Albertosaurus
Albertosaurus is a genus of large tyrannosaurid theropod dinosaur that lived in northwestern North America during the early to middle Maastrichtian age of the Late Cretaceous period, about 71 million years ago. The type species, A. sarcophagus, was apparently restricted in range to the modern-day Canadian province of Alberta, after which the genus is named, although an indeterminate species has been discovered in the Corral de Enmedio and Packard Formations of Mexico. Scientists disagree on the content of the genus and some recognize Gorgosaurus libratus as a second species.
As a tyrannosaurid, Albertosaurus was a bipedal predator with short arms, two-fingered hands, and a massive head with dozens of large, sharp teeth, a strong sense of smell, powerful binocular vision, and a bone crushing bite force. It may have even been the apex predator in its local ecosystem. While Albertosaurus was certainly large for a theropod, it was still much smaller than its larger and more famous relative Tyrannosaurus rex, growing up to in length and weighing.
Since the first discovery in 1884, fossils of more than 30 individuals have been recovered that provide scientists with a more detailed knowledge of Albertosaurus anatomy than what is available for most other tyrannosaurids. The discovery of 26 individuals in one particular site provides evidence of gregarious behavior and allows for studies of ontogeny and population biology. These are near impossible with lesser-known dinosaurs because their remains are rarer and more fragmentary when compared to those of Albertosaurus.
History of discovery
Naming
Albertosaurus was named by Henry Fairfield Osborn in a one-page note at the end of his 1905 description of Tyrannosaurus rex. Its namesake is Alberta, the Canadian province established the very same year where the first remains were found. The generic name also incorporates the Greek word σαυρος/sauros, meaning "lizard", which is the most common suffix in dinosaur names. The type species is Albertosaurus sarcophagus and the specific name is derived from the Ancient Greek term σαρκοφάγος, meaning "flesh-eating", and having the same etymology as the funeral container with which it shares its name, which is a combination of the Greek words σαρξ/sarx and φαγειν/phagein. More than 30 specimens of all ages are known to science.Early discoveries
The type specimen is a partial skull collected on June 9, 1884, from an outcrop of the Horseshoe Canyon Formation alongside the Red Deer River in Alberta. It was recovered by an expedition of the Geological Survey of Canada, led by the famous geologist Joseph Burr Tyrrell. Due to a lack of specialised equipment, the almost complete skull could only be partially secured. In 1889, Tyrrell's colleague Thomas Chesmer Weston found an incomplete smaller skull associated with some skeletal material at a location nearby. The two skulls were assigned to the preexisting species Laelaps incrassatus by Edward Drinker Cope in 1892. Although the name Laelaps was preoccupied by a genus of mite and had been changed to Dryptosaurus in 1877 by Othniel Charles Marsh, Cope stubbornly refused to recognize the new name created by his archrival. However, Lawrence Lambe used the name Dryptosaurus incrassatus instead of Laelaps incrassatus when he described the remains in detail in 1903 and 1904, which was a combination first coined by Oliver Perry Hay in 1902.Shortly later, Osborn pointed out that D. incrassatus was based on generic tyrannosaurid teeth, so the two Horseshoe Canyon skulls could not be confidently referred to that species. The Horseshoe Canyon skulls also differed markedly from the remains of D. aquilunguis, type species of Dryptosaurus, so Osborn gave them the new name Albertosaurus sarcophagus in 1905. He did not describe the remains in any great detail, citing Lambe's complete description the year before. Both specimens, the holotype CMN 5600 and the paratype CMN 5601, are stored in the Canadian Museum of Nature in Ottawa. By the early twenty-first century, some concerns had arisen that, due to the damaged state of the holotype, Albertosaurus might be a nomen dubium that could only be used for the type specimen itself because other fossils could not reliably be assigned to it. However, in 2010, Thomas Carr established that the holotype, the paratype, and comparable later finds all shared a single common unique trait, or autapomorphy. The possession of an enlarged pneumatic opening in the back rim of the side of the palatine bone proves that Albertosaurus is a valid taxon.
Dry Island bone bed
On August 11, 1910, American paleontologist Barnum Brown discovered 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 that they could identify in the bone bed. 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. Palaeontologist Philip J. Currie of 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. 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 long. None of these individuals are known from complete skeletons and most are represented by remains in both museums. Excavations continued until 2008, when the minimum number of individuals present had been established at 12 and at 26 if mirrored elements were counted when differing in size due to ontogeny. A total of 1,128 Albertosaurus bones had been secured, which is the largest concentration of large theropod fossils known from the Cretaceous.Other discoveries
In 1911, Barnum Brown, during the second year of the American Museum of Natural History's operations in Alberta, uncovered a fragmentary partial Albertosaurus skull at the Red Deer River near Tolman Bridge.William Parks described a new species in 1928, Albertosaurus arctunguis, based on a partial skeleton lacking a skull that was excavated by Gus Lindblad and Ralph Hornell near the Red Deer River in 1923, but this species has been considered identical to A. sarcophagus since 1970. Parks' specimen is housed in the Royal Ontario Museum in Toronto.
No Albertosaurus fossils were found from 1926 to 1972, but there has been an increase in findings since then. Apart from the Dry Island bonebed, six more skulls and skeletons have since been discovered in Alberta and are housed in various Canadian museums. Specimen RTMP 81.010.001 was found in 1978 by amateur paleontologist Maurice Stefanuk. RTMP 85.098.001 was found by Stefanuk on June 16, 1985. RTMP 86.64.001 was found in December 1985. RTMP 86.205.001 was found in 1986. RTMP 97.058.0001 was found in 1996 and then there is CMN 11315. Unfortunately, none of these skeletons were found with complete skulls. Fossils have also been reported from the American states of Montana, New Mexico, Wyoming, and Missouri, but they are doubted to be from A. sarcophagus and may not even belong to the genus Albertosaurus.
Two specimens from "cf Albertosaurus ".sp" have been found in Mexico.
''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. Other specimens were later found in Alberta and the US state of Montana. Finding no significant differences to separate the two taxa, 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. A species distinction was maintained because of the age difference. The addition extended the temporal range of the genus Albertosaurus earlier by several million years and its geographic range southwards by hundreds of kilometres.In 2003, Philip J. Currie, benefiting from much more extensive finds and a general increase in anatomical knowledge of theropods, compared several tyrannosaurid skulls and came to the conclusion that the two species are more distinct than previously thought. As the two species are sister taxa, they are more closely related to each other than to any other species of tyrannosaurid. Recognizing this, Currie nevertheless recommended that Albertosaurus and Gorgosaurus be kept as separate genera, as he concluded that they were no more similar than Daspletosaurus and Tyrannosaurus, which are almost always separated. In addition to this, several albertosaurine specimens have been recovered from Alaska and New Mexico. Currie suggested that the Albertosaurus-''Gorgosaurus'' situation may be clarified once these are fully described. Most authors have followed Currie's recommendation, but some have not.
Other species
In 1930, Anatoly Nikolaevich Riabinin named Albertosaurus pericolosus based on a tooth from China that probably belonged to Tarbosaurus. In 1932, Friedrich von Huene renamed Dryptosaurus incrassatus, not considered a nomen dubium by him, to Albertosaurus incrassatus. Because he had identified Gorgosaurus with Albertosaurus, in 1970, Russell also renamed Gorgosaurus sternbergi into Albertosaurus sternbergi and Gorgosaurus lancensis into Albertosaurus lancensis. The former species is today seen as a juvenile form of Gorgosaurus libratus and the latter is seen as either identical to Tyrannosaurus or representing a separate genus, Nanotyrannus. In 1988, Gregory S. Paul based Albertosaurus megagracilis on a small tyrannosaurid skeleton, specimen LACM 28345, from the Hell Creek Formation of Montana. It was renamed Dinotyrannus in 1995, but is now thought to represent a juvenile Tyrannosaurus rex. Also in 1988, Paul renamed Alectrosaurus olseni into Albertosaurus olseni, but this has found no general acceptance. In 1989, Gorgosaurus novojilovi was renamed by Bryn Mader and Robert Bradley as Albertosaurus novojilovi.On two occasions, species based on valid Albertosaurus material were reassigned to a different genus, Deinodon. In 1922, William Diller Matthew renamed A. sarcophagus into Deinodon sarcophagus. In 1939, German paleontologist Oskar Kuhn renamed A. arctunguis into Deinodon arctunguis.