Allosaurus Robustus Holotype

''Allosaurus'' ( ) is an extinct genus of theropod dinosaur that lived 155 to 145 million years ago during the Late Jurassic period (Kimmeridgian to late Tithonian ages). The first fossil remains that could definitively be ascribed to this genus were described in 1877 by Othniel C. Marsh. The name "''Allosaurus''" means "different lizard", alluding to its lightweight , which Marsh believed were unique. The genus has a very complicated taxonomy and includes at least three valid species, the best known of which is ''A. fragilis''. The bulk of ''Allosaurus'' remains come from North America's Morrison Formation, with material also known from the Alcobaça Formation and Lourinhã Formation in Portugal. It was known for over half of the 20th century as ''Antrodemus'', but a study of the abundant remains from the Cleveland-Lloyd Dinosaur Quarry returned the name "''Allosaurus''" to prominence. As one of the first well-known theropod dinosaurs, it has long attracted attention outside of paleontological circles.

''Allosaurus'' was a large bipedal predator for its time. Its skull was light, robust, and equipped with dozens of sharp, serrated teeth. It averaged in length for ''A. fragilis'', with the largest specimens estimated as being long. Relative to the large and powerful legs, its three-fingered hands were small and the body was balanced by a long, muscular tail. It is classified in the family Allosauridae. As the most abundant large predator of the Morrison Formation, ''Allosaurus'' was at the top of the food chain and probably preyed on large herbivorous dinosaurs such as ornithopods, stegosaurids, and sauropods. Scientists have debated whether ''Allosaurus'' had cooperative social behavior and hunted in packs or was a solitary predator that forms congregations, with evidence supporting either side.

Discovery and history

Early discoveries and research

The discovery and early study of ''Allosaurus'' is complicated by the multiplicity of names coined during the Bone Wars of the late 19th century. The first described fossil in this history was a bone obtained secondhand by Ferdinand V. Hayden in 1869. It came from Middle Park, near Granby, Colorado, probably from Morrison Formation rocks. The locals had identified such bones as "petrified horse hoofs". 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 ''Poicilopleuron'' ''valens''. He later decided it deserved its own genus, ''Antrodemus''.

''Allosaurus'' itself is based on YPM 1930, a small collection of fragmentary bones including parts of three vertebrae, a rib fragment, a tooth, a toe bone, and the shaft of the right (upper arm bone). Othniel Charles Marsh gave these remains the name ''Allosaurus fragilis'' in 1877. ''Allosaurus'' comes from the Greek words ''/'', meaning "strange" or "different", and ''/'', 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 their lightweight construction. The species epithet ''fragilis'' is Latin for "fragile", again referring to the lightening features in the vertebrae. The bones were collected from the Morrison Formation of Garden Park, north of Cañon City. Marsh and Edward Drinker Cope, who were in scientific competition with each other, went on to coin several other genera based on similarly sparse material that would later figure in the taxonomy of ''Allosaurus''. These include Marsh's ''Creosaurus'' and ''Labrosaurus'', as well as Cope's ''Epanterias''.

In their haste, Cope and Marsh did not always follow up on their discoveries (or, more commonly, those made by their subordinates). For example, after the discovery by Benjamin Mudge of the holotype specimen of ''Allosaurus'' in Colorado, Marsh elected to concentrate work in Wyoming. When work resumed at Garden Park in 1883, M. P. Felch found an almost complete ''Allosaurus'' and several partial skeletons. In addition, one of Cope's collectors, H. F. Hubbell, found a specimen in the Como Bluff area of Wyoming in 1879, but apparently did not mention its completeness and Cope never unpacked it. Upon unpacking it in 1903 (several years after Cope had died), it was found to be one of the most complete theropod specimens then known and the skeleton, now cataloged as AMNH 5753, was put on public view in 1908. This is the well-known mount poised over a partial ''Apatosaurus'' skeleton as if scavenging it, illustrated as such in a painting by Charles R. Knight. Although notable as the first free-standing mount of a theropod dinosaur and often illustrated and photographed, it has never been scientifically described.

The many names coined by Cope and Marsh complicated later research, with the situation further compounded by the terse descriptions they provided. Even at the time, authors such as Samuel W. Williston suggested that too many names had been coined. For example, Williston pointed out in 1901 that Marsh had never been able to adequately distinguish ''Allosaurus'' from ''Creosaurus''. The most influential early attempt to sort out the situation was produced by Charles W. Gilmore in 1920. He came to the conclusion that the tail vertebra named ''Antrodemus'' by Leidy was indistinguishable from those of ''Allosaurus'' and that ''Antrodemus'' should be the preferred name because, as the older name, it had priority. ''Antrodemus'' became the accepted name for this familiar genus 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. For example, the geological formation that the single bone of ''Antrodemus'' came from is unknown.

Cleveland-Lloyd discoveries and "Big Al"

Although sporadic work at what became known as Utah's Cleveland-Lloyd Dinosaur Quarry had taken place as early as 1927 and the fossil site itself was described by William L. Stokes in 1945, major operations did not begin there until 1960. Madsen led a cooperative effort between 1960 and 1965 involving nearly 40 institutions, during which thousands of bones were recovered from the site. The quarry is notable for the predominance of ''Allosaurus'' remains: the quarry preserves a minimum of 73 individual dinosaurs and at least 46 of those are ''A. fragilis.'' The bones found at the site were (no longer connected to each other) and well-mixed. The taphonomy of the site has been intensely debated, with many mutually exclusive explanations proposed as to how it may have formed. Regardless of the actual cause, the great quantity of well-preserved ''Allosaurus'' remains has allowed this genus to be known in great detail, making it among the best-known of all theropods. Skeletal remains from the quarry pertain to individuals of almost all ages and sizes, from less than to long, and the disarticulation is an advantage for describing aspects of bones that otherwise would have been concealed by other bones. Because its fossils are common at both this quarry and others in the state, ''Allosaurus'' was designated as the state fossil of Utah in 1988.

Significant finds would continue to be made after the Cleveland-Lloyd discoveries. In 1991, a Swiss team led by Kirby Siber discovered "Big Al" ( MOR 693): a 95% complete, partially articulated specimen that measured about 8 meters (about 26 ft) in length. The individual itself was below the average size for ''Allosaurus'', as it was a subadult estimated at only 87% grown. The excavation was taken over by a joint Museum of the Rockies and University of Wyoming Geological Museum team. The Swiss team later excavated a second ''Allosaurus'', "Big Al Two". Both specimens are known to have multiple preserved injuries.

Portuguese discoveries

''Allosaurus'' fossils are known from at least three localities in Portugal. These specimens come from the Lourinhã, Bombarral, and Alcobaça formations, and, like the North American occurrences, date to the Kimmerdigian to Tithonian ages. The first specimen ( MNHNUL/AND.001), a partial skeleton that includes an articulated hindlimb and pelvis, was found in 1988 near the village of Andrés in the District of Leiria during the construction of a warehouse. Reported in 1999 and assigned to the species ''A. fragilis'', it was the first ''Allosaurus'' specimen to be found outside of North America. ''A. fragilis'' became the first dinosaur species known from both Europe and North America, suggesting faunal exchange between the two continents. This site has been worked on again between 2005 and 2010, resulting in the discovery of at least two more ''Allosaurus'' individuals, including many skull bones. In 2005, a single maxilla was reported from the Guimarota coal mine, a locality well-known for its fossils of Mesozoic mammals; this fossil was assigned to ''Allosaurus'' but not to any particular species.

In 2006, Octávio Mateus and colleagues reported a partial skull and three neck vertebrae ( ML 415) from the Vale Frades beach in Lourinhã. As this specimen differed from North American ''Allosaurus'' fossils, they assigned it to a new species, ''A. europaeus''. These authors also assigned the Andrés specimen to this species, though solely based on the fact that it was found in Portugal. The presence of a separate ''Allosaurus'' species in Europe would suggest that faunal interchange between the continents was interrupted, possibly due to the opening of the Atlantic Ocean. 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'', a ''nomen dubium'' (doubtful name), or in need of re-evaluation. In 2024, André Burigo and Mateus re-described the Vale Frades specimen and carried out further fossil preparation that exposed additional bones. These authors identified nine unique features supporting the validity ''A. europaeus''. A 2025 study by Elisabete Malafaia and colleagues described the Andrés specimens in detail, including a newly discovered set of skull bones. Their analysis of the relationships between individual ''Allosaurus'' skulls instead suggested that ''A. europaeus'' is a synonym of ''A. fragilis'' because the Andrés specimens were more closely related to some North American specimens than to the Vale Frades specimen.

Taxonomy

Species

Seven species of ''Allosaurus'' have been named: ''A. anax'', ''A. amplus'', ''A. atrox'', ''A. europaeus'', the type species ''A. fragilis'', ''A. jimmadseni'' and ''A. lucasi''. Among these (excluding ''A. anax'', which was named in 2024), Chure and Loewen recovered only ''A. fragilis'', ''A. europaeus'', and the newly-named ''A. jimmadseni'' as being valid species. Some studies have suggested that ''A. europaeus'' does not show any unique characters compared to the North American species, though other authors have suggested that the species is valid and has a number of distinguishing characters.

''A. fragilis'' is the type species and was named by Marsh in 1877. It is known from the remains of at least 60 individuals, all found in the Kimmeridgian–Tithonian Upper Jurassic-age Morrison Formation of the United States, spread across Colorado, Montana, New Mexico, Oklahoma, South Dakota, Utah, and Wyoming. Details of the humerus (upper arm) of ''A. fragilis'' have been used as diagnostic among Morrison theropods, but ''A. jimmadseni'' indicates that this is no longer the case at the species level.

''A. jimmadseni'' has been scientifically described based on two nearly complete skeletons. The first specimen to wear the identification was unearthed in Dinosaur National Monument in northeastern Utah, with the original "Big Al" individual subsequently recognized as belonging to the same species. This species differs from ''A. fragilis'' in several anatomical details, including a jugal (cheekbone) with a straight lower margin. Fossils are confined to the Salt Wash Member of the Morrison Formation, with ''A. fragilis'' only found in the higher Brushy Basin Member. However, stratigraphic work done by Suzannah Maidment found that both species were actually coeval and were instead segregated by geography, with ''A. fragilis'' mostly found in the southern parts of the Morrison Formation, while ''A. jimmadseni'' is largely found in the northern parts. The specific name ''jimmadseni'' is named in honor of Madsen, for his contributions to the taxonomy of the genus, notably for his 1976 work.

''A. fragilis'', ''A. jimmadseni'', ''A. anax'', ''A. amplus'', and ''A. lucasi'' are all known from remains discovered in the Kimmeridgian–Tithonian Upper Jurassic-age Morrison Formation of the United States, spread across 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 60 individuals. For a while in the late 1980s and early 1990s, it was common to recognize ''A. fragilis'' as the short-snouted species, with the long-snouted taxon being ''A. atrox.'' However, subsequent analysis of specimens from the Cleveland-Lloyd Dinosaur Quarry, Como Bluff, and Dry Mesa Quarry showed that the differences seen in the Morrison Formation material could be 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 on such features as the shape of the lacrimal horns and the proposed differentiation of ''A. jimmadseni'' based on the shape of the jugal. ''A. anax'' was described and named in 2024 from several fossils representing various skeleton parts, the holotype being a postorbital numbered as OMNH 1771. This species is characterized by the lack of rugose ornamentation on the postorbital, the dorsal vertebrae with hourglass-shaped centra and pneumatic foramina, and other features of the postorbital, cervical vertebrae, and fibula. The specific name comes from the Ancient Greek ἄναξ (''anax'', "king", "lord" or "tribal chief"), and is intended to be an updated reference to the now dubious saurischian genus ''Saurophaganax'', to which the fossils were previously attributed.

The ''Allosaurus'' material from Portugal has a controversial taxonomic research history. The Andrés ''Allosaurus'' specimens, consisting of very complete cranial and post-cranial remains, have been attributed to ''A. fragilis'', ''A.'' sp'', A. europaeus'' and ''A.'' cf. ''europaeus''. The Vale Frades ''Allosaurus'', consisting of a partial skull and cervical vertebrae and ribs, is the type specimen of ''A. europaeus'', although the validity of that species has been previously questioned. In 2024, a revised diagnosis of ''A. europaeus'' was published, confirming the validity of the species. The specific affinities of the Andrés specimens are still unclear.

The issue of species and potential synonyms was historically complicated by the type specimen of ''Allosaurus fragilis'' ( YPM 1930) being extremely fragmentary, consisting of a few incomplete vertebrae, limb fragments, rib fragments, and a single tooth. Because of this, several scientists have interpreted the type specimen as potentially dubious, meaning 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 USNM 4734 (as a neotype), a decision that was ratified by the ICZN on December 29, 2023.

Teeth of indeterminate species of ''Allosaurus'' have been reported from Tönniesberg and Kahlberg in Saxony, Germany, dating to the upper Kimmeridigian.

Synonyms

''Creosaurus'', ''Epanterias'', and ''Labrosaurus'' are provisionally regarded as junior synonyms of ''Allosaurus'', though the latter two require new analyses to clarify their specific status. Most of the species that are regarded as synonyms of ''A. fragilis'', or that were misassigned to the genus, are obscure and based on very 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''.

In his 1988 book, ''Predatory Dinosaurs of the World'', the freelance artist & author Gregory S. Paul proposed that ''A. fragilis'' had tall pointed horns and a slender build compared to a postulated second species ''A. atrox'', as well as not being a different sex due to rarity. ''Allosaurus atrox'' was originally named by Marsh in 1878 as the type species of its own genus, ''Creosaurus'', and is based on YPM 1890, an assortment of bones that includes a couple of pieces of the skull, portions of nine tail vertebrae, two hip vertebrae, an ilium, and ankle and foot bones. Although the idea of two common Morrison allosaur species was followed in some semi-technical and popular works, the 2000 thesis on Allosauridae 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, AMNH 600, and AMNH 666.

"Allosaurus agilis", seen in Zittel, 1887, and Osborn, 1912, is a typographical error for ''A. fragilis.'' "Allosaurus ferox" is a typographical error by Marsh for ''A. fragilis'' in a figure caption for the partial skull YPM 1893 and YPM 1893 has been treated as a specimen of ''A fragilis''. Likewise, "Labrosaurus fragilis" is a typographical error by Marsh (1896) for ''Labrosaurus ferox''. "A. whitei" is a ''nomen nudum'' coined by Pickering in 1996 for the complete ''Allosaurus'' specimens that Paul referred to ''A. atrox''.

"Madsenius" was coined by David Lambert in 1990, being based on remains from Dinosaur National Monument assigned to ''Allosaurus'' or ''Creosaurus'' (a synonym of ''Allosaurus''), and was to be described by paleontologist Robert Bakker as "Madsenius trux". However, "Madsenius" is now seen as yet another synonym of ''Allosaurus'' because Bakker's action was predicated upon the false assumption of USNM 4734 being distinct from long-snouted ''Allosaurus'' due to errors in Gilmore's 1920 reconstruction of USNM 4734.

"Wyomingraptor" was informally coined by Bakker for allosaurid remains from the Morrison Formation of the Late Jurassic. The remains unearthed are labeled as ''Allosaurus'' and are housed in the Tate Geological Museum. However, there has been no official description of the remains and "Wyomingraptor" has been dismissed as a ''nomen nudum'', with the remains referable to ''Allosaurus''.

Formerly assigned species and fossils

Several species initially classified within or referred to ''Allosaurus'' do not belong within the genus. ''A. medius'' was named by Marsh in 1888 for various specimens from the Early Cretaceous Arundel Formation of Maryland, although most of the remains were removed by Richard Swann Lull to the new ornithopod species '' Dryosaurus grandis'', except for a tooth. It was transferred to ''Antrodemus'' by Oliver Hay in 1902, but Hay later clarified that this was an inexplicable error on his part. Gilmore considered the tooth nondiagnostic but transferred it to ''Dryptosaurus'', as ''D. medius''. The referral was not accepted in the most recent review of basal tetanurans, and ''Allosaurus medius'' was simply listed as a dubious species of theropod. It may be closely related to ''Acrocanthosaurus''.

''Allosaurus valens'' is a new combination for ''Antrodemus valens'' used by Friedrich von Huene in 1932; ''Antrodemus valens'' itself may also pertain to ''Allosaurus fragilis'', as Gilmore suggested in 1920.

''A. lucaris'', another Marsh name, was given to a partial skeleton in 1878. He later decided it warranted its own genus, ''Labrosaurus'', but this has not been accepted, and ''A. lucaris'' is also regarded as another specimen of ''A. fragilis''. ''Allosaurus lucaris'', is known mostly from vertebrae, sharing characters with ''Allosaurus''. Paul and Carpenter stated that the type specimen of this species, YPM 1931, was from a younger age than ''Allosaurus'', and might represent a different genus. However, they found that the specimen was undiagnostic, and thus ''A. lucaris'' was a ''nomen dubium''.

''Allosaurus sibiricus'' was described in 1914 by A. N. Riabinin on the basis of a bone, later identified as a partial fourth metatarsal, from the Early Cretaceous of Buryatia, Russia. It was transferred to ''Chilantaisaurus'' in 1990, but is now considered a ''nomen dubium'' indeterminate beyond Theropoda.

''Allosaurus meriani'' was a new combination by George Olshevsky for ''Megalosaurus meriani'' Greppin, 1870, based on a tooth from the Late Jurassic of Switzerland. However, a recent overview of ''Ceratosaurus'' included it in ''Ceratosaurus'' sp.

'' Apatodon mirus'', based on a scrap of vertebra Marsh first thought to be a mammalian jaw, has been listed as a synonym of ''Allosaurus fragilis''. However, it was considered indeterminate beyond Dinosauria by Chure, and Mickey Mortimer believes that the synonymy of ''Apatodon'' with ''Allosaurus'' was due to correspondence to Ralph Molnar by John McIntosh, whereby the latter reportedly found a paper saying that Othniel Charles Marsh admitted that the ''Apatodon'' holotype was actually an allosaurid dorsal vertebra.

''A. amplexus'' was named by Gregory S. Paul for giant Morrison allosaur remains, and included in his conception ''Saurophagus maximus'' (later ''Saurophaganax''). ''A. amplexus'' was originally coined by Cope in 1878 as the type species of his new genus ''Epanterias'', and is based on what is now AMNH 5767, parts of three vertebrae, a coracoid, and a metatarsal. Following Paul's work, this species has been accepted as a synonym of ''A. fragilis''. A 2010 neotype designation by Greogry S. Paul and Kenneth Carpenter, however, suggested that ''Epanterias'' holotype is temporally younger than the ''A. fragilis'' type specimen, and that it is not the same taxon as the ''Allosaurus'' holotype.

''A. maximus'' was a new combination by David K. Smith for Chure's ''Saurophaganax maximus'', a taxon created by Chure in 1995 for giant allosaurid remains from the Morrison of Oklahoma. These remains had been known as ''Saurophagus'', but that name was already in use, leading Chure to propose a substitute. Smith, in his 1998 analysis of variation, concluded that ''S. maximus'' was not different enough from ''Allosaurus'' to be a separate genus, but did warrant its own species, ''A. maximus''. This reassignment was rejected in a review of basal tetanurans. A 2024 reassessment of fossil material assigned to ''Saurophaganax'' suggested that the holotype neural arch of this taxon could not confidently be assigned to a theropod, but that it exhibited some similarities to sauropods. Other ''Saurophaganax'' bones could be referred to diplodocid sauropods. As such, the researchers assigned the remaining theropod bones to a new species of ''Allosaurus'', ''A. anax''.

There are also several species left over from the synonymizations of ''Creosaurus'' and ''Labrosaurus'' with ''Allosaurus''. '' Creosaurus potens'' was named by Lull in 1911 for a vertebra from the Early Cretaceous of Maryland. It is now regarded as a dubious theropod. ''Labrosaurus stechowi'', described in 1920 by Janensch based on isolated ''Ceratosaurus''-like teeth from the Tendaguru beds of Tanzania, was listed by Donald F. Glut as a species of ''Allosaurus'', is now considered a dubious ceratosaurian related to ''Ceratosaurus''. ''L. sulcatus'', named by Marsh in 1896 for a Morrison theropod tooth, which like ''L. stechowi'' is now regarded as a dubious ''Ceratosaurus''-like ceratosaur.

''A. tendagurensis'' was named in 1925 by Werner Janensch for a partial shin (MB.R.3620) found in the Kimmeridgian-age Tendaguru Formation in Mtwara, Tanzania. Although tabulated as a tentatively valid species of ''Allosaurus'' in the second edition of the Dinosauria, subsequent studies place it as indeterminate beyond Tetanurae, either a carcharodontosaurian or megalosaurid. Although obscure, it was a large theropod, possibly around long and in weight.

Kurzanov and colleagues in 2003 designated six teeth from Siberia as ''Allosaurus'' sp. (meaning the authors found the specimens to be most like those of ''Allosaurus'', but did not or could not assign a species to them). They were reclassified as an indeterminate theropod. Also, reports of ''Allosaurus'' in Shanxi, China go back to at least 1982. These were interpreted as ''Torvosaurus'' remains in 2012.

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. This identification was challenged by Samuel Welles, who thought it more resembled that of an ornithomimid, but the original authors defended their identification. 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 description of ''Fukuiraptor'', noted that the bone closely resembled that of their new genus. This specimen is sometimes referred to as " Allosaurus robustus", an informal museum name. It likely belonged to something similar to ''Australovenator'', although one study considered it to belong to an abelisaur.

Description

''Allosaurus'' was a typical large theropod, having a massive skull on a short neck, a long, slightly sloping tail, and reduced forelimbs. ''Allosaurus fragilis'', the best-known species, had an average length of and mass of , with the largest definitive ''Allosaurus'' specimen (AMNH 680) estimated at long, with an estimated weight of . In his 1976 monograph on ''Allosaurus'', James H. Madsen mentioned a range of bone sizes which he interpreted to show a maximum length of . As with dinosaurs in general, weight estimates are debatable, and since 1980 have ranged between , , and approximately for modal adult weight (not maximum). John Foster, a specialist on the Morrison Formation, suggests that is reasonable for large adults of ''A. fragilis'', but that is a closer estimate for individuals represented by the average-sized thigh bones he has measured. Using the subadult specimen nicknamed "Big Al", since assigned to the species ''Allosaurus jimmadseni'', researchers using computer modeling arrived at a best estimate of for the individual, but by varying parameters they found a range from approximately to approximately . A separate computational project estimated the adaptive optimum body mass in ''Allosaurus'' to be ~2,345 kg. ''A. europaeus'' has been measured up to in length and in body mass.

Several gigantic specimens have been attributed to ''Allosaurus'', but may in fact belong to other genera. The dubious genus ''Saurophaganax'' ( OMNH 1708) was estimated to reach around in length, and its single species was sometimes included in the genus ''Allosaurus'' as ''Allosaurus maximus''. However, a 2024 study concluded that some material assigned to ''Saurophaganax'' actually belonged to a diplodocid sauropod with the material confidently assigned to Allosauridae belonging to a new species of ''Allosaurus'', ''A. anax'', and the body mass of this species was tentatively estimated around based on fragmentary material. Another potential specimen of ''Allosaurus'', once assigned to the genus ''Epanterias'' (AMNH 5767), may have measured in length. A more recent discovery is a partial skeleton from the Peterson Quarry in Morrison rocks of New Mexico; this large allosaurid was suggested to be a potential specimen of ''Saurophaganax'' prior to this taxon's 2024 reassessment.

David K. Smith, examining ''Allosaurus'' fossils by quarry, found that the Cleveland-Lloyd Dinosaur Quarry (Utah) specimens are generally smaller than those from Como Bluff (Wyoming) or Brigham Young University's Dry Mesa Quarry (Colorado), but the shapes of the bones themselves did not vary between the sites. A later study by Smith incorporating Garden Park (Colorado) and Dinosaur National Monument (Utah) specimens found no justification for multiple species based on skeletal variation; skull variation was most common and was gradational, suggesting individual variation was responsible. Further work on size-related variation again found no consistent differences, although the Dry Mesa material tended to clump together on the basis of the astragalus, an ankle bone. Kenneth Carpenter, using skull elements from the Cleveland-Lloyd site, found wide variation between individuals, calling into question previous species-level distinctions based on such features as the shape of the horns, and the proposed differentiation of ''A. jimmadseni'' based on the shape of the jugal. A study published by Motani et al., in 2020 suggests that ''Allosaurus'' was also sexually dimorphic in the width of the femur's head against its length.

Skull

The skull and teeth of ''Allosaurus'' were modestly proportioned for a theropod of its size. Paleontologist Gregory S. Paul gives a length of for a skull belonging to an individual he estimates at long. Each premaxilla (the bones that formed the tip of the snout) held five teeth with D-shaped cross-sections, and each maxilla (the main tooth-bearing bones in the upper jaw) had between 14 and 17 teeth; the number of teeth does not exactly correspond to the size of the bone. Each dentary (the tooth-bearing bone of the lower jaw) had between 14 and 17 teeth, with an average count of 16. The teeth became shorter, narrower, and more curved toward the back of the skull. All of the teeth had saw-like edges. They were shed easily, and were replaced continually, making them common fossils. Its skull was light, robust and equipped with dozens of sharp, serrated teeth.

The skull had a pair of horns above and in front of the eyes. These horns were composed of extensions of the lacrimal bones, and varied in shape and size. There were also lower paired ridges running along the top edges of the nasal bones that led into the horns. The horns were probably covered in a keratin sheath and may have had a variety of functions, including acting as sunshades for the eyes, being used for display, and being used in combat against other members of the same species (although they were fragile). There was a ridge along the back of the skull roof for muscle attachment, as is also seen in tyrannosaurids.

Inside the lacrimal bones were depressions that may have held glands, such as salt glands. Within the maxillae were sinuses that were better developed than those of more basal theropods such as ''Ceratosaurus'' and ''Marshosaurus''; they may have been related to the sense of smell, perhaps holding something like Jacobson's organs. The roof of the braincase was thin, perhaps to improve thermoregulation for the brain. The skull and lower jaws had joints that permitted motion within these units. In the lower jaws, the bones of the front and back halves loosely articulated, permitting the jaws to bow outward and increasing the animal's gape. The braincase and frontals may also have had a joint.

Postcranial skeleton

''Allosaurus'' had nine vertebrae in the neck, 14 in the back, and five in the sacrum supporting the hips. The number of tail vertebrae is unknown and varied with individual size; James Madsen estimated about 50, while Gregory S. Paul considered that to be too many and suggested 45 or less. There were hollow spaces in the neck and anterior back vertebrae. Such spaces, which are also found in modern theropods (that is, the birds), are interpreted as having held air sacs used in respiration. The rib cage was broad, giving it a barrel chest, especially in comparison to less derived theropods like ''Ceratosaurus''. ''Allosaurus'' had gastralia (belly ribs), but these are not common findings, and they may have ossified poorly. In one published case, the gastralia show evidence of injury during life. A furcula (wishbone) was also present, but has only been recognized since 1996; in some cases furculae were confused with gastralia. The ilium, the main hip bone, was massive, and the pubic bone had a prominent foot that may have been used for both muscle attachment and as a prop for resting the body on the ground. Madsen noted that in about half of the individuals from the Cleveland-Lloyd Dinosaur Quarry, independent of size, the pubes had not fused to each other at their foot ends. He suggested that this was a sexual characteristic, with females lacking fused bones to make egg-laying easier. This proposal has not attracted further attention, however.

The forelimbs of ''Allosaurus'' were short in comparison to the hindlimbs (only about 35% the length of the hindlimbs in adults) and had three fingers per hand, tipped with large, strongly curved and pointed claws. The arms were powerful, and the forearm was somewhat shorter than the upper arm (1:1.2 ulna/humerus ratio). The wrist had a version of the semilunate carpal also found in more derived theropods like maniraptorans. Of the three fingers, the innermost (or thumb) was the largest, and diverged from the others. The phalangeal formula is 2-3-4-0-0, meaning that the innermost finger (phalange) has two bones, the next has three, and the third finger has four. The legs were not as long or suited for speed as those of tyrannosaurids, and the claws of the toes were less developed and more hoof-like than those of earlier theropods. Each foot had three weight-bearing toes and an inner dewclaw, which Madsen suggested could have been used for grasping in juveniles. There was also what is interpreted as the splint-like remnant of a fifth (outermost) metatarsal, perhaps used as a lever between the Achilles tendon and foot.

Skin

Skin impressions from ''Allosaurus'' have been described. One impression, from a juvenile specimen, measures 30 cm² and is associated with the anterior dorsal ribs/pectoral region. The impression shows small scales measuring 1–3 mm in diameter. A skin impression from the "Big Al Two" specimen, associated with the base of the tail, measures 20 cm × 20 cm and shows large scales measuring up to 2 cm in diameter. However, it has been noted that these scales are more similar to those of sauropods, and due to the presence of non-theropod remains associated with the tail of "Big Al Two" there is a possibility that this skin impression is not from ''Allosaurus''.

Another ''Allosaurus'' fossil features a skin impression from the mandible, showing scales measuring 1–2 mm in diameter. The same fossil also preserves skin measuring 20 × 20 cm from the ventral side of the neck, showing scutate scales measuring 0.5 cm wide and 11 cm long. A small skin impression from an ''Allosaurus'' skull has been reported but never described.

Classification

''Allosaurus'' was an allosaurid, a member of a family of large theropods within the larger group Carnosauria. The family name Allosauridae was created for this genus in 1878 by Othniel Charles Marsh, but the term was largely unused until the 1970s in favor of Megalosauridae, another family of large theropods that eventually became a wastebasket taxon. This, along with the use of ''Antrodemus'' for ''Allosaurus'' during the same period, is a point that needs to be remembered when searching for information on ''Allosaurus'' in publications that predate James Madsen's 1976 monograph. Major publications using the name "Megalosauridae" instead of "Allosauridae" include Gilmore, 1920, von Huene, 1926, Romer, 1956 and 1966, Steel, 1970, and Walker, 1964.

Following the publication of Madsen's influential monograph, Allosauridae became the preferred family assignment, but it too was not strongly defined. Semi-technical works used Allosauridae for a variety of large theropods, usually those that were larger and better-known than megalosaurids. Typical theropods that were thought to be related to ''Allosaurus'' included '' Indosaurus'', '' Piatnitzkysaurus'', '' Piveteausaurus'', '' Yangchuanosaurus'', ''Acrocanthosaurus'', ''Chilantaisaurus'', '' Compsosuchus'', ''Stokesosaurus'', and '' Szechuanosaurus''. Given modern knowledge of theropod diversity and the advent of cladistic study of evolutionary relationships, none of these theropods is now recognized as an allosaurid, although several, like ''Acrocanthosaurus'' and ''Yangchuanosaurus'', are members of closely related families.


Below is a cladogram based on the analysis of Benson et al. in 2010.

Allosauridae is one of four families in Allosauroidea; the other three are Neovenatoridae, Carcharodontosauridae and Sinraptoridae. Allosauridae has at times been proposed as ancestral to the Tyrannosauridae (which would make it paraphyletic), one example being Gregory S. Paul's ''Predatory Dinosaurs of the World'', but this has been rejected, with tyrannosaurids identified as members of a separate branch of theropods, the Coelurosauria. Allosauridae is the smallest of the carnosaur families, with only ''Saurophaganax'' and a currently unnamed French allosauroid accepted as possible valid genera besides ''Allosaurus'' in the most recent review. Another genus, ''Epanterias'', is a potential valid member, but it and ''Saurophaganax'' may turn out to be large examples of ''Allosaurus''. Some reviews have kept the genus ''Saurophaganax'' and included ''Epanterias'' with ''Allosaurus''.

The controversial ''Saurophaganax'', initially recognized as a large ''Allosaurus''-like theropod, has had a controversial taxonomic history. In 2019, Rauhut and Pol noted that its taxonomic placement within Allosauroidea is unstable, being recovered as a sister taxon of Metriacanthosauridae or Allosauria, or even as the basalmost carcharodontosaurian. In 2024, ''Saurophaganax'' was reassessed as a dubious, chimeric taxon with the holotype being so fragmentary that it could only be confidently referred to the Saurischia, and some specimens more likely belonging to a diplodocid sauropod.

Paleobiology

Life history

The wealth of ''Allosaurus'' fossils, from nearly all ages of individuals, allows scientists to study how the animal grew and how long its lifespan may have been. Remains may reach as far back in the lifespan as eggs—crushed eggs from Colorado have been suggested as those of ''Allosaurus''. Based on histological analysis of limb bones, bone deposition appears to stop at around 22 to 28 years, which is comparable to that of other large theropods like ''Tyrannosaurus''. From the same analysis, its maximum growth appears to have been at age 15, with an estimated growth rate of about 150 kilograms (330 lb) per year.

Medullary bone tissue (endosteally derived, ephemeral, mineralization located inside the medulla of the long bones in gravid female birds) has been reported in at least one ''Allosaurus'' specimen, a shin bone from the Cleveland-Lloyd Quarry. Today, this bone tissue is only formed in female birds that are laying eggs, as it is used to supply calcium to shells. Its presence in the ''Allosaurus'' individual has been used to establish sex and show it had reached reproductive age. However, other studies have called into question some cases of medullary bone in dinosaurs, including this ''Allosaurus'' individual. Data from extant birds suggested that the medullary bone in this ''Allosaurus'' individual may have been the result of a bone pathology instead. However, with the confirmation of medullary tissue indicating sex in a specimen of ''Tyrannosaurus'', it may be possible to ascertain whether or not the ''Allosaurus'' in question was indeed female.

The discovery of a juvenile specimen with a nearly complete hindlimb shows that the legs were relatively longer in juveniles, and the lower segments of the leg (shin and foot) were relatively longer than the thigh. These differences suggest that younger ''Allosaurus'' were faster and had different hunting strategies than adults, perhaps chasing small prey as juveniles, then becoming ambush hunters of large prey upon adulthood. The thigh bone became thicker and wider during growth, and the cross-section less circular, as muscle attachments shifted, muscles became shorter, and the growth of the leg slowed. These changes imply that juvenile legs has less predictable stresses compared with adults, which would have moved with more regular forward progression. Conversely, the skull bones appear to have generally grown isometrically, increasing in size without changing in proportion.

Feeding

Most paleontologists accept ''Allosaurus'' as an active predator of large animals. There is dramatic evidence for allosaur attacks on ''Stegosaurus'', including an ''Allosaurus'' tail vertebra with a partially healed puncture wound that fits a ''Stegosaurus'' tail spike, and a ''Stegosaurus'' neck plate with a U-shaped wound that correlates well with an ''Allosaurus'' snout. Sauropods seem to be likely candidates as both live prey and as objects of scavenging, based on the presence of scrapings on sauropod bones fitting allosaur teeth well and the presence of shed allosaur teeth with sauropod bones. However, as Gregory Paul noted in 1988, ''Allosaurus'' was probably not a predator of fully grown sauropods, unless it hunted in packs, as it had a modestly sized skull and relatively small teeth, and was greatly outweighed by contemporaneous sauropods. Another possibility is that it preferred to hunt juveniles instead of fully grown adults. Research in the 1990s and the first decade of the 21st century may have found other solutions to this question. Robert T. Bakker, comparing ''Allosaurus'' to Cenozoic saber-toothed carnivorous mammals, found similar adaptations, such as a reduction of jaw muscles and increase in neck muscles, and the ability to open the jaws extremely wide. Although ''Allosaurus'' did not have saber teeth, Bakker suggested another mode of attack that would have used such neck and jaw adaptations: the short teeth in effect became small serrations on a saw-like cutting edge running the length of the upper jaw, which would have been driven into prey. This type of jaw would permit slashing attacks against much larger prey, with the goal of weakening the victim.

Similar conclusions were drawn by another study using finite element analysis on an ''Allosaurus'' skull. According to their biomechanical analysis, the skull was very strong but had a relatively small bite force. By using jaw muscles only, it could produce a bite force of 805 to 8,724 N, but the skull could withstand nearly 55,500 N of vertical force against the tooth row. The authors suggested that ''Allosaurus'' used its skull like a machete against prey, attacking open-mouthed, slashing flesh with its teeth, and tearing it away without splintering bones, unlike ''Tyrannosaurus'', which is thought to have been capable of damaging bones. They also suggested that the architecture of the skull could have permitted the use of different strategies against different prey; the skull was light enough to allow attacks on smaller and more agile ornithopods, but strong enough for high-impact ambush attacks against larger prey like stegosaurids and sauropods. Their interpretations were challenged by other researchers, who found no modern analogs to a hatchet attack and considered it more likely that the skull was strong to compensate for its open construction when absorbing the stresses from struggling prey. The original authors noted that ''Allosaurus'' itself has no modern equivalent, that the tooth row is well-suited to such an attack, and that articulations in the skull cited by their detractors as problematic actually helped protect the palate and lessen stress. Another possibility for handling large prey is that theropods like ''Allosaurus'' were "flesh grazers" which could take bites of flesh out of living sauropods that were sufficient to sustain the predator so it would not have needed to expend the effort to kill the prey outright. This strategy would also potentially have allowed the prey to recover and be fed upon in a similar way later. An additional suggestion notes that ornithopods were the most common available dinosaurian prey, and that ''Allosaurus'' may have subdued them by using an attack similar to that of modern big cats: grasping the prey with their forelimbs, and then making multiple bites on the throat to crush the trachea. This is compatible with other evidence that the forelimbs were strong and capable of restraining prey. Studies done by Stephen Lautenschager et al. from the University of Bristol also indicate ''Allosaurus'' could open its jaws quite wide and sustain considerable muscle force. When compared with ''Tyrannosaurus'' and the therizinosaurid '' Erlikosaurus'' in the same study, it was found that ''Allosaurus'' had a wider gape than either; the animal was capable of opening its jaws to a 92-degree angle at maximum. The findings also indicate that large carnivorous dinosaurs, like modern carnivores, had wider jaw gapes than herbivores.

A biomechanical study published in 2013 by Eric Snively and colleagues found that ''Allosaurus'' had an unusually low attachment point on the skull for the longissimus capitis superficialis neck muscle compared to other theropods such as ''Tyrannosaurus''. This would have allowed the animal to make rapid and forceful vertical movements with the skull. The authors found that vertical strikes as proposed by Bakker and Rayfield are consistent with the animal's capabilities. They also found that the animal probably processed carcasses by vertical movements in a similar manner to falcons, such as kestrels: The animal could have gripped prey with the skull and feet, then pulled back and up to remove flesh. This differs from the prey-handling envisioned for tyrannosaurids, which probably tore flesh with lateral shakes of the skull, similar to crocodilians. In addition, ''Allosaurus'' was able to "move its head and neck around relatively rapidly and with considerable control", at the cost of power.

Other aspects of feeding include the eyes, arms, and legs. The shape of the skull of ''Allosaurus'' limited potential binocular vision to 20° of width, slightly less than that of modern crocodilians. As with crocodilians, this may have been enough to judge prey distance and time attacks. The arms, compared with those of other theropods, were suited for both grasping prey at a distance or clutching it close, and the articulation of the claws suggests that they could have been used to hook things. Finally, the top speed of ''Allosaurus'' has been estimated at per hour.

A paper on the cranio-dental morphology of ''Allosaurus'' and how it worked has deemed the hatchet jaw attack unlikely, reinterpreting the unusually wide gape as an adaptation to allow ''Allosaurus'' to deliver a muscle-driven bite to large prey, with the weaker jaw muscles being a trade-off to allow for the widened gape.

Sauropod carrion may also have been important to large theropods in the Morrison Formation. Forensic techniques indicate that sauropod carcasses were targeted by ''Allosaurus'' at all stages of decomposition, indicating that late-stage decay pathogens were not a significant deterrent. A survey of sauropod bones from the Morrison Formation also reported widespread bite marks on sauropod bones in low-economy regions, which suggests that large theropods scavenged large sauropods when available, with the scarcity of such bite marks on the remains of smaller bones being potentially attributable to much more complete consumption of smaller or adolescent sauropods and on ornithischians, which would have been more commonly taken as live prey. A single dead adult ''Barosaurus'' or ''Brachiosaurus'' would have had enough calories to sustain multiple large theropods for weeks or months, though the vast majority of the Morrison's sauropod fossil record consisted of much smaller-bodied taxa such as ''Camarasaurus lentus'' or ''Diplodocus''.

It has also been argued that disabled individuals such as Big Al and Big Al II were physically incapable of hunting due to their numerous injuries but were able to survive nonetheless as scavengers of giant sauropod-falls, Interestingly, a recent review of paleopathologies in theropods may support this conclusion. The researchers found a positive association between allosaurids and fractures to the appendicular skeleton, while tyrannosaurs had a statistically negative association with these types of injuries. The fact that allosaurs were more likely to survive and heal even when severe fractures limited their locomotion abilities can be explained, in part, by different resource accessibility paradigms for the two groups, as allosauroids generally lived in sauropod-inhabited ecosystems, some of which, including the Morrison, have been interpreted as arid and highly water-stressed environments; however, the water-stressed nature of the Morrison has been heavily criticized in several more recent works on the basis of fossil evidence for the presence of extensive forest cover and aquatic ecosystems.

Social behavior

It has been speculated since the 1970s that ''Allosaurus'' preyed on sauropods and other large dinosaurs by hunting in groups.
Such a depiction is common in semitechnical and popular dinosaur literature. Robert T. Bakker has extended social behavior to parental care, and has interpreted shed allosaur teeth and chewed bones of large prey animals as evidence that adult allosaurs brought food to lairs for their young to eat until they were grown, and prevented other carnivores from scavenging on the food. However, there is actually little evidence of gregarious behavior in theropods, and social interactions with members of the same species would have included antagonistic encounters, as shown by injuries to gastralia and bite wounds to skulls (the pathologic lower jaw named ''Labrosaurus ferox'' is one such possible example). Such head-biting may have been a way to establish dominance in a pack or to settle territorial disputes.

Although ''Allosaurus'' may have hunted in packs, it has been argued that ''Allosaurus'' and other theropods had largely aggressive interactions instead of cooperative interactions with other members of their own species. The study in question noted that cooperative hunting of prey much larger than an individual predator, as is commonly inferred for theropod dinosaurs, is rare among vertebrates in general, and modern diapsid carnivores (including lizards, crocodiles, and birds) rarely cooperate to hunt in such a way. Instead, they are typically territorial and will kill and cannibalize intruders of the same species, and will also do the same to smaller individuals that attempt to eat before they do when aggregated at feeding sites. According to this interpretation, the accumulation of remains of multiple ''Allosaurus'' individuals at the same site; e.g., in the Cleveland–Lloyd Quarry, are not due to pack hunting, but to the fact that ''Allosaurus'' individuals were drawn together to feed on other disabled or dead allosaurs, and were sometimes killed in the process. This could explain the high proportion of juvenile and subadult allosaurs present, as juveniles and subadults are disproportionally killed at modern group feeding sites of animals like crocodiles and Komodo dragons. The same interpretation applies to Bakker's lair sites. There is some evidence for cannibalism in ''Allosaurus'', including ''Allosaurus'' shed teeth found among rib fragments, possible tooth marks on a shoulder blade, and cannibalized allosaur skeletons among the bones at Bakker's lair sites. On the other hand, pathological analysis done by Foth ''et al.'' argued evidence of surviving serious injuries may support gregariousness in ''Allosaurus''.

Brain and senses

The brain of ''Allosaurus'', as interpreted from spiral CT scanning of an endocast, was more consistent with crocodilian brains than those of the other living archosaurs, birds. The structure of the vestibular apparatus indicates that the skull was held nearly horizontal, as opposed to strongly tipped up or down. The structure of the inner ear was like that of a crocodilian, indicating that ''Allosaurus'' was more adapted to hear lower frequencies and would have had difficulty hearing subtle sounds. The olfactory bulbs were large and well suited for detecting odors, but were typical for an animal of its size.

Paleopathology

The specimen "Big Al" is well-known for its numerous injuries. Nineteen of its bones were broken or showed signs of serious infection, which may have contributed to "Big Al's" death. Pathologic bones included five ribs, five vertebrae, and four bones of the feet. A particular problem for the living animal was infection and trauma to the right foot that probably affected movement and may have also predisposed the other foot to injury because of a change in gait. "Big Al" had an infection on the first phalanx on the third toe that was afflicted by an involucrum. The infection was long-lived, perhaps up to six months.

''Allosaurus'' was one of only two theropods examined in a 2001 study to exhibit a tendon avulsion, and in both cases the avulsion occurred on the forelimb. When the researchers looked for stress fractures, they found that ''Allosaurus'' had a significantly greater number of stress fractures than ''Albertosaurus'', ''Ornithomimus'' or '' Archaeornithomimus''. Of the 47 hand bones the researchers studied, three were found to contain stress fractures. Of the feet, 281 bones were studied and 17 were found to have stress fractures. The stress fractures in the foot bones "were distributed to the proximal phalanges" and occurred across all three weight-bearing toes in "statistically indistinguishable" numbers. Since the lower end of the third metatarsal would have contacted the ground first while an allosaur was running, it would have borne the most stress. If the allosaurs' stress fractures were caused by damage accumulating while walking or running this bone should have experience more stress fractures than the others. The lack of such a bias in the examined ''Allosaurus'' fossils indicates an origin for the stress fractures from a source other than running. The authors conclude that these fractures occurred during interaction with prey, like an allosaur trying to hold struggling prey with its feet. The abundance of stress fractures and avulsion injuries in ''Allosaurus'' provide evidence for "very active" predation-based rather than scavenging diets.

The left scapula and fibula of an ''Allosaurus fragilis'' specimen cataloged as USNM 4734 are both pathological, both probably due to healed fractures. The specimen USNM 8367 preserved several pathological gastralia which preserve evidence of healed fractures near their middle. Some of the fractures were poorly healed and "formed pseudoarthroses". A specimen with a fractured rib was recovered from the Cleveland-Lloyd Quarry. Another specimen had fractured ribs and fused vertebrae near the end of the tail. An apparent subadult male ''Allosaurus fragilis'' was reported to have extensive pathologies, with a total of fourteen separate injuries.

Additionally, a subadult ''Allosaurus'' individual that suffered from spondyloarthropathy has been discovered in Dana Quarry in Wyoming. This finding represents the first known fossil evidence of spondyloarthropathy occurring in a theropod.

Other pathologies reported in ''Allosaurus'' include:
* Willow breaks in two ribs
* Healed fractures in the humerus and radius
* Distortion of joint surfaces in the foot, possibly due to osteoarthritis or developmental issues
* Osteopetrosis along the endosteal surface of a tibia.
* Distortions of the joint surfaces of the tail vertebrae, possibly due to osteoarthritis or developmental issues
* " tensive 'neoplastic' ankylosis of caudals", possibly due to physical trauma, as well as the fusion of chevrons to centra
* Coossification of vertebral centra near the end of the tail
* Amputation of a chevron and foot bone, both possibly a result of bites
* " tensive exostoses" in the first phalanx of the third toe
* Lesions similar to those caused by osteomyelitis in two scapulae
* Bone spurs in a premaxilla, ungual, and two metacarpals
* Exostosis in a pedal phalanx possibly attributable to an infectious disease
* A metacarpal with a round depressed fracture

Paleoecology

''Allosaurus'' was the most common large theropod in the vast tract of Western American fossil-bearing rock known as the Morrison Formation, accounting for 70 to 75% of theropod specimens, and as such was at the top trophic level of the Morrison food chain. The Morrison Formation is interpreted as a semiarid environment with distinct wet and dry seasons, and flat floodplains. Vegetation varied from river-lining forests of conifers, tree ferns, and ferns (gallery forests), to fern savannas with occasional trees such as the ''Araucaria''-like conifer ''Brachyphyllum''.

The Morrison Formation has been a rich fossil hunting ground. The flora of the period has been revealed by fossils of green algae, fungi, mosses, horsetails, ferns, cycads, ginkgoes, and several families of conifers. Animal fossils discovered include bivalves, snails, ray-finned fishes, frogs, salamanders, turtles, sphenodonts, lizards, terrestrial and aquatic crocodylomorphs, several species of pterosaur, numerous dinosaur species, and early mammals such as Docodonta, docodonts, Multituberculata, multituberculates, Symmetrodonta, symmetrodonts, and Triconodonta, triconodonts. Dinosaurs known from the Morrison include the theropods ''Ceratosaurus'', ''Ornitholestes'', ''Tanycolagreus'', and ''Torvosaurus'', the sauropods ''Haplocanthosaurus'', ''Camarasaurus'', ''Cathetosaurus'', ''Brachiosaurus'', ''Suuwassea'', ''Apatosaurus'', ''Brontosaurus'', ''Barosaurus'', ''Diplodocus'', ''Supersaurus'', ''Amphicoelias'', and ''Maraapunisaurus'', and the ornithischians ''Camptosaurus'', ''Dryosaurus'', and ''Stegosaurus''. ''Allosaurus'' is commonly found at the same sites as ''Apatosaurus'', ''Camarasaurus'', ''Diplodocus'', and ''Stegosaurus''. The Late Jurassic formations of Portugal where ''Allosaurus'' is present are interpreted as having been similar to the Morrison, but with a stronger Ocean, marine influence. Many of the dinosaurs of the Morrison Formation are the same genera as those seen in Portuguese rocks (mainly ''Allosaurus'', ''Ceratosaurus'', ''Torvosaurus'', and ''Stegosaurus''), or have a close counterpart (''Brachiosaurus'' and ''Lusotitan'', ''Camptosaurus'' and ''Draconyx'').

''Allosaurus'' coexisted with fellow large theropods ''Ceratosaurus'' and ''Torvosaurus'' in both the United States and Portugal. The three appear to have had different ecological niches, based on anatomy and the location of fossils. ''Ceratosaurus'' and ''Torvosaurus'' may have preferred to be active around waterways, and had lower, thinner bodies that would have given them an advantage in forest and underbrush terrains, whereas ''Allosaurus'' was more compact, with longer legs, faster but less maneuverable, and seems to have preferred dry floodplains. ''Ceratosaurus'', better known than ''Torvosaurus'', differed noticeably from ''Allosaurus'' in functional anatomy by having a taller, narrower skull with large, broad teeth. ''Allosaurus'' was itself a potential food item to other carnivores, as illustrated by an ''Allosaurus'' Pubis (bone), pubic foot marked by the teeth of another theropod, probably ''Ceratosaurus'' or ''Torvosaurus''. The location of the bone in the body (along the bottom margin of the torso and partially shielded by the legs), and the fact that it was among the most massive in the skeleton, indicates that the ''Allosaurus'' was being scavenged.

A bone assemblage in the Upper Jurassic Mygatt-Moore Quarry preserves an unusually high occurrence of theropod bite marks, most of which can be attributed to ''Allosaurus'' and ''Ceratosaurus'', while others could have been made by ''Torvosaurus'' given the size of the striations. While the position of the bite marks on the herbivorous dinosaurs is consistent with predation or early access to remains, bite marks found on ''Allosaurus'' material suggest scavenging, either from the other theropods or from another ''Allosaurus''. The unusually high concentration of theropod bite marks compared to other assemblages could be explained either by a more complete utilization of resources during a dry season by theropods, or by a collecting bias in other localities.

Notes

References

External links

Specimens, discussion, and references pertaining to ''Allosaurus fragilis''
at The Theropod Database


(), from Pioneer: Utah's Online Library

Restoration of MOR 693 ("Big Al")
an

at Scott Hartman's Skeletal Drawing website


()

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Allosauridae
Dinosaur genera
Kimmeridgian dinosaurs
Tithonian dinosaurs
Lourinhã Formation
Morrison Formation
Dinosaurs of the United States
Fossil taxa described in 1877
Taxa named by Othniel Charles Marsh