Nearly complete skeleton of an Allosaurus fragilis known as "Big Al" on display at the Museum of the Rockies in Bozeman, Montana. This specimen was collected at the Howe Quarry in Big Horn County, Wyoming.

The name "Allosaurus" means "different lizard", and "fragilis" means "fragile". This refers to Allosaurus' relatively lightweight skull and slender teeth. It was first discovered in 1877 by paleontologist Othniel Charles Marsh, and used to be known as Antrodemus.

Allosaurus lived 155 to 150 million years ago. It averaged 28 feet in length, but could reach 40 feet. It had a relatively long and muscular tail, which helped it balance and turn quickly, and the skull had a pair of horns above and in front of the eyes.

One of the most important Allosaurus finds came in 1991 with the discovery of "Big Al" in Big Horn County, Wyoming. "Big Al" was 95 percent complete, and partially articulated (so scientists didn't have to guess how the bones fit together or moved). "Big Al" is about 26 feet long. It is a subadult and only about 87 percent fully grown. It suffered some pathologies (disease and injury) in its left foot, which made it compensate during hunting with its right foot. This caused injury to the right foot.