Gorgosaurus

*Gorgosaurus libratus*

* Jurassic Park Institute Artwork
* Robert F Walters Artwork

***Gorgosaurus*** (meaning "fierce lizard") is an extinct genus of theropod and carnivorous dinosaur that reached 8 to 9 meters (26 to 30 feet) in length, and weighed 2.8-3.4 tonnes. It was first described by paleontologist Lawrence Morris Lambe in 1914, and has been found in western Canada and the United States. It lived about 76-75 million years ago in the late Cretaceous period.

Over 20 *Gorgosaurus* skeletons have been recovered, making it the most well-represented tyrannosaurid in the fossil record. Generally similar to *Tyrannosaurus* and most other large tyrannosaurids (such as *Daspletosaurus*, and *Albertosaurus*), *Gorgosaurus* can be described as having a massive head, large, curved teeth, tiny two-fingered front limbs, and powerful legs. Compared to the other tyrannosaurids, *Gorgosaurus* is most similar to its very close relative *Albertosaurus*.

Although it has been suggested that *Gorgosaurus* was a scavenger, its co-existence with the similarly sized, but more robust tyrannosaurid *Daspletosaurus* casts doubt on this theory. Another hypothesis proposes that *Gorgosaurus*, which was rather lean for a tyrannosaurid, actively hunted fleet-footed animals such as duckbills and ornithomimids ("ostrich-mimic" dinosaurs). According to this proposition, the more troublesome ceratopsians and ankylosaurians (horned and heavily armoured dinosaurs) would have been left to *Daspletosaurus* and Gorgosaurus would have hunted on less armored hadrosaurs. An apex predator, it was at the top of the food chain.

For years, the species *Gorgosaurus libratus* (the only species of *Gorgosaurus* currently recognized) was assigned to the *Albertosaurus* genus. However, recent work done by paleontologists suggest that enough differences exist between *G. libratus* and the other *Albertosaurus* species, to justify the original genus name of *Gorgosaurus*.

## Discovery and naming

*Gorgosaurus libratus* was first described by Lawrence Lambe in 1914. Its name is derived from the Greek γοργος/*gorgos* ("fierce" or "terrible") and σαυρος/*saurus* ("lizard"). The type species is *G. libratus*; the specific epithet "balanced" is the past participle of the Latin verb *librare*, meaning "to balance".

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.

Matthew and Brown also described a fifth skeleton (AMNH 5664), which Charles H. Sternberg had collected in 1917 and sold to their museum. It was smaller than other *Gorgosaurus* specimens, with a lower, lighter skull and more elongate limb proportions. Many sutures between bones were unfused in this specimen as well. Matthew and Brown noted that these features were characteristic of juvenile tyrannosaurids, but still described it as the holotype of a new species, *G. sternbergi*. Today's paleontologists regard this specimen as a juvenile *G. libratus*. Dozens of other specimens have been excavated from the Dinosaur Park Formation and are housed in museums across the United States and Canada.*G. libratus* is the best-represented tyrannosaurid in the fossil record, known from a virtually complete growth series.

In 1856, Joseph Leidy described two tyrannosaurid premaxillary teeth from Montana. Although there was no indication of what the animal looked like, the teeth were large and robust, and Leidy gave them the name *Deinodon*. Matthew and Brown commented in 1922 that these teeth were indistinguishable from those of *Gorgosaurus*, but in the absence of skeletal remains of *Deinodon*, opted not to synonymize the two genera. Although *Deinodon* teeth are very similar to those of *Gorgosaurus*, tyrannosaurid teeth are extremely uniform, so it cannot be said for certain which animal they belonged to. *Deinodon* is regarded as a *nomen dubium* today. 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.

## Description

Gorgosaurus was smaller than Tyrannosaurus or Tarbosaurus, closer in size to Albertosaurus and Daspletosaurus. Adults reached 8 to 9.3 m (26 to 31 ft) from snout to tail and a body weight of around 3 tonnes. 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. Gorgosaurus has a proportionally shorter skull than Albertosaurus, 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.

## Classification and systematics

Gorgosaurus is classified in the theropod subfamily Albertosaurinae within the family Tyrannosauridae.

## Paleobiology

### Life history

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.

## Paleoecology

Most specimens of Gorgosaurus libratus have been recovered from the Dinosaur Park Formation in Alberta.

## JPInstitute.com Description

Gorgosaurus is considered by some scientists to be the same as Albertosaurus. This dinosaur looked like a smaller, slightly less muscular version of Tyrannosaurus. As an early member of the tyrannosaur family, Gorgosaurus was the final design for the large meat-eating dinosaurs. It had only two fingers on its hands, which were at the end of short arms. It had long legs and a wide skull with lots of power in its bite.

Gorgosaurus was named based on material that is now being disputed as requiring a genus separate from Albertosaurus.

## Dinosaur Field Guide Description

Gorgosaurus ("fierce lizard") was a tyrannosaur, or tyrant dinosaur, It was a very close relative of Tyrannosaurus- although Gorgosaurus was a little smaller and lived somewhat earlier. The tyrant dinosaurs were very specialized meat-eaters. Their arms were extremely short, and they had only two fingers (the thumb and the index finger). The arms of Gorgosaurus-like those of Tyrannosaurus- were so short they couldn't even reach its mouth! Tyrant dinosaurs' legs were long and slender, and their narrow, compact feet had a special shock-absorbing shape. A young Gorgosaurus was probably as fast as an ostrich dinosaur, and even an adult was probably faster than any of the duckbills and horned dinosaurs it hunted. Although its arms would be useless in catching prey, it could use its powerful jaws, filled with many strong teeth, to grab its victims. Tyrant dinosaurs' teeth, unlike those of most theropods, or meat-eating dinosaurs, were not shaped like blades. Instead, they were thick and round in cross-section. This meant that the teeth could be used to hold and grip tighter, and maybe even to crush bone. When tyrant dinosaurs were around, they were the largest predators in their environment. The only meat-eater a Gorgosaurus had to fear was another Gorgosaurus, or the even larger tyrant dinosaur Daspletosaurus, which lived at the same time. Of course, Gorgosaurus would have to be careful when attacking a Centrosaurus or other horned dinosaur. Although these ceratopsians ate plants, they could still defend themselves with their deadly horns.

### Fun Facts

Gorgosaurus was once considered e species of Albertosaurus, which is really a different (but closely related) tyrant dinosaur from later in the Cretaceous.

### Trivia

Almost every dinosaur labeled "Albertosaurus" in a museum is really a Gorgosaurus.

## Links

http://web.archive.org/web/20040214160606fw\_/http://www.jpinstitute.com/dinopedia/dinocards/dc\_gorgo.html#
https://web.archive.org/web/20080723142257/http://kids.yahoo.com/dinosaurs/283--Gorgosaurus

## References

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