Albertosaurus

Albertosaurus sarcophagus
Albertosaurus (pronounced /ælˌbɜrtɵˈsɔrəs/; meaning "Alberta lizard") is a genus of tyrannosaurid theropod dinosaur that lived in western North America during the Late Cretaceous Period, more than 70 million years ago. The type species, A. sarcophagus, was restricted in range to the modern-day Canadian province of Alberta, after which the genus is named. Scientists disagree on the content of the genus, with some recognizing Gorgosaurus libratus as a second species.As a tyrannosaurid, Albertosaurus was a bipedal predator with tiny, two-fingered hands and a massive head with dozens of large, sharp teeth. It may have been at the top of the food chain in its local ecosystem. Although relatively large for a theropod, Albertosaurus was much smaller than its more famous relative Tyrannosaurus, probably weighing less than 2 metric tons.

Since the first discovery in 1884, fossils of more than thirty individuals have been recovered, providing scientists with a more detailed knowledge of Albertosaurus anatomy than is available for most other tyrannosaurids. The discovery of 22 individuals at one site provides evidence of pack behavior and allows studies of ontogeny and population biology which are impossible with lesser-known dinosaurs.

Description
Albertosaurus was smaller than the truly gigantic tyrannosaurids like Tarbosaurus and Tyrannosaurus. Typical adults measured up to 9 meters (30 ft) long,[1] [2] while rare individuals of great age could grow to over 10 meters (33 ft) in length.[3] Several independent mass estimates, obtained by different methods, suggest that an adult Albertosaurus weighed between 1.3 tonnes (1.4 short tons)[4] and 1.7 tonnes (1.9 tons).[5]

The massive skull of Albertosaurus, perched on a short, S-shaped neck, was approximately 1 meter (3.3 ft) long in the largest adults.[6] Wide openings in the skull (fenestrae) reduced the weight of the head while also providing space for muscle attachment and sensory organs. Its long jaws contained more than 60 banana-shaped teeth; larger tyrannosaurids possessed fewer teeth. Unlike most theropods, Albertosaurus and other tyrannosaurids were heterodont, with teeth of different forms depending on their position in the mouth. The premaxillary teeth at the tip of the upper jaw were much smaller than the rest, more closely packed, and D-shaped in cross section.[2] Above the eyes were short bony crests that may have been brightly colored in life and used in courtship to attract a mate.[7]

All tyrannosaurids, including Albertosaurus, shared a similar body appearance. Typically for a theropod, Albertosaurus was bipedal and balanced the heavy head and torso with a long tail. However, tyrannosaurid forelimbs were extremely small for their body size and retained only two digits. The hind limbs were long and ended in a four-toed foot. The first digit, called the hallux, was short and only the other three contacted the ground, with the third (middle) digit longer than the rest.[2] Albertosaurus may have been able to reach speeds of 25–30 miles per hour.[7]

Classification and systematics
Albertosaurus is a member of the theropod family Tyrannosauridae, in the subfamily Albertosaurinae. Its closest relative is the slightly older Gorgosaurus libratus (sometimes called Albertosaurus libratus; see below).[8] These two species are the only described albertosaurines, although other undescribed species may exist.[9] [http://en.wikipedia.org/wiki/Thomas_R._Holtz,_Jr. Thomas Holtz] found Appalachiosaurus to be an albertosaurine in 2004,[2] but his more recent unpublished work locates it just outside Tyrannosauridae,[10] in agreement with other authors.[11]

The other major subfamily of tyrannosaurids is the Tyrannosaurinae, including Daspletosaurus, Tarbosaurus and Tyrannosaurus. Compared with these robust tyrannosaurines, albertosaurines had slender builds, with proportionately smaller skulls and longer bones of the lower leg (tibia) and feet (metatarsals and phalanges).[6] <sup class="reference" id="cite_ref-currieetal2003_7-1">[8]

Discovery and naming
Albertosaurus was named by Henry Fairfield Osborn in a very brief note at the end of his 1905 description of Tyrannosaurus rex. The name honors Alberta, the Canadian province in which the first remains were found. The generic name also incorporates the Greek term σαυρος/sauros ("lizard"), the most common suffix in dinosaur names. The type species is A. sarcophagus, which means "flesh-eater" and has the same etymology as the funeral container with which it shares its name: a combination of the Ancient Greek words σαρξ/sarx ("flesh") and Φαγειν/phagein ("to eat").<sup class="reference" id="cite_ref-osborn1905_11-0">[12] More than thirty specimens of all ages are known to science.<sup class="reference" id="cite_ref-ericksonetal2006_2-1">[3] <sup class="reference" id="cite_ref-currie2003b_8-1">[9]

Early discoveries
The type specimen is a partial skull, collected in 1884 from an outcrop of the Horseshoe Canyon Formation alongside the Red Deer River in Alberta. This specimen and a smaller skull associated with some skeletal material were recovered by expeditions of the Geological Survey of Canada, led by the famous geologist Joseph B. Tyrrell. The two skulls were assigned to the preexisting species "Laelaps incrassatus" by Edward Drinker Cope in 1892,<sup class="reference" id="cite_ref-cope1892_12-0">[13] despite the fact that the name Laelaps was preoccupied by a genus of mite and had been changed to Dryptosaurus in 1877 by Othniel Charles Marsh. Cope refused to recognize the new name created by his archrival Marsh, so it fell to Lawrence Lambe to change "Laelaps incrassatus" to Dryptosaurus incrassatus when he described the remains in detail in 1904.<sup class="reference" id="cite_ref-lambe1904_13-0">[14] Shortly later, Osborn pointed out that D. incrassatus was based on generic tyrannosaurid teeth, so the two Alberta skulls could not be confidently referred to that species. The Alberta skulls also differed markedly from the remains of D. aquilunguis, type species of Dryptosaurus, so Osborn created the new name Albertosaurus sarcophagus for them in 1905. He did not describe the remains in any great detail, citing Lambe's complete description the year before.<sup class="reference" id="cite_ref-osborn1905_11-1">[12] Both specimens (CMN 5600 and 5601) are stored in the Canadian Museum of Nature in Ottawa.

Dry Island bonebed
In 1910, American paleontologist Barnum Brown uncovered the remains of a large group of Albertosaurus at another quarry alongside the Red Deer River. Because of the large number of bones and the limited time available, Brown's party did not collect every specimen, but made sure to collect remains from all of the individuals they could identify in the bonebed. Among the bones deposited in the American Museum of Natural History collections in New York City are seven sets of right metatarsals, along with two isolated toe bones that did not match any of the metatarsals in size. This indicated the presence of at least nine individuals in the quarry. The Royal Tyrrell Museum of Palaeontology rediscovered the bonebed in 1997 and resumed fieldwork at the site, which is now located inside Dry Island Buffalo Jump Provincial Park.<sup class="reference" id="cite_ref-currie1998_14-0">[15] Further excavation from 1997 to 2005 turned up the remains of 13 more individuals of various ages, including a diminutive two-year-old and a very old individual estimated at over 10 meters (33 ft) in length. None of these individuals are known from complete skeletons, and most are represented by remains in both museums.<sup class="reference" id="cite_ref-ericksonetal2006_2-2">[3] <sup class="reference" id="cite_ref-ericksonetal2004_3-1">[4]

Gorgosaurus libratus
In 1913, paleontologist Charles H. Sternberg recovered another tyrannosaurid skeleton from the slightly older Dinosaur Park Formation in Alberta. Lawrence Lambe named this dinosaur Gorgosaurus libratus in 1914.<sup class="reference" id="cite_ref-lambe1914_15-0">[16] Other specimens were later found in Alberta and Montana. Finding few differences to separate the two genera, Dale Russell declared the name Gorgosaurus a junior synonym of Albertosaurus, which had been named first, and G. libratus was renamed Albertosaurus libratus in 1970. This addition extended the temporal range of the genus Albertosaurus backwards by several million years and its geographic range southwards by hundreds of kilometers.<sup class="reference" id="cite_ref-russell1970_0-1">[1]

In 2003, Phil Currie compared several tyrannosaurid skulls and came to the conclusion that the two species are more distinct than previously thought. The decision to use one or two genera is rather arbitrary, as the two species are sister taxa, more closely related to each other than to any other species. Recognizing this, Currie nevertheless recommended that Albertosaurus and Gorgosaurus be retained as separate genera, as they are no more similar than Daspletosaurus and Tyrannosaurus, which are almost always separated. In addition, several albertosaurine specimens have been recovered from Alaska and New Mexico, and Currie suggested that the Albertosaurus-Gorgosaurus situation may be clarified once these are described fully.<sup class="reference" id="cite_ref-currie2003b_8-2">[9] Most authors have followed Currie's recommendation,<sup class="reference" id="cite_ref-holtz2004_1-4">[2] <sup class="reference" id="cite_ref-ericksonetal2004_3-2">[4] <sup class="reference" id="cite_ref-ricklefs2007_16-0">[17] but some have not.<sup class="reference" id="cite_ref-carretal2005_10-1">[11]

Other discoveries
William Parks described a new species, Albertosaurus arctunguis, based on a partial skeleton excavated near the Red Deer River in 1928,<sup class="reference" id="cite_ref-parks1928_17-0">[18] but this species has been considered identical to A. sarcophagus since 1970.<sup class="reference" id="cite_ref-russell1970_0-2">[1] Parks' specimen (ROM 807) is housed in the Royal Ontario Museum in Toronto. Six more skulls and skeletons have since been discovered in Alberta and are housed in various Canadian museums. Fossils have also been reported from the American states of Montana, New Mexico, and Wyoming, but these probably do not represent A. sarcophagus and may not even belong to the genus Albertosaurus.<sup class="reference" id="cite_ref-holtz2004_1-5">[2] <sup class="reference" id="cite_ref-currie2003b_8-3">[9]

Albertosaurus megagracilis was based on a small tyrannosaurid skeleton from the Hell Creek Formation of Montana.<sup class="reference" id="cite_ref-paul1988_18-0">[19] It was renamed Dinotyrannus in 1995,<sup class="reference" id="cite_ref-olshevsky1995_19-0">[20] but is now thought to represent a juvenile Tyrannosaurus rex.<sup class="reference" id="cite_ref-currie2003a_5-2">[6]

Growth pattern
Most age categories of Albertosaurus are represented in the fossil record. Using bone histology, the age of an individual animal at the time of death can often be determined, allowing growth rates to be estimated and compared with other species. The youngest known Albertosaurus is a two-year-old discovered in the Dry Island bonebed, which would have weighed about 50 kilograms (110 lb) and measured slightly more than 2 meters (7 ft) in length. The 10 meter (33 ft) specimen from the same quarry is the oldest and largest known, at 28 years of age. When specimens of intermediate age and size are plotted on a graph, an S-shaped growth curve results, with the most rapid growth occurring in a four-year period ending around the sixteenth year of life, a pattern also seen in other tyrannosaurids. The growth rate during this phase was 122 kilograms (268 lb) per year, based on an adult 1.3 tonnes (1.4 short tons). Other studies have suggested higher adult weights; this would affect the magnitude of the growth rate but not the overall pattern. Tyrannosaurids similar in size to Albertosaurus had similar growth rates, although the much larger Tyrannosaurus rex grew almost five times faster (601 kilograms 1325 lb per year) at its peak.<sup class="reference" id="cite_ref-ericksonetal2006_2-3">[3] The end of the rapid growth phase suggests the onset of sexual maturity in Albertosaurus, although growth continued at a slower rate throughout the animals' lives.<sup class="reference" id="cite_ref-ericksonetal2006_2-4">[3] <sup class="reference" id="cite_ref-ericksonetal2004_3-4">[4] Sexual maturation while still actively growing appears to be a shared trait among small<sup class="reference" id="cite_ref-ericksonetal2007_20-0">[21] and large<sup class="reference" id="cite_ref-lee_werning2008_21-0">[22] dinosaurs as well as in large mammals such as humans and elephants.<sup class="reference" id="cite_ref-lee_werning2008_21-1">[22] This pattern of relatively early sexual maturation differs strikingly from the pattern in birds, which delay their sexual maturity until after they have finished growing.<sup class="reference" id="cite_ref-ricklefs2007_16-1">[17] <sup class="reference" id="cite_ref-lee_werning2008_21-2">[22]

Life history
Most known Albertosaurus individuals were aged 14 years or more at the time of death. Juvenile animals are rarely found as fossils for several reasons, mainly preservation bias, where the smaller bones of younger animals were less likely to preserved by fossilization than the larger bones of adults, and collection bias, where smaller fossils are less likely to be noticed by collectors in the field.<sup class="reference" id="cite_ref-roachbrinkman2007_22-0">[23] Young Albertosaurus are relatively large for juvenile animals, but their remains are still rare in the fossil record compared with adults. It has been suggested that this phenomenon is a consequence of life history, rather than bias, and that fossils of juvenile Albertosaurus are rare because they simply did not die as often as adults did.<sup class="reference" id="cite_ref-ericksonetal2006_2-5">[3]

A hypothesis of Albertosaurus life history postulates that hatchlings died in large numbers, but have not been preserved in the fossil record due to their small size and fragile construction. After just two years, juveniles were larger than any other predator in the region aside from adult Albertosaurus, and more fleet of foot than most of their prey animals. This resulted in a dramatic decrease in their mortality rate and a corresponding rarity of fossil remains. Mortality rates doubled at age twelve, perhaps the result of the physiological demands of the rapid growth phase, and then doubled again with the onset of sexual maturity between the ages of fourteen and sixteen. This elevated mortality rate continued throughout adulthood, perhaps due to high physiological demands, stress and injuries received during intraspecific competition for mates and resources, and eventually, the ever-increasing effects of senescence. The higher mortality rate in adults may explain their more common preservation. Very large animals were rare because few individuals survived long enough to attain such sizes. High infant mortality rates, followed by reduced mortality among juveniles and a sudden increase in mortality after sexual maturity, with very few animals reaching maximum size, is a pattern observed in many modern large mammals, including elephants, African buffalo, and rhinoceros. The same pattern is also seen in other tyrannosaurids. The comparison with modern animals and other tyrannosaurids lends support to this life history hypothesis, but bias in the fossil record may still play a large role, especially since more than two-thirds of all Albertosaurus specimens are known from one locality.<sup class="reference" id="cite_ref-ericksonetal2006_2-6">[3] <sup class="reference" id="cite_ref-ricklefs2007_16-2">[17]

Pack behavior
The Dry Island bonebed discovered by Barnum Brown and his crew contains the remains of 22 Albertosaurus, the most individuals found in one locality of any Cretaceous theropod, and the second-most of any large theropod dinosaur behind the Allosaurus assemblage at the Cleveland Lloyd Dinosaur Quarry in Utah. The group seems to be composed of one very old adult; eight adults between 17 and 23 years old; seven sub-adults undergoing their rapid growth phases at between 12 and 16 years old; and six juveniles between the ages of 2 and 11 years, who had not yet reached the growth phase.<sup class="reference" id="cite_ref-ericksonetal2006_2-7">[3]

The near-absence of herbivore remains and the similar state of preservation between the many individuals at the Albertosaurus bonebed quarry led Phil Currie to conclude that the locality was not a predator trap like the La Brea Tar Pits in California, and that all of the preserved animals died at the same time. Currie claims this as evidence of pack behavior.<sup class="reference" id="cite_ref-currie1998_14-1">[15] Other scientists are skeptical, observing that the animals may have been driven together by drought, flood or for other reasons.<sup class="reference" id="cite_ref-ericksonetal2006_2-8">[3] <sup class="reference" id="cite_ref-roachbrinkman2007_22-1">[23] <sup class="reference" id="cite_ref-eberthmccrea2001_23-0">[24]

There is abundant evidence for gregarious behavior among herbivorous dinosaurs, including ceratopsians and hadrosaurs.<sup class="reference" id="cite_ref-horner1997_24-0">[25] However, only rarely are so many dinosaurian predators found at the same site. Small theropods like Deinonychus,<sup class="reference" id="cite_ref-maxwellostrom1995_25-0">[26] Coelophysis and Megapnosaurus (Syntarsus) rhodesiensis<sup class="reference" id="cite_ref-raath1990_26-0">[27] have been found in aggregations, as have larger predators like Allosaurus and Mapusaurus.<sup class="reference" id="cite_ref-coriacurrie2006_27-0">[28] There is some evidence of gregarious behavior in other tyrannosaurids as well. Fragmentary remains of smaller individuals were found alongside "Sue," the Tyrannosaurus mounted in the Field Museum of Natural History in Chicago, and a bonebed in the Two Medicine Formation of Montana contains at least three specimens of Daspletosaurus, preserved alongside several hadrosaurs.<sup class="reference" id="cite_ref-currieetal2005_28-0">[29] These findings may corroborate the evidence for social behavior in Albertosaurus, although some or all of the above localities may represent temporary or unnatural aggregations.<sup class="reference" id="cite_ref-currie1998_14-2">[15] Others have speculated that instead of social groups, at least some of these finds represent Komodo dragon-like mobbing of carcasses, where aggressive competition leads to some of the predators being killed and cannibalized.<sup class="reference" id="cite_ref-roachbrinkman2007_22-2">[23]

Currie also offers speculation on the pack-hunting habits of Albertosaurus. The leg proportions of the smaller individuals were comparable to those of Ornithomimids, which were probably among the fastest dinosaurs. Younger Albertosaurus were probably equally fleet-footed, or at least faster than their prey. Currie hypothesized that the younger members of the pack may have been responsible for driving their prey towards the adults, who were larger and more powerful, but also slower.<sup class="reference" id="cite_ref-currie1998_14-3">[15] Juveniles may also have had different lifestyles than adults, filling predator niches between the enormous adults and the smaller contemporaneous theropods, the largest of which were two orders of magnitude smaller than adult Albertosaurus in mass.<sup class="reference" id="cite_ref-holtz2004_1-6">[2] A similar situation is observed in modern Komodo dragons, with hatchlings beginning life as small insectivores before growing to become the dominant predators on their islands.<sup class="reference" id="cite_ref-auffenberg1981_29-0">[30] However, as the preservation of behavior in the fossil record is exceedingly rare, these ideas cannot readily be tested.

Paleoecology
All identifiable fossils of Albertosaurus sarcophagus are known from the Horseshoe Canyon Formation in Alberta. This geologic formation dates to the early Maastrichtian stage of the Late Cretaceous Period, 73 to 70 Ma (million years ago). Immediately below this formation is the Bearpaw Shale, a marine formation representing a section of the Western Interior Seaway. The seaway was receding as the climate cooled and sea levels subsided towards the end of the Cretaceous, exposing land that had previously been underwater. It was not a smooth process, however, and the seaway would periodically rise to cover parts of the region throughout Horseshoe Canyon times before finally receding altogether in the years after. Due to the changing sea levels, many different environments are represented in the Horseshoe Canyon Formation, including offshore and near-shore marine habitats and coastal habitats like lagoons, estuaries and tidal flats. Numerous coal seams represent ancient peat swamps. Like most of the other vertebrate fossils from the formation, Albertosaurus remains are found in deposits laid down in the deltas and floodplains of large rivers during the later half of Horseshoe Canyon times.<sup class="reference" id="cite_ref-eberth1997_30-0">[31]

The fauna of the Horseshoe Canyon Formation is well-known, as vertebrate fossils, including those of dinosaurs, are quite common. Sharks, rays, sturgeons, bowfins, gars and the gar-like Aspidorhynchus made up the fish fauna. Mammals included multituberculates and the marsupial Didelphodon. The saltwater plesiosaur Leurospondylus has been found in marine sediments in the Horseshoe Canyon, while freshwater environments were populated by turtles, Champsosaurus, and crocodilians like Leidyosuchus and Stangerochampsa. Dinosaurs dominate the fauna, especially hadrosaurs, which make up half of all dinosaurs known, including the genera Edmontosaurus, Saurolophus and Hypacrosaurus. Ceratopsians and ornithomimids were also very common, together making up another third of the known fauna. Along with much rarer Ankylosaurians and pachycephalosaurs, all of these animals would have been prey for a diverse array of carnivorous theropods, including troodontids, dromaeosaurids, and caenagnathids. Adult Albertosaurus were the apex predators in this environment, with intermediate niches possibly filled by juvenile albertosaurs.<sup class="reference" id="cite_ref-eberth1997_30-1">[31]

Albertosaurus libratus
Albertosaurus libratus or "Gorgosaurus" (pronounced /ˌɡɔrɡɵˈsɔrəs/ GOR-go-SOR-əs, meaning "fierce lizard") is a genus of tyrannosaurid theropod dinosaur that lived in western North America during the Late Cretaceous Period, between about 77 and 74 million years ago. Fossil remains have been found in the Canadian province of Alberta and possibly the U.S. state of 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 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 inland sea. An apex predator, it 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 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
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.<sup class="reference" id="cite_ref-russell1970_0-0">[1] <sup class="reference" id="cite_ref-holtz2004_1-0">[2] Paleontologists have estimated full-grown adults to weigh more than 2.4 tonnes (2.7 short tons).<sup class="reference" id="cite_ref-seebacher2001_2-0">[3] The largest known skull measures 99 centimeters (39 in) long, just slightly smaller than that of Daspletosaurus.<sup class="reference" id="cite_ref-russell1970_0-1">[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 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 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.<sup class="reference" id="cite_ref-holtz2004_1-1">[2] Differences in the shape of bones surrounding the brain set Gorgosaurus apart from Albertosaurus.<sup class="reference" id="cite_ref-currie2003a_3-0">[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.<sup class="reference" id="cite_ref-holtz2004_1-2">[2] Along with the eight premaxillary teeth, Gorgosaurus had 26 to 30 maxillary teeth and 30 to 34 teeth in the dentary bones of the lower jaw. This number of teeth is similar to Albertosaurus and Daspletosaurus but is fewer than those of Tarbosaurus or Tyrannosaurus.<sup class="reference" id="cite_ref-currieetal2003_4-0">[5]

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.<sup class="reference" id="cite_ref-holtz2004_1-3">[2] 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 fast-running animals.<sup class="reference" id="cite_ref-russell1970_0-2">[1] The two bones were of equal length in the largest specimens.<sup class="reference" id="cite_ref-matthewbrown1923_5-0">[6] The long, heavy tail served as a counterweight to the head and torso and placed the center of gravity over the hips.<sup class="reference" id="cite_ref-holtz2004_1-4">[2]

Classification and systematics
Gorgosaurus is classified in the theropod subfamily Albertosaurinae within the family Tyrannosauridae. It is most closely related to the slightly younger Albertosaurus.<sup class="reference" id="cite_ref-currieetal2003_4-2">[5] These are the only two definite albertosaurine genera that have been described, although other undescribed species may exist.<sup class="reference" id="cite_ref-currie2003a_3-1">[4] Appalachiosaurus was described as a basal tyrannosauroid just outside Tyrannosauridae,<sup class="reference" id="cite_ref-carretal2005_6-1">[7] although American paleontologist [http://en.wikipedia.org/wiki/Thomas_R._Holtz,_Jr. Thomas Holtz] published a phylogenetic analysis in 2004 which indicated it was an albertosaurine.<sup class="reference" id="cite_ref-holtz2004_1-5">[2] More recent, unpublished work by Holtz agrees with the original assessment.<sup class="reference" id="cite_ref-holtzdml2005_7-0">[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).<sup class="reference" id="cite_ref-currieetal2003_4-3">[5] <sup class="reference" id="cite_ref-currie2003b_8-0">[9]

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 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.<sup class="reference" id="cite_ref-matthewbrown1922_9-0">[10] Gorgosaurus libratus was formally reassigned to Albertosaurus (as Albertosaurus libratus) by Dale Russell in 1970,<sup class="reference" id="cite_ref-russell1970_0-3">[1] and many subsequent authors followed his lead.<sup class="reference" id="cite_ref-carretal2005_6-2">[7] <sup class="reference" id="cite_ref-paul1988_10-0">[11] Combining the two greatly expands the geographical and chronological range of the genus Albertosaurus. Other experts maintain the two genera as separate.<sup class="reference" id="cite_ref-holtz2004_1-6">[2] 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.<sup class="reference" id="cite_ref-currie2003a_3-2">[4]

Discovery and naming
Gorgosaurus libratus was first described by Lawrence Lambe in 1914.<sup class="reference" id="cite_ref-lambe1914a_11-0">[12] <sup class="reference" id="cite_ref-lambe1914b_12-0">[13] Its name is derived from the Greek γοργος/gorgos ("fierce" or "terrible") and σαυρος/saurus ("lizard").<sup class="reference" id="cite_ref-liddellscott_13-0">[14] The type species is G. libratus; the specific epithet "balanced" is the past participle of the Latin verb librare, meaning "to balance".<sup class="reference" id="cite_ref-lambe1914b_12-1">[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.<sup class="reference" id="cite_ref-lambe1914a_11-1">[12] It was found in the Dinosaur Park Formation of Alberta and is housed in the Canadian Museum of Nature in Ottawa.<sup class="reference" id="cite_ref-currie2003a_3-3">[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.<sup class="reference" id="cite_ref-matthewbrown1923_5-1">[6]

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.<sup class="reference" id="cite_ref-matthewbrown1923_5-2">[6] Today's paleontologists regard this specimen as a juvenile G. libratus.<sup class="reference" id="cite_ref-holtz2004_1-7">[2] <sup class="reference" id="cite_ref-currie2003a_3-4">[4] Dozens of other specimens have been excavated from the Dinosaur Park Formation and are housed in museums across the United States and Canada.<sup class="reference" id="cite_ref-russell1970_0-4">[1] <sup class="reference" id="cite_ref-currie2003a_3-5">[4] G. libratus is the best-represented tyrannosaurid in the fossil record, known from a virtually complete growth series.<sup class="reference" id="cite_ref-holtz2004_1-8">[2] <sup class="reference" id="cite_ref-carr1999_14-0">[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, and Leidy gave them the name Deinodon.<sup class="reference" id="cite_ref-leidy1856_15-0">[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.<sup class="reference" id="cite_ref-matthewbrown1922_9-1">[10] 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.<sup class="reference" id="cite_ref-russell1970_0-5">[1] <sup class="reference" id="cite_ref-carr1999_14-1">[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 new species.<sup class="reference" id="cite_ref-currie2003a_3-6">[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.<sup class="reference" id="cite_ref-natlgeo2003_16-0">[17] <sup class="reference" id="cite_ref-tcmigorgo_17-0">[18]

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 younger, late Maastrichtian-age Hell Creek Formation of Montana, was named Gorgosaurus lancensis by Charles Whitney Gilmore in 1946.<sup class="reference" id="cite_ref-gilmore1946_18-0">[19] This specimen was renamed Nanotyrannus by Bob Bakker and colleagues in 1988.<sup class="reference" id="cite_ref-bakkeretal1988_19-0">[20] Today, most paleontologists recognize Nanotyrannus as a juvenile Tyrannosaurus rex.<sup class="reference" id="cite_ref-holtz2004_1-9">[2] <sup class="reference" id="cite_ref-carr1999_14-2">[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.<sup class="reference" id="cite_ref-maleev1955b_20-0">[21] Ken Carpenter renamed the smaller specimen Maleevosaurus novojilovi in 1992,<sup class="reference" id="cite_ref-carpenter1992_21-0">[22] but both are now considered juveniles of Tarbosaurus bataar.<sup class="reference" id="cite_ref-holtz2004_1-10">[2] <sup class="reference" id="cite_ref-carr1999_14-3">[15] <sup class="reference" id="cite_ref-rozhdestvensky1965_22-0">[23]

Coexistence with Tyrannosaurus torosus
In the Dinosaur Park Formation, Gorgosaurus lived alongside a rarer species of the tyrannosaurine Tyrannosaurus torosus or Daspletosaurus. This is one of the few examples of two tyrannosaur genera coexisting. Similar-sized predators in modern predator guilds are separated into different ecological niches by anatomical, behavioral or geographical differences that limit competition. Niche differentiation between the Dinosaur Park tyrannosaurids is not well-understood.<sup class="reference" id="cite_ref-farlowpianka2002_23-0">[24] In 1970, Dale Russell hypothesized the more common Gorgosaurus actively hunted fleet-footed hadrosaurs, while the rarer and more troublesome ceratopsians and ankylosaurians (horned and heavily armoured dinosaurs) were left to the more heavy-built Daspletosaurus.<sup class="reference" id="cite_ref-russell1970_0-6">[1] However, a specimen of Daspletosaurus (OTM 200) from the contemporaneous Two Medicine Formation of Montana preserves the digested remains of a juvenile hadrosaur in its gut region.<sup class="reference" id="cite_ref-varricchio2001_24-0">[25]

Unlike some other groups of dinosaurs, neither genus was more common at higher or lower elevations than the other.<sup class="reference" id="cite_ref-farlowpianka2002_23-1">[24] 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.<sup class="reference" id="cite_ref-holtz2004_1-11">[2]

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. 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.<sup class="reference" id="cite_ref-ericksonetal2004_25-0">[26]

Gorgosaurus spent as much as half its life in the juvenile phase before ballooning up to near-maximum size in only a few years.<sup class="reference" id="cite_ref-ericksonetal2004_25-1">[26] 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 dragons, where hatchlings start off as tree-dwelling insectivores and slowly mature into massive apex predators capable of taking down large vertebrates.<sup class="reference" id="cite_ref-holtz2004_1-12">[2] Other tyrannosaurids, including Albertosaurus, have been found in aggregations that some have suggested to represent mixed-age packs, but there is no evidence of gregarious behavior in Gorgosaurus.<sup class="reference" id="cite_ref-tankecurrie1998_26-0">[27] <sup class="reference" id="cite_ref-currieetal2005_27-0">[28]

Paleoecology
All known specimens of Gorgosaurus libratus have been recovered from the Dinosaur Park Formation in Alberta.<sup class="reference" id="cite_ref-currie2003a_3-7">[4] This formation dates to the middle of the Campanian, between 76.5 and 74 million years ago.<sup class="reference" id="cite_ref-eberthhamblin1993_28-0">[29] 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 floodplains along the coast.<sup class="reference" id="cite_ref-englishjohnston2004_29-0">[30] <sup class="reference" id="cite_ref-eberth1997_30-0">[31] 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. Conifers formed the forest canopy, while the understory plants consisted of ferns, tree ferns and angiosperms.<sup class="reference" id="cite_ref-bramankoppelhus2005_31-0">[32] 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 massive Bearpaw Shale.<sup class="reference" id="cite_ref-eberth1997_30-1">[31]

The Dinosaur Park Formation preserves a great wealth of vertebrate fossils. A wide variety of fish swam the rivers and estuaries, including gars, sturgeons, sharks and rays, among others. Frogs, salamanders, turtles, crocodilians and champsosaurs also dwelled in the aquatic habitats. Azhdarchid pterosaurs and neornithine birds like Apatornis flew overhead, while the enantiornithine bird Avisaurus lived on the ground alongside multituberculate, marsupial and placental mammals. A number of species of terrestrial lizards were also present, including whiptails, skinks, monitors and alligator lizards. Dinosaur fossils in particular are found with unrivaled abundance and diversity. Huge herds of ceratopsids roamed the floodplains alongside equally large groups of hadrosaurine and lambeosaurine hadrosaurs. Other herbivorous groups like ornithomimids, therizinosaurs, pachycephalosaurs, small ornithopods, nodosaurids and ankylosaurids were also represented. Small predatory dinosaurs like oviraptorosaurs, troodonts and dromaeosaurs hunted smaller prey than the huge tyrannosaurids, Daspletosaurus and Gorgosaurus, which were two orders of magnitude larger in mass.<sup class="reference" id="cite_ref-eberth1997_30-2">[31] Intervening predatory niches may have been filled by young tyrannosaurids.<sup class="reference" id="cite_ref-russell1970_0-7">[1] <sup class="reference" id="cite_ref-holtz2004_1-13">[2] <sup class="reference" id="cite_ref-farlow1976_32-0">[33] Retrieved from "http://tyrannopedia.wikia.com/wiki/Albertosaurus"