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Gorgosaurus
Fossil range: Late Cretaceous

Scientific classification
Kingdom: Animalia
Phylum: Chordata
Class: Sauropsida
Superorder: Dinosauria
Order: Saurischia
Suborder: Theropoda
Family: Tyrannosauridae
Genus: Gorgosaurus
Lambe, 1914
Species
  • G. libratus (type)
    Lambe, 1914

Gorgosaurus (pronounced /ˌgɔrgoʊˈsɔrəs/ or GOR-go-SAWR-us; meaning "fierce lizard") is a genus of tyrannosaurid theropod dinosaur that lived in western North America between 80 and 73 million years ago, during the Late Cretaceous Period. Fossils of Gorgosaurus are found in Alberta and possibly 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. Typically for members of its family, dozens of large, sharp teeth lined its jaws, while its two-fingered forelimbs were comparatively small. Gorgosaurus was the closest relative of 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, Gorgosaurus 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 about 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.

Contents

[edit] Description

Gorgosaurus libratus with a human for scale
Gorgosaurus libratus with a human for scale

Gorgosaurus was smaller than Tyrannosaurus or Tarbosaurus, but closer in size to Albertosaurus and Daspletosaurus. Adults reached 8 or 9 meters (26 to 30 ft) from snout to tail.[1][2] Paleontologists have estimated full-grown adults to weigh more than 2.4 tonnes (2.7 short tons).[3]

The largest known Gorgosaurus skull measures 99 centimeters (39 in) long, just slightly smaller than that of Daspletosaurus.[1] As in other tyrannosaurids, the skull was large compared to its body size, although chambers within the skull bones and large openings (fenestrae) between them reduced its weight. Albertosaurus and 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 seen 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.[2] Differences in the shape of bones surrounding the brain set Gorgosaurus apart from Albertosaurus.[4]

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.[2] Gorgosaurus had eight premaxillary teeth, 26 to 30 maxillary teeth and 30 to 34 teeth in the dentary bones of the lower jaw. This is similar to Albertosaurus and Daspletosaurus but less than in Tarbosaurus or Tyrannosaurus.[5]

The same general body plan was present in all tyrannosaurids, including Gorgosaurus. Its massive head was perched on the end of an S-shaped neck. In contrast to its large head, Gorgosaurus forelimbs were very small. The forelimbs only had 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 in comparison to other theropods.[2] The largest known femur measured 105 centimeters (41 in) long. In several smaller specimens of Gorgosaurus, the tibia was longer than the femur, a proportion seen mainly in fast-running animals.[1] The two bones are of equal length in the largest specimens.[6] The long, heavy tail served as a counterweight to the head and torso and placed the center of gravity over the hips.[2]

[edit] Classification and systematics

Cladogram of Tyrannosauridae highlighting the position of Gorgosaurus
Tyrannosauridae 

Tyrannosaurinae

 Albertosaurinae 

Albertosaurus


Gorgosaurus



Gorgosaurus is classified in the theropod subfamily Albertosaurinae within the family Tyrannosauridae. It is most closely related to the slightly younger Albertosaurus.[5] These are the only two described albertosaurine genera, although other undescribed species may exist.[4] Appalachiosaurus was described as a basal tyrannosauroid just outside Tyrannosauridae,[7] although Thomas Holtz published a phylogenetic analysis in 2004 which indicated that it was an albertosaurine.[2] More recent, unpublished work by Holtz agrees with the original assessment.[8] 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).[5][9]

The close similarities between Gorgosaurus libratus and Albertosaurus sarcophagus lead many experts to combine them into one genus. Albertosaurus was named first, so by convention it is given priority over the name Gorgosaurus, which is considered its junior synonym. William Diller Matthew and Barnum Brown doubted the distinction of the two genera as early as 1922.[10] Gorgosaurus libratus was reassigned to Albertosaurus (as Albertosaurus libratus) by Dale Russell in 1970,[1] and many subsequent authors followed his lead.[7][11] Combining the two greatly explands the geographical and chronological range of the genus Albertosaurus. Other experts maintain the two genera separately.[2] Phil Currie also splits these genera, claiming that there are as many anatomical differences between Albertosaurus and Gorgosaurus as there are between Daspletosaurus and Tyrannosaurus, which are almost always kept separate. Currie also notes that undescribed tyrannosaurids discovered in Alaska, New Mexico and elsewhere in North America may help clarify the situation.[4]

[edit] Discovery and naming

Type specimen of Gorgosaurus sternbergi (AMNH 5664), now recognized as a juvenile Gorgosaurus libratus
Type specimen of Gorgosaurus sternbergi (AMNH 5664), now recognized as a juvenile Gorgosaurus libratus

Gorgosaurus libratus was first described by Lawrence Lambe in 1914.[12][13] Its name is derived from the Greek γοργος/gorgos ("fierce" or "terrible") and σαυρος/saurus ("lizard").[14] The type species is G. libratus, which is the past participle of the Latin libro ("to balance").[13]

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.[12] It was found in the Dinosaur Park Formation of Alberta and is housed in the Canadian Museum of Nature in Ottawa.[4] 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.[6]

Matthew and Brown also described a fifth Gorgosaurus 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 named it the holotype of a new species, G. sternbergi.[6] Today's paleontologists regard this specimen as a juvenile G. libratus.[2][4] Dozens of other specimens have been excavated from the Dinosaur Park Formation and are housed in museums across the United States and Canada.[1][4] G. libratus is the best-represented tyrannosaurid in the fossil record, known from a virtually complete growth series.[2][15]

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, so Leidy gave them the name Deinodon.[16] 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.[10] Although Deinodon teeth are very similar to those of Gorgosaurus, tyrannosaurid teeth are extremely uniform, so it cannot be said with certainty which animal they belonged to. Deinodon is regarded as a nomen dubium today.[1][15] 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 separate species.[4] 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.[17][18]

[edit] Misassigned species

Several species were incorrectly assigned to Gorgosaurus in the twentieth century. A complete skull of a small tyrannosaurid CMNH 7541) , found in the Hell Creek Formation of Montana, was named Gorgosaurus lancensis by Charles Whitney Gilmore in 1946.[19] This specimen, which clearly did not belong to Gorgosaurus, was renamed Nanotyrannus by Bob Bakker and colleagues in 1988.[20] Today, most paleontologists recognize Nanotyrannus as a juvenile Tyrannosaurus rex.[2][15] Similarly, Evgeny Maleev created the names Gorgosaurus lancinator and Gorgosaurus novojilovi for two small tyrannosaurid specimens (PIN 553-1 and PIN 552-2) from the Nemegt Formation of Mongolia in 1955.[21] Ken Carpenter renamed the smaller specimen Maleevosaurus novojilovi in 1992,[22] but both are now considered juvenile Tarbosaurus bataar.[2][15][23]

[edit] Paleobiology

Although it has been suggested that Gorgosaurus was a scavenger, its co-existence with the similarly sized 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.

Gorgosaurus attacking a female Parasaurolophus
Gorgosaurus attacking a female Parasaurolophus

In the late Campanian of North America, Gorgosaurus was a contemporary of Daspletosaurus, also a tyrannosaurid. This is one of the few examples of two tyrannosaur genera coexisting. In modern predator guilds, similarly-sized predators are separated into different ecological niches by anatomical, behavioral or geographical differences that limit competition.[24] Several studies have attempted to explain niche differentiation in Gorgosaurus and Daspletosaurus.

Paleontologist Dale Russell hypothesized that the more common Gorgosaurus may have preyed on the abundant hadrosaurs of the time, while the less common Daspletosaurus may have specialized on the less prevalent but better-defended ceratopsids, which may have been more difficult to hunt.[1] However, a specimen of Daspletosaurus (OTM 200) from the Two Medicine Formation preserves the digested remains of a juvenile hadrosaur in its gut region.[25] The higher and broader muzzles of tyrannosaurines like Daspletosaurus are mechanically stronger than the lower snouts of albertosaurines like Gorgosaurus, although tooth strengths are similar between the two groups. This may indicate a difference in feeding mechanics or diet.[26]

Other authors have suggested that competition was limited by geographical separation. Unlike some other groups of dinosaurs, there appears to be no correlation with distance from the sea. Neither Gorgosaurus nor Daspletosaurus was more common at higher or lower elevations than the other.[24] However, while there is some overlap, Gorgosaurus appears to be more common at northern latitudes, 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 and in southwestern North America during the Campanian. Thomas Holtz has suggested that this pattern indicates shared ecological preferences between tyrannosaurines, chasmosaurines and hadrosaurines. Holtz notes that, at the end of the later Maastrichtian stage, tyrannosaurines like Tyrannosaurus rex, hadrosaurines and chasmosaurines like Triceratops were widespread throughout western North America, while albertosaurines and centrosaurines went extinct, and lambeosaurines were very rare.[2]

[edit] References

  1. ^ a b c d e f g Russell, Dale A. (1970). "Tyrannosaurs from the Late Cretaceous of western Canada". National Museum of Natural Sciences Publications in Paleontology 1: 1-34. 
  2. ^ a b c d e f g h i j k l Holtz, Thomas R. (2004). "Tyrannosauroidea", in Weishampel, David B.; Dodson, Peter; & Osmólska, Halszka (eds.).: The Dinosauria, Second Edition, Berkeley: University of California Press, 111-136. ISBN 0-520-24209-2. 
  3. ^ Seebacher, Frank. (2001). "A new method to calculate allometric length-mass relationships of dinosaurs". Journal of Vertebrate Paleontology 21 (1): 51–60. doi:10.1671/0272-4634(2001)021%5B0051:ANMTCA%5D2.0.CO;2. 
  4. ^ a b c d e f g Currie, Philip J. (2003). "Cranial anatomy of tyrannosaurids from the Late Cretaceous of Alberta". Acta Palaeontologica Polonica 48 (2): 191–226. 
  5. ^ a b c Currie, Philip J.; Hurum, Jørn H; & Sabath, Karol. (2003). "Skull structure and evolution in tyrannosaurid phylogeny". Acta Palaeontologica Polonica 48 (2): 227–234. 
  6. ^ a b c Matthew, William D.; & Brown, Barnum. (1923). "Preliminary notices of skeletons and skulls of Deinodontidae from the Cretaceous of Alberta". American Museum Novitates 89: 1-9. 
  7. ^ a b Carr, Thomas D.; Williamson, Thomas E.; & Schwimmer, David R. (2005). "A new genus and species of tyrannosauroid from the Late Cretaceous (middle Campanian) Demopolis Formation of Alabama". Journal of Vertebrate Paleontology 25 (1): 119–143. 
  8. ^ Holtz, Thomas R. (2005-09-20). RE: Burpee Conference (LONG). Retrieved on 2007-06-18.
  9. ^ Currie, Philip J. (2003). "Allometric growth in tyrannosaurids (Dinosauria: Theropoda) from the Upper Cretaceous of North America and Asia". Canadian Journal of Earth Sciences 40 (4): 651–665. 
  10. ^ a b Matthew, William D.; & Brown, Barnum. (1922). "The family Deinodontidae, with notice of a new genus from the Cretaceous of Alberta". Bulletin of the American Museum of Natural History 46 (6): 367-385. 
  11. ^ Paul, Gregory S. (1988). Predatory Dinosaurs of the World. New York: Simon & Schuster, 464pp. ISBN 978-0671619466. 
  12. ^ a b Lambe, Lawrence M. (1914). "On the fore-limb of a carnivorous dinosaur from the Belly River Formation of Alberta, and a new genus of Ceratopsia from the same horizon, with remarks on the integument of some Cretaceous herbivorous dinosaurs". Ottawa Naturalist 27: 129-135. 
  13. ^ a b Lambe, Lawrence M. (1914). "On a new genus and species of carnivorous dinosaur from the Belly River Formation of Alberta, with a description of Stephanosaurus marginatus from the same horizon". Ottawa Naturalist 28: 13-20. 
  14. ^ Liddell, Henry G.; & Scott, Robert (1980). Greek-English Lexicon, Abridged Edition, Oxford: Oxford University Press,. ISBN 0-19-910207-4. 
  15. ^ a b c d Carr, Thomas D. (1999). "Craniofacial ontogeny in Tyrannosauridae (Dinosauria, Coelurosauria)". Journal of Vertebrate Paleontology 19 (3): 497–520. 
  16. ^ Leidy, Joseph. (1856). "Notice of remains of extinct reptiles and fishes, discovered by Dr. F.V. Hayden in the badlands of the Judith River". Proceedings of the Academy of Sciences in Philadelphia 8: 72-73. 
  17. ^ Pickrell, John. (2003-11-24). First dinosaur brain tumor found, experts suggest. National Geographic News. Retrieved on 2008-02-07.
  18. ^ Meet the Gorgosaur. The Children's Museum of Indianapolis. Retrieved on 2008-02-07.
  19. ^ Gilmore, Charles W. (1946). "A new carnivorous dinosaur from the Lance Formation of Montana". Smithsonian Miscellaneous Collections 106: 1-19. 
  20. ^ Bakker, Robert T.; Williams, Michael; & Currie, Philip J. (1988). "Nanotyrannus, a new genus of pygmy tyrannosaur, from the latest Cretaceous of Montana". Hunteria 1 (5): 1-30. 
  21. ^ Maleev, Evgeny A. (1955). "New carnivorous dinosaurs from the Upper Cretaceous of Mongolia." (in Russian). Doklady, Academy of Sciences USSR 104 (5): 779–783. 
  22. ^ Carpenter, Ken. (1992). "Tyrannosaurids (Dinosauria) of Asia and North America", in Mateer, Niall J.; and Chen Peiji (eds.): Aspects of Nonmarine Cretaceous Geology. Beijing: China Ocean Press, 250–268. 
  23. ^ Rozhdestvensky, Anatoly K. (1965). "Growth changes in Asian dinosaurs and some problems of their taxonomy". Paleontological Journal 3: 95–109. 
  24. ^ a b Farlow, James O.; & Pianka, Eric R. (2002). "Body size overlap, habitat partitioning and living space requirements of terrestrial vertebrate predators: implications for the paleoecology of large theropod dinosaurs". Historical Biology 16 (1): 21-40. doi:10.1080/0891296031000154687. 
  25. ^ Varricchio, David J. (2001). "Gut contents from a Cretaceous tyrannosaurid: implications for theropod dinosaur digestive tracts". Journal of Paleontology 75 (2): 401-406.  DOI: 10.1666/0022-3360(2001)075<0401:GCFACT>2.0.CO;2
  26. ^ Snively, Eric; Henderson, Donald M.; & Phillips, Doug S. (2006). "Fused and vaulted nasals of tyrannosaurid dinosaurs: implications for cranial strength and feeding mechanics" (PDF). Acta Palaeontologica Polonica 51 (3): 435–454. 

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