Allosaurus

***Allosaurus*** was a large theropod dinosaur from the Jurassic period, a predatory carnivore. _Allosaurus_ averaged 8.5 metres (28 ft) in length, though some remains suggest it could reach over 12 meters (39 ft). Its three-fingered forelimbs were smaller than its large hind legs, and the body was balanced by a long, heavy tail. It weighed 2.3 tons. It is the most common large predator found in the Morrison Formation of Colorado. This formation was laid down 155 to 145 million years ago, in the Jurassic. _Allosaurus_ was at the top of the food chain. It probably preyed on large herbivorous dinosaurs and perhaps even other predators, including _Ceratosaurus_ and _Marshosaurus_. Potential prey included ornithopods, stegosaurs, and sauropods.

Some paleontologists think _Allosaurus_ had cooperative behavior, and hunted in packs. Others believe they may have been aggressive toward each other. Groups have been found together in the fossil record. This might be evidence of pack behavior, or just the result of lone individuals feeding on the same carcass. Remains of many individuals have been found, including some which are almost complete. Over sixty-nine individuals from one species have been found.

## Feeding

Sauropods, live or dead, seem to be likely candidates of prey. _Apatosaurus_ bones with holes fitting allosaur teeth, and the presence of shed allosaur teeth with _Camarasaurus_ bones have also been found.

There is dramatic evidence for allosaur attacks on _Hesperosaurus_ and _Stegosaurus_. An _Allosaurus_ tail vertebra has been found with a partially healed puncture which fits a _Stegosaurus_ thagomizer. Also, there is a _Hesperosaurus_ neck plate with a U-shaped wound that correlates well with an _Allosaurus_ snout.

An _Allosaurus_ was probably not a predator of a fully grown _Haplocanthosaurus_, unless it hunted in packs. It had a modestly sized skull and relatively small teeth, and was greatly outweighed by adult _Haplocanthosaurus_. Another possibility is that it preferred to hunt juvenile _Haplocanthosaurus_ instead of fully grown ones.

Researchers have made other suggestions. Robert T. Bakker compared the short teeth to serrations on a saw. This saw-like cutting edge runs the length of the upper jaw, and could have been driven into prey. This type of jaw would permit slashing attacks against much larger prey, with the goal of weakening the victim.

Another study showed the skull was very strong but had a relatively small bite force. The authors suggested that _Allosaurus_ used its skull like a hatchet against prey, attacking open-mouthed, slashing flesh with its teeth, and tearing it away without splintering bones, but this is disputed by other scientists.

Different strategies could be used against different prey. The skull was light enough to allow attacks on smaller and more agile ornithopods, but strong enough for high-impact ambush attacks against larger prey like stegosaurs and sauropods.

Their ideas were challenged by other researchers, who found no modern examples of a hatchet attack. They thought it more likely that the skull was strong to absorb stresses from struggling prey.

The original authors noted that _Allosaurus_ itself has no modern equivalent, so the absence of a modern 'hatchet attacker' was not significant. They thought the tooth row was well-suited to such an attack, and that articulations (joints) in the skull helped to lessen stress.

Another possibility for handling large prey is that theropods like _Allosaurus_ were flesh grazers that take bites of flesh out of living sauropods such as _Cetiosaurus_, sufficient to sustain the predator so it did not need to kill the prey outright. This strategy might have allowed the victim to recover and be fed upon again later.

Another idea is that _Camptosaurus_ and _Drinker_, the most common prey of _Allosaurus_, could be subdued by it grasping the prey with their forelimbs, and then making bites on the throat to crush the trachea. The forelimbs were strong and capable of restraining prey, and the articulation of the claws suggests that they could have been used to hook things.

The shape of the _Allosaurus_ skull limited binocular vision to 20° of width, slightly less than that of modern crocodilians. As with crocodiles, this may have been enough to judge prey distance and time attacks. The similar width of their field of view suggests that allosaurs, like modern crocodiles, were ambush hunters.

Finally, the top speed of _Allosaurus_ has been estimated at 30 to 55 kilometers per hour (19 to 34 miles per hour).

## Cleveland-Lloyd discoveries

The fossil site  known as the Cleveland-Lloyd Dinosaur Quarry in Emery County, Utah was known in 1927, but major operations did not begin there until 1960. An effort from nearly 40 institutions got thousands of bones between 1960 and 1965. The quarry is notable for the many _Allosaurus_ remains, the condition of the specimens, and our ignorance of its ancient origin. It is estimated that the remains of at least 46 _A. fragilis_ have been found there, out of at least 73 dinosaurs. The fossils found there are disarticulated (separate) and well-mixed.

Suggestions as to how it arose include animals getting stuck in a bog, to becoming trapped in deep mud, to falling victim to drought-induced mortality around a waterhole, to getting trapped in a spring-fed pond or seep. Regardless of the cause, the great quantity of well-preserved _Allosaurus_ remains means the animal is one of the best-known theropods. Individuals of almost all ages and sizes are found, from less than 1 meter (3.3 ft) to 12 meters (39 ft) long.

## Species

There are five species of _Allosaurus_:
- _Allosaurus fragilis_
- _Allosaurus tendagurensis_ (named as _Epanterias_)
- _Allosaurus atrox_
- _Allosaurus europaeus_
- _Allosaurus jimmadseni_
- _Allosaurus anax_

## Media

_Allosaurus,_ like _Tarbosaurus_, _Tyrannosaurus_ and _Torvosaurus_, has come to represent the fourth large, carnivorous dinosaur in popular culture.

It is a common dinosaur in museums. A number of museums cooperated in excavations at the Cleveland Lloyd Dinosaur Quarry. By 1976, museums in eight countries on three continents had Cleveland-Lloyd allosaur material or casts. _Allosaurus_ is the official 'state fossil' of Utah. _Allosaurus_ is top predator in both Arthur Conan Doyle's 1912 novel, _The Lost World._

_Allosaurus_ was depicted as a species of _Tyrannosaurus_ in the 1956 horror film _The Beast of Hollow Mountain_, and the 1969 film _The Valley of Gwangi_. Gwangi is billed as an _Allosaurus_, though Ray Harryhausen named it as a _Tyrannosaurus_. _Allosaurus_ appeared in the second episode of the 1999 BBC television series _Walking with Dinosaurs_ and the follow-up special _The Ballad of Big Al_, which speculated on the life of the 'Big Al' specimen, as revealed by the numerous injuries and pathologies in its skeleton.