Allosaurus

*Allosaurus* is a predatory dinosaur that lived in the Jurassic period about 155-145 million years ago in Tanzania, Portugal, Australia and the United States of America. It was one the most common carnivores of that peroid.

## Description

*Allosaurus* stood about 15 feet tall and measured about 28 feet though some remains suggest it could reach over 12 meters (39 ft) depending on which genus it is. 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. The skull was narrow and had a pair of horns above and in front of the eyes.

## Classification

There are five recognised species of *Allosaurus*:

* *A. fragilis*
* *A. tendagurensis*
* *A. atrox*
* *A. europaeus*
* *A. jimmadseni*
* *Saurophaganax. maximus (possibly an Allosaurus)*
* *Epanterias. amplexus (originally Allosaurus. amplexus)*
* *Antrodemus. valens (possibly an Allosaurus)*

## History

It was first discovered in 1869 by Ferdinand Hayden and then named in 1877 by its founder Othniel Marsh.

Remains of many individuals have been found, including some which are almost complete. Over sixty individuals from one species have been found.

## Paleobiology

### Feeding

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

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* tail spike. 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.

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.

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.

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).

### Social 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.

1. ↑ Madsen, James H, Jr. 1993 [1976]. *Allosaurus fragilis*: a revised osteology. 2nd ed, *Utah Geological Survey Bulletin* **109**. Salt Lake City: Utah Geological Survey.
2. ↑ Madsen, James H., Jr. (1993) [1976]. *Allosaurus fragilis: A Revised Osteology*. Utah Geological Survey Bulletin **109** (2nd ed.). Salt Lake City: Utah Geological Survey.
3. ↑ Holtz, Thomas R., Jr.; Molnar, Ralph E.; and Currie, Philip J. 2004. Weishampel, David B.; Dodson, Peter; and Osmólska, Halszka (eds) *The Dinosauria*. 2nd ed, Berkeley: University of California Press. pp. 71–110. ISBN 0-520-24209-2
4. ↑ Fastovsky, David E.; and Smith, Joshua B. 2004. "Dinosaur Paleoecology", in *The Dinosauria*, 2nd ed. 614–626.
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10. ↑ Holtz, Thomas R., Jr.; Molnar, Ralph E.; and Currie, Philip J. 2004. Weishampel, David B.; Dodson, Peter; and Osmólska, Halszka (eds) The Dinosauria. 2nd ed, Berkeley: University of California Press. pp. 71–110. ISBN 0-520-24209-2
11. ↑ Foster, John Wikipedia:2007. "Allosaurus fragilis". *Jurassic West: the dinosaurs of the Morrison Formation and their world*. Bloomington, Indiana: Indiana University Press. pp. 170–176. ISBN 978-0-253-34870-8 OCLC 77830875
12. ↑ Carpenter, Kenneth 2002. "Forelimb biomechanics of nonavian theropod dinosaurs in predation". *Senckebergiana lethaea* **82** (1): 59–76. doi:10.1007/BF03043773
13. ↑ Gilmore, Charles W. 1920. "Osteology of the carnivorous dinosauria in the United States National Museum, with special reference to the genera Antrodemus (Allosaurus) and Ceratosaurus". *Bulletin of the United States National Museum* **110**: 1–159.
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15. ↑ "Evolve: Eyes". History channel Evolve. Transcript.
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