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Allosaurus

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Allosaurus
Temporal range: Late Jurassic
Replica of an Allosaurus fragilis skull (San Diego Natural History Museum).
Scientific classification
Kingdom:
Animalia
Phylum:
Chordata
Class:
Sauropsida
Superorder:
Dinosauria
Order:
Saurischia
Suborder:
Theropoda
Family:
Allosauridae
Genus:
Allosaurus

Marsh, 1877
Species
  • A. fragilis Marsh, 1877 (type)
  • ?A. europaeus Mateus et al., 2006
  • A. "jimmadseni" Chure, 2000 vide Glut, 2003
  • ?A. tendagurensis Janensch, 1925
Synonyms

Allosaurus (IPA pronunciation: /ˌæləˈsɔɹəs/) is a genus of large theropod dinosaur that lived 155 to 145 million years ago, in the late Jurassic period. The name Allosaurus comes from the Greek allos/αλλος, meaning 'strange' or 'different' and saurus/σαυρος, meaning 'lizard' or 'reptile'.[1] It was named 'different lizard' because its vertebrae were different from those of other dinosaurs known at the time of its discovery.[2][3] The genus has a complicated taxonomy, and includes an uncertain number of valid species, the best known of which is A. fragilis. The bulk of Allosaurus remains have come from North America's Morrison Formation, with material also from Portugal and possibly Tanzania.

Allosaurus was a multi-ton bipedal predator with a large skull, equipped with dozens of large, sharp teeth. Its average length was 8.5 meters (30 ft) and it was up to 12-13 meters (39-43 ft) long. Relative to the large and powerful hindlimbs, its three fingered forelimbs were small, and the body was balanced by a long, heavy tail. As an apex predator and the most common large predator in the Morrison Formation, Allosaurus was at the top of the food chain, probably preying on dinosaurs such as Stegosaurus and Camptosaurus. It also shared the landscape with several giant sauropods such as Apatosaurus, Diplodocus and Camarasaurus, though it is unknown whether it would have attacked an adult sauropod.

Description

A replica Allosaurus skeleton in Canterbury Museum, Christchurch, New Zealand. The current view is that the animal normally stood in a more horizontal position.

Allosaurus was a typical large theropod, having a massive skull on a short neck, a long tail and reduced forelimbs. Allosaurus fragilis, the best-known species, had an average length of 8.5 meters (30 ft),[4] with the largest definitive Allosaurus specimen (AMNH 680) estimated at 9.7 meters long (32 ft)[5][6] and 2.3 metric tons in weight (2.5 tons).[5] In his 1976 monograph on Allosaurus, James Madsen mentioned a range of bone sizes which he interpreted to show a maximum length of 12 to 13 meters (40 to 43 ft).[7] As with dinosaurs in general, its weight is debatable, with estimates made since 1980 of 1500 kilograms (3300 lb), 1000 to 4000 kilograms (2200 to 8800 lb), and 1010 kilograms (2230 lb) for modal adult weight (not maximum).[8] Several gigantic specimens have been attributed to Allosaurus, but may in fact belong to other genera. The closely related genus Saurophaganax (OMNH 1708) reached perhaps 10.9 meters (36 ft) in length,[5] and has sometimes been included in the genus Allosaurus as Allosaurus maximus, though recent studies support it as a separate genus.[9] Another potential specimen of Allosaurus, once assigned to the genus Epanterias (AMNH 5767), may have measured 12.1 meters in length (39.7 ft).[5]

Skull

The skull and teeth of Allosaurus were modestly-proportioned for a theropod of its size. Gregory S. Paul gives a length of 845 millimeters (33.3 in) for a skull belonging to an individual he estimates at 7.9 meters long (26 ft).[10] 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, more narrow, and more curved toward the back of the skull. All of the teeth were serrated. They were shed easily, as they were replaced continually; thus, they are common fossils.[7]

The skull was ornamented by a pair of horns above and in front of the eyes. These horns were composed of extensions of the lacrimal bones.[7] The shape and size of the horns varied. There were also lower paired ridges running along the top edges of the nasal bones that led into the horns.[7][10] The horns were probably covered in a keratin sheath and may have had a variety of functions, including acting as sunshades for the eye,[7] being used for display, and being used in combat against other members of the same species[11][10] (although they were fragile).[7] There was a ridge along the back of the skull roof for muscle attachment, as is also seen in tyrannosaurids.[10]

Inside the lacrimals were depressions that may have held glands, such as salt glands.[12] Within the maxillae were sinuses, better developed than those of more basal theropods like Ceratosaurus and Marshosaurus, and which may have been related to the sense of smell, perhaps holding something like Jacobson's organ. The roof of the braincase was thin, perhaps to improve thermoregulation for the brain.[7]

The skull showed evidence of being composed of separate modules, which could be moved in relation to one another, allowing large pieces of meat to be swallowed.[citation needed] The braincase and frontals may have had one such joint.[7]

Postcranial skeleton

Allosaurus had nine vertebrae in the neck, fourteen in the back, and five in the sacrum supporting the hips.[13] The number of tail vertebrae is unknown and would vary by animal size; James Madsen estimated approximately 50,[7] while Gregory S. Paul considered that to be too many and suggested 45 or less.[10] There were hollow spaces in the neck and anterior back vertebrae;[7] such spaces, found in modern theropods (birds), are interpreted as holding similar, bird-like air sacs used in respiration.[14] The rib cage was broad, giving it a barrel chest, especially in comparison to less derived theropods like Ceratosaurus.[15] Allosaurus had gastralia (belly ribs), but these are not commonly found;[7] they may have ossified poorly.[10] In one published case, the gastralia show evidence of injury during life.[16] A furcula (wishbone) was also present, but only recognized since 1996; in some cases furculae were confused with gastralia.[17][16] The ilium, the main hip bone, was massive, and the pubic bones 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 in 1976 that in approximately 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.[7] 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)[18] and had three fingers per hand, tipped with large strongly curved and pointed claws.[7] The arms were powerful,[10] The forearm was somewhat shorter than the upper arm (1:1.2 humerus/ulna ratio).[19] The wrist had a version of the semilunate carpal,[20] also found in more derived theropods like maniraptorans. Of the three fingers, the innermost (or thumb) was the largest,[10] and it diverged from the other fingers at rest.[19] 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.[10] Each foot had three weight-bearing claws and an inner dewclaw, which Madsen suggested in juveniles could have been used for grasping.[7] 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.[21]

Classification

Life restoration of Allosaurus fragilis.

Allosaurus was an allosaurid, a member of a clade of large theropods. The name Allosauridae was created for this genus in 1878 by Othniel Charles Marsh,[22] but this family 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 monograph (1976) and the widespread use of cladistics in paleontology; major publications using the name Megalosauridae instead of Allosauridae include Gilmore, 1920,[19] von Huene, 1926,[23] Romer, 1956 and 1966,[24][25] Steel, 1970,[26] and Walker, 1964.[27] 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,[28] Acrocanthosaurus, Chilantaisaurus, Compsosuchus, Stokesosaurus, and Szechuanosaurus.[29] With the increase in dinosaur study improving 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 related families.[14]

Allosauridae is one of three families in Carnosauria, the other two being Carcharodontosauridae and Sinraptoridae.[14] In 1988 Gregory S. Paul proposed that the family Allosauridae led to the Tyrannosauridae (and thus would be paraphyletic),[30] but this has been rejected, with tyrannosaurids identified as members of a separate branch of theropods, the Coelurosauria.[31] Allosauridae is the smallest of the carnosaur families, with only Saurophaganax and an unnamed French allosauroid accepted as possible valid allosaurids besides Allosaurus in the most recent review.[14] Another genus, Epanterias, is a potential valid member, but it and Saurophaganax may turn out to be large examples of Allosaurus.[10] Recent reviews have kept the genus Saurophaganax and included Epanterias with Allosaurus.[8][14]

Discovery and history

Early discoveries and research

The discovery and early study of Allosaurus is complicated by a multiplicity of names coined during the Bone Wars of the late 1800s. The first described fossil in this history was a bone obtained secondhand by Ferdinand Vandiveer 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 [sic] valens.[32] He later decided it deserved its own genus, Antrodemus.[33]

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, most useful for later discussions, the shaft of the right humerus (upper arm). Othniel Charles Marsh gave these remains the formal name Allosaurus fragilis in 1877. The species epithet fragilis is Latin for 'fragile', referring to lightening features in the vertebrae. The bones were collected from the Morrison Formation of Garden Park, north of Cañon City.[2] Marsh and Edward Drinker Cope, who were in scientific competition, 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[22] and Labrosaurus,[34] and Cope's Epanterias.[35]

AMNH 5753 in a Charles R. Knight life restoration, scavenging an Apatosaurus.

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 type 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.[12] 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 in 1903 (several years after Cope had died), it was found to be one of the most complete theropod specimens then known, and in 1908 the skeleton, now cataloged as AMNH 5753, was put on public view.[36] This is the well-known mount poised over a partial Apatosaurus skeleton as if scavenging it, illustrated as such by Charles R. Knight. Although notable as the first free-standing mount of a theropod dinosaurs, and often illustrated and photographed, it has never been scientifically described.[37]

The multiplicity of names complicated research, with the situation compounded by the terse descriptions provided by Marsh and Cope. Even at the time, authors such as Samuel Wendell Williston suggested that too many names had been coined.[38] For example, Williston pointed out in 1901 that Marsh had never been able to adequately distinguish Allosaurus from Creosaurus.[39] The most influential early attempt to sort out the convoluted situation was produced by Charles W. Gilmore in 1920. He came to the conclusion that the tail vertebra dubbed Antrodemus by Leidy was indistinguishable from those of Allosaurus, and Antrodemus thus should be the preferred name because as the older name it had priority.[19] Antrodemus became the accepted name for this familiar genus for over fifty years, until James Madsen published on the Cleveland-Lloyd specimens and concluded that Allosaurus should be used because Antrodemus was based on undiagnostic material with poor locality information (for example, the geological formation that the single bone of Antrodemus came from is unknown).[7] "Antrodemus" has been used informally for convenience when distinguishing between the skull Gilmore restored and the composite skull restored by Madsen.[40]

The Cleveland-Lloyd discoveries

Although sporadic work at what became known as the Cleveland-Lloyd Dinosaur Quarry in Emery County, Utah had taken place as early as 1927, and the fossil site described by William J. Stokes in 1945,[41] major operations did not begin there until 1960. Under a cooperative effort involving nearly 40 institutions, thousands of bones were recovered between 1960 and 1965.[7] The quarry is notable for several reasons: the majority of bones belong to a single species of large theropod, Allosaurus fragilis (as of 2006, of the minimum 73 dinosaurs present to account for the bones, 46 are Allosaurus); the bones are disarticulated and well-mixed; and nearly a dozen scientific papers have been written on the taphonomy of the site, coming up with numerous contradictory explanations for how it formed. Suggestions have ranged from animals getting stuck in a bog, to becoming trapped in deep mud, to falling victim to drought-induced mortality around a waterhole, to getting trapped in a spring-fed pond or seep.[42] Regardless of the actual cause, the great quantity of well-preserved Allosaurus remains has allowed this genus to be known in detail, making it among the best-known theropods. Skeletal remains from the quarry pertain to individuals of almost all ages and sizes, from less than 1 meter (3.3 ft)[43] to 12 meters (39 ft) long, and the disarticulation is an advantage for describing bones usually found fused.[7]

Recent work: 1980s-present

The period since Madsen's monograph has been marked by a great expansion in studies dealing with topics concerning Allosaurus in life (paleobiological and paleoecological topics). Such studies have covered topics including skeletal variation,[44][45] growth,[46][47] skull construction[48] and other biomechanical topics,[20] hunting methods,[49][48] the possibility of gregarious living and parental care,[50][51] and vision.[52] Reanalysis of old material (particularly of large 'allosaur' specimens),[10][53] new discoveries in Portugal,[54][55] and several very complete new specimens[56][16][57] have also contributed to the growing knowledge base.

"Big Al"

File:Allo.JPG
The Allosaurus "Big Al" (MOR 693), as depicted in The Ballad Of Big Al.

One of the more significant Allosaurus finds was the 1991 discovery of "Big Al" (MOR 693), a 95% complete, partially articulated specimen that measured about 8 meters (about 26 ft) in length. MOR 693 was excavated near Shell, Wyoming by the Museum of the Rockies and the University of Wyoming Geological Museum.[58] This skeleton was discovered by a Swiss team, led by Kirby Siber. The same team later excavated a second Allosaurus, "Big Al Two", which is the best preserved skeleton of its kind to date.[57]

The completeness, preservation, and scientific importance of this skeleton gave "Big Al" its name; the individual itself was below the average size for Allosaurus fragilis,[58] and was a subadult estimated at only 87% grown.[59] The specimen was described by Breithaupt in 1996.[56] Nineteen of its bones were broken or showed signs of infection, which may have contributed to "Big Al"'s death. Pathologic bones included five ribs, five vertebrae, and four bones of the feet; several damaged bones had osteomyelitis, a bone infection. 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.[59] "Big Al" was featured in The Ballad of Big Al, a special program in the BBC's Walking with Dinosaurs series.

Taxonomy and species

Potentially valid species

It is unclear how many species of Allosaurus there were. Seven species have been considered potentially valid since 1988 (A. amplexus,[10] A. atrox,[10] A. europaeus,[55] the type species A. fragilis,[14] the as-yet not formally described "A. jimmadseni",[9] A. maximus,[44] and A. tendagurensis[14]), although only a fraction are usually considered valid at one time. Additionally, there are eleven or more dubious or undescribed species that have been assigned to Allosaurus over the years, along with species belonging to genera now sunk into Allosaurus. In the most recent review of basal tetanuran theropods, only A. fragilis (including A. amplexus and A. atrox), "A. jimmadseni" (as an unnamed species), and A. tendagurensis were accepted as potentially valid species, with A. europaeus not yet proposed and A. maximus assigned to Saurophaganax.[14]

A. fragilis is the type species and was named by Marsh in 1877.[22] It is known from the remains of at least sixty individuals, all found in the Kimmeridgian-Tithonian Upper Jurassic-age Morrison Formation of the United States, including the states of Colorado, Montana, New Mexico, Oklahoma, South Dakota, Utah, and Wyoming.[14] Details of the humerus (upper arm) of A. fragilis have been used as diagnostic among Morrison theropods,[7] but the discovery of "A. jimmadseni" indicates that this will no longer be the case at the species level.[9] A. fragilis has always been difficult to define on the basis of its fragmentary and limited holotype, and so authors have advanced different proposals for redefining the species on better material.[7][10]

A. amplexus was named by Paul for giant Morrison allosaur remains, and included in his conception Saurophagus maximus (later Saurophaganax).[10] A. amplexus was originally coined by Cope in 1878 as the type species of his new genus Epanterias,[35] and is based on what is now AMNH 5767, parts of three vertebrae, a coracoid, and a metatarsal.[60] Subsequent to Paul's work, this species has been accepted as a synonym of A. fragilis.[14]

Allosaurus material from Portugal was first reported in 1999 on the basis of MHNUL/AND.001, a partial skeleton including a quadrate, vertebrae, ribs, gastralia, chevrons, part of the hips, and hindlimbs. This specimen was referred to A. fragilis,[54] but the subsequent discovery of a partial skull and neck (ML 415) near Lourinhã, in the Kimmeridgian-age Porto Novo Member of the Lourinhã Formation, spurred the naming of new species A. europaeus. It differs from other species of Allosaurus in cranial details.[55] However, more material may show it to be A. fragilis, as originally described.[61]

Daniel Chure's work on Morrison allosaurid remains has been responsible, directly or indirectly, for "A. jimmadseni" and A. maximus. "A. jimmadseni" is the proposed name for a new species of Morrison allosaur, based on a nearly complete skeleton and skull.[9][62] A. sp. 2, as it is also known, differs from A. fragilis in several anatomical details including a jugal or cheekbone with a straight lower margin, and is also found only in the Salt Wash Member of the Morrison, with A. fragilis only present in the higher Brushy Basin Member.Cite error: A <ref> tag is missing the closing </ref> (see the help page). This species has not had strong support in recent years, with opinions on its identity ranging from a tentatively valid species of Allosaurus,[14] to a basal tetanuran,[63] to simply a dubious theropod.[4] Although obscure, it was a large theropod, possibly around 10 meters long (33 ft) and 2.5 metric tons (2.8 short tons) in weight.[5]

Biological variation, A. atrox, and A. fragilis

The perception that there were two common Allosaurus species in the Morrison was popularized in Gregory S. Paul's Predatory Dinosaurs of the World (1988). Paul proposed that A. fragilis had tall pointed horns and a slender build compared to postulated second species A. atrox, and was not a different gender due to rarity.[10] 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 including a couple of pieces of the skull, portions of nine tail vertebrae, two hip vertebrae and an ilium (bone), and ankle and foot bones.[22] Although the idea of two common Morrison allosaur species has had support in semitechnical and popular works,[64] it has generally been rejected in the technical literature.[14] 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.[65] 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.[44] 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.[45]

Invalid and synonymous species

A number of species assigned to Allosaurus are no longer recognized as valid, for one reason or another. Species "A. agilis", seen in Zittel, 1887, and Osborn, 1912, is a typographical error for A. fragilis.[9] Marsh's A. ferox (1896; not to be confused with his 1884 Labrosaurus ferox, also part of Allosaurus taxonomy) was coined for a partial skull in a footnote,[66] and has been recognized as a specimen of A fragilis.[14] A. lucaris, another Marsh name, was given to a partial skeleton in 1878.[22] He later decided it warranted its own genus, Labrosaurus,[34] but this has not been accepted, and A. lucaris is also regarded as another specimen of A. fragilis. "A. whitei", an informally-described species coined by Pickering in 1996, is a recasting of the A. atrox versus A. fragilis debate using a better specimen to represent the A. atrox form,[9] and has not been recognized.[14]

Several species coined in genera other than Allosaurus are also now thought to be synonymous with A. fragilis. Labrosaurus ferox was 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.[67] Later researchers suggested that the bone was pathologic, showing an injury to the living animal,[19] and that part of the unusual form of the rear of the bone was due to plaster reconstruction.[68] It is recognized as most likely a specimen of A. fragilis.[68][14] Allosaurus trihedrodon started as Laelaps trihedrodon, the name given to five theropod teeth by Cope in 1877.[69] Long lost but recently relocated, these specimens also appear to pertain to Allosaurus,[70] perhaps A. fragilis.[14] This species is also partly responsible for confusion surrounding Hypsirophus discursus, a species of stegosaurid named by Cope. Cope, who was considering Hypsirophus as a theropod at the time, suggested that it might turn out to be the same as Laelaps trihedrodon.[70] Along with a mysterious theropod femur that he associated with Hypsirophus and which may have later been associated with Epanterias, this uncertainty has led to the misconception that Hypsirophus is a chimera,[70] based in part on Allosaurus fragilis remains.[4][14] Allosaurus valens is a typo for Antrodemus valens accidentally used by Friedrich von Huene in 1932;[9] Antrodemus valens itself may also pertain to Allosaurus fragilis,[14] as Gilmore suggested in 1920.[19] Apatodon mirus, based on a scrap of vertebra Marsh first thought to be a mammal jaw, may[4] or may not[14] be the same as Allosaurus.

Misassigned species

Several species that started in Allosaurus or were referred to it do not belong to Allosaurus. A. medius was named by Marsh in 1888 for "various specimens" from the Early Cretaceous of Maryland,[71] although most of the remains were removed by Richard Swann Lull to the new ornithopod species Dryosaurus grandis, except for a tooth.[72] Gilmore considered the tooth nondiagnostic but transferred it to a new species, Dryptosaurus medius.[19] The referral was not accepted in the most recent review, and Allosaurus medius was simply listed as a dubious species of theropod.[14] 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.[73] It was transferred to Chilantaisaurus in 1990.[74] This is not the only report of Allosaurus from Siberia, or Asia in general. 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 assign a species).[75] Also, reports of Allosaurus in Shanxi, China go back to at least 1982,[76] and tail vertebrae from Datong in Shanxi have been assigned to A. sp..[77]

Allosaurus meriani was described in 1870 by Greppin as a species of Megalosaurus, based on a tooth from the Late Jurassic of Switzerland.[78] It has occasionally been referred to Allosaurus,[4] but recent reviews have listed it as dubious theropod species Megalosaurus meriani,[14] or included it in Ceratosaurus sp.[68] Allosaurus stechowi was described in 1920 by Janensch as Labrosaurus stechowi for isolated Ceratosaurus-like teeth from the Tendaguru beds of Tanzania.[79] With the synonymization of Labrosaurus and Allosaurus, Glut listed it as a species of Allosaurus,[4] but it is now either assigned to Ceratosaurus sp.[68] or considered a dubious ceratosaurian.[80]

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.[72] It is now regarded as a dubious theropod.[14] "Labrosaurus huene" is an informal name used by von Huene (1956, 1958) for a tooth from the Late Jurassic of Szechuan, China, and L. fragilis is a typographical error by Marsh (1896) for Labrosaurus ferox.[68] L. sulcatus, named by Marsh in 1896 for a Morrison theropod tooth,[66] which like L. stechowi is now regarded as either Ceratosaurus sp.[68] or a dubious ceratosaurian.[80]

Possible Australian Allosaurus

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.[81] This identification was challenged by Samuel Welles, who thought it more resembled that of an ornithomimid,[82] but the original authors defended their identification.[83] 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.[84] Similarly, Yoichi Azuma and Phil Currie, in their description of the basal allosauroid Fukuiraptor, noted that the bone closely resembled that of their new genus.[85] This specimen is sometimes referred to as "Allosaurus robustus", an informal museum name.[62]

Paleoecology

Allosaurus skull from Dinosaur National Monument, still partially encased in matrix.

Allosaurus is the most common theropod in the vast tract of dinosaur-bearing rock in the American west and southwest known as the Morrison Formation.[86] The Morrison Formation is interpreted as a semiarid environment with distinct wet and dry seasons, with flat floodplains.[87] Vegetation varied from gallery forests of conifers, tree ferns, and ferns, to fern savannas with rare trees.[88] The Morrison Formation has been a rich fossil hunting ground, holding fossils including green alage, fungi, mosses, horsetails, ferns, cycads, ginkgoes, several families of conifers, bivalves, snails, ray-finned fishes, frogs, salamanders, turtles, sphenodonts, lizards, terrestrial and aquatic crocodylomorphans, several species of pterosaur, numerous dinosaur species, and early mammals such as docodonts, multituberculates, symmetrodonts, and triconodonts. Such dinosaurs as the theropods Ceratosaurus, Ornitholestes, and Torvosaurus, the sauropods Apatosaurus, Brachiosaurus, Camarasaurus, and Diplodocus, and the ornithischians Camptosaurus, Dryosaurus, and Stegosaurus are known from the Morrison.[89] The Late Jurassic formations of Portugal where Allosaurus is present are interpreted as having been similar to the Morrison but with a stronger marine influence. Many of the dinosaurs of the Morrison Formation are the same genera as those seen in Portuguese rocks (Allosaurus, Ceratosaurus, Torvosaurus, and Apatosaurus), or have a close counterpart (Brachiosaurus and Lusotitan, Camptosaurus and Draconyx).[90]

Allosaurus coexisted with fellow large theropods Ceratosaurus and Torvosaurus in both the Morrison and Portugal.[90] The three appear to have had different ecological niches, based on anatomy and where fossils have been found. Ceratosaurs and torvosaurs 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 allosaurs were more compact, with longer legs, were faster but less maneuverable, and seem to have preferred dry floodplains.[50] Ceratosaurus, better known than Torvosaurus, differed noticeably from Allosaurus in functional terms by having a taller, narrower skull with large broad teeth.[40]

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, as crushed eggs from Colorado have been suggested as those of Allosaurus.[4] Based on histological analysis of limb bones, the upper age limit for Allosaurus is estimated at 22 to 28 years, which is comparable to that of other large theropods like Tyrannosaurus. Its maximum growth appears to have been at age 15, putting on 148 kilograms (326 lb) in a year at that point.[46] 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.[47] 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 to adults, which would have moved with more regular forward progression.[91]

Feeding

Allosaurus is accepted as an active predator of large animals. Sauropods seem to be likely candidates as live prey and 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.[92] 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.[93] As Gregory Paul noted in 1988, Allosaurus seems an unlikely predator of sauropods, unless it hunted in packs, as it had a modestly-sized skull and relatively small teeth, and was greatly outweighed by contemporaneous sauropods.[10] Research in the 1990s and 2000s may have found other solutions to this question. Robert T. Bakker, comparing Allosaurus to Tertiary sabre-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 sabre teeth, Bakker suggested another mode of attack that would have used such neck and and jaw adapatations: the short teeth in effect became small serrations on a saw-like cutting edge running the length of the upper jaw, which would be driven into prey. This type of jaw would permit slashing attacks against much larger prey, with the goal of weakening the victim.[49]

Similar conclusions were drawn by Emily Rayfield et al 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 2,148 N, less than the values for alligators (13,000 N), lions (4,167 N), and leopards (2,268 N), but the skull could withstand nearly 55,500 N of vertical force against the tooth row. Rayfield et al. suggested that Allosaurus used its skull like a hatchet against prey, attacking open-mouthed, slashing flesh with its teeth, and tearing it away, without splintering bones like Tyrannosaurus is thought to be capable of doing. 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 attacking smaller and more agile ornithopods, but strong enough for high-impact ambush attacks against larger prey like stegosaurids and sauropods.[48] Their interpretations were challenged by Frazzetta and Kardong, who found no modern analogues 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.[94] In response, Rayfield et al noted that Allosaurus itself has no modern equivalent, the tooth row is well-suited to such an attack, and articulations in the skull cited by Frazzetta and Kardong as problematic actually helped protect the palate and lessen stress.[95] Another possibility for handling large prey is that theropods like Allosaurus were "flesh grazers", that could take bites of flesh out of living sauropods that were sufficient to sustain the predator so it didn't need to expend the effort to kill the prey outright; this strategy would also potentially allow the prey to recover and be fed upon in a similar way in the future.[14]

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.[96] The arms, compared to those of other theropods, were suited for both grasping prey at a distance or clutching it close,[20] and the articulation of the claws suggests that they could have been used to hook things.[19] Finally, the top speed of Allosaurus has been estimated at 30 to 55 kilometers per hour (19 to 34 miles per hour).[97]

Social behavior

Allosaurus has long been regarded in the semitechnical and popular literature as an animal that preyed on sauropods and other large dinosaurs by hunting in groups.[28][12][64] Robert T. Bakker has extended social behavior to parental care, interpreting shed allosaur teeth and chewed bones of large prey animals as showing adult allosaurs bringing food to lairs for their young to eat until they were grown, and prevented other carnivores from scavenging on the food.[98][50] However, there is actually little evidence of gregarious behavior in theropods,[14] and social interactions with members of the same species would have included antagonistic encounters, as shown by injuries to gastralia[16] 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.[97]

Recent research suggests that Allosaurus and other theropods were like other diapsids and had largely aggressive instead of cooperative interactions with other members of their own species. As noted by Brian Roach and Daniel Brinkman, cooperative hunting to kill prey much larger than an individual predator, as is commonly inferred for theropod dinosaurs, is rare among all animals, and modern diapsids (including lizards, crocodiles, and birds) very rarely cooperate to hunt in such a way. Many modern diapsid predators are 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 them when aggregated at feeding sites. They suggest that, for example, the Cleveland-Lloyd quarry shows Allosaurus individuals drawn together to feed on other disabled or dead allosaurs, sometimes getting killed in the process, and thus accumulating. 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. Roach and Brinkman interpret Bakker's lair sites similarly.[51] There is some evidence for cannibalism in Allosaurus, including Allosaurus shed teeth found among rib fragments, and possible tooth marks on a shoulder blade,[99] and cannibalized allosaur skeletons among the bones at Bakker's lair sites.[50]

Culture depictions of Allosaurus

File:Gwangi vs Styracosaurus.jpg
Gwangi, an Allosaurus from the Ray Harryhausen film The Valley of Gwangi, fights and kills a Styracosaurus.

Along with Tyrannosaurus, Allosaurus has come to represent the quintessential large, carnivorous dinosaur in popular culture. It is a common dinosaur in museums, due in particular to the excavations at the Cleveland Lloyd Dinosaur Quarry; by 1976, as a result of cooperative operations, 38 museums in eight countries on three continents had Cleveland-Lloyd allosaur material or casts.[7] Allosaurus is the official state dinosaur of Utah.[100]

Allosaurus has been depicted in popular culture since the early years of the 20th century. It is top predator in both Arthur Conan Doyle's 1912 novel, The Lost World, and the 1925 film adaptation, the first full-length motion picture to feature dinosaurs (it is not to be confused with Tyrannosaurus, which appears in one scene in the film).[4] It later became the starring dinosaur of the 1956 film The Beast of Hollow Mountain,[29] and the 1969 film The Valley of Gwangi, two genre combinations of living dinosaurs with Westerns. In The Valley of Gwangi, Gwangi is billed as an Allosaurus, although Ray Harryhausen based his model for the creature on Tyrannosaurus. Harryhausen often confuses the two, stating in a DVD interview "They're both meat eaters, they're both tyrants... one was just a bit larger than the other."[citation needed] His mentor Willis O'Brien had similar confusion, reportedly referring to the theropod in King Kong as Allosaurus although it appears to be based on a Charles R. Knight restoration of Tyrannosaurus, and is widely accepted as Tyrannosaurus (the shooting script and novelization identify it as a large carnivorous dinosaur).[4]

Allosaurus appears in the second and fifth episodes of the BBC television series Walking with Dinosaurs. The Walking with Dinosaurs special The Ballad of Big Al chronicles the life of the Allosaurus specimen nicknamed "Big Al".

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