| Gorgosaurus | |
|---|---|
 | |
| Skeletal mount,Royal Tyrrell Museum of Palaeontology | |
Scientific classification | |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | Reptilia |
| Clade: | Dinosauria |
| Clade: | Saurischia |
| Clade: | Theropoda |
| Superfamily: | †Tyrannosauroidea |
| Clade: | †Eutyrannosauria |
| Family: | †Tyrannosauridae |
| Subfamily: | †Albertosaurinae |
| Genus: | †Gorgosaurus Lambe, 1914 |
| Type species | |
| †Gorgosaurus libratus Lambe, 1914 | |
| Synonyms | |
List
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Gorgosaurus (/ˌɡɔːrɡəˈsɔːrəs/GOR-gə-SOR-əs;lit. 'dreadful lizard') is agenus oftyrannosauridtheropoddinosaur that lived in western North America during theLate CretaceousPeriod (Campanian), between about 76.5 and 75 million years ago.[1]Fossil remains have been found in theCanadian province ofAlberta and theU.S. state ofMontana.Paleontologists recognize only thetype species,G. libratus, although other species have been erroneously referred to the genus.
Like most known tyrannosaurids,Gorgosaurus was a largebipedalpredator, measuring 8–9 metres (26–30 ft) in length and 2–3 metric tons (2.2–3.3 short tons) in body mass. Dozens of large, sharp teeth lined its jaws, while its two-fingered forelimbs were comparatively small.Gorgosaurus was most closely related toAlbertosaurus, and more distantly related to the largerTyrannosaurus.Gorgosaurus andAlbertosaurus are extremely similar, distinguished mainly by subtle differences in the teeth and skull bones. Some experts considerG. libratus to be a species ofAlbertosaurus; this would makeGorgosaurus ajunior synonym of that genus.
Gorgosaurus lived in a lushfloodplain environment along the edge of aninland sea. It was anapex predator, preying upon abundantceratopsids andhadrosaurs. In some areas,Gorgosaurus coexisted with another tyrannosaurid,Daspletosaurus. Although these animals were roughly the same size, there is some evidence ofniche 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 itsontogeny,life history and other aspects of itsbiology.
Gorgosaurus libratus was first described byLawrence Lambe in 1914.[2][3] Its name is derived from theGreekγοργος (gorgos – "fierce" or "terrible") andσαυρος (saurus – "lizard").[4] Thetype species isG. libratus; the specific epithet "balanced" is thepast participle of theLatin verblibrare, meaning "to balance".[3]
Theholotype ofGorgosaurus libratus (NMC 2120) is a nearly complete skeleton associated with a skull, discovered in 1913 byCharles M. Sternberg. This specimen was the first tyrannosaurid found with a complete hand.[2] It was found in theDinosaur Park Formation of Alberta and is housed in theCanadian Museum of Nature inOttawa.[5] Prospectors from theAmerican Museum of Natural History inNew York City were active along theRed Deer River in Alberta at the same time, collecting hundreds of spectacular dinosaur specimens, including four completeG. 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 skeleton (AMNH 5664), whichCharles H. Sternberg had collected in 1917 and sold to their museum. It was smaller than otherGorgosaurus specimens, with a lower, lighter skull and more elongate limb proportions. Manysutures 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.[6] Today's paleontologists regard this specimen as a juvenileG. libratus.[7][5] Dozens of other specimens have been excavated from the Dinosaur Park Formation and are housed in museums across the United States and Canada.[8][5]G. libratus is the best-represented tyrannosaurid in the fossil record, known from a virtually complete growth series.[7][9]
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 nameDeinodon.[10] Matthew and Brown commented in 1922 that these teeth were indistinguishable from those ofGorgosaurus, but in the absence of skeletal remains ofDeinodon, opted not to unequivocally synonymize the two genera, provisionally naming a?Deinodon libratus.[11] AlthoughDeinodon teeth are very similar to those ofGorgosaurus, tyrannosaurid teeth are extremely uniform, so it cannot be said for certain which animal they belonged to.Deinodon is usually regarded as anomen dubium today.[9] Additional likely synonyms ofG. libratus and/orD. horridus includeLaelaps falculus,Laelaps hazenianus,Laelaps incrassatus, andDryptosaurus kenabekides.[12]
Several tyrannosaurid skeletons from theTwo Medicine Formation andJudith River Formation ofMontana probably belong toGorgosaurus, although it remains uncertain whether they belong toG. libratus or a new species.[5] One specimen from Montana (TCMI 2001.89.1), housed in theChildren's Museum of Indianapolis, shows evidence of severepathologies, including healed leg, rib, and vertebralfractures,osteomyelitis (infection) at the tip of the lower jaw resulting in permanent tooth loss, and possibly abrain tumor.[13][14]
Several species were incorrectly assigned toGorgosaurus in the 20th century. A complete skull of a small tyrannosaurid (CMNH 7541), found in the younger, lateMaastrichtian-ageHell Creek Formation of Montana, was namedGorgosaurus lancensis byCharles Whitney Gilmore in 1946.[15] This specimen was renamedNanotyrannus byBob Bakker and colleagues in 1988.[16] Currently, many paleontologists regardNanotyrannus as a juvenileTyrannosaurus rex.[7][9] Similarly,Evgeny Maleev created the namesGorgosaurus lancinator andGorgosaurus novojilovi for two small tyrannosaurid specimens (PIN 553-1 and PIN 552–2) from theNemegt Formation ofMongolia in 1955.[17]Kenneth Carpenter renamed the smaller specimenMaleevosaurus novojilovi in 1992,[18] but both are now considered juveniles ofTarbosaurus bataar.[7][9][19]
Gorgosaurus was smaller thanTyrannosaurus orTarbosaurus, close in size toAlbertosaurus. Adults reached 8 to 9 m (26 to 30 ft) in length from snout to tail,[8][7][20] and weighed 2–3 metric tons (2.2–3.3 short tons) in body mass.[21][22][23] The largest known skull measures 99 cm (39 in) long, just slightly smaller than that ofDaspletosaurus.[8] 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.Albertosaurus andGorgosaurus share proportionally longer and lower skulls thanDaspletosaurus and other tyrannosaurids. The end of the snout was blunt, and thenasal andparietal bones were fused along the midline of the skull, as in all other members of the family. Theeye socket was circular rather than oval or keyhole-shaped as in other tyrannosaurid genera. A tall crest rose from thelacrimal bone in front of each eye, similar toAlbertosaurus andDaspletosaurus.[7] Differences in the shape of bones surrounding the brain setGorgosaurus apart fromAlbertosaurus.[5]
Gorgosaurus teeth were typical of all known tyrannosaurids. The eightpremaxillary teeth at the front of the snout were smaller than the rest, closely packed andD-shaped incross section. InGorgosaurus, the first tooth in themaxilla 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.[7] Along with the eight premaxillary teeth,Gorgosaurus had 26 to 30 maxillary teeth and 30 to 34 teeth in thedentary bones of the lower jaw. This number of teeth is similar toAlbertosaurus andDaspletosaurus but is fewer than those ofTarbosaurus orTyrannosaurus.[24]
Gorgosaurus shared its general body plan with all other tyrannosaurids. Its massive head was perched on the end of anS-shaped neck. In contrast to its large head, its forelimbs were very small. The forelimbs had only two digits, although a thirdmetacarpal is known in some specimens, thevestigial 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.[7] The largest knownGorgosaurusfemur measured 105 cm (41 in) long. In several smaller specimens ofGorgosaurus, thetibia was longer than the femur, a proportion typical offast-running animals.[8] The two bones were of equal length in the largest specimens.[6] The long, heavy tail served as acounterweight to the head and torso and placed thecenter of gravity over the hips.[7]
In 2001, paleontologistPhil Currie reported skin impressions from the holotype specimen ofG. libratus. He originally reported the skin as being essentially smooth and lacking the scales found in other dinosaurs, similar to the secondarily featherless skin found in large modern birds.[25] Scales of some sort were present in this specimen, but they are reportedly widely dispersed from each other and very small. Other patches of isolatedGorgosaurus skin shows denser, and larger though still relatively fine scales (smaller thanhadrosaurid scales and approximately as fine as a Gila monster's).[26] Neither of these specimens was associated with any particular bone or specific body area.[26] In theEncyclopedia of DinosaursKenneth Carpenter pointed out that traces of skin impressions from the tail ofGorgosaurus showed similar small rounded or hexagonal scales.[27]
Gorgosaurus isclassified in the theropodsubfamily Albertosaurinae within thefamily Tyrannosauridae. It is most closely related to the slightly youngerAlbertosaurus.[24] These are the only two definite albertosaurine genera that have been described, although other undescribed species may exist.[5]Appalachiosaurus was described as abasaltyrannosauroid just outside Tyrannosauridae,[28] although American paleontologistThomas Holtz published a phylogenetic analysis in 2004 which indicated it was an albertosaurine.[7] More recent, unpublished work by Holtz agrees with the original assessment.[29] All other tyrannosaurid genera, includingDaspletosaurus,Tarbosaurus andTyrannosaurus, 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 andphalanges).[24][30]
The close similarities betweenGorgosaurus libratus andAlbertosaurus sarcophagus have led many experts to combine them into one genus over the years.Albertosaurus was named first, so by convention it is givenpriority over the nameGorgosaurus, which is sometimes considered itsjunior synonym.William Diller Matthew andBarnum Brown doubted the distinction of the two genera as early as 1922.[11]Gorgosaurus libratus was formally reassigned toAlbertosaurus (asAlbertosaurus libratus) byDale Russell in 1970,[8] and many subsequent authors followed his lead.[28][31] Combining the two greatly expands the geographical and chronological range of the genusAlbertosaurus. Other experts maintain the two genera as separate.[7] Canadian paleontologistPhil Currie claims there are as many anatomical differences betweenAlbertosaurus andGorgosaurus as there are betweenDaspletosaurus andTyrannosaurus, which are almost always kept separate. He also notes that undescribed tyrannosaurids discovered inAlaska,New Mexico and elsewhere in North America may help clarify the situation.[5]Gregory S. Paul has suggested thatGorgosaurus libratus is ancestral toAlbertosaurus sarcophagus.[32]
Below is the cladogram of Tyrannosauridae based on thephylogenetic analysis conducted by Loewenet al. in 2013.[33]
Just like other tyrannosaurids, bite force ofGorgosaurus andAlbertosaurus increases slowly among young individuals, and then it increases exponentially when they reach the late juvenile stage.[34] In 2012, Jovannelly and Lane estimated thatGorgosaurus could exert a bite force of at least 22,000, possibly up to 42,000 newtons.[35] Other paleontologists have produced significantly lower bite force estimates. In 2021, given that the largest knownGorgosaurus had a similar bite force to the similar-sizedTyrannosaurus, Therrien and colleagues proposed that the maximum bite force that could be produced by adult albertosaurines is around 12,200 to 21,800 newtons.[34] In 2022, Sakamoto estimated thatGorgosaurus had an anterior bite force of 6,418 newtons and a posterior bite force of 13,817 newtons.[36]
In 2023, a juvenileGorgosaurus (TMP 2009.12.14) with itsin situ stomach contents containing twoCitipes juveniles about a year old intact was reported from theDinosaur Park Formation. This juvenile would have been 5-7 years old at the time of death, measuring about 4 metres (13 ft) long and weighing around 335 kilograms (739 lb). It is much larger than the twoCitipes juveniles that weigh about 9–12 kilograms (20–26 lb), contrary to the assumption that tyrannosaurids fed on prey of their size once they reached 16–32 kilograms (35–71 lb), indicating that juvenile tyrannosaurids still consumed much smaller prey after exceeding a certain size threshold. It's a direct dietary evidence that reinforces the theory of 'ontogenetic dietary shift' for tyrannosaurids, as previously inferred by ecological modeling and anatomical features among different age groups. Only the remains of the hindlimbs and caudal vertebrae of juvenileCitipes were present in the tyrannosaurid's stomach cavity, suggesting that a juvenileGorgosaurus may have had preferential consumption of the muscular hindlimbs.[37][38]Thomas R. Holtz Jr., a paleontologist who also previously theorized that tyrannosaurs underwent a big dietary shift with maturation, said that the fossil "looks like it was Thanksgiving," as the juvenileGorgosaurus was mostly eating the legs ofCitipes.[39]
Gregory Erickson and colleagues have studied the growth and life history of tyrannosaurids using bonehistology, 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 fiveGorgosaurus specimens of various sizes, Erickson calculated a maximum growth rate of about 50 kg (110 lb) per year during the rapid growth phase, slower than in tyrannosaurines likeDaspletosaurus andTyrannosaurus, but comparable toAlbertosaurus.[40]
Gorgosaurus spent as much as half its life in the juvenile phase before ballooning up to near-maximum size in only a few years.[40] This, along with the complete lack of predators intermediate in size between huge adult tyrannosaurids and other small theropods, suggests that these niches may have been filled by juvenile tyrannosaurids. This pattern is seen in modernKomodo dragons, whose hatchlings start off as tree-dwellinginsectivores and slowly mature into massiveapex predators capable of taking down large vertebrates.[7] Other tyrannosaurids, includingAlbertosaurus, have been found in aggregations that some have suggested to represent mixed-agepacks, but there is no evidence of gregarious behavior inGorgosaurus.[41][42]
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The discovery of two exceptionally preserved juvenile skulls fromGorgosaurus suggests thatGorgosaurus underwent the morphological shift from gracile juveniles to robust adults at an earlier age thanTyrannosaurus, to which it was compared in a study published by Jared Voriset al., suggests that the ontogenetic changes occurred at roughly 5-7 years of age inGorgosaurus; much earlier than its larger and later relative. However, both tyrannosaur genera underwent these ontogenetic transformations at a similar percent of skull length relative to the large known adult individuals. The study's results likewise indicate that there is a dissociation between body size and cranial development in tyrannosaurs, while simultaneously allowing better identification of juvenile remains that may have been misidentified in museum fossil collections.[43] It is estimated that an ontogenetic dietary shift ofGorgosaurus andAlbertosaurus occurs when the mandibular length reaches 58 cm (1.90 ft), indicating that this is the stage when their bite force increases exponentially and when they begin to pursuit large prey.[34]
Several pathologies have been documented in theGorgosaurus libratusholotype, NMC 2120. These include the third right dorsal rib, as well as healed fractures on the 13th and 14th gastralia and left fibula. Its fourth left metatarsal bore roughenedexostoses both in the middle and at the far end. The third phalanx of the third right toe is deformed, as the claw on that digit has been described as "quite small and amorphous". The three pathologies may have been received in a single encounter with another dinosaur.[44]
Another specimen cataloged asTMP94.12.602 bears multiple pathologies. A 10 cm (3.9 in)longitudinal fracture is present in the middle of the right fibula's shaft. Multiple ribs bear healed fractures and the specimen had apseudoarthorticgastralium. Lesions from a bite received to the face were present and showed evidence that the wounds were healing before the animal died.[44]
Another specimen has a poorly healed fracture of the rightfibula, which left a largecallus on the bone. In a 2001 study conducted by Bruce Rothschild and other paleontologists, 54 foot bones referred toGorgosaurus were examined for signs ofstress fracture, but none were found.[44][45]
As with many tyrannosaurids, severalGorgosaurus specimens show evidence ofintraspecific face biting.[46]
Specimen TMP 1994.143.1, a juvenile skull from theDinosaur Park Formation with several tyrannosaur bite marks, was previously believed to beDaspletosaurus sp.[47][48][49] but was later assigned toGorgosaurus libratus.[50][51]
TMP 2017.012.0002 is aGorgosaurus right maxilla with five raised scars; healed injuries from face biting byconspecifics.[46]
TMP91.36.500 is a very complete sub-adultGorgosaurus with preserved face bite injuries and also has a thoroughly healed fracture in the right fibula. Also present was a healed fracture in the dentary and what the authors describing the specimen referred to as "a mushroom-likehyperostosis of a right pedal phalanx." Ralph Molnar has speculated that this may be the same kind of pathology afflicting an unidentifiedornithomimid discovered with a similar mushroom shaped growth on a toe bone.[44] TMP91.36.500 is also preserved in a characteristicdeath pose.[52]
Most specimens ofGorgosaurus libratus have been recovered from theDinosaur Park Formation in Alberta.[5] This formation dates to the middle of theCampanian, between 76.5 and 74.8 million years ago,[53] andGorgosaurus libratus fossils are known specifically from the lower to middle section of the formation, between 76.6 and 75.1 million years ago.[54] TheTwo Medicine Formation andJudith River Formation of Montana have also yielded possibleGorgosaurus remains. At this time, the area was a coastal plain along the western edge of theWestern Interior Seaway, which divided North America in half. TheLaramide Orogeny had begun uplifting theRocky Mountains to the west, from which flowed great rivers that deposited eroded sediment in vastfloodplains along the coast.[55][56] The climate wassubtropical with marked seasonality, and periodic droughts sometimes resulted in massive mortality among the great herds of dinosaurs, as represented in the numerousbonebed deposits preserved in the Dinosaur Park Formation.Conifers formed theforest canopy, while theunderstory plants consisted offerns,tree ferns andangiosperms.[57] Around 73 million years ago, the seaway began to expand,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 massiveBearpaw Shale.[56]
The Dinosaur Park Formation preserves a great wealth of vertebrate fossils. A wide variety of fish swam the rivers andestuaries, includinggars,sturgeons,sharks andrays, among others.Frogs,salamanders,turtles,crocodilians andchampsosaurs also dwelled in the aquatic habitats.Azhdarchidpterosaurs andornithuran birds likeApatornis flew overhead, while theenantiornithine birdAvisaurus lived on the ground alongsidemultituberculate,metatherian andeutherianmammals. A number of species of terrestrial lizards were also present, includingwhiptails,skinks,monitors andalligator lizards. Dinosaur fossils in particular are found with unrivaled abundance and diversity. Huge herds ofceratopsids roamed the floodplains alongside equally large groups of saurolophine and lambeosaurinehadrosaurs. Other herbivorous groups likeornithomimids,pachycephalosaurs, smallornithopods,nodosaurids andankylosaurids were also represented. Small predatory dinosaurs likeoviraptorosaurs,troodonts anddromaeosaurs hunted smaller prey than the huge tyrannosaurids;Daspletosaurus andGorgosaurus, which were two orders of magnitude larger in mass.[56] Intervening predatory niches may have been filled by young tyrannosaurids.[8][7][58] ASaurornitholestesdentary has been discovered in the Dinosaur Park Formation that bore tooth marks left by the bite of a young tyrannosaur, possiblyGorgosaurus.[59]
In the middle stages of the Dinosaur Park Formation,Gorgosaurus lived alongside a rarer species of tyrannosaurid,Daspletosaurus. This is one of the few examples of two tyrannosaur genera coexisting. Similar-sized predators in modern predatorguilds are separated into differentecological niches by anatomical, behavioral or geographical differences that limit competition. Niche differentiation between the Dinosaur Park tyrannosaurids is not well understood.[60] In 1970, Dale Russellhypothesized that the more commonGorgosaurus actively hunted fleet-footedhadrosaurs, while the rarer and more troublesomeceratopsians andankylosaurians (horned and heavilyarmoured dinosaurs) were left to the more heavy builtDaspletosaurus.[8] However, a specimen ofDaspletosaurus (OTM 200) from the contemporaneousTwo Medicine Formation of Montana preserves the digested remains of a juvenile hadrosaur in its gut region,[61] and another bonebed contains the remains of threeDaspletosaurus along with the remains of at least five hadrosaurs.[41]
Unlike some other groups of dinosaurs, neither genus was more common at higher or lower elevations than the other.[60] However,Gorgosaurus appears more common in northern formations like Dinosaur Park, with species ofDaspletosaurus being more abundant to the south. The same pattern is seen in other groups of dinosaurs. Chasmosaurine ceratopsians and saurolophine hadrosaurs are also more common in the Two Medicine Formation of Montana and in southwestern North America during the Campanian, while centrosaurines and lambeosaurines dominate in northern latitudes. Holtz has suggested this pattern indicates shared ecological preferences between tyrannosaurines, chasmosaurines and saurolophines. At the end of the later Maastrichtian stage, tyrannosaurines likeTyrannosaurus rex,saurolophines likeEdmontosaurus andKritosaurus and chasmosaurines likeTriceratops andTorosaurus were widespread throughout western North America, while lambeosaurines were rare, consisting of a few species likeHypacrosaurus, and albertosaurines and centrosaurines had gone extinct.[7] However, in the case of the centrosaurines, they had thrived inAsia with genera likeSinoceratops.[62] While albertosaurine remains have been found in theHell Creek Formation, it is most likely these are indeterminate remains belonging to a species ofTyrannosaurus.[63]
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