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 (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 (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 /, meaning or , and /, meaning or . Marsh chose the name 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 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 (a pair of bony struts that extend backwards and sidewards from the at the back of the skull) are extending ventrally (downwards) beyond the level of the (tubercle-like extensions on the underside of the braincase). Another distinguishing feature is found in the pelvis, where the lower end of the is suboval in side view.

Inner systematics The relationships between individual Allosaurus specimens can be assessed using specimen-level phylogenetic analysis, in which a software calculates which of all possible cladograms (evolutionary trees) requires the least number of evolutionary steps to explain the differences between specimens. In 2024, Burigo and colleagues analyzed five skulls to conclude that A. europaeus is a separate species most closely related to A. jimmadseni. In 2025, Malafaia and colleagues used 17 skulls and instead concluded that the two A. europaeus skulls are more closely related to some specimens of A. fragilis than to each other, suggesting that A. europaeus is a synonym of A. fragilis.

Outer systematics

Allosaurus is the eponymous member several higher-level groups: Allosaurinae, Allosauridae, Allosauria, and Allosauroidea. The subfamily Allosaurinae was first used by Gregory S. Paul in a 1988 popular book and included Allosaurus and Chilantaisaurus, but has not been widely used. Allosauridae is a family that Marsh had named in 1878, one year after his description of A. fragilis. Originally, Allosauridae contained only Allosaurus itself. Although additional genera were later added, recent reviews have restricted the family to Allosaurus and Saurophaganax, with the latter no longer being recognized as a distinct genus. Allosauria is a higher-level group that typically includes Allosauridae and Carcharodontosauria. Allosauroidea is an even more inclusive group that also encompasses Metriacanthosauridae.

Currently recognized species

Allosaurus fragilis The type species of Allosaurus, A. fragilis, was named by Marsh in 1877, together with the genus Allosaurus. The name fragilis is Latin for and refers to the lightening features in the vertebrae. Because the specimen on which Marsh based the species on (the holotype, YPM 1930) is not diagnostic, the substantially more complete specimen USNM 4734 was designated as the neotype in 2023. USNM 4734 was discovered by Felch in the same quarry in which the holotype was discovered; it is therefore likely that both represent the same species.

Malafaia and colleagues, in their 2025 study, proposed that A. fragilis can be diagnosed by four features found in the skull. There are two rows of (small openings for blood vessels, nerves, and similar tissues) on the outer surface of the (upper jaw bone). The (bone of the cheek area) has a bulging on its lower edge, and in the braincase, the fossa (depression) on the below the (the bony projection that connects with the vertebral column) has parallel margins and is less than 60% of the width of the occipital condyle. The lacrimal horn is triangular in shape.

Allosaurus europaeus

A. europaeus was named in 2006 by Octávio Mateus and colleagues based on a partial skull and three neck vertebrae (ML 415) from the Vale Frades beach in Lourinhã, Portugal. These authors also assigned a second specimen to this species, a partial skeleton that includes an hind limb and pelvis (MNHNUL/AND.001), which was found in 1988 near the village of Andrés in the District of Leiria. The specific name europaeus alludes to Europe. Mateus and colleagues defined their new species based on 17 features in the skull, such as a narrow lacrimal horn and a bifurcated rear end of the maxilla. The status of A. europaeus is controversial, and some studies have argued that the species is a synonym of A. fragilis, a nomen dubium (dubious name), or in need of re-evaluation. In a 2025 analysis, Malafaia and colleagues argued that all but one of the diagnostic features of A. europaeus fall within the variation range of A. fragilis.

In 2025, André Burigo and Mateus re-described the Vale Frades specimen and identified nine unique features in the skull and neck vertebrae that support the validity A. europaeus. These authors further proposed that the species differs from A. fragilis in its longer lacrimal horns, a narrow crest on the nasal bone, the sizes of small pneumatic foramina (small openings) in the nasal bone, and the presence of additional laminae (sheets of bone) in the neck vertebrae. These results were questioned by Malafaia and colleagues in 2025, although these authors cautioned that most of the diagnostic features listed by Burigo and Mateus were new and still have to be evaluated in a wider range of Allosaurus specimens.

Allosaurus jimmadseni

The name A. jimmadseni was first used in 2000, in the unpublished PhD thesis of Daniel Chure, and has since been used informally as a nomen nudum (, a name that was invalidly published). A. jimmadseni was formally described by Chure and Mark Loewen in 2020. The name honors James Madsen for his work on the Cleveland-Lloyd Dinosaur Quarry and his influential 1976 monograph on the genus. A nearly complete specimen from Dinosaur National Monument, DINO 11541, was chosen as the holotype specimen, while several other specimens were assigned to the species, including MOR 693 ("Big Al") and SMA 0005 ("Big Al II"). Chure and Mark diagnosed the species by a unique combination of seven anatomical details, including a low and narrow crest that runs on either side of the skull on along the nasal bones, lacrimal horns that are higher than those of A. fragilis, and the straight lower margin of the jugal. In 2024, Susannah Maidment found that A. fragilis and A. jimmadseni appear to have been contemporaneous but separated geographically, with A. fragilis concentrated in the south and east of the Morrison basin and A. jimmadseni in the north and west. Borth species occur together only at Dry Mesa quarry.

Allosaurus anax