Allosaurus - Species and Taxonomy

Species and Taxonomy

It is unclear how many species of Allosaurus there were. Seven species have been considered potentially valid since 1988 (A. amplexus, A. atrox, A. europaeus, the type species A. fragilis, the as-yet not formally described "A. jimmadseni", A. maximus, and A. tendagurensis), although only a fraction are usually considered valid at any given time. Additionally, there are at least ten dubious or undescribed species that have been assigned to Allosaurus over the years, along with the species belonging to genera now sunk into Allosaurus. In a recent review of basal tetanuran theropods, only A. fragilis (including A. amplexus and A. atrox as synonyms), "A. jimmadseni" (as an unnamed species), and A. tendagurensis were accepted as potentially valid species, with A. europaeus not yet proposed and A. maximus assigned to Saurophaganax.

A. amplexus, A. atrox, A. fragilis, "A. jimmadseni", and A. maximus 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 sixty individuals. Debate has gone on since the 1980s regarding the possibility that there are two common Morrison Formation species of Allosaurus, with the second known as A. atrox; recent work has followed a "one species" interpretation, with the differences seen in the Morrison Formation material 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 such features as the shape of the lacrimal horns, and the proposed differentiation of "A. jimmadseni" based on the shape of the jugal. A. europaeus was found in the Kimmeridgian-age Porto Novo Member of the Lourinhã Formation, but may be the same as A. fragilis. A. tendagurensis was found in Kimmeridgian-age rocks of Tendaguru, in Mtwara, Tanzania. It may be a more basal tetanuran, a carcharodontosaurid, or simply a dubious theropod. Although obscure, it was a large theropod, possibly around 10 meters long (33 ft) and 2.5 metric tons (2.8 short tons) in weight.

Allosaurus is regarded as a probable synonym of the genera Antrodemus, Creosaurus, Epanterias, and Labrosaurus. Most of the species that are regarded as synonyms of A. fragilis, or that were misassigned to the genus, are obscure and were based on scrappy remains. One exception is Labrosaurus ferox, named in 1884 by Marsh for an oddly formed partial lower jaw, with a prominent gap in the tooth row at the tip of the jaw, and a rear section greatly expanded and turned down. Later researchers suggested that the bone was pathologic, showing an injury to the living animal, and that part of the unusual form of the rear of the bone was due to plaster reconstruction. It is now regarded as an example of A. fragilis. Other remains thought to pertain to Allosaurus have come from across the world, including Australia, Siberia, and Switzerland, but these fossils have been reassessed as belonging to other dinosaurs.

The issue of synonyms is complicated by the type specimen of Allosaurus fragilis (catalog number YPM 1930) being extremely fragmentary, consisting of a few incomplete vertebrae, limb bone fragments, rib fragments, and a tooth. Because of this, several scientists have interpreted the type specimen as potentially dubious, and thus the genus Allosaurus itself or at least the species A. fragilis would be a nomen dubium ("dubious name", based on a specimen too incomplete to compare to other specimens or to classify). To address this situation, Gregory S. Paul and Kenneth Carpenter (2010) submitted a petition to the ICZN to have the name A. fragilis officially transferred to the more complete specimen USNM4734 (as a neotype). This request is currently pending review.