Gorgosaurus
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{{Taxobox
| name = ''Gorgosaurus''
| fossil_range = [[Late Cretaceous]]
| image=Gorgosaurus_BW.jpg
| image_width = 200px
| regnum = [[Animal]]ia
| phylum = [[Chordate|Chordata]]
| classis = [[Reptile|Sauropsida]]
| superordo = [[Dinosaur]]ia
| ordo = [[Saurischia]]
| subordo = [[Theropod]]a
| familia = [[Tyrannosaurid]]ae
| genus = '''''Gorgosaurus'''''
| genus_authority = [[Lawrence Lambe|Lambe]], 1914
| subdivision_ranks = [[Species]]
| subdivision =
* '''''G. libratus''''' ([[Type species|type]])<br/><small>Lambe, 1914 </small>
}}
'''''Gorgosaurus''''' ({{PronEng|ˌgɔrgoʊˈsɔrəs}} or GOR-go-SAWR-us; meaning "fierce lizard") is a [[genus]] of [[tyrannosaurid]] [[theropod]] [[dinosaur]] that lived in western [[North America]] during the Late [[Cretaceous]] [[Period (geology)|Period]], between about 77 and 74 [[million years ago]]. [[Fossil]] remains have been found in the [[Canada|Canadian]] province of [[Alberta]] and possibly the [[U.S. state]] of [[Montana]]. [[Paleontologist]]s recognize only the [[type species|type]] [[species]], ''G. libratus'', although other species have been erroneously referred to the genus.
Like most known tyrannosaurids, ''Gorgosaurus'' was a [[biped]]al [[predation|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 [[Western Interior Seaway|inland sea]]. An [[apex predator]], it was at the top of the [[food chain]], preying upon abundant [[ceratopsid]]s and [[hadrosaur]]s. 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==
[[Image:Gorgosaurusscale.png|left|thumb|200px|''Gorgosaurus libratus'' with a human for scale]]
''Gorgosaurus'' was smaller than ''Tyrannosaurus'' or ''[[Tarbosaurus]]'', closer in size to ''Albertosaurus'' and ''Daspletosaurus''. Adults reached 8 or 9 meters (26 to 30 ft) from snout to tail.<ref name=russell1970>{{cite_journal |last=Russell |first=Dale A. |authorlink=Dale Russell |year=1970 |title=Tyrannosaurs from the Late Cretaceous of western Canada |journal=National Museum of Natural Sciences Publications in Paleontology |volume=1 |pages=1–34}}</ref><ref name=holtz2004>{{cite_book |last=Holtz |first=Thomas R. |authorlink=Thomas R. Holtz, Jr. |year=2004 |chapter=Tyrannosauroidea |editor= [[David B. Weishampel|Weishampel, David B.]]; [[Peter Dodson|Dodson, Peter]]; & Osmólska, Halszka (eds.). |title=The Dinosauria |edition=Second Edition |publisher=University of California Press |location=Berkeley |pages=111–136 |isbn=0-520-24209-2}}</ref> Paleontologists have estimated full-grown adults to weigh more than 2.4 [[tonne]]s (2.7 [[short ton]]s).<ref name=seebacher2001>{{cite_journal |last=Seebacher |first=Frank |year=2001 |title=A new method to calculate allometric length-mass relationships of dinosaurs |journal=Journal of Vertebrate Paleontology |volume=21 |issue=1 |pages=51–60 |doi=10.1671/0272-4634(2001)021[0051:ANMTCA]2.0.CO;2}}</ref> The largest known skull measures 99 centimeters (39 [[inch|in]]) long, just slightly smaller than that of ''Daspletosaurus''.<ref name=russell1970/> As in other tyrannosaurids, the skull was large compared to its body size, although chambers within the skull bones and large openings ([[fenestra]]e) between bones 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 bone|nasal]] and [[parietal bone]]s were fused along the midline of the skull, as in all other members of the family. The [[orbit (anatomy)|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''.<ref name=holtz2004/> Differences in the shape of bones surrounding the brain set ''Gorgosaurus'' apart from ''Albertosaurus''.<ref name=currie2003a>{{cite_journal |last=Currie |first=Philip J. |authorlink=Phil Currie |year=2003 |title=Cranial anatomy of tyrannosaurids from the Late Cretaceous of Alberta |journal=Acta Palaeontologica Polonica |volume=48 |issue=2 |pages=191–226 |url=http://app.pan.pl/acta48/app48-191.pdf |format={{dead link|date=July 2008}} – <sup>[http://scholar.google.co.uk/scholar?hl=en&lr=&q=author%3ACurrie+intitle%3ACranial+anatomy+of+tyrannosaurids+from+the+Late+Cretaceous+of+Alberta&as_publication=Acta+Palaeontologica+Polonica&as_ylo=2003&as_yhi=2003&btnG=Search Scholar search]</sup>}}</ref>
''Gorgosaurus'' teeth were typical of all known tyrannosaurids. The eight [[premaxilla]]ry teeth at the front of the snout were smaller than the rest, closely packed and ''D''-shaped in [[Cross section (geometry)|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.<ref name=holtz2004/> Along with the eight premaxillary teeth, ''Gorgosaurus'' had 26 to 30 maxillary teeth and 30 to 34 teeth in the [[dentary bone]]s of the lower jaw. This number of teeth is similar to ''Albertosaurus'' and ''Daspletosaurus'' but is fewer than those of ''Tarbosaurus'' or ''Tyrannosaurus''.<ref name=currieetal2003>{{cite_journal |last=Currie |first=Philip J. |coauthors=Hurum, Jørn H; & Sabath, Karol. |authorlink=Phil Currie |year=2003 |title=Skull structure and evolution in tyrannosaurid phylogeny |journal=Acta Palaeontologica Polonica |volume=48 |issue=2 |pages=227–234 |url=http://app.pan.pl/acta48/app48-227.pdf |format={{dead link|date=July 2008}} – <sup>[http://scholar.google.co.uk/scholar?hl=en&lr=&q=author%3ACurrie+intitle%3ASkull+structure+and+evolution+in+tyrannosaurid+phylogeny&as_publication=Acta+Palaeontologica+Polonica&as_ylo=2003&as_yhi=2003&btnG=Search Scholar search]</sup>}}</ref>
''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.<ref name=holtz2004/> 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 [[cursorial|fast-running]] animals.<ref name=russell1970/> The two bones were of equal length in the largest specimens.<ref name=matthewbrown1923>{{cite_journal |last=Matthew |first=William D. |authorlink=William Diller Matthew |coauthors=& [[Barnum Brown|Brown, Barnum]]. |year=1923 |title=Preliminary notices of skeletons and skulls of Deinodontidae from the Cretaceous of Alberta |journal=American Museum Novitates |volume=89 |pages=1–9 |url=http://digitallibrary.amnh.org/dspace/handle/2246/3207}}</ref> The long, heavy tail served as a [[counterweight]] to the head and torso and placed the [[center of gravity]] over the hips.<ref name=holtz2004/>
==Classification and systematics==
{{userboxtop|align=right|bordercolor=lightgray|toptext=[[Cladistics#Cladograms|Cladogram]] of [[Tyrannosauridae]] highlighting the position of ''Gorgosaurus''}}
<br/><u><center>'''[[Thomas Carr (paleontologist)|Carr]] ''et al.'' 2005'''<ref name=carretal2005/></center></u>
{{clade| style=font-size:80%;line-height:80%
|label1=Tyrannosauridae
|1={{clade
|label1= Albertosaurinae
|1={{clade
|1=''[[Albertosaurus]]''
|2='''''Gorgosaurus'''''* }}
|label2= Tyrannosaurinae
|2={{clade
|1={{clade
|label1=<font color="white">void</font>
|1=''[[Daspletosaurus]]''
|2={{clade
|label1= <font color="white">void</font>
|1=''[[Tarbosaurus]]''*
|2=''[[Tyrannosaurus]]''
}} }} }} }} }}
<small>*<u>Note</u>: Carr ''et al.'' regard ''Gorgosaurus libratus'' as a species of ''Albertosaurus'' and ''Tarbosaurus bataar'' as a species of ''Tyrannosaurus''</small>
<br/><u><center>'''[[Philip Currie|Currie]] ''et al.'' 2003'''<ref name=currieetal2003/></center></u>
{{clade| style=font-size:80%;line-height:80%
|label1=Tyrannosauridae
|1={{clade
|label1=<font color="white">void</font>
|label1= Albertosaurinae
|1={{clade
|1=''[[Albertosaurus]]''
|2='''''Gorgosaurus''''' }}
|label2= Tyrannosaurinae
|2={{clade
|label1=<font color="white">void</font>
|1={{clade
|label1=<font color="white">void</font>
|1=''[[Daspletosaurus]]''
|2={{clade
|label1= <font color="white">void</font>
|1=''[[Tarbosaurus]]''
|2=''[[Alioramus]]''}} }}
|2={{clade
|label1=<font color="white">void</font>
|1=''[[Nanotyrannus]]''
|2=''[[Tyrannosaurus]]''
}} }} }} }}
{{userboxbottom}}
''Gorgosaurus'' is [[biological classification|classified]] in the theropod [[subfamily]] Albertosaurinae within the [[family (biology)|family]] Tyrannosauridae. It is most closely related to the slightly younger ''Albertosaurus''.<ref name=currieetal2003/> These are the only two definite albertosaurine genera that have been described, although other undescribed species may exist.<ref name=currie2003a/> ''[[Appalachiosaurus]]'' was described as a [[basal (phylogenetics)|basal]] [[tyrannosauroid]] just outside Tyrannosauridae,<ref name=carretal2005>{{cite_journal |last=Carr |first=Thomas D. |authorlink=Thomas Carr (paleontologist) |coauthors=Williamson, Thomas E.; & Schwimmer, David R. |year=2005 |title=A new genus and species of tyrannosauroid from the Late Cretaceous (middle Campanian) Demopolis Formation of Alabama |journal=Journal of Vertebrate Paleontology |volume=25 |issue=1 |pages=119–143 |doi=10.1671/0272-4634(2005)025[0119:ANGASO]2.0.CO;2}}</ref> although American paleontologist [[Thomas R. Holtz, Jr.|Thomas Holtz]] published a phylogenetic analysis in 2004 which indicated it was an albertosaurine.<ref name=holtz2004/> More recent, unpublished work by Holtz agrees with the original assessment.<ref name=holtzdml2005>{{cite web |url=http://dml.cmnh.org/2005Sep/msg00345.html |title=RE: Burpee Conference (LONG) |accessdate=2007-06-18 |last=Holtz |first=Thomas R. |authorlink=Thomas R. Holtz, Jr. |date=2005-09-20}}</ref> 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 ([[metatarsal]]s and [[phalanges]]).<ref name=currieetal2003/><ref name=currie2003b>{{cite_journal|last=Currie |first=Philip J. |authorlink=Phil Currie |year=2003 |title=Allometric growth in tyrannosaurids (Dinosauria: Theropoda) from the Upper Cretaceous of North America and Asia |journal=Canadian Journal of Earth Sciences |volume=40 |issue=4 |pages=651–665 |url=http://www.ingentaconnect.com/content/nrc/cjes/2003/00000040/00000004/art00014 |doi=10.1139/e02-083}}</ref>
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 [[Principle of Priority|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.<ref name=matthewbrown1922>{{cite_journal |last=Matthew |first=William D. |authorlink=William Diller Matthew |coauthors=& [[Barnum Brown|Brown, Barnum]]. |year=1922 |title=The family Deinodontidae, with notice of a new genus from the Cretaceous of Alberta |journal=Bulletin of the American Museum of Natural History |volume=46 |issue=6 |pages=367–385 |url=http://digitallibrary.amnh.org/dspace/handle/2246/1300}}</ref> ''Gorgosaurus libratus'' was formally reassigned to ''Albertosaurus'' (as ''Albertosaurus libratus'') by [[Dale Russell]] in 1970,<ref name=russell1970/> and many subsequent authors followed his lead.<ref name=carretal2005/><ref name=paul1988>{{cite_book |last=Paul |first=Gregory S. |authorlink=Gregory S. Paul |year=1988 |title=Predatory Dinosaurs of the World |publisher=Simon & Schuster |location=New York |pages=464pp |isbn=978-0671619466}}</ref> Combining the two greatly expands the geographical and chronological range of the genus ''Albertosaurus''. Other experts maintain the two genera as separate.<ref name=holtz2004/> 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.<ref name=currie2003a/>
==Discovery and naming==
[[Image:Albertosaurus skeleton.jpg|thumb|left|''Gorgosaurus'' skeleton (AMNH 5428).]]
''Gorgosaurus libratus'' was first described by [[Lawrence Lambe]] in 1914.<ref name=lambe1914a>{{cite_journal |last=Lambe |first=Lawrence M. |authorlink=Lawrence Lambe |year=1914 |title=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 |journal=Ottawa Naturalist |volume=27 |pages=129–135}}</ref><ref name=lambe1914b>{{cite_journal |last=Lambe |first=Lawrence M. |authorlink=Lawrence Lambe |year=1914 |title=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 |journal=Ottawa Naturalist |volume=28 |pages=13–20}}</ref> Its name is derived from the [[Ancient Greek|Greek]] γοργος/''gorgos'' ("fierce" or "terrible") and σαυρος/''saurus'' ("lizard").<ref name=liddellscott>{{cite book |last=Liddell |first=Henry G. |authorlink=Henry Liddell |coauthors=& [[Robert Scott (philologist)|Scott, Robert]] |year=1980 |title=Greek-English Lexicon |edition=Abridged Edition |publisher=Oxford University Press, |location=Oxford |isbn=0-19-910207-4}}</ref> The [[type species]] is ''G. libratus''; the specific epithet "balanced" is the [[past participle]] of the [[Latin]] verb ''librare'', meaning "to balance".<ref name=lambe1914b/>
The [[holotype]] of ''Gorgosaurus libratus'' ([[Canadian Museum of Nature|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.<ref name=lambe1914a/> It was found in the [[Dinosaur Park Formation]] of Alberta and is housed in the [[Canadian Museum of Nature]] in [[Ottawa]].<ref name=currie2003a/> 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.<ref name=matthewbrown1923/>
[[Image:Sharp naturalhistory1920 deinodon.jpg|thumb|left|[[Type specimen]] of ''Gorgosaurus sternbergi'' ([[American Museum of Natural History|AMNH]] 5664), now recognized as a juvenile ''Gorgosaurus libratus'']]
Matthew and Brown also described a fifth skeleton ([[American Museum of Natural History|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 [[Suture (anatomical)|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''.<ref name=matthewbrown1923/> Today's paleontologists regard this specimen as a juvenile ''G. libratus''.<ref name=holtz2004/><ref name=currie2003a/> Dozens of other specimens have been excavated from the Dinosaur Park Formation and are housed in museums across the United States and Canada.<ref name=russell1970/><ref name=currie2003a/> ''G. libratus'' is the best-represented tyrannosaurid in the fossil record, known from a virtually complete growth series.<ref name=holtz2004/><ref name=carr1999>{{cite_journal |last=Carr |first=Thomas D. |authorlink=Thomas Carr (paleontologist) |year=1999 |title=Craniofacial ontogeny in Tyrannosauridae (Dinosauria, Coelurosauria) |journal=Journal of Vertebrate Paleontology |volume=19 |issue=3 |pages=497–520 |url=http://www.vertpaleo.org/publications/jvp/19-497-520.cfm}}</ref>
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]]''.<ref name=leidy1856>{{cite_journal |last=Leidy |first=Joseph. |authorlink=Joseph Leidy |year=1856 |title=Notice of remains of extinct reptiles and fishes, discovered by Dr. F.V. Hayden in the badlands of the Judith River |journal=Proceedings of the Academy of Sciences in Philadelphia |volume=8 |pages=72–73}}</ref> 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.<ref name=matthewbrown1922/> 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.<ref name=russell1970/><ref name=carr1999/> 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.<ref name=currie2003a/> One specimen from Montana ([[Children's Museum of Indianapolis|TCMI]] 2001.89.1), housed in the [[Children's Museum of Indianapolis]], shows evidence of severe [[pathology|pathologies]], including healed leg, rib, and vertebral [[Bone fracture|fractures]], [[osteomyelitis]] (infection) at the tip of the lower jaw resulting in permanent tooth loss, and possibly a [[brain tumor]].<ref name=natlgeo2003>{{cite_web |author=Pickrell, John. |date=2003-11-24 |url=http://news.nationalgeographic.com/news/2003/11/1124_031124_dinocancer.html |title=First dinosaur brain tumor found, experts suggest |publisher=National Geographic News |accessdate=2008-02-07}}</ref><ref name=tcmigorgo>{{cite_web |url=http://www.childrensmuseum.org/themuseum/dinosphere/profiles/gorgo.html |title=Meet the Gorgosaur |publisher=The Children's Museum of Indianapolis |accessdate=2008-02-07}}</ref>
===Misassigned species===
Several species were incorrectly assigned to ''Gorgosaurus'' in the twentieth century. A complete skull of a small tyrannosaurid ([[Cleveland Museum of Natural History|CMNH]] 7541), found in the younger, late [[Maastrichtian]]-age [[Hell Creek Formation]] of Montana, was named ''Gorgosaurus lancensis'' by [[Charles Whitney Gilmore]] in 1946.<ref name=gilmore1946>{{cite_journal |last=Gilmore |first=Charles W. |authorlink=Charles Whitney Gilmore |year=1946 |title=A new carnivorous dinosaur from the Lance Formation of Montana |journal=Smithsonian Miscellaneous Collections |volume=106 |pages=1–19}}</ref> This specimen was renamed ''[[Nanotyrannus]]'' by [[Bob Bakker]] and colleagues in 1988.<ref name=bakkeretal1988>{{cite_journal |last=Bakker |first=Robert T. |authorlink=Robert T. Bakker |coauthors=Williams, Michael; & [[Phil Currie|Currie, Philip J.]] |year=1988 |title=''Nanotyrannus'', a new genus of pygmy tyrannosaur, from the latest Cretaceous of Montana |journal=Hunteria |volume=1 |issue=5 |pages=1–30}}</ref> Today, most paleontologists recognize ''Nanotyrannus'' as a juvenile ''Tyrannosaurus rex''.<ref name=holtz2004/><ref name=carr1999/> Similarly, [[Evgeny Maleev]] created the names ''Gorgosaurus lancinator'' and ''Gorgosaurus novojilovi'' for two small tyrannosaurid specimens ([[Paleontological Institute of Russian Academy of Sciences|PIN]] 553-1 and PIN 552-2) from the [[Nemegt Formation]] of [[Mongolia]] in 1955.<ref name=maleev1955b>{{cite_journal |last=Maleev |first=Evgeny A. |authorlink=Evgeny Maleev |year=1955 |title=New carnivorous dinosaurs from the Upper Cretaceous of Mongolia |journal=Doklady, Academy of Sciences USSR |volume=104 |issue=5 |pages=779–783 |language=Russian}}</ref> [[Ken Carpenter]] renamed the smaller specimen ''Maleevosaurus novojilovi'' in 1992,<ref name=carpenter1992>{{cite_book |last=Carpenter |first=Ken. |authorlink=Kenneth Carpenter |year=1992 |chapter=Tyrannosaurids (Dinosauria) of Asia and North America |editor=Mateer, Niall J.; and Chen Peiji (eds.) |title=Aspects of Nonmarine Cretaceous Geology |location=Beijing |publisher=China Ocean Press |pages=250–268 |url=https://scientists.dmns.org/sites/kencarpenter/PDFs%20of%20publications/pdfs.aspx}}</ref> but both are now considered juveniles of ''Tarbosaurus bataar''.<ref name=holtz2004/><ref name=carr1999/><ref name=rozhdestvensky1965>{{cite_journal |last=Rozhdestvensky |first=Anatoly K. |authorlink=Anatoly Konstantinovich Rozhdestvensky |year=1965 |title=Growth changes in Asian dinosaurs and some problems of their taxonomy |journal=Paleontological Journal |volume=3 |pages=95–109}}</ref>
==Paleobiology==
===Coexistence with ''Daspletosaurus''===
[[Image:Gorgosaurus.JPG|thumb|left|Mounted ''Gorgosaurus'' skeleton.]]
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. Similarly-sized predators in modern predator [[Guild (ecology)|guilds]] are separated into different [[ecological niche]]s by anatomical, behavioral or geographical differences that limit competition. Niche differentiation between the Dinosaur Park tyrannosaurids is not well-understood.<ref name=farlowpianka2002>{{cite_journal |last=Farlow |first=James O. |authorlink=James Farlow |coauthors=& Pianka, Eric R. |year=2002 |title=Body size overlap, habitat partitioning and living space requirements of terrestrial vertebrate predators: implications for the paleoecology of large theropod dinosaurs |journal=Historical Biology |volume=16 |issue=1 |pages=21–40 |doi=10.1080/0891296031000154687}} </ref> In 1970, Dale Russell [[hypothesis|hypothesized]] the more common ''Gorgosaurus'' actively hunted fleet-footed [[hadrosaurs]], while the rarer and more troublesome [[ceratopsia]]ns and [[ankylosauria]]ns (horned and heavily [[Armour (zoology)|armoured]] dinosaurs) were left to the more heavily-built ''[[Daspletosaurus]]''.<ref name=russell1970/> However, a specimen of ''Daspletosaurus'' ([[Old Trail Museum|OTM]] 200) from the contemporaneous [[Two Medicine Formation]] of Montana preserves the digested remains of a juvenile hadrosaur in its gut region.<ref name=varricchio2001>{{cite_journal |last=Varricchio |first=David J. |year=2001 |title=Gut contents from a Cretaceous tyrannosaurid: implications for theropod dinosaur digestive tracts |journal=Journal of Paleontology |volume=75 |issue=2 |pages=401–406 |doi=10.1666/0022-3360(2001)075<0401:GCFACT>2.0.CO;2}}</ref>
Unlike some other groups of dinosaurs, neither genus was more common at higher or lower elevations than the other.<ref name=farlowpianka2002/> 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.<ref name=holtz2004/>
===Life history===
[[Image:Tyrantgraph.png|thumb|380px|right|A graph showing the hypothesized growth curves (body mass versus age) of four tyrannosaurids. ''Gorgosaurus'' is shown in blue. Based on Erickson ''et al''. 2004.]]
[[Gregory Erickson (paleontologist)|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. A growth curve can be developed when the ages of various individuals are plotted against their sizes on a graph. Tyrannosaurids grew throughout their lives, but underwent tremendous growth spurts for about four years, after an extended juvenile phase. [[Sexual maturity]] may have ended this rapid growth phase, after which growth slowed down considerably in adult animals. Examining five ''Gorgosaurus'' specimens of various sizes, Erickson calculated a maximum growth rate of about 110 kilograms (50 lb) during the rapid growth phase, slower than in tyrannosaurines like ''Daspletosaurus'' and ''Tyrannosaurus'', but comparable to ''Albertosaurus''.<ref name=ericksonetal2004>{{cite_journal |last=Erickson, Gregory M. |coauthors=Makovicky, Peter J.; [[Phil Currie|Currie, Philip J.]]; Norell, Mark A.; Yerby, Scott A.; & Brochu, Christopher A. |year=2004 |title=Gigantism and comparative life-history parameters of tyrannosaurid dinosaurs |journal=Nature |volume=430 |pages=772–775 |doi=10.1038/nature02699}}</ref>
''Gorgosaurus'' spent as much as half its life in the juvenile phase before ballooning up to near-maximum size in only a few years.<ref name=ericksonetal2004/> 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 dragon]]s, where hatchlings start off as tree-dwelling [[insectivore]]s and slowly mature into massive [[apex predator]]s capable of taking down large vertebrates.<ref name=holtz2004/> Other tyrannosaurids, including ''Albertosaurus'', have been found in aggregations that some have suggested to represent mixed-age [[pack hunter|packs]], but there is no evidence of gregarious behavior in ''Gorgosaurus''.<ref name=tankecurrie1998>{{cite_journal |last=Tanke |first=Darren H. |coauthors=& [[Phil Currie|Currie, Philip J.]] |year=1998 |title=Head-biting behavior in theropod dinosaurs: paleopathological evidence |journal=Gaia |volume=15 |pages=167–184 |url=http://www.mnhn.ul.pt/geologia/gaia/12.pdf|format=PDF}} [not printed until 2000]</ref><ref name=currieetal2005>{{cite_book |last=Currie |first=Philip J. |authorlink=Phil Currie |coauthors=Trexler, David; [[Eva Koppelhus|Koppelhus, Eva B.]]; Wicks, Kelly; & Murphy, Nate. |year=2005 |chapter=An unusual multi-individual tyrannosaurid bonebed in the Two Medicine Formation (Late Cretaceous, Campanian) of Montana (USA) |editor=[[Kenneth Carpenter|Carpenter, Kenneth]] (ed.). |title=The Carnivorous Dinosaurs |publisher=Indiana University Press |location=Bloomington |pages=313–324 |isbn=978-0253345394}}</ref>
==Paleoecology==
[[Image:GorgosaurusDB.jpg|thumb|left|''Gorgosaurus'' attacking a ''[[Parasaurolophus|Parasaurolophus cyrtocristatus]]'']]
All known specimens of ''Gorgosaurus libratus'' have been recovered from the [[Dinosaur Park Formation]] in Alberta.<ref name=currie2003a/> This formation dates to the middle of the [[Campanian]], between 76.5 and 74 [[Mega-annum|million years ago]].<ref name=eberthhamblin1993>{{cite_journal |last=Eberth |first=David A. |coauthors=& Hamblin, Anthony P. |year=1993 |title=Tectonic, stratigraphic, and sedimentologic significance of a regional discontinuity in the Upper Judith River Group (Belly River wedge) of southern Alberta, Saskatchewan, and northern Montana |journal=Canadian Journal of Earth Sciences |volume=30 |pages=174–200}}</ref> At this time, the area was a coastal plain along the western edge of the [[Western Interior Seaway]], which divided North America in half. The [[Laramide Orogeny]] had begun uplifting the [[Rocky Mountains]] to the west, from which flowed great rivers that deposited eroded sediment in vast [[floodplain]]s along the coast.<ref name=englishjohnston2004>{{cite_journal |last=English |first=Joseph M. |coauthors=& Johnston, Stephen T. |year=2004 |title=The Laramide Orogeny: what were the driving forces? |journal=International Geology Review |volume=46 |issue=9 |pages=833–838 |url=http://www.ingentaconnect.com/content/bell/igr/2004/00000046/00000009/art00004 |doi=10.2747/0020-6814.46.9.833}}</ref><ref name=eberth1997>{{cite_book |last=Eberth |first=David A. |year=1997 |chapter=Judith River Wedge |editor=[[Phil Currie|Currie, Philip J.]] & Padian, Kevin (eds.). |title=Encyclopedia of Dinosaurs |publisher= Academic Press |location=San Diego |pages=199–204 |isbn=0-12-226810-5}}</ref> The climate was [[subtropical]] with marked seasonality, and periodic droughts sometimes resulted in massive mortality among the great herds of dinosaurs, as represented in the numerous [[bonebed]] deposits preserved in the Dinosaur Park Formation. [[Conifer]]s formed the [[forest canopy]], while the [[understory]] plants consisted of [[fern]]s, [[tree fern]]s and [[angiosperm]]s.<ref name=bramankoppelhus2005>{{cite_book |last=Braman |first=Dennis R. |coauthors=and Koppelhus, Eva B. |year=2005 |chapter=Campanian palynomorphs |title=Dinosaur Provincial Park: A Spectacular Ancient Ecosystem Revealed |editors=[[Phil Currie|Currie, Phillip J.]] and Koppelhus, Eva B. (eds.) |publisher=Indiana University Press |location=Bloomington |pages=101–130 |isbn=0-253-34595-2}}</ref> Around 73 million years ago, the seaway began to expand, [[Transgression (geology)|transgressing]] into areas formerly above sea level and drowning the Dinosaur Park ecosystem. This transgression, called the Bearpaw Sea, is recorded by the marine sediments of the massive [[Bearpaw Shale]].<ref name=eberth1997/>
The Dinosaur Park Formation preserves a great wealth of vertebrate fossils. A wide variety of fish swam the rivers and [[estuary|estuaries]], including [[gar]]s, [[sturgeon]]s, [[shark]]s and [[ray]]s, among others. [[Frog]]s, [[salamander]]s, [[turtle]]s, [[crocodilia]]ns and [[champsosaur]]s also dwelled in the aquatic habitats. [[Azhdarchid]] [[pterosaur]]s and [[neornithes|neornithine birds]] like ''[[Apatornis]]'' flew overhead, while the [[Enantiornithes|enantiornithine bird]] ''[[Avisaurus]]'' lived on the ground alongside [[multituberculate]], [[marsupial]] and [[placental]] [[mammal]]s. A number of species of terrestrial lizards were also present, including [[Teiidae|whiptail]]s, [[skink]]s, [[varanid|monitors]] and [[Anguidae|alligator lizards]]. Dinosaur fossils in particular are found with unrivaled abundance and diversity. Huge herds of [[Ceratopsidae|ceratopsids]] roamed the floodplains alongside equally large groups of hadrosaurine and lambeosaurine [[hadrosaur]]s. Other herbivorous groups like [[ornithomimid]]s, [[therizinosaur]]s, [[pachycephalosaur]]s, small [[ornithopod]]s, [[nodosaurid]]s and [[ankylosaurid]]s were also represented. Small predatory dinosaurs like [[oviraptorosaur]]s, [[troodont]]s and [[dromaeosaur]]s hunted smaller prey than the huge tyrannosaurids, ''Daspletosaurus'' and ''Gorgosaurus'', which were two orders of magnitude larger in mass.<ref name=eberth1997/> Intervening predatory niches may have been filled by young tyrannosaurids.<ref name=russell1970/><ref name=holtz2004/><ref name=farlow1976>{{cite_journal |last=Farlow |first=James O. |authorlink=James Farlow |year=1976 |title=Speculations about the diet and foraging behavior of large carnivorous dinosaurs |journal=American Midland Naturalist |volume=95 |issue=1 |pages=186–191 |url=http://links.jstor.org/sici?sici=0003-0031%28197601%2995%3A1%3C186%3ASATDAF%3E2.0.CO%3B2-5 |doi=10.2307/2424244}}</ref>
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==References==
{{reflist|2}}
== External links ==
{{wikispecies|Gorgosaurus}}
{{portalpar|Dinosaurs}}
*[http://www.palaeos.com/Vertebrates/Units/340Theropoda/340.760.html#Gorgosaurus ''Gorgosaurus''] from [http://www.palaeos.com Palaeos.com]
[[Category:Cretaceous dinosaurs]]
[[Category:Dinosaurs of North America]]
[[Category:Tyrannosaurs]]
[[cs:Gorgosaurus]]
[[es:Gorgosaurus]]
[[eo:Gorgosaŭro]]
[[fa:گورگوسورس]]
[[fr:Gorgosaurus]]
[[it:Gorgosaurus libratus]]
[[he:גורגוזאור ליברטוס]]
[[nl:Gorgosaurus]]
[[ja:ゴルゴサウルス]]
[[pl:Gorgozaur]]
[[pt:Gorgossauro]]
[[fi:Gorgosaurus]]
[[sv:Gorgosaurus]]
[[zh:蛇髮女怪龍]]
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