Old Earth Ministries Online Dinosaur Curriculum

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Lesson 23 - Gorgosaurus

Gorgosaurus (meaning "fierce lizard") is a genus of tyrannosaurid theropod dinosaur that lived in western North America during the Late Cretaceous Period, between about 76.5 and 75 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 a metric ton 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. libratus to 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. An apex predator, it was at the top of the food chain, preying upon abundant ceratopsids and hadrosaurs. In some areas, Gorgosaurus coexisted with another tyrannosaurid, Daspletosaurus. Though 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.

Description

     Gorgosaurus teeth were typical of all known tyrannosaurids. The eight premaxillary teeth at the front of the snout were smaller than the rest, closely packed and D-shaped in cross section. In Gorgosaurus, the first tooth in the maxilla was also shaped like the premaxillary teeth. The rest of the teeth were oval in cross section, rather than blade-like as in most other theropods. Along with the eight premaxillary teeth, Gorgosaurus had 26 to 30 maxillary teeth and 30 to 34 teeth in the dentary bones of the lower jaw. This number of teeth is similar to Albertosaurus and Daspletosaurus but is fewer than those of Tarbosaurus or Tyrannosaurus.

     Gorgosaurus shared its general body plan with all other tyrannosaurids. Its massive head was perched on the end of an S-shaped neck. In contrast to its large head, its forelimbs were very small. The forelimbs had only two digits, although a third metacarpal is known in some specimens, the vestigial remains of the third digit seen in other theropods. Gorgosaurus had four digits on each hindlimb, including a small first toe (hallux) which did not contact the ground. Tyrannosaurid hindlimbs were long relative to overall body size compared with other theropods. The largest known Gorgosaurus femur measured 105 centimeters (41 in) long. In several smaller specimens of Gorgosaurus, the tibia was longer than the femur, a proportion typical of fast-running animals. The two bones were of equal length in the largest specimens. The long, heavy tail served as a counterweight to the head and torso and placed the center of gravity over the hips.

Classification and Systematics

Gorgosaurus is classified in the theropod subfamily Albertosaurinae within the family Tyrannosauridae. It is most closely related to the slightly younger Albertosaurus. These are the only two definite albertosaurine genera that have been described, although other undescribed species may exist.  All other tyrannosaurid genera, including Daspletosaurus, Tarbosaurus and Tyrannosaurus, are classified in the subfamily Tyrannosaurinae. Compared to the tyrannosaurines, albertosaurines had slender builds, with proportionately smaller, lower skulls and longer bones of the lower leg (tibia) and feet (metatarsals and phalanges).

The close similarities between Gorgosaurus libratus and Albertosaurus sarcophagus have led many experts to combine them into one genus over the years. Albertosaurus was named first, so by convention it is given priority over the name Gorgosaurus, which is sometimes considered its junior synonym. William Diller Matthew and Barnum Brown doubted the distinction of the two genera as early as 1922. Gorgosaurus libratus was formally reassigned to Albertosaurus (as Albertosaurus libratus) by Dale Russell in 1970, and many subsequent authors followed his lead. Combining the two greatly expands the geographical and chronological range of the genus Albertosaurus. Other experts maintain the two genera as separate. 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. He also notes that undescribed tyrannosaurids discovered in Alaska, New Mexico and elsewhere in North America may help clarify the situation.

Discovery and Naming

Gorgosaurus libratus was first described by Lawrence Lambe in 1914. The type species is G. libratus.  The holotype of Gorgosaurus libratus (NMC 2120) is a nearly complete skeleton associated with a skull, discovered in 1913 by Charles M. Sternberg. This specimen was the first tyrannosaurid found with a complete hand. It was found in the Dinosaur Park Formation of Alberta and is housed in the Canadian Museum of Nature in Ottawa. 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. Matthew and Brown described four of these specimens in 1923.

Several tyrannosaurid skeletons from the Judith River Formation of Montana probably belong to Gorgosaurus, although it remains uncertain whether they belong to G. libratus or a new species. 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.

Paleobiology

Coexistence with Daspletosaurus

In the Dinosaur Park Formation, Gorgosaurus lived alongside a rarer species of the tyrannosaurine Daspletosaurus. This is one of the few examples of two tyrannosaur genera coexisting. Similar-sized predators in modern predator guilds are separated into different ecological niches by anatomical, behavioral or geographical differences that limit competition. Niche differentiation between the Dinosaur Park tyrannosaurids is not well-understood. In 1970, Dale Russell hypothesized 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. However, a specimen of Daspletosaurus (OTM 200) from the contemporaneous Two Medicine Formation of Montana preserves the digested remains of a juvenile hadrosaur in its gut region, and another bonebed contains the remains of three Daspletosaurus along with the remains of at least five hadrosaurs.

Unlike some other groups of dinosaurs, neither genus was more common at higher or lower elevations than the other. However, Gorgosaurus appears more common in northern formations like the Dinosaur Park, with species of Daspletosaurus more abundant to the south. The same pattern is seen in other groups of dinosaurs. Chasmosaurine ceratopsians and hadrosaurine hadrosaurs are also more common in the Two Medicine Formation of Montana and in southwestern North America during the Campanian, while centrosaurine and lambeosaurines dominate in northern latitudes. Holtz has suggested this pattern indicates shared ecological preferences between tyrannosaurines, chasmosaurines and hadrosaurines. At the end of the later Maastrichtian stage, tyrannosaurines like Tyrannosaurus rex, hadrosaurines like Edmontosaurus and chasmosaurines like Triceratops were widespread throughout western North America, while albertosaurines and centrosaurines went extinct, and lambeosaurines were rare.

Life History

Gorgosaurus spent as much as half its life in the juvenile phase before ballooning up to near-maximum size in only a few years. This, along with the complete lack of predators intermediate in size between huge adult tyrannosaurids and other small theropods, suggests these niches may have been filled by juvenile tyrannosaurids. This is seen in modern Komodo dragons, where hatchlings start off as tree-dwelling insectivores and slowly mature into massive apex predators capable of taking down large vertebrates. 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.

 Paleoecology

The Dinosaur Park Formation preserves a great wealth of vertebrate fossils. A wide variety of fish swam the rivers and estuaries, including gars, sturgeons, sharks and rays, among others. Frogs, salamanders, turtles, crocodilians and champsosaurs also dwelled in the aquatic habitats. Azhdarchid pterosaurs and neornithine birds like Apatornis flew overhead, while the enantiornithine bird Avisaurus lived on the ground alongside multituberculate, marsupial and placental mammals. A number of species of terrestrial lizards were also present. Dinosaur fossils in particular are found with unrivaled abundance and diversity. Huge herds of ceratopsids roamed the floodplains alongside equally large groups of hadrosaurine and lambeosaurine hadrosaurs. Other herbivorous groups like ornithomimids, therizinosaurs, pachycephalosaurs, small ornithopods, nodosaurids and ankylosaurids were also represented. 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. Intervening predatory niches may have been filled by young tyrannosaurids.

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