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Allosaurus
Temporal range: U.J.-L.K. 155–120Ma
Allosaurus skull SDNHM.jpg
Replica of Allosaurus skull (San Diego Natural History Museum).
Conservation status
???
Scientific classification
Kingdom:
Phylum:
Superorder:
Order:
Suborder:
Family:
Allosauridae
Genus:
Allosaurus

Marsh, 1877
AllosaurusAMNH5753
The skeletal mount of AMNH 5753, posed as scavenging an Apatosaurus, based on Charles R. Knight's life restoration.

Allosaurus was a large theropod dinosaur from the Jurassic period, a predatory carnivore. It is the most common large predator found in the Morrison Formation of North America. This formation was laid down 155 to 145 million years ago, in the Jurassic. Remains of many individuals have been found, including some which are almost complete. Over sixty-nine individuals from one species have been found.

Characteristics

Allosaurus4
Allosaurus in a Charles R. Knight life restoration

The Allosaurus was a large carnivorous dinosaur that walked on two legs and had two arms on the front of its body. Its arms were longer than those of the T.Rex with large claws. The skin was green or brown with fading red stripes on the back. This dinosaur 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 had a very long head, 90 cm (3 ft), with 5 to 10 cm teeth. Allosaurus's teeth were thin and serrated not made to puncture bone but to strip bits of meat off a carcass.

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.

Allosaurus was bipedal. The top speed of Allosaurus has been estimated at 30 to 55 kilometers per hour (19 to 34 miles per hour).

Name

The Allosaurus did not look like the other dinosaurs that existed at that time. Because of this, the word Allosaurus means "different lizard". The Allosaurus is in the theropod (meaning "beast foot") family.

Discovery

The first fossil of an Allosaurus to be discovered was found in the USA state of Colorado, in 1869. At first, the people who found it thought it was a petrified horse hoof instead of a dinosaur bone. In 1991 a complete Allosaurus skeleton was discovered. This Allosaurus was named "Big Al".

"Big Al" was made famous by a TV programme called The Ballad of Big Al also called Walking with Dinosaurs: Allosaurus. This program showed what Big Al's life could have been like.

They are also one of the most common dinosaurs in museums from the Jurassic time period.

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 found thousands of bones between 1960 and 1965.

Suggestions as to how so many fossils were found include animals getting stuck in a bog, to becoming trapped in deep mud and falling victim to drought. 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.

Species

There are five recognised species of Allosaurus:

  • A. fragilis
  • A. tendagurensis (?)
  • A. atrox (?)
  • A. europaeus
  • A. jimmadseni

Diet

DMSN dinosaurs
Allosaurus and Stegosaurus skeletons, Denver Museum of Nature and Science

Allosaurus was at the top of the food chain. It probably preyed on large herbivorous dinosaurs and perhaps even other predators (e.g. Ceratosaurus). Potential prey included ornithopods, stegosaurs, and sauropods.

Young baby Allosaurus may eaten insects like dragonflies and centipedes, and other small animals, then around two years old, Allosaurus probably would have eaten small dinosaurs.

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

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

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 many scientists.

They suggested that 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' which could take bites of flesh out of living sauropods, sufficient to sustain the predator so it did not need to kill the prey outright. This strategy might have allowed the prey to recover and be fed upon again later.

Allosaurus skull front
Allosaurus skull cast in front view at the Museum für Naturkunde, Berlin

Another idea is that ornithopods, the most common available prey, could be subdued by Allosaurus 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.

Hunting behavior

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.

In popular culture

Allosaurus in Baltow 20060916 1500
Allosaurus model in Bałtów, Poland.

Allosaurus, like Tyrannosaurus, has come to represent the quintessential 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, and its 1925 film adaptation, the first full-length motion picture to feature dinosaurs.

Allosaurus was used as the starring dinosaur of the 1956 film The Beast of Hollow Mountain, and the 1969 film The Valley of Gwangi. Gwangi is billed as an Allosaurus, though Ray Harryhausen based his model for the creature on Charles R. Knight's depiction of 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.

Images for kids

See also

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