Albertosaurus
| Albertosaurus Temporal range: Late Cretaceous
| |
|---|---|
| File:Albertosaurus model.jpg | |
| Scientific classification | |
| Kingdom: | |
| Phylum: | |
| Class: | |
| Superorder: | |
| Order: | |
| Suborder: | |
| Family: | |
| Genus: | Albertosaurus
|
| Species: | A. sarcophagus
|
| Binomial name | |
| Albertosaurus sarcophagus Osborn, 1905
| |
| Synonyms | |
|
A. arctunguis Parks, 1928 | |
Albertosaurus (IPA pronunciation: [æl,bɜr.to'soʊr.əs]; "Alberta lizard") is a genus of tyrannosaurid theropod dinosaur, which lived in western North America during the the Late Cretaceous Period, over 70 million years ago. Scientists currently disagree on the number of species represented within the genus, recognizing either one or two species. The genus was restricted to the modern-day Canadian province of Alberta, if only the type species is considered but may have ranged farther north and south, if additional species are recognized.
As a tyrannosaur, Albertosaurus was a bipedal predator with a massive head, lined with dozens of sharp teeth, and tiny, two-fingered hands. Like other tyrannosaurs, this animal may have been the top carnivore in its environment. Although relatively large for a theropod, Albertosaurus was much smaller than the better-known Tyrannosaurus, probably weighing only as much as a modern black rhinoceros, albeit taller and longer.
Fossils of over 20 individuals have been recovered, making Albertosaurus one of the most completely known tyrannosaurs. Ten of these individuals were found at a single site, providing evidence of possible pack behavior and allowing studies of developmental biology.
Description
Albertosaurus was a smaller tyrannosaur than truly gigantic predators such as Tyrannosaurus and Tarbosaurus. Adults measured approximately 9 meters (30 ft) long.[1][2] Several independent mass estimates have been obtained using different methods, suggesting that a full-grown Albertosaurus weighed between 1.282 tonnes (1.4 tons)[3] and 1.685 tonnes (1.85 tons).[4]
The massive skull of Albertosaurus, perched on a short, S-shaped neck, was approximately 1 meter (3.3 ft) long in the largest adults.[5] Large openings (fenestrae) in the skull reduced its weight and provided areas for muscle attachment and sensory organs. Long jaws contained a total of over 60 banana-shaped teeth, more than in larger tyrannosaurs such as Tyrannosaurus and Tarbosaurus. Like other tyrannosaurs but unlike most other theropods, these teeth were formed differently in different parts of the mouth, with the premaxillary teeth much smaller, more closely-packed and D-shaped in cross-section.[2]
All tyrannosaurs, including Albertosaurus, shared the same basic body plan. Typically for theropods, they were bipedal and balanced the heavy head and torso with a long tail. However, the 'arms' of tyrannosaurs were extremely small for their body size and retained only two digits. The hindlimbs were long and ended in a four-toed foot. Of the four digits on the hindlimb, the first digit (the hallux) was very short and only the latter three contacted the ground, with the third (middle) digit being longer than the rest.[2]
Taxonomy
Albertosaurus was named by Henry Fairfield Osborn of the American Museum of Natural History (AMNH) 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 fossil remains were found. Coincidentally, Alberta was granted provincial status the same year. The generic name also incorporates the Greek term σαυρος (sauros; "lizard"), the most common suffix used in dinosaur names.[6]
Classification
Albertosaurus is a member of the theropod family Tyrannosauridae. Within this family, Albertosaurus is usually considered to be most closely related to Gorgosaurus libratus (sometimes called Albertosaurus libratus; see below), and the two species are often classified together in the subfamily Albertosaurinae.[7] Albertosaurines were more slender than the robust tyrannosaurines, the other major subfamily of tyrannosaurids. Appalachiosaurus has also been placed as an albertosaurine in at least one study,[2] although this is disputed.[8]
Albertosaurus sarcophagus
The type species of Albertosaurus is A. sarcophagus, also named by Osborn in 1905. This name has the same etymology as the funeral container of the same name: a combination of the Greek words σαρξ (sarx; "flesh") and Φαγειν (phagein; "to eat"). This refers to the dinosaur's quite obvious carnivorous habits.[6] Over 20 specimens are known to science, from very young juveniles to full-grown adults.[9]
?Albertosaurus libratus
In 1913, paleontologist Charles Hazelius Sternberg recovered another tyrannosaur skeleton from slightly older sediments in Alberta. This dinosaur was named Gorgosaurus libratus in 1914 by Lawrence Lambe.[10] Finding very few differences to separate the two genera, Dale Russell declared Gorgosaurus a junior synonym of Albertosaurus in 1970, creating the new combination Albertosaurus libratus and extending the temporal range of the genus backwards by several million years and its geographic range southwards by hundreds of kilometers.[1]
More recent examination of Albertosaurus and Gorgosaurus has cast some doubt on Russell's proposed synonymy. In 2003, Phil Currie and colleagues examined skulls of the two species and came to the conclusion that the two species do not belong to the same genus, but that two separate genera should be retained. Currie does acknowledge that the two genera are sister taxa and that the distinction is therefore rather arbitrary. However, according to Currie, Albertosaurus and Gorgosaurus are no more similar than Daspletosaurus and Tyrannosaurus, which are almost always retained as separate genera. In addition, several undescribed albertosaurine specimens have been recovered from other parts of North America, including Alaska and New Mexico, so Currie recommends leaving the two genera separate until more of this diversity is explained.[9] Most authors have since followed Currie's recommendation,[2][3] while others have not.[8]
Invalid species
Other species of Albertosaurus have been named and later invalidated. William Parks described a partial skeleton from Alberta as A. arctunguis in 1928,[11] but this is universally considered a junior synonym of A. sarcophagus.[1] A. megagracilis (later renamed Dinotyrannus[12]) was based on a small tyrannosaur skeleton from the Hell Creek Formation of Montana in the United States.[13] However, it is now thought to be a juvenile Tyrannosaurus.[5]
History of discovery
The holotype or original specimen, is a partial skull collected in 1884 from what is now known as the Horseshoe Canyon Formation of Alberta. This specimen, along with an additonal smaller skull and some skeletal material, was recovered by a Geological Survey of Canada team led by famous Canadian geologist Joseph B. Tyrrell and is now stored in the Canadian Museum of Nature. The two skulls were referred to the species Laelaps incrassatus by Edward Drinker Cope in 1892.[14] Several years later, the name was changed to Dryptosaurus incrassatus as Laelaps was found to be preoccupied by a genus of mite. Finally, realizing that the skulls were not definitively referrable to the species D. incrassatus, which was based solely on non-diagnostic tyrannosaur teeth nor even to the genus Dryptosaurus, Osborn created the name Albertosaurus sarcophagus for the material in 1905.[6]
In 1910, American paleontologist Barnum Brown uncovered the remains of a large group of Albertosaurus at a quarry alongside the Red Deer River in Alberta. Due to the large number of bones and the limited amount of time available, Brown's party did not collect every specimen, but made sure to collect bones from all the individuals present. Among many other bones deposited in the AMNH collections in New York City, seven sets of right metatarsals were collected, along with two isolated toe bones that did not match any of the metatarsals in size. This indicates the presence of at least nine individuals in the quarry. A team from the Royal Tyrrell Museum of Palaeontology (RTMP) relocated the site in 1997 and the museum continues to excavate the quarry.[15] This further excavation turned up a tenth, very young individual in 2002.[3]
The specimen originally named A. arctunguis was also excavated near the Red Deer River and is now housed in the Royal Ontario Museum in Ottawa.[11] Six other relatively complete skulls and skeletons have since been discovered and are housed in Canadian museums.[9]
All firmly identifiable fossils of Albertosaurus sarcophagus are known from the Horseshoe Canyon Formation in Alberta. This formation is from the early Maastrichtian stage of the Late Cretaceous Period, about 73 to 70 million years ago. Many other dinosaurs have been found there, including smaller theropods like Ornithomimus, Chirostenotes and several dromaeosaurids, as well as a wide variety of herbivores like ankylosaurians, ceratopsians, pachycephalosaurs and hadrosaurids. Fossils of Albertosaurus have also been reported from the American states of Montana, New Mexico, and Wyoming but these are not recognizable as A. sarcophagus and may not even belong to Albertosaurus.[2][9]
Paleobiology
Like many tyrannosaurids, Albertosaurus is well known from several different specimens, allowing detailed studies of its life history to be performed.
Growth pattern
Most age categories of Albertosaurus are represented in the fossil record. Using bone histology, the age of individual dinosaurs can often be determined, allowing growth rates to be estimated and compared to other species. The youngest known individual is a two-year-old, recently found in the re-opened bonebed quarry, which would have weighed around 50 kilograms (110 lb). A 24-year-old specimen from the RTMP collection is the oldest known individual, weighing approximately 1.14 tonnes (1.25 ton), although the largest is a 1.28 tonne (1.4 ton) individual in the AMNH collection, which is estimated to be 22 years old. When specimens of intermediate age and size are considered, this results in an S-shaped growth curve, with the most rapid growth during a four-year period ending around the sixteenth year of life, as seen in other tyrannosaurs. The growth rate during this phase was 122 kilograms (268 lb) per year. This rate is close to that seen in other similar-sized tyrannosaurs, although it is several times lower than Tyrannosaurus, which grew at about 767 kilograms (1687 lb) per year, during its growth phase. Skeletal maturity, marked by the end of the rapid growth phase, appears to have arrived at around 16 years in Albertosaurus, although growth continued at a slower rate throughout the animals' lives.[3]
Pack behavior
The Albertosaurus bonebed discovered by Barnum Brown and his crew contains the remains of at least 10 individuals. The pack as whole seems to be composed of two or three full-grown individuals over 21 years of age, one younger adult around 17 years old, four subadults undergoing their rapid growth phases at between 12 and 16 years old and a ten-year-old juvenile, who had not yet reached the growth phase. A very young juvenile estimated at around two years of age was also present.[3]
Only rarely are so many predators found at the same fossil site, although several examples exist. Over a hundred Coelophysis bauri were found at the Ghost Ranch site in New Mexico, apparently congregating around a water source during a severe drought, while a large accumulation of its close relative Megapnosaurus (Syntarsus) rhodesiensis, numbering over 30 individuals, was also recovered from a single locality alongside the Chitake River in northern Zimbabwe.[16] Concerning larger theropods, the Cleveland Lloyd Dinosaur Quarry in Utah contains the remains of over 70 Allosaurus fragilis in what appears to have been a predator trap similar to the La Brea Tar Pits in California, and at least seven individuals of Mapusaurus roseae were found together in Argentina.[17] Tyrannosaurs have also been found in groups. Fragmentary remains of smaller individuals were found alongside "Sue," the Tyrannosaurus rex now mounted in the Field Museum of Natural History in Chicago and a bonebed containing three Daspletosaurus associated with several hadrosaurs was recently described from Montana, in the United States.[18]
The near-complete lack of herbivore remains and the similar state of preservation between the many different individuals at the Albertosaurus bonebed quarry led Currie to conclude that it was not a predator trap and that all of the preserved animals died at the same time, providing some of the best evidence of pack behavior among large carnivorous dinosaurs. The leg proportions of the smaller individuals were comparable to those of ornithomimids, probably among the fastest dinosaurs. Younger Albertosaurus were probably equally fleet-footed, much faster than their prey. Currie hypothesized that the younger members of the pack may have been responsible for driving the prey towards the larger and more powerful, but slower, adults.[15] However, other scientists are more skeptical, believing that the animals may have been driven together by flood conditions or for other reasons.[19]
References
- ^ a b c Russell, D.A. 1970. Tyrannosaurs from the Late Cretaceous of western Canada. National Museum of Natural Sciences Publications in Paleontology 1: 1-34.
- ^ a b c d e f Holtz, T.R. 2004. Tyrannosauroidea. In: Weishampel, D.B., Dodson, P. & Osmolska, H. (Eds.). The Dinosauria (2nd Edition). Berkeley: University of California Press. Pp. 111-136.
- ^ a b c d e Erickson, G.M., Makovicky, P.J., Currie, P.J., Norell, M.A., Yerby, S.A., & Brochu, C.A. 2004. Gigantism and comparative life-history parameters of tyrannosaurid dinosaurs. Nature 430: 772-775.
- ^ Christiansen, P. & Fariña, R.A. Mass prediction in theropod dinosaurs. Historical Biology 16: 85-92.
- ^ a b Currie, P.J. 2003a. Allometric growth in tyrannosaurids (Dinosauria: Theropoda) from the Upper Cretaceous of North America and Asia. Canadian Journal of Earth Sciences 40: 651-665.
- ^ a b c Osborn, H.F. 1905. Tyrannosaurus and other Cretaceous carnivorous dinosaurs. Bulletin of the American Museum of Natural History. 21: 259-265.
- ^ Currie, P.J., Hurum, J.H. & Sabath, K. 2003. Skull structure and evolution in tyrannosaurid phylogeny. Acta Palaeontologica Polonica 48(2): 227-234.
- ^ a b Carr, T.D., Williamson, T.E. & Schwimmer, D.R. 2005) A new genus and species of tyrannosauroid from the Late Cretaceous (middle Campanian) Demopolis Formation of Alabama. Journal of Vertebrate Paleontology 25(1): 119-143.
- ^ a b c d Currie, P.J. 2003b. Cranial anatomy of tyrannosaurids from the Late Cretaceous of Alberta. Acta Palaeontologica Polonica 48(2): 191-226.
- ^ Lambe, L.M. 1914. On a new genus and species of carnivorous dinosaur from the Belly River Formation of Alberta, with a description of the skull of Stephanosaurus marginatus from the same horizon. Ottawa Naturalist 28: 13-20.
- ^ a b Parks, W.A. 1928. Albertosaurus arctunguis, a new species of therapodous dinosaur from the Edmonton Formation of Alberta. University of Toronto Studies, Geological Series 25: 1-42.
- ^ Olshevsky, G. 1995. [The origin and evolution of the tyrannosaurids.] Kyoryugaku Saizensen [Dino Frontline] 9: 92–119. [in Japanese]
- ^ Paul, G.S. Predatory Dinosaurs of the World New York: Simon & Schuster. 464pp.
- ^ Cope, E.D. On the skull of the dinosaurian Laelaps incrassatus Cope. American Philosophical Society, Proceedings 30: 240-245.
- ^ a b Currie, P.J. 1998. Possible evidence of gregarious behavior in tyrannosaurids. Gaia 15: 271-277. [not printed until 2000]
- ^ Raath, M.A. Morphological variation in small theropods and its meaning in systematics: evidence from Syntarsus rhodesiensis. In: Carpenter, K. & Currie, P.J. Dinosaur Systematics: Approaches and Perspectives. Cambridge: Cambridge University Press. Pp. 91-105.
- ^ Coria, R.A. & Currie, P.J. 2006. A new carcharodontosaurid (Dinosauria, Theropoda) from the Upper Cretaceous of Argentina. Geodiversitas. 28(1): 71-118.
- ^ Currie, P.J., Trexler, D., Koppelhus, E.B., Wicks, K., & Murphy, N. 2005. An unusual multi-individual, tyrannosaurid bonebed in the Two Medicine Formation (Late Cretaceous, Campanian) of Montana (USA). In: Carpenter, K. (Ed.). The Carnivorous Dinosaurs. Bloomington: Indiana University Press. Pp. 313-324.
- ^ Eberth, D.A. & McCrea, R.T. 2001. Were large theropods gregarious? Journal of Vertebrate Paleontology 21(3): 46A. [published abstract only]
External Links
- A brief history of Joseph B. Tyrrell and his discovery of the holotype skull of Albertosaurus sarcophagus.
- Operation Dino-Lift, detailing the discovery, excavation, and preparation of a possible Albertosaurus from Bureau of Land Management land in New Mexico.