Albertosaurus (al-BUR-tuh-SAW-rus)
Albertosaurus (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 apparently 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 behaviour and allows studies of ontogeny and population biology which are impossible with lesser-known dinosaurs.
Description
Albertosaurus was smaller than some other tyrannosaurids, such as Tarbosaurus and Tyrannosaurus. Typical adults of Albertosaurus measured up to 9 metres (30 ft) long, while rare individuals of great age could grow to over 10 metres (33 ft) in length. Several independent mass estimates, obtained by different methods, suggest that an adult Albertosaurus weighed between 1.3 tonnes (1.4 short tons) and 1.7 tonnes (1.9 tons).
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. Albertosaurus may have been able to reach speeds of 14−21 kilometres per hour (8−13 miles per hour).
Discovery and naming
Naming
Albertosaurus was named by Henry Fairfield Osborn in a one-page note at the end of his 1905 description of Tyrannosaurus rex.:265 The name honours Alberta, the present-day 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"). More than thirty specimens of all ages are known to science.
Early discoveries
Almost three-quarters of all Albertosaurus remains have been discovered alongside the river, in outcrops like the ones on either side of this picture.The type specimen is a partial skull, collected in 1884 from an outcrop of the Horseshoe Canyon Formation alongside the Red Deer River, in present-day 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, 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.
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 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.
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. 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 metres (33 ft) in length. None of these individuals are known from complete skeletons, and most are represented by remains in both museums.
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 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 kilometres.
In 2003, Philip J. 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. Most authors have followed Currie's recommendation, but some have not.
Other discoveries
William Parks described a new species, Albertosaurus arctunguis, based on a partial skeleton excavated near the Red Deer River in 1928, but this species has been considered identical to A. sarcophagus since 1970. 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.
Albertosaurus megagracilis was based on a small tyrannosaurid skeleton from the Hell Creek Formation of Montana. It was renamed Dinotyrannus in 1995, but is now thought to represent a juvenile Tyrannosaurus rex.
No comments:
Post a Comment