Gorgosaurus

Credit: Sergey Meleshin.

*Gorgosaurus* is an extinct genus of tyrannosaurid theropod dinosaur that lived in North America during the Late Cretaceous.

## History

Lawrence Lambe (1914) named G. libratus, with the the Greek librare, the past participle "balanced". NMC 2120 is the holotype, near-complete postcrania and crania discovered by Charles Sternberg the year before. This is the first tyrannosaur with a full hand, fund in the Dinosaur Park Formation and now housed at Canadian Museum of Nature. Four complete *G. libratus* skulls and 3 associated skeletons were found by prospectors among other finds, and were described by Matthew and Brown (1923). AMNH 5664 was collected by Sternberg and was the fifth specimen described by Matthew and Brown, but was smaller than the others; they noticed it had juvenile characters such as a lower and lighter skull, more elongate limbs and many sutures, but still assigned it the holotype of G. sternbergi. Today, it is reclassified as a juvenile *G. libratus*. Many more specimens are known, being the best represented tyrannosaurid in the fossil record. Thus, a virtually-complete ontogenetic series is understood.

Joseph Leidy (1856) named two tyrannosaurids from Montana based on premaxillary teeth; naming it *Deinodon*. They were robust and large, with the lack of skeletal remnants leading most to reject synonymizing. Since they share teeth, the connection has been proposed, but tyrannosaur teeth are relatively uniform, so it is generally considered dubious and of an unknown animal. Specimens from the Two Medicine and Judith River Formations likely represent *Gorgosaurus*, but it is uncertain if they are *G. libratus* or distinct. TCMI 2001.89.1 of the Children's Museum displays some paleopathologies including a healed leg, rib, vertebral fractures, osteomyelitis at the mandibular teeth resulting in complete tooth loss and possibly a brain tumour.

## Description

*Gorgosaurus* is smaller than *Tyrannosaurus* and *Tarbosaurus*, matching the size of *Albertosaurus* and *Daspletosaurus*. The largest skull is 99 centimeters (39 inches) long, slightly smaller than *Daspletosaurus*. It and *Albertosaurus* bear proportionally longer and lower skulls than other tyrannosaurids. Unlike other tyrannosaurids, the orbit is shaped like a keyhole. The lacrimal supports a tall crest over each socket. The braincase is one of the distinguishing characteristics. Gorgosaurus has standard tyrannosaurid teeth, having 8 premaxillary being smaller than the others and packed in a D-shaped cross section. The first maxillary tooth is shaped like the premaxillary teeth; the rest of the teeth have an ovular cross section. The teeth count is similar to *Albertosaurus* and *Daspletosaurus* but fewer than Tarbosaurus or *Tyrannosaurus*. It's body shape is general. It has a large head atop an S-shaped neck, small forelimbs with two digits (with some specimens having a third metacarpal), four digits on each hindlimb including the hallux that always remained raised from the ground and a large tail. The hindlimbs are proportionally long, with the longest femur being 105 centimeters (41 inches), with some specimens having a longer tibia than femur. This is a character seen in fast animals. In the largest specimens, they were of equal length. Phil Currie (2001) reports ski impressions on a *G. libratus*, finding a relatively smooth surface lacking scales similar to the secondarily featherless skin of some birds. Scales are present, but are widely dispersed and tiny. Other isolate skin impressions show a denser and larger, but still fine, skin. They are similar to hadrosaurid scales and about as fine as a Gila monster, and are not associated with any specific body part. In the Encyclopedia of Dinosaurs, Kenneth Carpenter finds the skin traces from the tail region are similar, being small, rounded and hexagonal.

## Classification

*Gorgosaurus* is often considered an albertosaurine, which is supporter by Thomas Holtz Jr., who has presented unpublished materials that reconsider it from a basal tyrannosauroid outside Tyrannosauridae then to an albertosaurine tyrannosaurid. Similarity between it and *Albertosaurus* have led to confusion and the occasionally synonymization. However, both have been realized as distinct. Loewen et al. (2013) conclude:

## Paleoecology

In 2012 it was found *Gorgosaurus* has a bite force of ~42,000 newtons.

### Life History

Gregory Erikson et. al. have been studying the life history of tyrannosaurids based on bone histology determining age in life. He plotted the age and weights of tyrannosaurids on a graph to determine the growth rate of different tyrannosaur genera. The graph tells scientists that juvenile tyrannosaurs underwent massive growth spurts, for about four years, in the middle of their lives. After the individuals hit sexual maturity, they then tremendously slowed their growth. Gorgosaurus showed a faster growth rate due to having a higher mature weight. *Gorgosaurus*' maximum growth rate is about 50 kilograms (110 pounds) per year. The lack of predators that allowed for it to grow large suggests the predator niche was inhabited by small juveniles, seen in Komodo dragons. There is no evidence for gregarious behaviour.

### Paleopathology

Several pathologies from NMC 2120 are recorded, including: the third right dorsal rib, healed fractures of the 13th and 14th gastralia and left fibula, a 4th metatarsal with rough exostoses in the middle and far end, a deformed third phalanx of the right toe and a small and amorphous claw. Three may have been received via a single confrontation with another dinosaur. TMP91.36.500 bears craniofacial bites, a healed dentary fracture and a mushroom-like hyperostosis on the right pedal phalanx and a thoroughly healed fracture of the right fibula. Such a hyperostosis may be similar to a similar growth an unidentified ornithomimosaur suffered. It is preserved in the death pose. Another specimen bears a poorly-healed fracture on the right fibula. Bruce Rothschild et al. (2001) tested 54 foot bones for stress fractures, but none were recovered. The specimen "Ruth" was the first dinosaur to have evidence of a brain tumour, which would have weakened motor skills and caused the individual to fall down often.

## Paleoecology

Most *Gorgosaurus* are known from the Campanian-aged Dinosaur Park Formation. Many vertebrates are known from here, with fish swimming the rivers and dinosaurs roaming the land. It lived with *Daspletosaurus*. One *Saurornitholestes* dentary bears the bites of a juvenile tyrannosaur, possibly being of *Gorgosaurus*.

## Notable Specimens

* TCMI 2001.89.1 "Ruth": A specimen unveiled at the Museum of Ancient Life at Thanksgiving Point, Utah   on June 22, 2018 and presented by paleontologist Rick Hunter. The skull is 90% complete and the skeleton is about 75% complete, discovered by the Linster family's son, 14 at the time, on their ranch in Montana in 1993 or 1997. The specimen was photographed, documented and catalogued by the family over the next 15 summers. The skull was discovered by the father, Cliff, and took 3 years to fully excavate. The skull was nearly intact and very well preserved, allowing molds to be taken of the cranium which revealed that this individual suffered a brain tumour, being the first evidence of this in dinosaurs. This affliction likely weakened motor skills and caused the animal to have fell down often. The specimen was initially supposed to have gone to a university, but the family insisted it go to a local museum because they wanted "non-traditional" paleontologists and locals to explore deep time. The specimen resides in the lobby of the museum. Ruth also bears a shattered shoulder, broken fibula, crushed caudals and broken ribs.

## Synonyms

* *Deinodon horridus*?
* *Laelaps falculus*?
* *Laelaps hazenianus*?
* *Laelaps incrassatus*
* *Dryptosaurus kenabekides*?
* *G. sternbergi*
* *Albertosaurus*?
* *G. lancensis*
* *G. lancinator*
* *G. novojilovi*

## References

* https://en.wikipedia.org/wiki/Gorgosaurus
* https://lehifreepress.com/2018/06/28/ruth-the-gorgosaurus-unveiled-at-the-museum-of-ancient-life/
* https://www.deseret.com/2018/6/22/20647754/youth-welcome-ruth-to-her-new-digs

1. ↑ https://www.researchgate.net/publication/359957045\_Two\_exceptionally\_preserved\_juvenile\_specimens\_of\_Gorgosaurus\_libratus\_Tyrannosauridae\_Albertosaurinae\_provide\_new\_insight\_into\_the\_timing\_of\_ontogenetic\_changes\_in\_tyrannosaurids