Allosaurus (/ˌæləˈsɔːrəs/AL-o-SAWR-us)[1] is agenus oftheropod dinosaur that lived 155 to 145 million years ago during theLate Jurassicperiod (Kimmeridgian to lateTithonianages). The first fossil remains that could definitively be ascribed to thisgenus were described in1877 byOthniel C. Marsh. The name "Allosaurus" means "different lizard", alluding to its lightweightvertebrae, which Marsh believed were unique. The genus has a very complicatedtaxonomy and includes at least three validspecies, the best known of which isA. fragilis. The bulk ofAllosaurus remains come from North America'sMorrison Formation, with material also known from theAlcobaça, Bombarral, andLourinhã formations in Portugal. It was known for over half of the 20th century asAntrodemus, but a study of the abundant remains from theCleveland-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 largebipedal predator for its time. Its skull was light, robust, and equipped with dozens of sharp,serrated teeth. It averaged 8.5 meters (28 ft) in length forA. fragilis, with the largest specimens estimated as being 9.7 meters (32 ft) 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 familyAllosauridae. 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 asornithopods,stegosaurids, andsauropods. Scientists have debated whetherAllosaurus had cooperativesocial behavior and hunted in packs or was a solitary predator that forms congregations, with evidence supporting either side.
Allosaurus was discovered during theBone Wars, a feud between two American paleontologists,Othniel Charles Marsh andEdward Drinker Cope, that led to a surge offossil discoveries in the Western US.[2] The first described fossil in the taxonomic history ofAllosaurus was a bone obtained secondhand byFerdinand V. Hayden in1869.[3]: 11 [4] It came fromMiddle Park, nearGranby, Colorado, probably fromMorrison Formation rocks. The locals had identified such bones as "petrified horse hoofs". Hayden sent his specimen toJoseph Leidy, who identified it as half of a tail vertebra and tentatively assigned it to the European dinosaur genusPoekilopleuron asPoicilopleuron [sic]valens.[5] He later decided it deserved its own genus,Antrodemus.[4]
Allosaurus itself isbased onYPM 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 righthumerus (upper arm bone). Marsh gave these remains the nameAllosaurus fragilis in 1877.Allosaurus comes from theGreek wordsallos/αλλος, meaning "strange" or "different", andsauros/σαυρος, meaning "lizard" or "reptile".[6] Marsh chose the name 'different lizard' because he believed that the vertebrae were different from those of other dinosaurs due to their lightweight construction.[a][7][1] The species epithetfragilis isLatin for "fragile", again referring to the lightening features in the vertebrae.[1] The bones were uncovered by two of Marsh's collectors,Benjamin Mudge andSamuel W. Williston, in the autumn of 1877 at Felch Quarry, in theGarden Park area of Colorado. Marsh and his collectors were unsatisfied with the quality of the collected fossils, so he ordered to close the quarry that same autumn. Yet, Marsh named two new dinosaurs from these remains:Diplodocus andAllosaurus. In 1883, Marsh hired the original discoverer of the quarry, Marshall P. Felch, to continue excavations. Felch's subsequent discoveries made the quarry one of the prime sites of the Morrison, and included the holotype specimens ofCeratosaurus nasicornis,Stegosaurus stenops, and a mostly completeAllosaurus skeleton (USNM 4734) that would later be selected as theneotype specimen ofAllosaurus fragilis (the single specimen the species is based on, replacing the inadequateholotype).[2]
In 1879, one of Cope's collectors, H. F. Hubbell, found a specimen in the Como Bluff area, 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 asAMNH 5753, was put on public view in1908.[8] This is the well-known mount poised over a partialApatosaurus skeleton as ifscavenging it, illustrated as such in a painting byCharles 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.[9]
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 asSamuel W. Williston suggested that too many names had been coined.[11] For example, Williston pointed out in1901 that Marsh had never been able to adequately distinguishAllosaurus fromCreosaurus.[12] The most influential early attempt to sort out the situation was produced byCharles W. Gilmore in1920. He came to the conclusion that the tail vertebra namedAntrodemus by Leidy was indistinguishable from those ofAllosaurus and thatAntrodemus should be the preferred name because, as the older name, it had priority.[13]Antrodemus became the accepted name for this familiar genus for over 50 years, untilJames H. Madsen published on the Cleveland-Lloyd specimens and concluded thatAllosaurus should be used becauseAntrodemus was based on material with poor, if any, diagnostic features and locality information. For example, thegeological formation that the single bone ofAntrodemus came from is unknown.[3]
In 1909,Earl Douglass from theCarnegie Museum discovered what should later becomeDinosaur National Monument in Utah. Until 2022, Douglass and his team excavated over 700,000 lb (320,000 kg) of fossils of multiple dinosaur species from a single quarry, including severalAllosaurus specimens.[14] Among these finds is CM 11844, which was collected between 1913 and 1915 and comprises much of the skeleton and a fragmentary skull. Since 1938, this skeleton is on display at the Carnegie Museum.[15] During the summer of 1924, theUniversity of Utah uncovered DINO 2560, the best preservedAllosaurus specimen known at that time. The skull of this particularly large individual is on exhibit at the Dinosaur National Monument.[14][16][17]
Map showing the thousands of dinosaur bones excavated at the Cleveland-Lloyd Dinosaur Quarry
Although sporadic work at what became known as Utah'sCleveland-Lloyd Dinosaur Quarry had taken place as early as1927 and the fossil site itself was described byWilliam L. Stokes in1945,[18] major operations did not begin there until1960. Madsen led a cooperative effort between 1960 and1965 involving nearly 40 institutions, during which thousands of bones were recovered from the site.[3] The quarry is notable for the predominance ofAllosaurus remains: the quarry preserves a minimum of 73 individual dinosaurs and at least 46 of those areA. fragilis. The great quantity of well-preservedAllosaurus 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 1 metre (3.3 feet)[19] to 12 metres (39 feet) long.[3] Because its fossils are common at both this quarry and others in the state,Allosaurus was designated as thestate fossil of Utah in1988.[20]
In the early 1990s, a Swiss team led by Kirby Siber set out for commercial fossil excavations at Howe Ranch Quarry, Wyoming. This quarry had originally been worked on in 1934 byBarnum Brown and his crew, who collected more than 30 tons of bones, mostly of sauropods. Because the Swiss team could not locate additional specimens in the quarry, they explored the surrounding area, where they discovered "Big Al" (MOR 693) in 1991: a 95% complete, partially articulatedAllosaurus specimen. However, because the new site was located on public land, the excavation was taken over by a jointMuseum of the Rockies andUniversity of Wyoming Geological Museum team.[21][22] The specimen, now on exhibit at the Museum of the Rockies, belonged to an individual of about 8 m (26 ft) in length. This was below the average size forAllosaurus,[21] as it was a subadult estimated at only 87% grown.[23] The Swiss team later excavated a secondAllosaurus, "Big Al II" (SMA 0005), on private land on Howe Ranch, which is exhibited at theAathal Dinosaur Museum in Switzerland.[22]
In 1991, Brooks Britt argued that there were at least two species ofAllosaurus: A robust species with a short and high skull and pointed lacrimal horns, and a moregracile species with a long and low skull and rounded lacrimal horns. The robust species is geologically younger from localities such asDry Mesa Quarry andGarden Park, while the gracile species, found at the Cleveland-Lloyd and atDinosaur National Monument, is older.[24]: 59 Already in 1988,Gregory S. Paul made a similar distinction in a popular book, in which he referred to the gracile species asA. fragilis and to the robust species asA. atrox, using a species originally described by Marsh asCreosaurus atrox.[25]: 310 However, a series of statistical analyses by David K. Smith between 1996 and 1999[26][27] suggested that the differences seen in the Morrison Formation material can be attributed to individual variation.[28]
Cliffs ofLourinhã Formation outcrops, Portugal. The Vale Frades beach, where theholotype ofA. europaeus was found, is labelled "2"
Allosaurus is known from at least three localities in Portugal, from rocks of theLourinhã,Bombarral, andAlcobaça formations.[29] The first specimen (MNHNUL/AND.001), a partial skeleton that includes an articulated hind limb and pelvis, was found in 1988 near the village of Andrés in theDistrict of Leiria during the construction of a warehouse.[30][31] Reported in 1999 and assigned to the speciesA. fragilis, it was the firstAllosaurus 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.[29][30] This site has been worked on again between 2005 and 2010, resulting in the discovery of at least two moreAllosaurus individuals, including many skull bones.[29] In 2005, a single maxilla was reported from theGuimarota coal mine, a locality well-known for its fossils of Mesozoic mammals; this fossil was assigned toAllosaurus but not to any particular species.[32]
A. europaeus holotype skull with diagram showing preserved elements
In 2006,Octávio Mateus and colleagues reported a partial skull and three neck vertebrae (ML 415) from the Vale Frades beach inLourinhã. As this specimen differed from North AmericanAllosaurus 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 separateAllosaurus species in Europe would suggest that faunal interchange between the continents was interrupted, possibly due to the opening of the Atlantic Ocean.[29][33] The status ofA. europaeus was controversially discussed in the subsequent years, with different studies arguing that the species is a synonym ofA. fragilis,[34] anomen dubium (doubtful name),[31] or in need of re-evaluation.[35] In 2024, André Burigo and Mateus re-described the Vale Frades specimen and carried out furtherfossil preparation that exposed additional bones. These authors identified nineunique features supporting the validityA. europaeus.[36] 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 individualAllosaurus skulls instead suggested thatA. europaeus is a synonym ofA. fragilis because the Andrés specimens were more closely related to some North American specimens than to the Vale Frades specimen.[29]
Diagram comparing skulls of three recognized species;A. fragilis (A),A. jimmadseni (B),A. europaeus (C)
On July 15, 1990, George Engelmann discovered toe bones and some tail vertebrae weathering out of the rock while conducting an inventory of fossils at Dinosaur National Monument. Staff of the National Monument began excavating the new specimen later that year; the conditions were difficult because of its location in a steeply inclined rock face that, with ongoing excavation, became vertical. In 1994, much of the excavated skeleton was flown out via helicopter in single block weighing 2,700 kg (6,000 lb).[19] The skull, which was still missing, was only located two years later, in 1996, with the help of novel radiological surveying techniques that detectgamma radiation fromradioactive minerals which accumulate in bones duringfossilization.[37] The specimen, DINO 11541, is one of the most complete theropod skeletons recovered from the Late Jurassic.[37] In 2000, Daniel Chure described the specimen in hisPhDthesis, arguing that it represents a new species,A. jimmadseni. However, as the thesis did not meet the requirements of theInternational Commission on Zoological Nomenclature (ICZN), it remained an invalidnomen nudum ("naked name") until Chure and Mark Loewen formally described the species in 2020.[38]: 223 [22] These authors assigned several other specimens toA. jimmadseni, including the original "Big Al" individual (MOR 693).[22] The namejimmadseni honors Madsen for his contributions to the taxonomy of the genus, notably his 1976 work.[22]
The issue of species and potential synonyms was historically complicated by thetype specimen ofAllosaurus fragilis (YPM 1930) being extremely fragmentary. Because of this, several scientists have interpreted the type specimen as potentially dubious, meaning the genusAllosaurus itself or at least the speciesA. fragilis would be anomen dubium ("dubious name", based on a specimen too incomplete to compare to other specimens or to classify). To address this situation,Gregory S. Paul andKenneth Carpenter submitted a petition to theICZN in 2010 to have the more complete specimen USNM 4734 selected as a neotype,[17] a decision that was ratified by the ICZN on December 29, 2023.[39] In 2014, Sebastian Dalman named the new speciesAllosaurus lucasi based on two specimens from the Tithonian of Colorado, but this species was not accepted by later authors.[40][22][29]
In 1995, Chure created the taxonSaurophaganax maximus for giant allosaurid remains from western Oklahoma. These remains had been known asSaurophagus, but that name was already in use, leading Chure to propose a substitute.[41] Smith, in his 1998 analysis of variation, concluded thatS. maximus was not different enough fromAllosaurus to be a separate genus, but did warrant its own species,A. maximus.[42] This reassignment was rejected in a review of basal tetanurans.[43] A 2024 reassessment by Andy Danison and colleagues suggested that theSaurophaganax specimen is achimera that combines the bones ofAllosaurus with those of a sauropod. The holotype ofSaurophaganax itself, aneural arch, cannot be confidently be assigned to a theropod, making it anomen dubium. TheAllosaurus bones, however, were found to be distinct enough to warrant a new species ofAllosaurus,A. anax. The nameanax is Greek for'king', and also alludes to the name change fromSaurophagus toSaurophaganax.[44]
Allosaurus was a typical largetheropod, having a massive skull on a short neck, a long, slightly sloping tail, and reduced forelimbs. The average length ofA. fragilis, the best-known species, has been estimated at 8.5 m (28 ft) and its average mass at 1.7 t (1.9 short tons),[45][46][47] with the largest definitiveAllosaurus specimen (AMNH 680) estimated at 9.7 m (32 ft) long,[48] with an estimated weight of 2.3–2.7 t (2.5–3.0 short tons).[48][49] In his 1976monograph onAllosaurus, James H. Madsen mentioned a range of bone sizes which he interpreted to show a maximum length of 12 to 13 m (39 to 43 ft).[3] As with dinosaurs in general, weight estimates are debatable, and since 1980 have ranged between 1 and 4 t (1.1 and 4.4 short tons) formodal adult weight (not maximum).[50]John Foster, a specialist on the Morrison Formation, suggests that 1 t (1.1 short tons) is reasonable for large adults ofA. fragilis, but that 700 kg (1,500 lb) is a closer estimate for average individuals.[51] Using the subadult specimen nicknamed "Big Al", since assigned to the speciesAllosaurus jimmadseni,[22] researchers using computer modeling arrived at a best estimate of 1.5 t (1.7 short tons) for the individual, but by varying parameters they found a range from approximately 1.4–2 t (1.5–2.2 short tons).[52] A separate computational project estimated the adaptive optimum body mass inAllosaurus to be 2.3 t (2.5 short tons).[53]A. europaeus has been measured up to 6–8 m (20–26 ft) in length and 1 t (1.1 short tons) in body mass.[46][54]
A. jimmadseni skeletal reconstruction
Several gigantic specimens have been attributed toAllosaurus, but may in fact belong to other genera . The dubious genusSaurophaganax (OMNH 1708) was estimated to reach around 10.5 m (34 ft) in length,[46] and its single species has sometimes been included in the genusAllosaurus asA. maximus.[44] However, a 2024 study questioned the referral of materials beyond the holotype toSaurophaganax, with the material they could confidently assign toAllosauridae belonging to the new speciesAllosaurus anax. The body mass of this species was estimated around 3.8–4.6 metric tons (4.2–5.1 short tons) based on fragmentary material.[44] Another potential specimen ofAllosaurus, once assigned to the genusEpanterias (AMNH 5767), may have measured 12.1 m (40 ft) in length.[48] A more recent discovery is a partial skeleton from the Peterson Quarry in Morrison rocks ofNew Mexico; this large allosaurid was suggested to be a potential specimen ofSaurophaganax prior to this taxon's 2024 reassessment.[55]
David K. Smith, examiningAllosaurus fossils by quarry, found that specimens from Utah'sCleveland-Lloyd Dinosaur Quarry are generally smaller than those from Wyoming'sComo Bluff or theBYUDry Mesa Quarry in Colorado, but the shapes of the bones themselves did not vary between the sites.[26] A later study by Smith incorporating specimens from the Garden Park andDinosaur National Monument sites found no justification for multiple species based on skeletal variation; skull variation was most common and was gradational, suggesting individual variation was responsible.[42] Further work on size-related variation again found no consistent differences, although the Dry Mesa material tended to clump together on the basis of theastragalus, an ankle bone.[27]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 ofA. jimmadseni based on the shape of thejugal.[28]
A. jimmadseni skull with diagram highlighting individual bones
The skull ofAllosaurus was light and equipped with dozens of sharp,serrated teeth, and both were modestly proportioned for a theropod of its size. PaleontologistGregory S. Paul gives a length of 845 mm (33.3 in) for a skull belonging to an individual he estimates at 7.9 m (26 ft) long.[25] Eachpremaxilla (the bones that formed the tip of the snout) held five teeth with D-shaped cross-sections, and eachmaxilla (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. Eachdentary (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 continually replaced, making them common fossils.[3]
Thelacrimal bones ofAllosaurus extend above and in front of the eyes to form a pair ofhorns which varied in shape and size.[3] Leading into the horns, the skull also features a pair of ridges running along the top of thenasal bones.[3] The horns were probably covered in akeratin sheath and may have had a variety of functions, including acting as sunshades for the eyes,[3] being used for display, and being used in combat against other members of the same species (although they were fragile).[3][25][56] There was a ridge along the back of the skull roof for muscle attachment, as is also seen intyrannosaurids.[25]
Inside the lacrimal bones were depressions that may have heldglands, such assalt glands.[57] Within the maxillae weresinuses that were better developed than those of morebasal theropods such asCeratosaurus andMarshosaurus; they may have been related to thesense of smell, perhaps holding something likeJacobson's organs. The roof of the braincase was thin, perhaps to improvethermoregulation for the brain.[3] 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.[58] Thebraincase andfrontals may also have had a joint.[3]
Allosaurus had ninevertebrae in the neck, 14 in the back, and five in thesacrum supporting the hips.[b] The number of tail vertebrae is unknown and varied with individual size;James Madsen estimated about 50,[3] whileGregory S. Paul considered that to be too many and suggested 45 or less.[25] There were hollow spaces in the neck andanterior back vertebrae.[3] Such spaces, which are also found in modern theropods (that is, the birds), are interpreted as having heldair sacs used inrespiration.[43] The rib cage was broad, giving it a barrel chest, especially in comparison to lessderived theropods likeCeratosaurus.[59]Allosaurus hadgastralia (belly ribs), but these are not common findings,[3] and they may haveossified poorly.[25] In one published case, the gastralia show evidence of injury during life.[60] Afurcula (wishbone) was also present, but has only been recognized since 1996; in some cases furculae were confused with gastralia.[60][61] Theilium, the main hip bone, was massive, and thepubic 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 theCleveland-Lloyd Dinosaur Quarry, independent of size, the pubes had not fused to each other at their foot ends. He suggested that this was asexual characteristic, with females lacking fused bones to make egg-laying easier.[3] This proposal has not attracted further attention, however.
Hand and claws ofA. fragilis
The forelimbs ofAllosaurus were short in comparison to the hindlimbs (only about 35% the length of the hindlimbs in adults)[62] and had three fingers per hand, tipped with large, strongly curved and pointedclaws.[3] The arms were powerful,[25] and the forearm was somewhat shorter than the upper arm (1:1.2ulna/humerus ratio).[13] The wrist had a version of the semilunatecarpal[63] also found in more derived theropods likemaniraptorans. Of the three fingers, the innermost (or thumb) was the largest,[25] and diverged from the others.[13] 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.[64] The legs were not as long or suited for speed as those oftyrannosaurids, and the claws of the toes were less developed and morehoof-like than those of earlier theropods.[25] Each foot had three weight-bearing toes and an innerdewclaw, which Madsen suggested could have been used for grasping in juveniles.[3] There was also what is interpreted as the splint-like remnant of a fifth (outermost)metatarsal, perhaps used as a lever between theAchilles tendon and foot.[65] A 2020 study by Motani and colleagues suggests thatAllosaurus was also sexually dimorphic in the width of the femur's head against its length.[66]
Skin impressions fromAllosaurus have been described. One impression, from a juvenile specimen, measures 30 square centimeters and is associated with the anterior dorsal ribs/pectoral region. The impression shows smallscales measuring 1 to 3 millimeters in diameter. A skin impression from the "Big Al Two" specimen, associated with the base of the tail, measures 20 centimeters by 20 centimeters and shows large scales measuring up to 2 centimeters in diameter. However, it has been noted that these scales are more similar to those ofsauropods, 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 fromAllosaurus.[67]
AnotherAllosaurus fossil features a skin impression from themandible, showing scales measuring 1 to 2 millimeters in diameter. The same fossil also preserves skin measuring 20 by 20 centimeters from the ventral side of the neck, showing scutate scales measuring 0.5 centimeters wide and 11 centimeters long. A small skin impression from anAllosaurus skull has been reported but never described.[67] Additional undescribed skin impressions are known from various parts of the body in one specimen.[68]
Illustrations showing the skull ofA. jimmadseni from the side (A), top (B), and back (C)
Allosaurus is the eponymous member of theAllosauridae, afamily that Marsh had named in 1878, one year after his description ofA. fragilis.[69] Originally, Allosauridae contained justAllosaurus itself. Marsh named two additional allosaurids in 1879 –Creosaurus andLabrosaurus – but these were later found to be synonyms ofAllosaurus.[70][3]: 10 In 1890,Karl Alfred von Zittel instead classifiedAllosaurus within the familyMegalosauridae,[71] while Marsh classified it withinDryptosauridae in 1895 and 1896. Most studies followed Zittel's classification within Megalosauridae until Madsen, in his 1976 monograph, argued that Megalosauridae has been used for any kind of carnivorous dinosaur with recurved and pointed teeth. BecauseAllosaurus differed fromMegalosaurus in multiple aspects, Madsen argued that Allosauridae should be kept as a separate family.[38]: 213 [3]: 10 Some studies in the 1980s and 1990s proposed thatAllosaurus was more closely related to theTyrannosauridae than to most other theropods,[72][38]: 216 but this has been rejected, with tyrannosaurids identified as members of a separate branch of theropods, theCoelurosauria.[73] Various genera, such asPiatnitzkysaurus orAcrocanthosaurus, have been classified within Allosauridae by different authors, but two later reviews from 2004 and 2012 restricted the family to justAllosaurus andSaurophaganax.[43][38] In 2024, Andy Danison and colleagues showed thatSaurophaganax did not exist as a separate genus of allosaurid, as it consisted of a mixture ofAllosaurus and sauropod fossils.[44]
Together with the groupCarcharodontosauria, Allosauridae is often classified within the groupAllosauria. Allosauria, in turn, forms the group Allosauroidea together with the familyMetriacanthosauridae in many analyses.[38] Historically,Allosaurus has often been classified in the groupCarnosauria, which originally encompassed any large theropod that was not aceratosaur or a tyrannosaurid. Carnosauria is still sometimes used, either to encompass the groups Allosauroidea andMegalosauroidea, or as an approximate synonym of Allosauroidea.[38]: 266 In a 1988 popular book,Gregory S. Paul used the subfamily Allosaurinae to unite the generaAllosaurus andChilantaisaurus.[25] This group has not been widely used.[74] Thecladogram shown below is from a 2015 study of Christophe Hendrickx and colleagues:[75]
The wealth ofAllosaurus 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 aseggs—crushed eggs from Colorado have been suggested as those ofAllosaurus.[45] Based onhistological analysis of limb bones, bone deposition appears to stop at around 22 to 28 years, which is comparable to that of other large theropods likeTyrannosaurus. From the same analysis, its maximum growth appears to have been at age 15, with an estimated growth rate of about 148 kilograms (326lb) per year.[76]
Medullary bone tissue (endosteally derived, ephemeral, mineralization located inside themedulla of the long bones in gravid female birds) has been reported in at least oneAllosaurus specimen, ashin bone from theCleveland-Lloyd Quarry. Today, this bone tissue is only formed in female birds that are laying eggs, as it is used to supplycalcium to shells. Its presence in theAllosaurus individual has been used to establish sex and show it had reached reproductive age.[77] However, other studies have called into question some cases of medullary bone in dinosaurs, including thisAllosaurus individual. Data from extant birds suggested that the medullary bone in thisAllosaurus individual may have been the result of a bone pathology instead.[78] However, with the confirmation of medullary tissue indicating sex in a specimen ofTyrannosaurus, it may be possible to ascertain whether or not theAllosaurus in question was indeed female.[79]
Restoration of a juvenileAllosaurus
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 youngerAllosaurus were faster and had different hunting strategies than adults, perhaps chasing small prey as juveniles, then becoming ambush hunters of large prey upon adulthood.[80] Thethigh 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.[81] Conversely, the skull bones appear to have generally grownisometrically, increasing in size without changing in proportion.[28]
BittenStegosaurus plate close-up, showing how well the damage matches the front of anAllosaurus "mouth"
Most paleontologists acceptAllosaurus as an active predator of large animals. There is dramatic evidence for allosaur attacks onStegosaurus, including anAllosaurus tail vertebra with a partially healed puncture wound that fits aStegosaurustail spike, and aStegosaurus neck plate with a U-shaped wound that correlates well with anAllosaurus snout.[82]Sauropods seem to be likely candidates as both live prey and as objects ofscavenging, based on the presence of scrapings on sauropod bones fitting allosaur teeth well and the presence of shed allosaur teeth with sauropod bones.[83] 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.[25] Another possibility is that it preferred to hunt juveniles instead of fully grown adults.[84][51] Research in the 1990s and the first decade of the 21st century may have found other solutions to this question.Robert T. Bakker, comparingAllosaurus toCenozoic 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. AlthoughAllosaurus 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 asaw-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.[85]
A. fragilis showing its maximum possible gape, based onBakker (1998) andRayfield et al. (2001)
Similar conclusions were drawn by another study usingfinite element analysis on anAllosaurus 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 8,724N,[86][87] but the skull could withstand nearly 55,500 N of vertical force against the tooth row.[86] The authors suggested thatAllosaurus used its skull like a machete against prey, attacking open-mouthed, slashing flesh with its teeth, and tearing it away without splintering bones, unlikeTyrannosaurus, 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.[86] 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.[88] The original authors noted thatAllosaurus 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 thepalate and lessen stress.[89] Another possibility for handling large prey is that theropods likeAllosaurus 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.[43] An additional suggestion notes that ornithopods were the most common available dinosaurian prey, and thatAllosaurus 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.[51] This is compatible with other evidence that the forelimbs were strong and capable of restraining prey.[63] Studies done by Stephen Lautenschager et al. from the University of Bristol also indicateAllosaurus could open its jaws quite wide and sustain considerable muscle force. When compared withTyrannosaurus and the therizinosauridErlikosaurus in the same study, it was found thatAllosaurus 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.[90][91]
Abiomechanical study published in 2013 by Eric Snively and colleagues found thatAllosaurus had an unusually low attachment point on the skull for thelongissimus capitis superficialis neck muscle compared to other theropods such asTyrannosaurus. 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 tofalcons, such askestrels: 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.[92] In addition,Allosaurus was able to "move its head and neck around relatively rapidly and with considerable control", at the cost of power.[93]
Other aspects of feeding include the eyes, arms, and legs. The shape of the skull ofAllosaurus limited potentialbinocular vision to 20° of width, slightly less than that of moderncrocodilians. As with crocodilians, this may have been enough to judge prey distance and time attacks.[94][95][96] The arms, compared with those of other theropods, were suited for both grasping prey at a distance or clutching it close,[63] and the articulation of the claws suggests that they could have been used to hook things.[13] Finally, the top speed ofAllosaurus has been estimated at 30–55 km (19–34 mi) per hour.[97]
A paper on the cranio-dental morphology ofAllosaurus and how it worked has deemed the hatchet jaw attack unlikely, reinterpreting the unusually wide gape as an adaptation to allowAllosaurus to deliver a muscle-driven bite to large prey, with the weaker jaw muscles being a trade-off to allow for the widened gape.[98]
Restoration ofBarosaurus rearing to defend itself against a pair ofA. fragilis
Sauropod carrion may also have been important to large theropods in the Morrison Formation. Forensic techniques indicate that sauropod carcasses were targeted byAllosaurus at all stages of decomposition, indicating that late-stage decay pathogens were not a significant deterrent.[99][100] 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.[53][101] A single dead adultBarosaurus orBrachiosaurus would have had enough calories to sustain multiple large theropods for weeks or months,[102] though the vast majority of the Morrison's sauropod fossil record consisted of much smaller-bodied taxa such asCamarasaurus lentus orDiplodocus.[103]
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,[104] 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.[105] 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.[103]
The holotype dentary ofLabrosaurus ferox, which may have been injured by the bite of anotherA. fragilis
It has been speculated since the 1970s thatAllosaurus preyed on sauropods and other large dinosaurs by hunting in groups.[106]Such a depiction is common in semitechnical and popular dinosaur literature.[57][84][107]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.[108] However, there is actually little evidence of gregarious behavior in theropods,[43] and social interactions with members of the same species would have included antagonistic encounters, as shown by injuries to gastralia[60] and bite wounds to skulls (the pathologic lower jaw namedLabrosaurus 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.[109]
AlthoughAllosaurus may have hunted in packs,[110] it has been argued thatAllosaurus 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 moderndiapsid 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 multipleAllosaurus individuals at the same site; e.g., in theCleveland–Lloyd Quarry, are not due to pack hunting, but to the fact thatAllosaurus 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 andKomodo dragons. The same interpretation applies to Bakker's lair sites.[111] There is some evidence for cannibalism inAllosaurus, includingAllosaurus shed teeth found among rib fragments, possible tooth marks on a shoulder blade,[112] and cannibalized allosaur skeletons among the bones at Bakker's lair sites.[113] On the other hand, pathological analysis done by Fothet al. argued evidence of surviving serious injuries may support gregariousness inAllosaurus.[114]
The brain ofAllosaurus, as interpreted from spiralCT scanning of anendocast, was more consistent withcrocodilian brains than those of the other livingarchosaurs, birds. The structure of thevestibular apparatus indicates that the skull was held nearly horizontal, as opposed to strongly tipped up or down. The structure of theinner ear was like that of a crocodilian, indicating thatAllosaurus was more adapted to hear lower frequencies and would have had difficulty hearing subtle sounds.[94] Theolfactory bulbs were large and well suited for detecting odors,[115] but were typical for an animal of its size.[116]
MountedA. fragilis skeleton (USNM 4734), which has several healed injuries
The specimen "Big Al" is well-known for its numerous injuries. Nineteen of its bones were broken or showed signs ofserious 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 aninvolucrum. The infection was long-lived, perhaps up to six months.[23][117]
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 thatAllosaurus had a significantly greater number of stress fractures thanAlbertosaurus,Ornithomimus orArchaeornithomimus. 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 theproximalphalanges" 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 examinedAllosaurus 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 inAllosaurus provide evidence for "very active" predation-based rather than scavenging diets.[118]
The leftscapula andfibula of anAllosaurus 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 theCleveland-Lloyd Quarry. Another specimen had fractured ribs and fused vertebrae near the end of the tail. An apparent subadult maleAllosaurus fragilis was reported to have extensive pathologies, with a total of fourteen separate injuries.
Additionally, a subadultAllosaurus individual that suffered fromspondyloarthropathy has been discovered in Dana Quarry in Wyoming. This finding represents the first known fossil evidence of spondyloarthropathy occurring in a theropod.[119]
Skeletal restoration of "Big Al II" showing bones with pathologies
Other pathologies reported inAllosaurus include:[78][120]
The Morrison Formation covers an area of 1.2 million km², andAllosaurus is found across this range. It occurs in the Salt Wash and Brushy Basin members and their equivalents, being absent only in the oldest part of the formation. Most specimens are not yet assigned to any particular species. According to a 2024 study by Susannah Maidment,A. fragilis andA. jimmadseni appear to have been contemporaneous but separated geographically, withA. fragilis concentrated in the south and east of the Morrison basin andA. jimmadseni in the north and west. Both species occur together only at Dry Mesa quarry.[126] The speciesA. anax is only known from the Kenton 1 Quarry in western Oklahoma from rocks of the Kenton Member, a locally recognised member equivalent to the Brushy Basin Member.[44]Allosaurus is the most common theropod in the Morrison Formation, accounting for 70 to 75% of theropod specimens.[51]
Allosaurus was at the toptrophic level of the Morrison food chain.[131] Calcium isotopic values showAllosaurus was an opportunistic predator that ateCamarasaurus,Camptosaurus, andDiplodocus, although it is unclear if it was hunting or scavenging on the sauropods. It also revealedAllosaurus primarily consumed flesh with only small amounts of bone being consumed compared to tyrannosaurids likeTyrannosaurus.[132] It coexisted with fellow large theropodsCeratosaurus andTorvosaurus in both the United States and Portugal.[33] According toRobert Bakker, the three appear to have had differentecological niches, based on anatomy and the location of fossils.Ceratosaurus andTorvosaurus 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, whereasAllosaurus was more compact, with longer legs, faster but less maneuverable, and seems to have preferred dry floodplains.[113]Ceratosaurus, better known thanTorvosaurus, differed noticeably fromAllosaurus in functional anatomy by having a taller, narrower skull with large, broad teeth.[133]
Dry season at the Mygatt-Moore Quarry showingCeratosaurus (center) andAllosaurus fighting over the desiccated carcass of another theropod
Utah's Cleveland-Lloyd quarry is unusual in that about 66% of the vertebrate fossils belong to a single species of carnivore,A. fragilis, and that the bones aredisarticulated (no longer connected to each other) and well-mixed. More papers have been published on how this sitemay have formed than on any other fossiltetrapod site. A popular idea is that the site was apredator trap to which theAllosaurus were attracted and became mired. In this case, the carcasses could have accumulated over a longer period of time. Other studies postulated a single catastrophe as the cause, such as a severe drought during which anAllosaurus population congregated around a water hole and, by their presence, deterred other dinosaurs.[134][135] The disarticulation of the bones has been explained with trampling by other dinosaurs; movements of the sediment; scavenging; up-welling water; or repeatedreworking and re-deposition of the bones.[134][136] It is also possible that theAllosaurus population died elsewhere for unknown reasons and that their bones were washed into the site.[136]
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 toAllosaurus andCeratosaurus, while others could have been made byTorvosaurus 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 onAllosaurus material suggest scavenging, either from the other theropods or from anotherAllosaurus. 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.[137]
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