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Albertosaurus

2007 Schools Wikipedia Selection. Related subjects: Dinosaurs

   iAlbertosaurus

                        Fossil range: Late Cretaceous

               Scientific classification

   Kingdom:    Animalia
   Phylum:     Chordata
   Class:      Sauropsida
   Superorder: Dinosauria
   Order:      Saurischia
   Suborder:   Theropoda
   Family:     Tyrannosauridae
   Subfamily:  Albertosaurinae
   Genus:      Albertosaurus
   Species:    A. sarcophagus

                                Binomial name

   Albertosaurus sarcophagus
   Osborn, 1905

                                  Synonyms

   A. arctunguis Parks, 1928

   Albertosaurus ( IPA: [æl,bɜr.toʊ'soʊr.əs]; 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 number of species represented in the
   genus, recognizing either one or two species.

   As a tyrannosaurid, Albertosaurus was a bipedal predator with a massive
   head, jaws lined with dozens of large teeth and tiny, two-fingered
   'hands'. 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 only as much as a modern black rhinoceros.

   Fossils of more than twenty individuals have been recovered, providing
   scientists with a more detailed knowledge of Albertosaurus anatomy than
   is available for other tyrannosaurids. The discovery of ten individuals
   at one site provides evidence of pack behaviour and allows studies of
   developmental biology which are impossible with lesser-known animals.

Description

   Albertosaurus was smaller than the truly gigantic tyrannosaurids like
   Tarbosaurus and Tyrannosaurus. Adults measured approximately 9 m
   (30 ft) long. Several independent mass estimates, obtained by different
   methods, suggest that a full-grown Albertosaurus weighed between 1.282
   tonnes (1.4  short tons) and 1.685 tonnes (1.85 short tons).

   The massive skull of Albertosaurus, perched on a short, S-shaped neck,
   was approximately 1 m (3.3 ft) long in the largest adults. Wide
   openings in the skull ( fenestrae) reduced the potential weight of the
   head, and provided areas for muscle attachment and sensory organs. Its
   long jaws contained more than 60 banana-shaped teeth; larger
   tyrannosaurids possessed fewer teeth. Unlike most other theropods,
   tyrannosaurids were heterodont; the teeth took 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.

   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 hindlimbs were long and ended in a
   four-toed foot. The first of these digits, called the hallux, was very
   short and only the other three contacted the ground, with the middle
   digit longer than the rest.

Taxonomy

   Albertosaurus was named in 1905 by Henry Fairfield Osborn of the
   American Museum of Natural History in a very brief note at the end of
   his description of Tyrannosaurus rex. The name honours Alberta, the
   Canadian province in which the first remains were found. The generic
   name also incorporates the Greek term σαυρος (sauros, meaning
   "lizard"), the most common suffix in dinosaur names.
   An albertosaurine skull in the midst of preparation, discovered in New
   Mexico. This undescribed species may belong to Albertosaurus.
   Enlarge
   An albertosaurine skull in the midst of preparation, discovered in New
   Mexico. This undescribed species may belong to Albertosaurus.

Classification

   Albertosaurus is a member of the theropod family Tyrannosauridae.
   Within this family, Albertosaurus sarcophagus is usually classified
   with Gorgosaurus libratus (sometimes called Albertosaurus libratus; see
   below) in the subfamily Albertosaurinae. Albertosaurines were more
   slender than the robust tyrannosaurines, the other major subfamily of
   tyrannosaurids. Appalachiosaurus has been called an albertosaurine in
   at least one study, although this is disputed.

Albertosaurus sarcophagus

   The type species of Albertosaurus is A. sarcophagus, also named by
   Osborn in 1905. The name 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, meaning "flesh") and Φαγειν
   (phagein, meaning "to eat"). More than twenty specimens of all ages are
   known to science.

Gorgosaurus libratus

   In 1913, paleontologist Charles Hazelius Sternberg recovered another
   tyrannosaurid skeleton from slightly older sediments in Alberta. This
   dinosaur was named Gorgosaurus libratus in 1914 by Lawrence Lambe.
   Finding few differences to separate the two genera, Dale Russell
   declared Gorgosaurus a junior synonym of Albertosaurus in 1970,
   creating the new combination Albertosaurus libratus. This extended the
   temporal range of the genus backwards by several million years and its
   geographic range southwards by hundreds of kilometers.

   More recent examination of Albertosaurus and Gorgosaurus has cast 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 distinct genera should be retained, although they acknowledged 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 has recommended leaving the two genera separate until more of
   this diversity is clarified. Most authors have since followed Currie's
   recommendation, but some have not.

Invalid species

   Other species of Albertosaurus have been named but later invalidated.
   William Parks described a partial skeleton from Alberta as
   Albertosaurus arctunguis in 1928, but this is universally considered a
   junior synonym of A. sarcophagus. Albertosaurus megagracilis (later
   renamed Dinotyrannus) was based on a small tyrannosaurid skeleton from
   the Hell Creek Formation of Montana in the United States. However, it
   is now thought to be a juvenile Tyrannosaurus.

History of discovery

   The type specimen is a partial skull, collected in 1884 from an outcrop
   alongside the Red Deer River in Alberta. This specimen, along with an
   additional smaller skull and some skeletal material, was recovered by
   an expedition of the Geological Survey of Canada, led by the famous
   Canadian geologist Joseph B. Tyrrell. The specimen is now stored in the
   Canadian Museum of Nature. The two skulls were assigned to the existing
   species Laelaps incrassatus by Edward Drinker Cope in 1892. However, as
   early as 1877 the name Laelaps had been found to be preoccupied by a
   genus of mite, and it was changed to Dryptosaurus by Cope's rival,
   Othniel Charles Marsh. (Cope refused to recognize the new name.)
   Lawrence Lambe moved Laelaps incrassatus to the genus Dryptosaurus in
   1904. Finally, because D. incrassatus was only based on generic
   tyrannosaurid teeth that could not be said to belong to any particular
   species, and because the Alberta skull material differed markedly from
   that of Dryptosaurus, Osborn named it Albertosaurus sarcophagus in
   1905.
   The Red Deer River near Drumheller, Alberta. More than half of all
   Albertosaurus remains have been discovered alongside the river, in
   outcrops like the ones on either side of this picture.
   Enlarge
   The Red Deer River near Drumheller, Alberta. More than half of all
   Albertosaurus remains have been discovered alongside the river, in
   outcrops like the ones on either side of this picture.

   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 bones from all of the individuals present. Among many other
   bones deposited in the American Museum of Natural History 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. The Royal Tyrrell Museum of Palaeontology rediscovered the
   site in 1997 and resumed fieldwork there. This further excavation
   turned up a tenth, very young individual in 2002.

   The specimen originally named A. arctunguis was also excavated near the
   Red Deer River and is housed in the Royal Ontario Museum in Toronto,
   Canada. Six more skulls and skeletons have since been discovered in
   Alberta and are housed in other Canadian museums.

   All identifiable fossils of Albertosaurus sarcophagus are known from
   the Horseshoe Canyon Formation in Alberta. This formation dates to the
   early Maastrichtian stage of the Late Cretaceous period, 70 to 73
   million years ago. Many other dinosaurs have been found there,
   including smaller theropods like Ornithomimus, Chirostenotes and
   several dromaeosaurids, and 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 probably do not
   represent A. sarcophagus and may not even belong to the Albertosaurus
   genus.

Paleobiology

   Like many tyrannosaurids, Albertosaurus is well known from several
   different specimens, allowing detailed studies of its life history and
   providing evidence of its behaviour.

Growth pattern

   A graph showing the hypothesized growth curves (body mass versus age)
   of four tyrannosaurids. Albertosaurus is drawn in red. Based on
   Erickson et al. 2004.
   Enlarge
   A graph showing the hypothesized growth curves (body mass versus age)
   of four tyrannosaurids. Albertosaurus is drawn in red. Based on
   Erickson et al. 2004.

   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 to other species. The youngest known
   Albertosaurus is a two-year-old, recently found in the re-opened
   bonebed quarry, and would have weighed about 50 kg (110 lb). A
   24-year-old specimen from the collection of the Royal Tyrrell Museum of
   Palaeontology is the oldest known, weighing approximately 1.14 tonnes
   (1.25 tons), although the largest is a 1.28-tonne (1.4-ton) individual
   in the American Museum of Natural History collection. This specimen is
   estimated to have been 22 years old when it died. When specimens of
   intermediate age and size are considered, an S-shaped growth curve
   results, with the most rapid growth occurring in a four-year period
   ending around the sixteenth year of life, as seen in other
   tyrannosaurids. The growth rate during this phase was 122 kg (268 lb)
   per year. Other, similarly-sized tyrannosaurids showed a similar growth
   rate, but the rate is much lower than that of Tyrannosaurus, which grew
   almost eight times faster (767 kg [1687 lb] per year) during its growth
   phase. At about 16 years of age, skeletal maturity—marked by the end of
   the rapid growth phase—appears to have arrived in Albertosaurus,
   although growth continued at a slower rate throughout the animals'
   lives.

Pack behaviour

   The Albertosaurus bonebed discovered by Barnum Brown and his crew
   contains the remains of at least 10 individuals. This group seems to be
   composed of two or three full-grown individuals over 21 years of age;
   one younger adult about 17 years old; four sub-adults 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.

   The near-absence of herbivore remains and the similar state of
   preservation between the many individuals at the Albertosaurus bonebed
   quarry led 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, providing evidence of pack behaviour.
   Other scientists are skeptical, observing that the animals may have
   been driven together by flood conditions or for other reasons.

   There is abundant evidence for gregarious behaviour among herbivorous
   dinosaurs, including ceratopsians and hadrosaurs. However, only rarely
   are so many dinosaurian predators found at the same site. Small
   theropods like Coelophysis bauri, Deinonychus antirrhopus, and
   Megapnosaurus (Syntarsus) rhodesiensis have been found in aggregations,
   as have larger predators like Allosaurus fragilis and Mapusaurus
   roseae. There is some evidence of gregarious behaviour in other
   tyrannosaurids as well. Fragmentary remains of smaller individuals were
   found alongside "Sue," the Tyrannosaurus rex mounted in the Field
   Museum of Natural History in Chicago. A bonebed in the Two Medicine
   Formation of Montana contains at least three specimens of an unnamed
   species of Daspletosaurus, preserved alongside several hadrosaurs.
   These findings may corroborate the evidence for social behaviour in
   Albertosaurus, although some or all of the above localities may
   represent temporary or unnatural aggregations.

   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, which may have
   included ceratopsians and hadrosaurs. 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. However, as the preservation of behaviour in the fossil record
   is exceedingly rare, this idea cannot readily be tested.
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