Species of Allosaurus - Biological Variation, A. Atrox, and A. Fragilis

Biological Variation, A. Atrox, and A. Fragilis

The perception that there were two common Allosaurus species in the Morrison Formation was popularized in Gregory S. Paul's 1988 book Predatory Dinosaurs of the World. Paul proposed that A. fragilis had tall pointed horns and a slender build compared to a postulated second species A. atrox, and was not a different gender due to rarity. Allosaurus atrox was originally named by Marsh in 1878 as the type species of its own genus, Creosaurus, and is based on YPM 1890, an assortment of bones including a couple of pieces of the skull, portions of nine tail vertebrae, two hip vertebrae, an illium, and ankle and foot bones. Although the idea of two common Morrison allosaur species has had support in semi-technical and popular works, it has generally been rejected in the technical literature.

David K. Smith, examining Allosaurus fossils by quarry, found that the Cleveland Lloyd Dinosaur Quarry (Utah) specimens are generally smaller than those from Como Bluff (Wyoming) or Brigham Young University's Dry Mesa Quarry (Colorado), but the shapes of the bones themselves did not vary between the sites. A later study by Smith incorporating Garden Park (Colorado) and Dinosaur National Monument (Utah) specimens found no justification for multiple species based on skeletal variation; skull variation was most common and was gradational, suggesting individual variation was responsible. Further work on size-related variation again found no consistent differences, although the Dry Mesa material tended to clump together on the basis of the astragalus, an ankle bone. Kenneth Carpenter, using skull elements from the Cleveland Lloyd site, found wide variation between individuals, calling into question previous species-level distinctions based such features as the shape of the horns, and the proposed differentiation of "A. jimmadseni" based on the shape of the jugal.