Albertosaurus

*Albertosaurus sacrophagus*

*Albertosaurus* (/ælˌbɜːrtəˈsɔːrəs/; meaning "Alberta lizard") is a genus of tyrannosaurid theropod dinosaurs that lived in western North America during the Late Cretaceous Period, about 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.

## Description

*Albertosaurus* was smaller than some other tyrannosaurids, such as *Tarbosaurus* and *Tyrannosaurus*. Typical *Albertosaurus* adults measured up to 9 m (30 ft) long and weighing over 3 tonnes.

*Albertosaurus* shared a similar body appearance with all other tyrannosaurids. Like all theropods, *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 on which the first digit, called the hallux, was short and did not reach the ground. The third digit was longer than the rest. *Albertosaurus* may have been able to reach walking speeds of 14−21 km/hour (8−13 mi/hour). At least for the younger individuals, a high running speed is plausible. This was probably to chase or pursue after tough prey, or to be nimble when confrontation. Its thick tail might've helped with balance, and forward facing eyes help the *Albertosaurus* in keeping its vision on its prey and not losing sight through the thick woodland environment the *Albertosaurus* is thought to have lived in.

Two skin impressions from *Albertosaurus* are known, both showing scales. One patch is found with some gastralic ribs and the impression of a long, unknown bone, indicating that the patch is from the belly. The scales are pebbly and gradually become larger and somewhat hexagonal in shape. Also preserved are two larger feature scales, placed 4,5 cm apart from each other. Another skin impression is from an unknown part of the body. These scales are small, diamond-shaped and arranged in rows, possibly a thick scaly hide reminiscent to tyrannosaurids of the Northern Hemisphere.

## Discovery and naming

Although the small flesh-eating dinosaurs were diverse and dangerous, Cretaceous Alberta was ruled by members of the family tyrannosauridae.

All tyrannosaurs had hind legs that were long and powerful, with each hind foot having three toes ending in enormous claws. The two-fingered front limbs were small, not much larger than a mature human arm. The function of the front limbs is not known. A recent study conducted by Tyrannosaur-researchers have suggested that the stubby arms of Carnosaurs and Tyrannosaurs might've acted as meathooks when grappling prey. Of course, the huge maw of these creatures would've done at least 90% of the job, but the front hooked claws might've grappled the hide of its prey to prevent from the prey easily escaping.

*Albertosaurus*, the "lizard from Alberta," was among the most fearsome predators in Cretaceous Alberta. 9 meters long and 3 meters high at the hip, it was the most common of the large carnivores found here. Smaller but longer-limbed than *T. rex*, *Albertosaurus* would have been a mobile hunter. Like modern carnivores, it probably fed on the carcasses of already dead animals as well, something iconic to all carnivores, extinct and modern.

*Albertosaurus* weighed up to 3 tons yet may have been capable of attaining speeds of more than 40 mph. This fleetness, combined with obvious physical strength, would have made *Albertosaurus* a fierce hunter, but less than the more massive *T. rex*. *Albertosaurus* neck was strong and muscular, supporting a large but lightly built head. The teeth were long and recurved with saw-like edges, perfect for tearing flesh. They were not adapted for chewing, making it likely than *Albertosaurus* swallowed flesh in large chunks, using gravity for the iconic Tyrannosaur feeding motion, by ripping off a chunk of meat, and throwing it in the air, and swallowing it.

*Albertosaurus* bones were among the earliest dinosaur remains collected in Alberta. A skull found by J.B. Tyrrell in 1884 was the first important dinosaur fossil to be discovered along the Red Deer River. It was named in 1905, the same year that Alberta became a province. Since then, many *Albertosaurus* fossils have been discovered. the smallest documented *Albertosaurus*, a juvenile less than a quarter of the size of a full grown adult, was collected from Sandy Point on the South Saskatchewan River in 1986. As you can see, Albertosaurus was a very informal and important discovery to the current knowledge we have on dinosaurs and the rich beds of North America. [1]

J.B. Tyrrell spent most of his long career as a geologist, explorer and entrepreneur on the Canadian Shield. However, in 1884 his first field work was in Cretaceous strata along the Red Deer River where his discovery of a skull of the tyrannosaur *Albertosaurus* provided a name to the paleontological museum in Drumheller. At the time, several discoveries were made to what was 10 Albetosaurus species (Gorgosaurus, Deinodon, Tarbosaurus, Alectrosaurus, Dryptosaurus, Dinotyrannus/Tyrannosaurus) ; including the Nanotyrannus, Jane, by courtesy of von Huene, Riabinin, and most noticeably Gilmore, Matthew, Kuhn, Paul and Russell. But recent research proved that the other species were sperate genus, and there being only one genus of Albertosaurus.

## 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). These two species are the only described albertosaurines; other undescribed species may exist. Thomas Holtz found *Appalachiosaurus* to be an albertosaurine in 2004, but his more recent unpublished work locates it just outside Tyrannosauridae, in agreement with other authors.

The other major subfamily of tyrannosaurids is the Tyrannosaurinae, which includes *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).

## Paleobiology

### 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 metres (6.6 feet) in length. The 10 metres (33 feet) 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 (269 pounds) per year, based on an adult 1.3 tonnes. 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 at almost five times this rate (601 kilograms [1,325 pounds] per year) at its peak. 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. Sexual maturation while still actively growing appears to be a shared trait among small and large dinosaurs as well as in large mammals such as humans and elephants. 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.

During growth, through thickening the tooth morphology changed so much that, had the association of young and adult skeletons on the Dry Island bonebed not proven they belonged to the same taxon, the teeth of juveniles would likely have been identified by statistical analysis as those of a different species.

### 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 be 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. 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.

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 the high physiological demands of procreation, including 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.

### Pack behaviour

The Dry Island bonebed discovered by Barnum Brown and his crew contains the remains of 26 *Albertosaurus*, the most individuals found in one locality of any large 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.

The near-absence of herbivore remains and the similar state of preservation common to 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. Currie claims this as evidence of pack behaviour. Other scientists are skeptical, observing that the animals may have been driven together by drought, flood or for other reasons.

There is plentiful 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 *Deinonychus* and *Coelophysis* have been found in aggregations, as have larger predators like *Allosaurus* and *Mapusaurus*. There is some evidence of gregarious behaviour 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. 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. 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.

Currie has also speculated 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. 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. 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. However, as the preservation of behaviour in the fossil record is exceedingly rare, these ideas cannot readily be tested. In 2010, Currie, though still favouring the hunting pack hypothesis, admitted that the concentration could have been brought about by other causes, such as a slowly rising water level during an extended flood.

### Paleopathology

Tyrannosaur jaw-bones with trichomonosis-type lesions; D (upper right) is *Albertosaurus*
In 2009, researchers hypothesized that smooth-edged holes found in the fossil jaws of tyrannosaurid dinosaurs such as *Albertosaurus* were caused by a parasite similar to *Trichomonas gallinae*, which infects birds. They suggested that tyrannosaurids transmitted the infection by biting each other, and that the infection impaired their ability to eat food.

In 2001, Bruce Rothschild and others published a study examining evidence for stress fractures and tendon avulsions in theropod dinosaurs and the implications for their behavior. They found that only one of the 319 *Albertosaurus* foot bones checked for stress fractures actually had them and none of the four hand bones did. The scientists found that stress fractures were "significantly" less common in *Albertosaurus* than in the carnosaur *Allosaurus*. ROM 807, the holotype of *A. arctunguis* (now referred to *A. sarcophagus*), had a 2.5 by 3.5 cm (0.98 by 1.38 in) deep hole in the iliac blade, although the describer of the species did not recognize this as pathological. The specimen also contains some exostosis on the fourth left metatarsal. In 1970, two of the five *Albertosaurus sarcophagus* specimens with humeri were reported by Dale Russel as having pathological damage to them.

In 2010, the health of the Dry Island *Albertosaurus* assembly was reported upon. Most specimens showed no sign of disease. On three phalanges of the foot strange bony spurs, consisting of abnormal ossifications of the tendons, so-called enthesophytes, were present, their cause unknown. Two ribs and a belly-rib showed signs of breaking and healing. One adult specimen had a left lower jaw showing a puncture wound and both healed and unhealed bite marks. The low number of abnormalities compares favourably with the health condition of a *Majungasaurus* population of which it in 2007 was established that 19% of individuals showed bone pathologies.

## Paleoecology

The Albertosaurus lived in the northern Canadian regions as its name suggests. It likely shared its environment with other carnivores, and much herbivores. One specific paleo-art, a painting shows an Albertosaurus pursuing after a juvenile Edmontosaurus in the Northern regions of Alberta. It probably shared its environment with Pachyrhinosaurus, Edmontosaurus, Troodon, and all sorts of Pterosaurs. The Albertosaurus might've shared its Tyrannosaur-ceratopsid rivalry with Pachyrhinosaurus, and probably pursued Edmontosaurus as well.

## JPInstitute.com Description

Albertosaurus was the grandfather of T. rex. This dinosaur looked like a smaller, less muscular version of Tyrannosaurus. As an early member of the tyrannosaur family, Albertosaurus was the final design for the large meat-eating dinosaurs. It only had two fingers on its hands, which were at the end of short arms. It had long legs and it a wide skull with lots of power in its bite.

Albertosaurus is considered by many scientists to be the best known of the tyrannosaurs. A number of nearly complete skeletons, two adults and a juvenile, have been found. There are two recognized species and several in dispute. It is sometimes referenced as a transitory animal, a creature on its way to evolving into T. rex, but it appears that it was extremely successful and longer lived than the Tyrannosaurus genus.

## Links

http://web.archive.org/web/20040420200121fw\_/http://www.jpinstitute.com/dinopedia/dinocards/dc\_alber.html#

## References

General Information Credits to "ROMTECH" Computer CD Dinosaur Discovery

History credits to http://www.cbv.ns.ca/marigold/history/dinosaurs/datafiles/albertosaurus.html and http://gsc.nrcan.gc.ca/paleochron/31\_e.php

http://en.wikipedia.org/wiki/Albertosaurus

http://animals.howstuffworks.com/dinosaurs/albertosaurus.htm

http://www.prehistoric-wildlife.com/species/a/albertosaurus.html

http://archaeology.knoji.com/albertosaurus-the-tyrants-cretaceous-cousin/

http://www.absoluteastronomy.com/topics/Albertosaurus

http://www.enchantedlearning.com/subjects/dinosaurs/dinos/Albertosaurus.shtml

1. ↑ Template:Cite journal