Allosaurus
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[Photo] Life restoration of Allosaurus fragilis, an allosaurid theropod from the Late Jurassic pf North America, pencil drawing. Date Dec 17, 2006; modified Apr 26, 2007. Author Arthur Weasley http://commons.wikimedia.org/wiki/User:ArthurWeasley

Allosaurus (IPA pronunciation: /??æl????s??????s/) is a genus of large (up to 9.7 m long) theropod dinosaurs. The name Allosaurus comes from the Greek allos/αλλο??, meaning 'strange' or 'different' and saurus/σαυρο??, meaning 'lizard' or 'reptile'. It was named 'different lizard' because its vertebrae were different from those of other dinosaurs known at the time of its discovery. The genus contains three described species, the best known of which is A. fragilis. Several other dinosaurs originally classified as Allosaurus are now in separate genera.

Allosaurus was a bipedal carnivorous dinosaur with a large skull balanced by a long, heavy tail. Relative to the large and powerful hindlimbs, its three fingered forelimbs were small. It was the most common large predator in the Morrison Formation of what is now North America, 155 to 145 million years ago, in the late Jurassic period. It shared the landscape with several genera of giant sauropods such as Apatosaurus, Diplodocus and Camarasaurus as well as other herbivores such as Stegosaurus and Camptosaurus, all of which may have been potential prey.

Description
Allosaurus was a typical large theropod, having a massive skull on a short neck, a long tail and reduced forelimbs. Its most distinctive feature was a pair of blunt horns, just above and in front of the eyes. Although short in comparison to the hindlimbs, the forelimbs were massive and bore large, eagle-like claws. The skull showed evidence of being composed of separate modules, which could be moved in relation to one another, allowing large pieces of meat to be swallowed. The skeleton of Allosaurus, like other theropods, displayed bird-like features, such as a furcula (wishbone) and neck vertebrae hollowed by air sacs.

Allosaurus fragilis had an average length of 7-9 meters (~30 feet), with the largest definitive Allosaurus specimen (AMNH 680) measuring 9.7 m (32 ft). Several gigantic specimens have been attributed to Allosaurus, but may in fact belong to other genera. The closely related genus Saurophaganax (OMNH 1708) reached 10.9 m (36 ft) in length, and has sometimes been included in the genus Allosaurus as Allosaurus maximus, though recent studies lend support to the idea that it does belong in a separate genus. Another specimen, once assigned to the genus Epanterias (AMNH 5767), may have measured 12.1 m in length. Epanterias may be a species of Allosaurus or Saurophaganax.

Finds
Allosaurus is the most common theropod in the vast tract of dinosaur-bearing rock in the American Southwest known as the Morrison Formation. Remains have been recovered in Montana, Wyoming, South Dakota, Colorado, Oklahoma, New Mexico and Utah, in the United States. There have also been finds in Portugal. Allosaurus shared the Jurassic landscape with several other theropods, including Ceratosaurus and the massive Torvosaurus.

A famous fossil bed can be found in the Cleveland Lloyd Dinosaur Quarry in Utah. This fossil bed contains over 10,000 bones, mostly of Allosaurus, intermixed with the remains of other dinosaurs, such as Stegosaurus and Ceratosaurus. It is still a mystery how the remnants of so many animals can be found in one place. The ratio of fossils of carnivorous animals over fossils of plant eaters is normally very small. Findings like these can be explained by pack hunting, although this is difficult to prove. Another possibility is that the Cleveland Lloyd site formed a 'predator trap', similar to the La Brea Tar Pits, that caused large numbers of predators to become mired in an inescapable sediment.

"Big Al"
One of the more significant finds was the 1991 discovery of "Big Al" (MOR 593), a 95% complete, partially articulated, specimen that measured 7.5-8 meters (24-26 feet) in length. Nineteen bones were broken or showed signs of infection, which probably contributed to Big Al's death. It was featured in "The Ballad of Big Al", a special programme in the BBC's Walking with Dinosaurs series. The fossils were excavated near Shell, Wyoming by the Museum of the Rockies and the University of Wyoming Geological Museum. The completeness of this skeleton gave Big Al its name ??? the individual itself was below the average size for Allosaurus fragilis, and may have been a subadult or a new, smaller species. The specimen was described by Breithaupt in 1996.

This skeleton was initially discovered by a Swiss team, led by Kirby Siber. The same team later excavated a second Allosaurus, "Big Al Two", which is the best preserved skeleton of its kind to date.

Classification and history
The first Allosaurus fossil to be described was a 'petrified horse hoof' given to Ferdinand Vandiveer Hayden in 1869, by the natives of Middle Park, near Granby, Colorado. It was actually a caudal vertebra (a tail bone), which Joseph Leidy tentatively assigned first to the Poekilopleuron genus and later to a new genus, Antrodemus. However, it was Othniel Charles Marsh who gave the formal name Allosaurus fragilis to the genus and type species in 1877, based on much better material, including a partial skeleton, from Garden Park, north of Ca??on City, Colorado.

The species epithet fragilis is Latin for 'fragile', referring to lightening features in the vertebrae.

It is unclear how many species of Allosaurus there were. The material from the Cleveland Lloyd Dinosaur Quarry specimen is much smaller and more lightly-built than the huge and robust Allosaurus from Brigham Young University's Dry Mesa Quarry. One species of Allosaurus has been described from Portugal, A. europaeus.

Allosaurid relatives
An allosaurid astragalus (ankle bone) was found at Cape Patterson, Victoria in early Cretaceous beds in Southeastern Australia. This is notable as this part of Australia lay within the Antarctic Circle at the time.

In popular culture
Allosaurus is the official state dinosaur of Utah, in the United States. Along with Tyrannosaurus, Allosaurus has come to represent the quintessential large, carnivorous dinosaur in popular culture.

Allosaurus is top predator in both Arthur Conan Doyle's novel, The Lost World, and the 1925 film adaptation (not to be confused with Tyrannosaurus, which also appears in the film). In the 1969 film The Valley of Gwangi, Gwangi is billed as an Allosaurus (however, Ray Harryhausen based his model for the creature on Tyrannosaurus. Harryhausen often confuses the two, stating in a DVD interview "They're both meat eaters, they're both Tyrants... one was just a bit larger than the other.")

Allosaurus appears in the second and fifth episodes of the BBC television series Walking with Dinosaurs. The Walking with Dinosaurs special The Ballad of Big Al chronicles the life of a specimen of Allosaurus nicknamed "Big Al".

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