VARIATION AND STRATIGRAPHIC DISTRIBUTION OF ALLOSAURUS WITHIN THE LATE JURASSIC MORRISON FORMATION

The taxonomy of the Late Jurassic theropod dinosaur Allosaurus has long been a contentious subject of debate, with 16 species assigned over the past 125 years. A recent study suggests that there are only two valid species of Allosaurus within the Morrison Formation. New specimens and recent work refining the stratigraphic relationships of the Morrison Formation allow an assessment of the variation and distribution of Allosaurus in the western U.S.

Variation within the species Allosaurus fragilis was assessed using thousands of specimens from the Cleveland-Lloyd Dinosaur Quarry (CLDQ) of central Utah. Morphometric analysis of the cranial and postcranial elements within this large sample document changes related to ontogeny as well as individual variation within the species. Analysis of Allosaurus specimens housed in institutions throughout North America combined with CLDQ variation data support the view of only two distinct species of Allosaurus. The two species are distinguishable on the basis of several characters, including the morphology of the ventral jugal margin. A. fragilis (i.e. UUVP 6000 and CLDQ specimens) possesses a strongly sigmoidal ventral margin of the jugal, while in A. sp. 2 (i.e. DINO 11541 and MOR 693) the ventral jugal margin is relatively straight. In addition, the nasals of the two species are distinct. This variation in cranial elements between the two species is well beyond the range of ontogenetic variation, and can be documented in different sized animals from juveniles through adults.

The stratigraphic distribution of Allosaurus specimens further supports the distinction between these two species. A. sp. 2 has only been recovered from the Salt Wash Member in Utah and its lateral equivalents in the lower part of the Morrison Formation in northern and southern Wyoming. Specimens of A. fragilis are restricted to the overlying Brushy Basin Member of the Morrison. Furthermore, where the two species co-occur in the same section, they are stratigraphically separated. This study documents the range of variation (ontogenetic, individual and interspecific) within Allosaurus and suggests that there are two temporally and morphologically distinct species within the Morrison Formation.