Allosaurus

Allosaurus is a genus of theropod dinosaur that lived about 155 to 143 million years ago, during the late Jurassic period. The first fossil remains that could clearly be linked to this genus were described by Othniel C. Marsh. The name "Allosaurus" means "different lizard," because Marsh thought its lightweight vertebrae were unusual.

The genus has a complicated taxonomy. It includes at least three valid species, and the best known is A. fragilis. Most Allosaurus remains come from the Morrison Formation in North America, and more remains have been found in Portugal, in the Alcobaça, Bombarral, and Lourinhã formations. For over half of the 20th century, it was called Antrodemus. Later, a study of many remains from the Cleveland-Lloyd Dinosaur Quarry brought back the name "Allosaurus." As one of the first well-known theropod dinosaurs, it has long been popular outside of science.

Allosaurus was a large bipedal predator for its time. It walked on two legs. Its light, strong skull had dozens of sharp, serrated teeth. The species A. fragilis was about 7.5 to 8 meters (25 to 26 feet) long, and the largest known specimens were estimated to be about 9 meters (30 feet) long. Its three-fingered hands were small compared to its big, strong legs. A long, muscular tail helped balance its body. It belongs to the family Allosauridae.

As the most common large predator of the Morrison Formation, Allosaurus was an apex predator, meaning it sat at the top of the food chain. It probably hunted large plant-eating dinosaurs such as ornithopods, stegosaurids, and sauropods. Scientists still debate whether Allosaurus hunted together in groups or hunted alone and sometimes gathered in small groups. Evidence supports both ideas.

History of discovery

Main article: Taxonomy of Allosaurus

Allosaurus was found during the Bone Wars, a rivalry between two American scientists who studied fossils, Othniel Charles Marsh and Edward Drinker Cope. The rivalry led to many fossil discoveries in the Western US. The first fossil described was a bone that Ferdinand V. Hayden got from another person in 1869. It came from Middle Park, near Granby, Colorado, probably from Morrison Formation rocks. Hayden sent it to Joseph Leidy. Leidy thought it was part of a tail bone and called it Poekilopleuron sic valens. Later, he decided it needed its own genus, Antrodemus. In 1877, Marsh named Allosaurus fragilis from a small group of bones called YPM 1930, which included a humerus (upper arm bone). The name comes from Greek words meaning "different lizard." Marsh thought its light bones were unlike those of other dinosaurs. The bones were found by Benjamin Mudge and Samuel W. Williston at a quarry in Garden Park, Colorado. Marsh also named Diplodocus from the same remains. Later, Marshall P. Felch found a nearly complete skeleton, which became the neotype for the species. Cope's collector found another skeleton in 1879. It was later displayed as AMNH 5753, in a pose scavenging an Apatosaurus, as painted by Charles R. Knight.

Many names given by Cope and Marsh were confusing, so later scientists worked to sort them out. Charles W. Gilmore decided that Antrodemus should be the preferred name. Later, James H. Madsen chose Allosaurus instead. In 1909, Earl Douglass found the site that became Dinosaur National Monument. Major digs at Cleveland-Lloyd began in 1960, and many Allosaurus bones were found there. Big Al was discovered at Howe Ranch in 1991 by a Swiss team. It was later studied by the Museum of the Rockies. Scientists still disagree about how many species exist. Gregory S. Paul proposed A. atrox. In Portugal, Octávio Mateus and his team described A. europaeus from Lourinhã. In 2020, A. jimmadseni was formally named. In 2024, Andy Danison and colleagues proposed A. anax.

Description

Allosaurus was a large theropod. It had a big skull on a short neck, a long tail that sloped a little, and small front legs. The best-known species, A. fragilis, was about 7 to 9 m (23 to 30 ft) long and weighed about 1.7 to 2.7 t (1.9 to 3.0 short tons). In 1976, James H. Madsen wrote a long report on Allosaurus (monograph). He thought some bones showed a length of 12 to 13 m (39 to 43 ft). Scientists still disagree about how much these animals weighed. Since 1980, estimates for a typical adult have ranged from 1 to 4 t (1.1 to 4.4 short tons). John Foster, an expert on the Morrison Formation, thinks 1 t (1.1 short tons) is reasonable for large adults of A. fragilis. He thinks average adults were closer to 700 kg (1,500 lb).

A young animal nicknamed "Big Al" is now called Allosaurus jimmadseni. It is estimated to have been 7.6 metres (25 ft) long. A. europaeus grew up to about 6 to 8 m (20 to 26 ft) long and weighed about 1 t (1.1 short tons). Some very large fossils may belong to other kinds of dinosaur. The Saurophaganax (OMNH 1708) was about 10.5 m (34 ft) long. Sometimes it is placed within Allosaurus as A. maximus. A 2024 study linked other remains from Allosauridae to a new species, Allosaurus anax. Another possible fossil, once called Epanterias (AMNH 5767), may have been 12.1 m (40 ft) long. A large partial skeleton has also been found at the Peterson Quarry in New Mexico.

David K. Smith found that Allosaurus fossils from Utah's Cleveland-Lloyd Dinosaur Quarry were generally smaller than those from Wyoming's Como Bluff or the BYU Dry Mesa Quarry in Colorado. However, the shapes of the bones were the same. Later work at Dinosaur National Monument and other sites found no good reason to split them into separate species based on their skeletons. Kenneth Carpenter found a lot of variation between skulls. He questioned some earlier species that were based on horn shape and the jugal bone.

The skull was light and had dozens of sharp, serrated teeth. Gregory S. Paul gives a skull length of 845 mm (33.3 in) for one animal he estimates was 7.9 m (26 ft) long. Each premaxilla, the bone at the tip of the snout, held five teeth. Each dentary, the tooth-bearing bone of the lower jaw, had 14 to 17 teeth. The lacrimal bones reached above and in front of the eyes to form a pair of horns. The horns varied in shape and size. They were probably covered in keratin. They may have shaded the eyes, been used to show off, or been used in fights, although they were fragile. Inside the lacrimal bones were small hollows that may have held salt glands. The roof of the braincase was thin, which may have helped control the temperature of the brain (thermoregulation).

Allosaurus had nine vertebrae in the neck, 14 in the back, and five in the sacrum, the part that supports the hips. James Madsen estimated about 50 tail vertebrae. Gregory S. Paul suggested 45 or fewer. Hollow spaces in the neck and front back vertebrae may have held air sacs used in respiration, as in birds. The forelimbs were short compared with the hindlimbs. Each hand had three fingers tipped with large, curved, pointed claws. The wrist had a version of the semilunate carpal that is also found in maniraptorans. Skin impressions show small scales on one young animal. Another specimen had larger scales that look more like those of sauropods, so that impression may not be from Allosaurus.

Classification

Allosaurus is the main member of the Allosauridae family. Marsh named this family in 1878, one year after he described A. fragilis. At first, the family had only Allosaurus in it. In 1879, Marsh named two more allosaurids, Creosaurus and Labrosaurus, but later scientists found they were the same as Allosaurus.^{: 10} In 1890, Karl Alfred von Zittel placed Allosaurus in the Megalosauridae family. Marsh placed it in Dryptosauridae in 1895 and 1896. Most studies followed Zittel until 1976, when Madsen argued that Allosauridae should be its own family, because Allosaurus was different from Megalosaurus in many ways.^{: 213 : 10} Some studies in the 1980s and 1990s said Allosaurus was closely related to the Tyrannosauridae. Scientists later rejected this idea. Tyrannosaurids are now placed in a different group of theropods, the Coelurosauria. Some scientists have placed other animals, such as Piatnitzkysaurus or Acrocanthosaurus, in Allosauridae. However, two later reviews from 2004 and 2012 kept only Allosaurus and Saurophaganax in the family. In 2024, Andy Danison and colleagues found that Saurophaganax was not a separate allosaurid. Its fossils were a mix of Allosaurus and sauropod fossils.

Allosauridae is often placed in the group Allosauria, together with Carcharodontosauria. In many studies, Allosauria forms the group Allosauroidea along with the family Metriacanthosauridae. In the past, Allosaurus was often placed in the group Carnosauria. This group first included any large theropod that was not a ceratosaur or a tyrannosaurid. Some scientists still use the name Carnosauria, either to include Allosauroidea and Megalosauroidea together, or as a close match for Allosauroidea. In a 1988 popular book, Gregory S. Paul grouped Allosaurus and Chilantaisaurus into the subfamily Allosaurinae. Few other scientists have used this group. The cladogram shown below comes from a 2015 study by Christophe Hendrickx and colleagues:

Paleobiology

Scientists have learned how Allosaurus grew and lived from its many fossils. Some crushed eggs from Colorado have been suggested as Allosaurus eggs. A histological study of its leg bones suggests it stopped growing at about 22 to 28 years old, similar to other large theropods like Tyrannosaurus. Its fastest growth seems to have been at age 15, when it grew about 148 kilograms (326 lb) per year. One shin bone from the Cleveland-Lloyd Quarry contains medullary bone tissue. This tissue is found in egg-laying female birds, where it provides calcium for shells. Some scientists used this to suggest the animal was an adult female, but other studies question this idea. A young Allosaurus with a nearly complete hind leg shows that juveniles had relatively longer legs and may have been faster than adults, similar to the later tyrannosaurids. Its thigh bone changed shape as it grew, while its skull grew bigger without changing its proportions.

Most scientists agree that Allosaurus hunted large animals. One tail bone has a healed puncture that fits a tail spike from Stegosaurus, and a Stegosaurus neck plate has a U-shaped wound that matches an Allosaurus snout. Sauropods were likely both prey and animals it ate after finding them already dead, a behavior called scavenging. Robert T. Bakker compared Allosaurus to Cenozoic saber-toothed mammals and suggested its short teeth formed a saw-like edge for cutting prey. A finite element analysis of its skull found it was strong and could bite with force between 805 and 8,724 N. It may have used its skull like a hatchet, though other researchers disagreed. Its jaws could open very wide, and it could move its head quickly, which may have helped it handle prey. Its skull limited its binocular vision to about 20 degrees, similar to crocodilians. Its top speed was estimated at 30–55 km (19–34 mi) per hour.

Other studies suggest Allosaurus may have eaten sauropod remains at all stages of decay. Whether it hunted in groups is still debated. Some researchers, including Bakker, have suggested adults brought food to lairs for their young, but there is little evidence of group living in theropods. Many fossils of several Allosaurus individuals were found together, which may mean they gathered to feed on other injured or dead allosaurs, similar to how Komodo dragons and crocodiles gather at feeding sites. The CT scanning of an endocast showed a brain more like that of crocodilians than of birds, the archosaurs that are alive today. Its inner ear suggests it heard low sounds best, and its olfactory bulbs were large but typical for its size.

Injuries and illnesses are common in large theropods, and Allosaurus is especially well documented. Fossils from the Cleveland-Lloyd Quarry show willow breaks in ribs, ankylosis in tail bones, exostoses in a toe bone, and possible infections. A 2001 study found stress fractures in hand and foot bones more often than in some related dinosaurs. This suggests these injuries came from struggling prey rather than walking. The famous "Big Al" specimen has 19 injuries, including an osteomyelitis infection in its right foot that may have caused a limp, and a chevron bone that healed after an impact to its tail. A second specimen, "Big Al II," shows a pelvis fracture that did not heal and may have affected how it moved. Another specimen from Wyoming showed signs of spondyloarthropathy, the first evidence of this condition in a theropod. Some infections were suppurative, meaning they produced pus. Osteoarthritis and other avulsions have also been studied.

Paleoecology

Allosaurus is known from the Morrison Formation of the American West and from the Alcobaça, Bombarral, and Lourinhã formations in Portugal. Both areas date to the Kimmeridgian to Tithonian ages of the Late Jurassic. It has also been reported from Russia (A. sibiricus), Tanzania (A. tendagurensis), and Switzerland (A. meriani). These specimens are no longer placed in this genus. Most Morrison specimens do not yet have a known species name. Allosaurus is the most common theropod (meat-eating dinosaur) in the Morrison Formation, making up 70 to 75% of theropod fossils.

The Morrison Formation was likely a semiarid area, which means it was fairly dry. It had clear wet and dry seasons and flat floodplains. Plants included forests of conifers and tree ferns along rivers, and ferns (gallery forests). There were also fern savannas with a few trees, such as the Araucaria-like conifer Brachyphyllum. Animal fossils include bivalves, snails, ray-finned fishes, frogs, salamanders, turtles, sphenodonts, lizards, crocodylomorphs, several kinds of pterosaur, many dinosaurs, and early mammals such as docodonts, multituberculates, symmetrodonts, and triconodonts. Other Morrison theropods include Ceratosaurus, Ornitholestes, Tanycolagreus, and Torvosaurus. The sauropods include Haplocanthosaurus, Camarasaurus, Cathetosaurus, Brachiosaurus, Suuwassea, Apatosaurus, Brontosaurus, Barosaurus, Diplodocus, Supersaurus, Amphicoelias, and Maraapunisaurus. The ornithischians include Camptosaurus, Dryosaurus, and Stegosaurus. The Portuguese formations were probably similar to the Morrison but had a stronger marine influence. Many of their dinosaurs belong to the same genera as Morrison ones, or have close relatives such as Lusotitan and Draconyx.

Allosaurus was at the top of the Morrison food chain, at the highest trophic level. Studies of calcium in its remains suggest it was an opportunistic predator, meaning it took whatever food it could find. It ate Camarasaurus, Camptosaurus, and Diplodocus, though it is unclear whether it hunted these animals or scavenged them. It mostly ate meat and consumed much less bone than Tyrannosaurus. It lived alongside Ceratosaurus and Torvosaurus in both the United States and Portugal. According to Robert Bakker, these three large theropods seem to have had different ecological niches. Allosaurus was more compact, with longer legs, and it seems to have preferred dry floodplains. Ceratosaurus and Torvosaurus may have stayed closer to waterways. They had lower, thinner bodies that helped them move through forests and thick plants. Ceratosaurus differed from Allosaurus by having a taller, narrower skull with large, broad teeth.

At Utah's Cleveland-Lloyd quarry, about 66% of the vertebrate fossils belong to one meat-eater, A. fragilis. The bones are disarticulated (no longer connected to each other) and mixed together. More papers have been written about how this site may have formed than about any other fossil tetrapod site. A popular idea is that it was a predator trap, where Allosaurus were drawn in and became stuck. Other studies suggest a single event, such as a severe drought that brought many Allosaurus to a water hole. The scattered bones may have been caused by trampling, moving sediment, scavenging, moving water, or repeated reworking and re-deposition. Another possibility is that the population died elsewhere and its bones were washed into the site. At the Upper Jurassic Mygatt-Moore Quarry, many theropod bite marks have been found. Most of these can be linked to Allosaurus and Ceratosaurus.