suggested to represent mixed-age packs, but there is no evidence of gregarious behavior in Gorgosaurus.

However, both tyrannosaur genera underwent these ontogenetic transformations at a similar percent of skull length relative to the large known adult individuals.

The study's results likewise indicate that there is a dissociation between body size and cranial development in tyrannosaurs, while simultaneously allowing better identification of juvenile remains that may have been misidentified in museum fossil collections.

[43] It is estimated that an ontogenetic dietary shift of Gorgosaurus and Albertosaurus occurs when the mandibular length reaches 58 cm (1.90 ft), indicating that this is the stage when their bite force increases exponentially and when they begin to pursuit large prey.

In a 2001 study conducted by Bruce Rothschild and other paleontologists, 54 foot bones referred to Gorgosaurus were examined for signs of stress fracture, but none were found.

[46] Specimen TMP 1994.143.1, a juvenile skull from the Dinosaur Park Formation with several tyrannosaur bite marks, was previously believed to be Daspletosaurus sp.

Also present was a healed fracture in the dentary and what the authors describing the specimen referred to as "a mushroom-like hyperostosis of a right pedal phalanx."

Ralph Molnar has speculated that this may be the same kind of pathology afflicting an unidentified ornithomimid discovered with a similar mushroom shaped growth on a toe bone.

The Laramide Orogeny had begun uplifting the Rocky Mountains to the west, from which flowed great rivers that deposited eroded sediment in vast floodplains along the coast.

[57] Around 73 million years ago, the seaway began to expand, transgressing into areas formerly above sea level and drowning the Dinosaur Park ecosystem.

Huge herds of ceratopsids roamed the floodplains alongside equally large groups of saurolophine and lambeosaurine hadrosaurs.

Small predatory dinosaurs like oviraptorosaurs, troodonts and dromaeosaurs hunted smaller prey than the huge tyrannosaurids; Daspletosaurus and Gorgosaurus, which were two orders of magnitude larger in mass.

[8][7][58] A Saurornitholestes dentary has been discovered in the Dinosaur Park Formation that bore tooth marks left by the bite of a young tyrannosaur, possibly Gorgosaurus.

[59] In the middle stages of the Dinosaur Park Formation, Gorgosaurus lived alongside a rarer species of tyrannosaurid, Daspletosaurus.

[60] In 1970, Dale Russell hypothesized that the more common Gorgosaurus actively hunted fleet-footed hadrosaurs, while the rarer and more troublesome ceratopsians and ankylosaurians (horned and heavily armoured dinosaurs) were left to the more heavy built Daspletosaurus.

Chasmosaurine ceratopsians and saurolophine hadrosaurs are also more common in the Two Medicine Formation of Montana and in southwestern North America during the Campanian, while centrosaurines and lambeosaurines dominate in northern latitudes.

At the end of the later Maastrichtian stage, tyrannosaurines like Tyrannosaurus rex, saurolophines like Edmontosaurus and Kritosaurus and chasmosaurines like Triceratops and Torosaurus were widespread throughout western North America, while lambeosaurines were rare, consisting of a few species like Hypacrosaurus, and albertosaurines and centrosaurines had gone extinct.