Taxonomy of Allosaurus

The dinosaur genus Allosaurus has a complex taxonomic history, including multiple proposed species of which only a few are considered valid. The genus was first described during the Bone Wars by Othniel Charles Marsh from a very fragmentary specimen from Colorado, US. 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 on).

Four species are considered as 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 considered valid, and a number of other genera, such as Epanterias, Creosaurus, and Labrosaurus, have been considered as synonyms of Allosaurus.^[1] Several fossils from outside of North America and Portugal were assigned to Allosaurus, including finds from Tanzania, Siberia, and Australia, but these are no longer thought to belong to this genus.

The genus Allosaurus

Casts of the holotype of Allosaurus fragilis, YPM 1930, including a foot bone (1) and parts of two vertebrae (2 and 3)

Allosaurus is among the best-known dinosaurs, both scientifically and in public perception.^[2] It is the most common theropod in the Morrison Formation, accounting for 70 to 75% of theropod specimens,^[3] with additional specimens discovered from Portugal.^[4] Besides the type species, A. fragilis, several additional species have been proposed, but only three of these are currently accepted as potentially valid.^[5] Most specimens are not yet assigned to any particular species.^[3]

Allosaurus was discovered during the Bone Wars, a feud between two American paleontologists, Othniel Charles Marsh and Edward Drinker Cope that led to a surge of fossil discoveries in the Western US.^[6] The holotype specimen of A. fragilis (the specimen on which the species is based on) includes a tooth, the centra (bodies) of two dorsal vertebrae, and a bone from the third toe.^[2] These fossils are part of the collection of the Peabody Museum of Natural History and have the specimen number YPM 1930. Marsh gave them the name Allosaurus fragilis in 1877.^[7] Allosaurus comes from the Greek words allos/αλλος, meaning "strange" or "different", and sauros/σαυρος, meaning "lizard" or "reptile".^[8] Marsh chose the name 'different lizard' because he believed that the vertebrae were different from those of other dinosaurs due to deep concavities in their sides, giving them a lightweight construction. More complete specimens have later shown that these concavities were in fact internal cavities.^[9][7] The bones were collected from the Morrison Formation by Marshall Felch in what is now known as Felch Quarry, in the Garden Park area, Colorado.^[6]

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 he thought should have priority. Antrodemus then because the accepted name for the animal for more than 50 years, until James Henry Madsen published his 1976 monograph "Allosaurus Fragilis: A Revised Osteology", in which he returned to Marsh's name. Madsens 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.^[10][11]

Neotype designation and diagnosis

The neotype specimen of A. fragilis, USNM 4734

The holotype specimen of Allosaurus fragilis is too fragmentary to distinguish it from related genera and therefore undiagnostic, and the same is true for 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 designation did not comply with the nomenclatural rules of the ICZN. In 2010, Gregory S. Paul and Kenneth Carpenter submitted a formal case to the ICZN to have the name A. fragilis officially transferred to USNM 4734, which was then to become the neotype, to warrant stability of the genus. These researchers argued that USNM 4734 is the ideal replacement because it is from the same quarry as the holotype, and therefore probably of the same species.^[2] The decision has subsequently supported by two other comments and was ratified by the ICZN on December 29, 2023.^[12][13][14]

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.^[4]

Currently recognized species

Allosaurus was a variable genus, one such area of variability being size.

Allosaurus fragilis

The type species of Allosaurus, A. fragilis, was named by Marsh in 1877, together with the genus Allosaurus.^[7] The name fragilis is Latin for "fragile" and refers to the lightening features in the vertebrae.^[9] Because the specimen Marsh based the species on (the holotype, YPM 1930) is undiagnostic, 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.^[2]

Malafaia and colleagues, in their 2025 study, proposed that A. fragilis can be diagnosed by four features found in the skull. These includes the presence of two rows of nutrient foramina (small openings for blood vessels, nerves, and similar tissues) on the outer surface of the maxilla (upper jaw bone). Some other Allosaurus specimens only show a single row. The jugal (bone of the cheek area) has a bulging on its lower edge, and in the braincase, the fossa (depression) on the basioccipital below the occipital condyle (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.^[4]

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 articulated hindlimb 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.^[15] The status of A. europaeus was controversially discussed in the subsequent years, with different studies arguing that the species is a synonym of A. fragilis,^[16] a nomen dubium (doubtful name),^[17] or in need of re-evaluation.^[18] 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.^[4]

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.^[19] 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.^[4]

Allosaurus jimmadseni

The name A. jimmadseni was first used in 2000 the unpublished PhD thesis of Chure, and has since been used informally as a nomen nudum ('nacked name', a name that was invalidly published).^{[20]: 223 [21]} A. jimmadseni was formally described by Daniel Chure And Mark Loewen in 2020. The name honors James H. Madsen, Jr 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 from the premaxilla at the tip of the snout over the nasals and until the lacrimal bone, lacrimal horns that are higher than those of A. fragilis, and the straight lower margin of the jugal.^[21]

Allosaurus anax

A. anax was described by Andy Danison and colleagues in 2024, based on a few bones that were previously included in the taxon Saurophaganax maximus, which had been regarded as an allosaurid separate from Allosaurus. Danison and colleagues found that Saurophaganax is a chimera, comprising the fossils of an diplodocid sauropod as well as Allosaurus fossils. One of these Allosaurus fossils, a postorbital bone (OMNH 1771), became the holotype of A. anax, while six additional specimens consisting of parts of cervical and dorsal vertebrae as well as fibulae (calf bones) were assigned to the species. The name anax is Greek for 'king', and also alludes to the name Saurophaganax. According to Danison and colleagues, the A. anax fossils are significantly larger than those of other Allosaurus species. Unique features of the species include the lack of pronounced ornamentation of the postorbital and the hourglass-shaped dorsal centra (the vertebral bodies of the back vertebrae) that are penetrated by pneumatic foramina (small openings), among other features.^[5]

Previously assigned species and synonyms

Poikilopleuron valens (Antrodemus)

Main article: Antrodemus

Antrodemus valens holotype tail vertebra (above) compared to same of Allosaurus (below)

In 1869, Ferdinand Vandeveer Hayden obtained a fossil secondhand from Middle Park, near Granby, Colorado, probably from Morrison Formation rocks. Hayden sent his specimen to Joseph Leidy, who identified it as half of a tail vertebra, and tentatively assigned it to the European dinosaur genus Poekilopleuron, as the new species Poicilopleuron [sic] valens, based on the shared presence of a large medullary cavity. He noted that should more differences to P. bucklandii be found, the new species might be assigned to its own genus, Antrodemus.^[22] In 1873, he amended his description and identified the species as Antrodemus valens.^[23] In his 1920 monograph, Gilmore concluded that Antrodemus valens was indistinguishable from Allosaurus, and that Antrodemus valens should be the preferred name because, as the older name, it had priority.^[11] Antrodemus became the accepted name over Allosaurus for over 50 years, until James H. Madsen published on the Cleveland-Lloyd specimens and concluded that Allosaurus should be used because Antrodemus was based on material with poor, if any, diagnostic features and locality information.^[10] Antrodemus has since been considered as a nomen dubium (doubtful name).^[2][24] The specimen is catalogued as USNM 218.^[2] Allosaurus valens is a new combination for Antrodemus valens used by Friedrich von Huene in 1932.^[25]: 230

Allosaurus medius

A. medius was named by Marsh in 1888 for various specimens from the Early Cretaceous Arundel Formation of Maryland.^{[26][27]: 556} These include teeth, limb, and foot bones, and, according to Marsh, indicate an animal 10 to 12 ft (3.0 to 3.7 m) in length. In 1911, most of the remains were moved by Richard Swann Lull to the new ornithopod species Dryosaurus grandis, except for a tooth.^[28] Gilmore, in 1920, considered the tooth nondiagnostic but transferred it to Dryptosaurus, as D. medius.^[11] This assignment was not accepted in a 2004 review, where A. medius was simply listed as a dubious species of theropod.^[29]

Allosaurus lucaris

A. lucaris, another Marsh name, was given to a fragmentary skeleton in 1878.^[30] This specimen (YPM 1931) stems from the Morrison Formation of southern Wyoming and comprises vertebrae and parts of the shoulder girdle and forelimbs.^[2] In 1879, Marsh decided it warranted its own genus, Labrosaurus, as L. lucaris.^[31] Marsh later placed the genus within its own family, Labrosauridae, and named a second species, L. ferox.^[32] Holtz and colleagues, in their 2004 review, listed L. lucaris as another synonym of A. fragilis.^[29] Paul and Carpenter stated that the species is from a younger age than Allosaurus, and might therefore represent a different genus. However, they found that the specimen was undiagnostic, and thus A. lucaris is a nomen dubium.^[2]

Epanterias amplexus

The new genus and species Epanterias amplexus was described by Cope in 1878 based on a specimen from Quarry 2 in Garden Park, the same region in which Felch Quarry, the type locality of Allosaurus, is located.^[33][34] The name Epanterias translates to "buttressed (vertebrae)" and is derived from the Ancient Greek epi 'upon, on' and anteris 'buttress', while the suffix ias means 'characterized by'. The name amplexus is Latin for 'embracing'.^[35] The genus is based on AMNH 5767, which includes two anterior dorsals (trunk vertebrae), one of which preserves a mostly complete neural arch; the fourth or fifth cervical (neck vertebra); an axis (the second cervical); a coracoid (a bone of the shoulder girdle); and a possible metatarsal (foot bone).^{[36]: 283–284 [2]}

Cope thought that Epanterias was related to Camarasaurus und Streptospondylus, and assigned it to the Camarasauridae,^[33][36] a group that was later assigned to Sauropoda. Subsequently, Epanterias appeared in lists of North American sauropods. In 1921, Henry Fairfield Osborn and Charles Craig Mook instead found Epanterias to be a large theropod, the largest known from the Morrison Formation at that time. These authors were the first to publish figures the fossils.^[36] In 1988, Paul argued that Epanterias was probably a large species of Allosaurus, which he classified as Allosaurus amplexus. He also suggested that large fossils from Oklahoma that would later become known as Saurophaganax maximus may be assigned to the same species.^[37] Other authors have considered E. amplexus as a synonym of Allosaurus fragilis^[29] or Allosaurus^[20]. In 2010, Paul and Carpenter noted that the E. amplexus specimen comes from higher in the Morrison Formation than the type specimen of Allosaurus fragilis, and is therefore "probably a different taxon". They also considered its holotype specimen not diagnostic and classified it as a nomen dubium.^[2]

Creosaurus atrox

Holotype material of Creosaurus atrox, more recently known as Allosaurus atrox

Creosaurus atrox was described as a new genus and species by Marsh in 1878, in the same paper that introduced A. lucaris.^[30] It is based on YPM 1890, which was collected by Samuel Wendell Williston at Como, Wyoming, and includes two skull bones (a jugal and a premaxilla with teeth), two sacrals (hip vertebrae), and parts of the pelvis and hind limb, including an illium.^{[2][30][11]: 118} Marsh found these fossils to be "in excellent preservation". He speculated that the animal preyed upon the Atlantosauridae (sauropods), and estimated its body length at about 20 ft (6.1 m).^[30] Marsh found Creosaurus to be closely related to Dryptosaurus,^[30] and in 1884 classified these and some other genera including Allosaurus within the Megalosauridae.^[32]

In 1901, Williston argued that Marsh has never been able to adequately distinguish Creosaurus from Allosaurus, and that Marsh labelled the drawing of a lumbar vertebra as C. atrox in a 1884 paper^[32] even though he had labelled the same drawing as A. fragilis in a 1879 paper.^[31][38] Osborn, in 1903, assigned two fragmentary skulls to Creosaurus, but re-assigend them to Allosaurus in 1912.^{[39][40][11]: 9} Oliver Perry Hay, in 1908, again questioned the validity of Creosaurus, as the ilium, which he assumed to be the only bone belonging to the holotype, is indistinguishable from that of Allosaurus.^[41] In 1911, Richard Swann Lull named a second species, Creosaurus potens, based on a series of vertebrae from the Arundel Formation of Maryland. In his 1920 monograph, Gilmore considered C. atrox as a synonym of Antrodemus (=Allosaurus), but assigned C. potens to the genus Dryptosaurus, as Dryptosaurus? potens.^[11]

In his 1988 popular book, Paul assigned C. atrox to Allosaurus, as Allosaurus atrox. He suggested that A. fragilis had tall pointed horns and a slender build compared A. atrox, and that these differences were probably not due to sexual dimorphism because A. atrox is much rarer.^[37] Although the idea of two common Morrison allosaur species was followed in some semi-technical and popular works,^[42] the 2000 thesis by Chure noted that Charles Gilmore mistakenly reconstructed USNM 4734 as having a shorter skull than the specimens referred by Paul to atrox, refuting supposed differences between USNM 4734 and putative A. atrox specimens like DINO 2560.^[19] Robert T. Bakker, in a 1998 paper, referred to a "creosaur-type allosaurid" that is distinct from A. fragilis, although this informal taxon did not include the Creosaurus holotype.^[20] Matthew Carrano and colleagues, in a 2012 review, suggested that Bakkers taxon is mostly equivalent to the subsequently named species A. jimmadseni.^[20]

Labrosaurus ferox

The holotype dentary of Labrosaurus ferox, which may have been injured by the bite of another theropod

Labrosaurus ferox was named in 1884 by Marsh as a species of Labrosaurus,^[32] a genus that he had named in 1879 for a species he originally referred to as A. lucaris.^[31] L. ferox is based on a lower jaw (USNM 2315) discovered in Felch Quarry, the same quarry in which the Allosaurus holotype specimen was found.^[2] This jaw is oddly formed with a prominent gap in the tooth row at the tip of the jaw, and a rear section greatly expanded and turned down.^[32] Later researchers suggested that the bone was pathologic, showing an injury to the living animal,^[11] and that part of the unusual form of the rear of the bone was due to plaster reconstruction.^[43] It has been recognized as most likely a specimen of A. fragilis,^[29][43] or as an indeterminate specimen that may or may not belong to Allosaurus.^[2]

Allosaurus sibiricus

Allosaurus sibiricus was described in 1914 by Anatoly Riabinin on the basis of a bone, later identified as a partial fourth metatarsal, from the Early Cretaceous of Buryatia, Russia.^[44][20] In 1990, Ralph Molnar and colleagues transferred it to Chilantaisaurus, as Chilantaisaurus? sibiricus, based on its age and locality.^[45] It is now considered a nomen dubium indeterminate beyond Theropoda.^[20]

Megalosaurus meriani

Allosaurus meriani was a new combination by George Olshevsky for Megalosaurus meriani Greppin, 1870, based on a tooth from the Late Jurassic of Switzerland.^[46][47] It was later assigned to Ceratosaurus sp.^[43] and as an indeterminate member of Ceratosauria.^[20]: 258

Allosaurus tendagurensis

A. tendagurensis tibia, Naturkunde Museum Berlin

A. tendagurensis was named in 1925 by Werner Janensch for a tibia (MB.R.3620) found in the Kimmeridgian-Tithonian Tendaguru Formation in Mtwara, Tanzania.^[48] Missing portions of the bone had been restored, which possibly made the bone more elongate than it originally was.^[20]: 252 Janensch also assigned caudal vertebrae to this species, which was not accepted by later studies.^[20] Although tabulated as a tentatively valid species of Allosaurus in the second edition of The Dinosauria,^[29] subsequent studies placed it as indeterminate beyond Tetanurae, either a carcharodontosaurian or megalosaurid.^{[49][50][20]: 252} Carrano and colleagues, in 2012, noted that the tibia is not very similar to that of Allosaurus.^[20]: 252

Saurophaganax maximus

Main article: Saurophaganax

The fossils that would later become known as Saurophaganax were discovered by John Willis Stovall in 1931–1931 in the Morrison Formation of Oklahoma. In a popular magazine, Stovall named these fossils Saurophagus maximus. Later authors suggested that the name Saurophagus had already been given to a modern bird, the great kiskadee, and that Stovall's description was too terse to comply with the nomenclatural rules of the ICZN. Consequently, Saurophagus maximus was considered a nomen nudum.^[5] In 1995, Chure properly described the taxon and renamed it to Saurophaganax.^[51]

David K. Smith, in a 1998 analysis of variation within Allosaurus, concluded that S. maximus was not different enough from Allosaurus to be a separate genus, but did warrant its own species, A. maximus.^[52] This reassignment was rejected in a 2004 review of basal tetanurans.^[29] A 2024 reassessment by Andy Danison and colleagues suggested that the holotype neural arch of Saurophaganax might have been from a sauropod. Other Saurophaganax bones were assigned to diplodocid sauropods. The remaining bones were confirmed as theropodan, and assigned to a new species of Allosaurus, A. anax.^[5]

Other misassigned specimens

Several fragmentary specimens from Europe and Asia have been assigned to Allosaurus, but Burigo and colleagues, in a 2025 review, found that only the Portuguese material as well as some teeth from Germany described in 2016 are indeed assignable to the genus. Other teeth from Germany and France, as well as tracks from England, cannot be assigned to any particular genus.^[19] A specimen from the middle Cretaceous Mifune Group of Kumamoto, Japan, consisting of vertebrae, limb bones, and teeth, has been assigned to Allosaurus by Minoru Tamura and colleagues in 1991.^[53] Burigo and colleagues instead assigned this specimen to Segnosaurus.^[19] In 2003, Kurzanov and colleagues assigned six teeth from Siberia to Allosaurus.^[54]

An astragalus (ankle bone) thought to belong to a species of Allosaurus was found at Cape Paterson, Victoria in Early Cretaceous beds in southeastern Australia. It was thought to provide evidence that Australia was a refugium for animals that had gone extinct elsewhere.^[55] This identification was challenged by Samuel Welles, who thought it more resembled that of an ornithomimid,^[56] but the original authors defended their identification.^[57] With fifteen years of new specimens and research to look at, Daniel Chure reexamined the bone and found that it was not Allosaurus, but could represent an allosauroid.^[58] Similarly, Yoichi Azuma and Phil Currie, in their description of Fukuiraptor, noted that the bone closely resembled that of their new genus.^[59] It may have belonged to something similar to, or the same as, Australovenator,^[60] or it may represent an abelisaur.^[61]