Allosaurus jimmadseni

Artwork depicting an allosuarus jimmadseni dinosaur

NPS/Bob Walters Tess Kissinger

Allosaurus jimmadseni slab cast

Allosaurus jimmadseni is the second species of Allosaurus found at Dinosaur National Monument. The first specimen of A. jimmadseni was found in the monument. It is much rarer than Allosaurus fragilis and they differ in their skeletal details.

Allosaurus, meaning "different reptile," is a theropod (meat eating dinosaur) that probably ate other smaller dinosaurs. Its teeth were serrated like steak knives for cutting flesh. Adults hunted by overpowering their prey, possibly in small groups. Juvenile Allosaurus relied on their longer limb bones that were better adapted to running in order to hunt small vertebrates. Allosaurus was not the largest predator of the time, but its sharp teeth and hook-shaped claws made Allosaurus a nightmare for many creatures of the Jurassic.


Only half of an Allosaurus jimmadseni skull has been found so far. However, the skull separated along its natural midline. Although only the left half of the skull is preserved, it shows both the outside shape and the internal structures, which are rarely seen. The entire braincase is present as well as both ear bones. For the first time in any Allosaurus the wishbone is in place. This Allosaurus jimmadseni is most complete Allosaurus of any species ever found.

Exterior and interior of an Allosaurus jimmadseni skull.
With one side completely missing from this specimen, paleontologists can study both the internal and external structures of Allosaurus jimmadseni. (Left) The exterior of the specimen - when the lower jaw is closed, the upper teeth extend alongside it. (Right) The internal structure of Allosaurus jimmadseni including the brain case and the entire inside of the left side in the skull can be seen.

Jurassic Fact: A year after the body was discovered, the skull was found thanks to equipment that detects radiation in the bone. Without that equipment, the skull might never have been found.

For the recent paper published on Allosaurus jimmadseni, click here.

Last updated: February 4, 2020

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