?A. amplexus
?A. ferox
A. fragilis (type)

Allosaurus (AL-oh-sore-us) was a large carnivorous dinosaur with a length of up to 12 meters (39 feet). It was the most common large predator in North America, 140 million years ago, in the Jurassic period.

Allosaurus is the official state fossil of Utah, in the United States.

Description

Allosaurus is a classic big theropod: a big skull on a short neck, a long tail, and reduced forelimbs. Its most distinctive feature is a pair of blunt horns just above and in front of the eyes. Although short in comparison to the hindlimbs, the forelimbs are massive and bear large, eagle-like claws. The skull shows 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 and of all theropods shows birdlike features like fragile, hollow bones.

Life and death

Allosaurus is the most common theropod in the huge section 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; and in Portugal. Curiously, 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 Quarry in Utah. This fossil bed contains over 10,000 bones, mostly of Allosaurus, with other dinosaurs like Stegosaurus and Ceratosaurus thrown in. It is still a mystery how the remnants of so many animals can be found in one place: normally the ratio of fossils of carnivorous animals over fossils of plant eaters is very small. Findings like these can be explained by pack hunting, although this is difficult to prove.

One of the more significant finds was the 1991 discovery of "Big Al" (MOR 593), a 95% complete, partially articulated, juvenile specimen that measured 8 meters (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 BBC's Walking with Dinosaurs series in the "Ballad of Big Al". The fossils were excavated near Shell, Wyoming by the Museum of the Rockies and the University of Wyoming Geological Museum.

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 a caudal vertebrae (a tail bone), which Joseph Leidy tentatively assigned first to the Poicilopleuron 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 Canon City, Colorado.

The name Allosaurus comes from the Greek allos, meaning "strange" or "different"; and sauros, meaning "lizard" or "reptile". The species epithet fragilis is Latin for "fragile". Both refer to lightening features in the vertebrae.

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

Allosaurus's closest relative is probably the Lower Cretaceous Acrocanthosaurus.

External links

• Allosaurus, the story of "Big Al", from the University of Wyoming Geological Museum in Laramie.
• Public Pioneer, Utah State Fossil, Allosaurus, from Utah.gov.
• Allosaurus fragilis, Smithsonian National Museum of Natural History, Department of Paleobiology.
• Discovery Channel - Walking with Dinosaurs - Fact Files - Allosaurus, from BBC Online.
• List of the many possible Allosaurus species...