Allosaurus

Initial finds and naming of Allosaurus Allosaurus was discovered during the Bone Wars, a feud between two American paleontologists, Othniel Charles Marsh and Edward Drinker Cope, that led to a surge of fossil discoveries in the Western US. Renaming to Antrodemus and early discoveries at Dinosaur National Monument The many names coined by Cope and Marsh complicated later research, with the situation further compounded by the terse descriptions they provided. Even at the time, authors such as Samuel W. Williston suggested that too many names had been coined. For example, Williston pointed out in 1901 that Marsh had never been able to adequately distinguish Allosaurus from Creosaurus. The most influential early attempt to sort out the situation was produced by Charles W. Gilmore in 1920. He came to the conclusion that the tail vertebra named Antrodemus by Leidy was indistinguishable from those of Allosaurus and that Antrodemus should be the preferred name because, as the older name, it had priority. Antrodemus became the accepted name for this familiar genus for over 50 years, until James H. Madsen published on the Cleveland-Lloyd specimens and concluded that Allosaurus should be used because Antrodemus was based on material with poor, if any, diagnostic features and locality information. For example, the geological formation that the single bone of Antrodemus came from is unknown. 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. Cleveland-Lloyd discoveries and "Big Al" Although sporadic work at what became known as Utah's Cleveland-Lloyd Dinosaur Quarry had taken place as early as 1927 and the fossil site itself was described by William L. Stokes in 1945, major operations did not begin there until 1960. Madsen led a cooperative effort between 1960 and 1965 involving nearly 40 institutions, during which thousands of bones were recovered from the site. The quarry is notable for the predominance of Allosaurus remains: the quarry preserves a minimum of 73 individual dinosaurs and at least 46 of those are A. fragilis. The great quantity of well-preserved Allosaurus remains has allowed this genus to be known in great detail, making it among the best-known of all theropods. Skeletal remains from the quarry pertain to individuals of almost all ages and sizes, from less than to long. Because its fossils are common at both this quarry and others in the state, Allosaurus was designated as the state fossil of Utah in 1988. 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 A. 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. Portuguese and other European discoveries outcrops, Portugal. The Vale Frades beach, where the holotype of A. europaeus was found, is labelled "2" Allosaurus is known from at least three localities in Portugal, from rocks of the Lourinhã, Bombarral, and Alcobaça formations. 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 the District of Leiria during the construction of a warehouse. Reported in 1999 and assigned to the species A. fragilis, it was the first Allosaurus specimen to be found outside of North America. A. fragilis became the first dinosaur species known from both Europe and North America, suggesting faunal exchange between the two continents. This site has been worked on again between 2005 and 2010, resulting in the discovery of at least two more Allosaurus individuals, including many skull bones. In 2005, a single maxilla was reported from the Guimarota coal mine, a locality well-known for its fossils of Mesozoic mammals; this fossil was assigned to Allosaurus but not to any particular species. In 2006, Octávio Mateus and colleagues reported a partial skull and three neck vertebrae (ML 415) from the Vale Frades beach in Lourinhã. As this specimen differed from North American Allosaurus fossils, they assigned it to a new species, A. europaeus. These authors also assigned the Andrés specimen to this species, though solely based on the fact that it was found in Portugal. The presence of a separate Allosaurus species in Europe would suggest that faunal interchange between the continents was interrupted, possibly due to the opening of the Atlantic Ocean. The status of A. europaeus was controversially discussed in the subsequent years, with different studies arguing that the species is a synonym of A. fragilis, a nomen dubium (doubtful name), or in need of re-evaluation. In 2024, André Burigo and Mateus re-described the Vale Frades specimen and carried out further fossil preparation that exposed additional bones. These authors identified nine unique features supporting the validity A. europaeus. A 2025 study by Elisabete Malafaia and colleagues described the Andrés specimens in detail, including a newly discovered set of skull bones. Their analysis of the relationships between individual Allosaurus skulls instead suggested that A. europaeus is a synonym of A. fragilis because the Andrés specimens were more closely related to some North American specimens than to the Vale Frades specimen. A. jimmadseni and A. anax 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 . The skull, which was still missing, was only located two years later, in 1996, with the help of novel radiological surveying techniques that detect gamma radiation from radioactive minerals which accumulate in bones during fossilization. The specimen, DINO 11541, is one of the most complete theropod skeletons recovered from the Late Jurassic. In 2000, Daniel Chure described the specimen in his PhD thesis, arguing that it represents a new species, A. jimmadseni. However, as the thesis did not meet the requirements of the International Commission on Zoological Nomenclature (ICZN), it remained an invalid nomen nudum ("naked name") until Chure and Mark Loewen formally described the species in 2020. These authors assigned several other specimens to A. jimmadseni, including the original "Big Al" individual (MOR 693). The name jimmadseni honors Madsen for his contributions to the taxonomy of the genus, notably his 1976 work. 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 is Greek for , and also alludes to the name change from Saurophagus to Saurophaganax. ==Description==

Allosaurus was a typical large theropod, having a massive skull on a short neck, a long, slightly sloping tail, and reduced forelimbs. The average length of A. fragilis, the best-known species, has been estimated at and its average mass at , with the largest definitive Allosaurus specimen (AMNH 680) estimated at long, Several gigantic specimens have been attributed to Allosaurus, but may in fact belong to other genera . The dubious genus Saurophaganax (OMNH 1708) was estimated to reach around in length, ==Classification==

Allosaurus is the eponymous member of the Allosauridae, a family that Marsh had named in 1878, one year after his description of A. fragilis. Originally, Allosauridae contained just Allosaurus itself. Marsh named two additional allosaurids in 1879 – Creosaurus and Labrosaurus – but these were later found to be synonyms of Allosaurus. In 1890, Karl Alfred von Zittel instead classified Allosaurus within the family Megalosauridae, while Marsh classified it within Dryptosauridae 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. Because Allosaurus differed from Megalosaurus in multiple aspects, Madsen argued that Allosauridae should be kept as a separate family. Some studies in the 1980s and 1990s proposed that Allosaurus was more closely related to the Tyrannosauridae than to most other theropods, but this has been rejected, with tyrannosaurids identified as members of a separate branch of theropods, the Coelurosauria. Various genera, such as Piatnitzkysaurus or Acrocanthosaurus, have been classified within Allosauridae by different authors, but two later reviews from 2004 and 2012 restricted the family to just Allosaurus and Saurophaganax. In 2024, Andy Danison and colleagues showed that Saurophaganax did not exist as a separate genus of allosaurid, as it consisted of a mixture of Allosaurus and sauropod fossils. 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 ceratosaur 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: }} ==Paleobiology==

Life history The wealth of Allosaurus fossils, from nearly all ages of individuals, allows scientists to study how the animal grew and how long its lifespan may have been. Remains may reach as far back in the lifespan as eggs—crushed eggs from Colorado have been suggested as those of Allosaurus. while the traditional view of cooperative hunting being largely restricted to mammals and being relatively rare in modern diapsid carnivores (including lizards, crocodiles, and birds) has come into question as a result of more recent research, suggesting that true cooperative hunting was possible even in the most basal non-avian theropods. Pathological analysis done by Foth et al. argued that evidence of surviving serious injuries may support gregariousness in Allosaurus. Paleopathology The specimen "Big Al" is well-known for its numerous injuries. Nineteen of its bones were broken or showed signs of serious infection, which may have contributed to "Big Al's" death. Pathologic bones included five ribs, five vertebrae, and four bones of the feet. A particular problem for the living animal was infection and trauma to the right foot that probably affected movement and may have also predisposed the other foot to injury because of a change in gait. "Big Al" had an infection on the first phalanx on the third toe that was afflicted by an involucrum. The infection was long-lived, perhaps up to six months. Allosaurus was one of only two theropods examined in a 2001 study to exhibit a tendon avulsion, and in both cases the avulsion occurred on the forelimb. When the researchers looked for stress fractures, they found that Allosaurus had a significantly greater number of stress fractures than Albertosaurus, Ornithomimus or Archaeornithomimus. Of the 47 hand bones the researchers studied, three were found to contain stress fractures. Of the feet, 281 bones were studied and 17 were found to have stress fractures. The stress fractures in the foot bones "were distributed to the proximal phalanges" and occurred across all three weight-bearing toes in "statistically indistinguishable" numbers. Since the lower end of the third metatarsal would have contacted the ground first while an allosaur was running, it would have borne the most stress. If the allosaurs' stress fractures were caused by damage accumulating while walking or running this bone should have experience more stress fractures than the others. The lack of such a bias in the examined Allosaurus fossils indicates an origin for the stress fractures from a source other than running. The authors conclude that these fractures occurred during interaction with prey, like an allosaur trying to hold struggling prey with its feet. The abundance of stress fractures and avulsion injuries in Allosaurus provide evidence for "very active" predation-based rather than scavenging diets. The left scapula and fibula of an Allosaurus fragilis specimen cataloged as USNM 4734 are both pathological, both probably due to healed fractures. The specimen USNM 8367 preserved several pathological gastralia which preserve evidence of healed fractures near their middle. Some of the fractures were poorly healed and "formed pseudoarthroses". A specimen with a fractured rib was recovered from the Cleveland-Lloyd Quarry. Another specimen had fractured ribs and fused vertebrae near the end of the tail. An apparent subadult male Allosaurus fragilis was reported to have extensive pathologies, with a total of fourteen separate injuries. Additionally, a subadult Allosaurus individual that suffered from spondyloarthropathy has been discovered in Dana Quarry in Wyoming. This finding represents the first known fossil evidence of spondyloarthropathy occurring in a theropod. Other pathologies reported in Allosaurus include: • Willow breaks in two ribs • Healed fractures in the humerus and radius • Distortion of joint surfaces in the foot, possibly due to osteoarthritis or developmental issues • Osteopetrosis along the endosteal surface of a tibia. • Distortions of the joint surfaces of the tail vertebrae, possibly due to osteoarthritis or developmental issues • "[E]xtensive 'neoplastic' ankylosis of caudals", possibly due to physical trauma, as well as the fusion of chevrons to centra • Coossification of vertebral centra near the end of the tail • Amputation of a chevron and foot bone, both possibly a result of bites • "[E]xtensive exostoses" in the first phalanx of the third toe • Lesions similar to those caused by osteomyelitis in two scapulae • Bone spurs in a premaxilla, ungual, and two metacarpals • Exostosis in a pedal phalanx possibly attributable to an infectious disease • A metacarpal with a round depressed fracture ==Paleoecology==

Distribution (yellow) where Allosaurus remains have been found Allosaurus is known from the Morrison Formation of the American West and from the Alcobaça, Bombarral, and Lourinhã formations in Portugal. 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 ecological niches, based on anatomy and the location of fossils. Ceratosaurus and Torvosaurus may have preferred to be active around waterways, and had lower, thinner bodies that would have given them an advantage in forest and underbrush terrains, whereas Allosaurus was more compact, with longer legs, faster but less maneuverable, and seems to have preferred dry floodplains. Ceratosaurus, better known than Torvosaurus, differed noticeably from Allosaurus in functional anatomy by having a taller, narrower skull with large, broad teeth. Taphonomy 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 are (no longer connected to each other) and well-mixed. More papers have been published on how this site may have formed than on any other fossil tetrapod site. A popular idea is that the site was a predator trap to which the Allosaurus 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 an Allosaurus population congregated around a water hole and, by their presence, deterred other dinosaurs. The disarticulation of the bones has been explained with trampling by other dinosaurs; movements of the sediment; scavenging; up-welling water; or repeated reworking and re-deposition of the bones. It is also possible that the Allosaurus population died elsewhere for unknown reasons and that their bones were washed into the site. A bone assemblage in the Upper Jurassic Mygatt-Moore Quarry preserves an unusually high occurrence of theropod bite marks, most of which can be attributed to Allosaurus and Ceratosaurus, while others could have been made by Torvosaurus given the size of the striations. While the position of the bite marks on the herbivorous dinosaurs is consistent with predation or early access to remains, bite marks found on Allosaurus material suggest scavenging, either from the other theropods or from another Allosaurus. The unusually high concentration of theropod bite marks compared to other assemblages could be explained either by a more complete utilization of resources during a dry season by theropods, or by a collecting bias in other localities. ==Notes==