The dinosaur genus Allosaurus has a complex taxonomic history, including multiple proposed species of which only a few are considered valid today. The genus was first described in 1877 by Othniel Charles Marsh from a very fragmentary specimen found in Colorado. For several decades, Allosaurus was known under the name Antrodemus, until a 1976 publication re-established the name Allosaurus. In 2023, another, much more complete specimen found in the same quarry was selected as the neotype (the specimen on which the taxon is based, replacing the inadequate original specimen). Four species are considered potentially valid. Besides the well-known type species A. fragilis, these are A. europaeus, A. jimmadseni, and A. anax. A. europaeus, based on a specimen from Portugal, is the only species found outside of North America. Its validity is contested, and some researchers found it to be a synonym of A. fragilis. A. jimmadseni was described in 2020 and is known from multiple complete specimens. A. anax was described in 2024 based on a few bones that were previously assigned to the genus Saurophaganax. Several other species are no longer accepted, and several other genera, such as Epanterias, Creosaurus, and Labrosaurus, have been considered as synonyms of Allosaurus. Fossils from Tanzania, Siberia, and Australia have been previously assigned but are no longer thought to belong to this genus.
The genus Allosaurus
Allosaurus is among the best-known dinosaurs, both scientifically and in public perception. It is the most common theropod in the Morrison Formation, an important fossil deposit in the Western US, accounting for 70 to 75% of theropod specimens. Additional specimens have been discovered in Portugal. Besides the type species, A. fragilis, several additional species have been proposed, three of which are currently in use. Most specimens are not yet assigned to any particular species. Allosaurus was discovered during the Bone Wars, a feud between the two paleontologists Othniel Charles Marsh and Edward Drinker Cope that led to a surge of fossil discoveries in the Western US. Marsh coined the name Allosaurus fragilis in 1877 for a fragmentary specimen (the holotype specimen; the specimen on which the species was based). According to Marsh, this specimen includes a lumbar vertebra, later identified as a middle dorsal vertebra (back vertebra); a tail vertebra; and "feet bones", later identified as the proximal phalanx (finger bone) of digit III of the right foot. James Henry Madsen, in his 1976 monograph "Allosaurus Fragilis: A Revised Osteology", noted that a tooth and a humerus (upper arm bone) fragment are part of the same specimen. These fossils are part of the collection of the Peabody Museum of Natural History and have the specimen number YPM 1930. The genus name Allosaurus comes from the Greek words allos/αλλος, meaning 'strange' or 'different', and sauros/σαυρος, meaning 'lizard' or 'reptile'. Marsh chose the name 'different lizard' because he believed that the vertebrae were different from those of other dinosaurs due to the deep concavities in their sides, giving them a lightweight construction. More complete specimens have later shown that these concavities were in fact internal chambers that were exposed due to damage of the bone. The bones were collected in 1877 from the Morrison Formation by Marshall Felch in what is now known as Felch Quarry, in the Garden Park area, Colorado.
In a 1920 monograph, Charles W. Gilmore published the first comprehensive description of Allosaurus, mostly based on the much more complete specimen USNM 4734. This specimen includes a skeleton with skull that Felch had discovered in the same quarry in which he already found the holotype. However, Gilmore used the name Antrodemus valens instead of Allosaurus fragilis, an older name that was based on a single vertebra of uncertain origin, and which he thought had priority. Antrodemus then became the accepted name for the animal for more than 50 years, until Madsen published his 1976 monograph in which he returned to Marsh's name. Madsen's account was based on immense amounts of material from Cleveland-Lloyd Dinosaur Quarry as well as DINO 2560, a well-preserved skeleton from Dinosaur National Monument.
Neotype designation and diagnosis
The holotype specimen of Allosaurus fragilis is too fragmentary to distinguish it from related genera and therefore not diagnostic, and the same is true of the holotype of Antrodemus valens, which consists of a part of a tail vertebra. Therefore, Madsen designated DINO 2560 as the neotype (replacement) specimen in his 1976 monograph, but this was not recognized because such a designation requires a formal ruling by the International Commission on Zoological Nomenclature (ICZN). In 2010, Gregory S. Paul and Kenneth Carpenter argued that the younger age and the proportionally longer skull of this specimen suggest that it might belong to a taxon other than A. fragilis. Paul and Carpenter instead argued that USNM 4734 should become the neotype, and submitted a formal case to the ICZN to have the name A. fragilis officially transferred to this specimen in order to warrant stability of the genus. They argued that USNM 4734 is the ideal replacement specimen because it is from the same quarry as the holotype and therefore probably of the same species. The proposal has subsequently been supported by other comments and was ratified by the ICZN on December 29, 2023. The exact set of anatomical features used to distinguish Allosaurus from other genera varies between studies. Among the commonly cited features is the large horn that is formed by the lacrimal bone and is situated above and in front of the eye. The lower jaw contains a bone known as the antarticular, which newly evolved in Allosaurus and is not found in other genera. The paroccipital processes (a pair of bony struts that extend backwards and sidewards from the braincase at the back of the skull) are extending ventrally (downwards) beyond the level of the basal tubera (tubercle-like extensions on the underside of the braincase). Another distinguishing feature is found in the pelvis, where the lower end of the ischium is suboval in side view.
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