of Allosaurus have been named: A. amplus, A. atrox, A. europaeus, the type species A. fragilis, A. jimmadseni and A. lucasi. Among these, Daniel Chure and Mark Loewen in 2020 only recognized the species A. fragilis, A. europaeus, and the newly-named A. jimmadseni as being valid species. However, A. europaeus does not show any unique characters compared to the North American species, so a reevaluation of its validity is required. A. fragilis is the type species and was named by Marsh in 1877. It is known from the remains of at least 60 individuals, all found in the Kimmeridgian–Tithonian Upper Jurassic-age Morrison Formation of the United States, spread across the states of Colorado, Montana, New Mexico, Oklahoma, South Dakota, Utah, and Wyoming. Details of the humerus (upper arm) of A. fragilis have been used as diagnostic among Morrison theropods, but A. jimmadseni indicates that this is no longer the case at the species level. A. jimmadseni has been scientifically described based on two nearly complete skeletons. The first specimen to wear the identification was unearthed in Dinosaur National Monument in northeastern Utah, with the original "Big Al" individual subsequently recognized as belonging to the same species. This species differs from A. fragilis in several anatomical details, including a jugal or cheekbone with a straight lower margin. Fossils are confined to the Salt Wash Member of the Morrison Formation, with A. fragilis only found in the higher Brushy Basin Member. A. fragilis, A. jimmadseni, A. amplus, and A. lucasi are all known from remains discovered in the Kimmeridgian–Tithonian Upper Jurassic-age Morrison Formation of the United States, spread across the states of Colorado, Montana, New Mexico, Oklahoma, South Dakota, Utah and Wyoming. A. fragilis is regarded as the most common, known from the remains of at least 60 individuals. For a while in the late 1980s and early 1990s, it was common to recognize A. fragilis as the short-snouted species, with the long-snouted taxon being A. atrox; however, subsequent analysis of specimens from the Cleveland-Lloyd Dinosaur Quarry, Como Bluff, and Dry Mesa Quarry showed that the differences seen in the Morrison Formation material could be attributed to individual variation. A study of skull elements from the Cleveland-Lloyd site found wide variation between individuals, calling into question previous species-level distinctions based on such features as the shape of the lacrimal horns, and the proposed differentiation of A. jimmadseni based on the shape of