Taxonomy of Allosaurus

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. 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. 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 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 a diplodocid sauropod as well as Allosaurus fossils. One of these Allosaurus fossils, a (OMNH 1771), became the holotype of A. anax, while six additional specimens consisting of parts of cervical and dorsal vertebrae as well as (calf bones) were assigned to the species. The name is Greek for , 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 a 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. ==Previously assigned species and synonyms==

Poikilopleuron valens (Antrodemus) In 1869, Ferdinand Vandeveer Hayden obtained a fossil secondhand from Middle Park, near Granby, Colorado, 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 theropod genus Poekilopleuron, as the new species Poicilopleuron [sic] valens, based on the shared presence of a large medullary cavity. He noted that if more differences from P. bucklandii were to be found, the new species might be assigned to its own genus, Antrodemus. The remaining bones were confirmed as those of a theropod, and assigned to a new species of Allosaurus, A. anax. Allosaurus lucasi Allosaurus lucasi was named in 2014 by Sebastian Dalman for two specimens from the top of the Morrison Formation (Tithonian) in Colorado. The name lucasi honors Spencer G. Lucas for his contributions to vertebrate paleontology. The specimens were excavated in 1953 by a field crew of the Yale Peabody Museum led by Joseph T. Gregory at McElmo Canyon in what is now Canyons of the Ancients National Monument, Colorado. The holotype specimen (YPM VP 57589) is a fragmentary skeleton of an adult individual that includes disarticulated bones of the skull and postcranium, including an almost complete left foot. The second specimen (YPM VP 57726) consists of two skull bones (dentary and ) of a juvenile individual. According to Dalman, A. lucasi differs from A. fragilis in several features, such as its shorter premaxilla and the shorter branches of the that contacted the jugal and the . Dalman suggested that A. lucasi was comparatively large, and possibly the geologically youngest known species of Allosaurus. Later authors did not accept this proposed species. Chure and Loewen declared the species as "invalid" in 2020, while Burigo and colleagues considered it a nomen dubium in 2025. ==Typographical errors and nomina nuda==

"Allosaurus agilis", seen in Osborn, 1912, is a typographical error for A. fragilis. "Allosaurus ferox" is a typographical error by Marsh for A. fragilis made in a footnote in 1896. Likewise, "Labrosaurus fragilis" is a typographical error by Marsh (1896) for Labrosaurus ferox. "Labrosaurus huene" is a nomen nudum used by von Huene in 1956 and 1958 for a tooth from the Upper Jurassic of Sichuan, China. "Allosaurus whitei" was coined by Pickering in 1995, and has been listed as a synonym of A. fragilis. ==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. A specimen from the Late Cretaceous Jobu Formation of Kumamoto, Japan, consisting of vertebrae, limb bones, and teeth, was assigned to Allosaurus by Minoru Tamura and colleagues in 1991. Burigo and colleagues instead assigned this specimen to Segnosaurus. In 2003, Kurzanov and colleagues assigned six teeth (PIN 4874/2) from Siberia to Allosaurus sp. Carrano and colleagues, in 2012, noted that these teeth are indeterminate beyond Theropoda. 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. In their 1981 description of the specimen, Ralph Molnar and colleagues suggested that it might indicate that Australia was a refugium for animals that had gone extinct elsewhere. This identification was challenged by Samuel Welles in 1983, who thought it more resembled that of an ornithomimid, but the original authors defended their identification in 1985. In 1998, 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. Similarly, Yoichi Azuma and Phil Currie, in their 2000 description of Fukuiraptor, noted that the bone closely resembled that of their new genus. It may have belonged to Australovenator or a similar taxon, or it may represent an abelisaur. ==References==