Allosaurus Marsh 1877

Allosaurus Marsh, 1877

Included taxa. Allosaurus fragilis Marsh, 1877; Allosaurus maximus (Chure, 1995); Allosaurus sp. nov. (to be described by D. Chure).

Temporal range. Kimmeridgian-Tithonian.

Occurrence. Morrison Formation, Wyoming, Utah, Colorado, New Mexico, South Dakota, Oklahoma, all USA; Lourinha Formation, Portugal.

Diagnosis. Distinct ‘step’ in the ventral margin of the jugal, leading to a significant ventral displacement of the posterior part in relation to the anterior part; neomorph element present in lower jaw (antarticular in Madsen 1976); well-developed notch in the anteroventral margin of the prearticular.

Remarks. Although it is one of the best known and best represented of theropod dinosaurs, the taxonomy of Allosaurus (Text-fig. 5e) is problematic. Originally described by Marsh (1877) on the basis of rather poor material, it was later often synonymized with the genus Antrodemus (e.g. Gilmore 1920), but Madsen (1976) argued that the latter taxon represents a nomen dubium. This view is followed here. Another matter of debate is the number and taxonomy of the species included in the genus Allosaurus. Pending a detailed revision of the genus, I recognize three different species within the genus: Allosaurus fragilis, which is the most common large theropod of the Late Jurassic Morrison Formation; Allosaurus maximus, which is based on the remains of at least two individuals of a very large theropod from the Morrison Formation of Oklahoma that differ in some anatomical details from A. fragilis (Chure 1995; Smith 1998), and a new, undescribed species from Dinosaur National Monument, which exhibits all the synapomorphies of the genus, but differs from both of the other species in several morphological details (Chure, pers. comm. 1998).