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Allosaurus ( ) is a genus of theropoda 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, Bombarral, and Lourinhã formations 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 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 , , and . 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.
Allosaurus itself is Holotype 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). Marsh gave these remains the name Allosaurus fragilis in 1877. Allosaurus comes from the Ancient Greek words allos/αλλος, meaning "strange" or "different", and sauros/σαυρος, 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 uncovered by two of Marsh's collectors, Benjamin Mudge and Samuel W. Williston, in the autumn of 1877 at Felch Quarry, in the Garden 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 and Allosaurus. 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 of Ceratosaurus nasicornis, Stegosaurus stenops, and a mostly complete Allosaurus skeleton (USNM 4734) that would later be selected as the neotype specimen of Allosaurus fragilis (the single specimen the species is based on, replacing the inadequate holotype).
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 as AMNH 5753, was put on public view in 1908. This is the well-known mount poised over a partial Apatosaurus skeleton as if Scavenger 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.
In 1925, Werner Janensch described Allosaurus tendegurensis from the Tendeguru Beds of Tanzania. It was based on a sole tibia. A study from June 2012 by Carrano et. al. dismissed the remains of A. tendegurensis as undiagnostic, rendering it an indeterminate Tetanurae theropod.
In 1909, Earl Douglass from the Carnegie Museum discovered what should later become Dinosaur National Monument in Utah. Until 2022, Douglass and his team excavated over of fossils of multiple dinosaur species from a single quarry, including several Allosaurus specimens. 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. During the summer of 1924, the University of Utah uncovered DINO 2560, the best preserved Allosaurus specimen known at that time. The skull of this particularly large individual is on exhibit at the Dinosaur National Monument.
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 by Barnum 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 articulated Allosaurus specimen. However, because the new site was located on public land, the excavation was taken over by a joint Museum of the Rockies and University of Wyoming Geological Museum team. The specimen, now on exhibit at the Museum of the Rockies, belonged to an individual of about in length. This was below the average size for Allosaurus, as it was a subadult estimated at only 87% grown. The Swiss team later excavated a second Allosaurus, "Big Al II" (SMA 0005), on private land on Howe Ranch, which is exhibited at the Aathal Dinosaur Museum in Switzerland.
In 1991, Brooks Britt argued that there were at least two species of Allosaurus: A robust species with a short and high skull and pointed lacrimal horns, and a more gracile species with a long and low skull and rounded lacrimal horns. The robust species is geologically younger from localities such as Dry Mesa Quarry and Garden Park, while the gracile species, found at the Cleveland-Lloyd and at Dinosaur National Monument, is older. Already in 1988, Gregory S. Paul made a similar distinction in a popular book, in which he referred to the gracile species as A. fragilis and to the robust species as Allosaurus atrox, using a species originally described by Marsh as Creosaurus atrox. However, a series of statistical analyses by David K. Smith between 1996 and 1999 suggested that the differences seen in the Morrison Formation material can be attributed to individual variation.
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 Autapomorphy 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.
The issue of species and potential synonyms was historically complicated by the type specimen of Allosaurus fragilis (YPM 1930) being extremely fragmentary. 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 submitted a petition to the ICZN in 2010 to have the more complete specimen USNM 4734 selected as a neotype, a decision that was ratified by the ICZN on December 29, 2023. In 2014, Sebastian Dalman named the new species Allosaurus lucasi based on two specimens from the Tithonian of Colorado, but this species was not accepted by later authors.
In 1995, Chure created the taxon Saurophaganax maximus for giant allosaurid remains from western 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 by Andy Danison and colleagues suggested that the Saurophaganax specimen is a chimera that combines the bones of Allosaurus with those of a sauropod. The holotype of Saurophaganax itself, a neural arch, cannot be confidently be assigned to a theropod, making it a nomen dubium. The Allosaurus bones, however, were found to be distinct enough to warrant a new species of Allosaurus, A. anax. The name anax is Greek for , and also alludes to the name change from Saurophagus to Saurophaganax.
David K. Smith, examining Allosaurus fossils by quarry, found that specimens from Utah's Cleveland-Lloyd Dinosaur Quarry are generally smaller than those from Wyoming's Como Bluff or the BYU Dry Mesa Quarry in Colorado, but the shapes of the bones themselves did not vary between the sites. A later study by Smith incorporating specimens from the Garden Park and Dinosaur 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. 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 Talus bone, 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.
The of Allosaurus extend above and in front of the eyes to form a pair of horns which varied in shape and size. Leading into the horns, the skull also features a pair of ridges running along the top of the . 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 Tyrannosauridae.
Inside the lacrimal bones were depressions that may have held , such as . Within the maxillae were Maxillary sinus 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 Frontal bone may also have had a joint.
Another Allosaurus fossil features a skin impression from the mandible, 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 an Allosaurus skull has been reported but never described. Additional undescribed skin impressions are known from various parts of the body in one specimen.
Together with the group Carcharodontosauria, Allosauridae is often classified within the group Allosauria. Allosauria, in turn, forms the group Allosauroidea together with the family Metriacanthosauridae in many analyses. Historically, Allosaurus has often been classified in the group Carnosauria, which originally encompassed any large theropod that was not a Ceratosauria or a tyrannosaurid. Carnosauria is still sometimes used, either to encompass the groups Allosauroidea and Megalosauroidea, or as an approximate synonym of Allosauroidea. In a 1988 popular book, Gregory S. Paul used the subfamily Allosaurinae to unite the genera Allosaurus and Chilantaisaurus. This group has not been widely used. The cladogram shown below is from a 2015 study of Christophe Hendrickx and colleagues:
Medullary bone tissue (endosteally derived, ephemeral, mineralization located inside the Bone marrow of the long bones in gravid female birds) has been reported in at least one Allosaurus specimen, a Tibia 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 allometry, increasing in size without changing in proportion.
A Biomechanics 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 , such as : 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 . 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.
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 . 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.
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:
The Morrison Formation covers an area of 1.2 million km², and Allosaurus 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 and A. jimmadseni appear to have been contemporaneous but separated geographically, with A. fragilis concentrated in the south and east of the Morrison basin and A. jimmadseni in the north and west. Both species occur together only at Dry Mesa quarry. The species A. 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. Allosaurus is the most common theropod in the Morrison Formation, accounting for 70 to 75% of theropod specimens.
Allosaurus was at the top trophic level of the Morrison food chain. Calcium isotopic values show Allosaurus was an opportunistic predator that ate Camarasaurus, Camptosaurus, and Diplodocus, although it is unclear if it was hunting or scavenging on the sauropods. It also revealed Allosaurus primarily consumed flesh with only small amounts of bone being consumed compared to tyrannosaurids like Tyrannosaurus. It coexisted with fellow large theropods Ceratosaurus and Torvosaurus in both the United States and Portugal. According to Robert Bakker, the three appear to have had different , 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.
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.
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