Allosaurus

*Allosaurus fragilis*

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***Allosaurus*** (pronounced /ˌæləˈsɔːrəs/; meaning "different lizard") is a genus of large theropod dinosaur that lived 155 to 150 million years ago during the late Jurassic period (Kimmeridgian to early Tithonian).

*Allosaurus* was first described in 1877 as *Antrodemus*, but it has since been renamed to *Allosaurus* after it was decided the original classification was based upon poor material.

## Description

*Allosaurus* stood about 8 feet (2.4 metres) tall and measured about 28 feet (8.5 metres) long, though some remains suggest it could reach up to 12 meters (40 feet) depending on the species. It's 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.

## History

*Allosaurus* was first discovered in 1869 by Ferdinand Hayden and then named in 1877 by its founder Othniel Charles 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.

As the top predator of its age and habitat, *Allosaurus* hunted and scavenged whatever smaller or larger prey was abundant. However, it had a modestly-sized skull filled with slightly small teeth and was greatly outweighed by sauropods and probably preferred to hunt young, weak, old, sick, injured, or trapped sauropods over healthy adults. As sauropods were the dominant species of the time, they would be preyed upon by Allosaurus more often than not, with a common speculation being that this was done through hunting in packs, which it did to bring down any large prey that could put up a fight.

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, lifting its head up, with its mouth open, and crashing down on its prey. This 'Axe-bite' weakened the prey severely.

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 palaeontologists 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 behaviour, or just the result of lone individuals feeding on the same carcass.

## JPInstitute.com Description

One of the most popular and well known dinosaurs, Allosaurus was the most ferocious meat-eater of the Jurassic. It was the first of the really big meat-eaters which had huge heads filled with lots of sharp teeth. It could run very fast, close to 40 mph, and had very strong arms and long, sharp, curved claws on its hands. One of its hand claws was more than one foot long!

Allosaurus lived among some of the largest dinosaurs to ever walk the earth and it is likely that it hunted some of smaller members of these families. Even the smaller members would have been huge, and this required Allosaurus to have great strength.

Allosaurus is the namesake of the very successful allosaurid family of dinosaurs. This family includes what may be the very largest carnivore, the Giganotosaurus, and other creatures such as the Acrocanthosaurus and Carcharodontosaurus. Its more distant relatives are the raptors and tyrannosaurs of the late Cretaceous.

Allosaurus had relatively small teeth for such a large dinosaur, especially when compared to later big carnivores. It probably had a weak bite compared to its size, but its mouth could open very wide. Allosaurus most likely used its very heavily muscled arms with its huge claws as potent weapons in bringing down and holding its prey. It also was close to having true stereo vision - its big eyes had some forward vision that would have allowed for a degree of depth perception.

There is a great deal known about this dinosaur's life style. Footprint evidence indicates it hunted in groups and that it cared for its young in large nests. It inhabited floodplains, meadows and some forests.

## Dinosaur Field Guide Description

Allosaurus ("other lizard") was the most common predator of the Late jurassic and probably one of the most dangerous. Like most meat-eating dinosaurs, its jaws were filled with serrated teeth shaped like blades. Computer studies suggest that Allosaurus attacked by using its upper jaw like a battle ax to hack at its victim, then it used its lower jaw to bite out a slice of meat. It might have been easy for an Allosaurus to kill a fairly defenseless dinosaur like Camptosaurus, but Stegosaurus could certainly put up a fight, and the sauropods (giant long-necks) were so huge that they could easily crush even an adult Allosaurus. We know that Allosaurus led a dangerous life. The Allosaurus on display at
the Smithsonian Institution has a smashed shouldeb blade, many broken ribs, and a lower jaw so damaged that paleontologists didn't realize it was an Allosaurus jaw for over 100 years! But these were tough dinosaurs: Their bones show that they lived long enough for their wounds to heal.

### Fun Facts

The first fossil of Allosaurus ever found- the broken half of a backbone--was called a petrified horse hoof by its discoverers!

### Trivia

Allosaurus is the official state dinosaur of Utah.

## Links

http://web.archive.org/web/20040420200713fw\_/http://www.jpinstitute.com/dinopedia/dinocards/dc\_allos.html#

## References

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.
5. ↑ Template:Cite book
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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.
14. ↑ Template:Cite journal
15. ↑ "Evolve: Eyes". History channel Evolve. Transcript.
16. ↑ Template:Cite journal