Allosaurus

Allosaurus is a genus of theropod dinosaur that lived 155 to 143 million years ago, during the late Jurassic period. The first fossil remains that could clearly be assigned to this genus were described in (/wiki/1877_in_paleontology) by Othniel C. Marsh. The name "Allosaurus" means "different lizard," because Marsh believed its lightweight vertebrae were unique.

The genus has a very complicated taxonomy. It includes at least three valid species, and the best known is A. fragilis. Most Allosaurus remains come from North America's Morrison Formation, and more material is known from the Alcobaça, Bombarral, and Lourinhã formations in Portugal. For over half of the 20th century, it was known as Antrodemus. However, a study of the 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 attracted attention outside of scientific circles.

Allosaurus was a large bipedal predator for its time. Its light, sturdy skull had dozens of sharp, serrated teeth. The species A. fragilis averaged 7.5 to 8 meters (25 to 26 feet) in length, and the largest specimens were estimated to be 9 meters (30 feet) long. Its three-fingered hands were small compared to its large, powerful legs, and a long, muscular tail balanced the body. It is classified in the family Allosauridae.

As the most common large predator of the Morrison Formation, Allosaurus was an apex predator at the top of the food chain. It probably hunted large plant-eating dinosaurs such as ornithopods, stegosaurids, and sauropods. Scientists have debated whether Allosaurus had cooperative social behavior and hunted in packs, or whether it was a solitary predator that formed groups. Evidence supports both ideas.

History of discovery

Main article: Taxonomy of Allosaurus

Allosaurus was found during the Bone Wars, a feud between two American paleontologists, Othniel Charles Marsh and Edward Drinker Cope, which led to many fossil discoveries in the Western US. The first described fossil was a bone that Ferdinand V. Hayden got secondhand in 1869. It came from Middle Park, near Granby, Colorado, probably from Morrison Formation rocks. Hayden sent it to Joseph Leidy, who thought it was half of a tail bone and called it Poekilopleuron sic valens. Later, he decided it needed its own genus, Antrodemus. Marsh named Allosaurus fragilis in 1877 from a small group of bones called YPM 1930, including a humerus (upper arm bone). The name comes from Greek words meaning "different lizard," because Marsh thought its lightweight 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, where Marsh named Diplodocus from the same remains. Later, Marshall P. Felch found a nearly complete skeleton that became the neotype for the species. Cope's collector found another skeleton in 1879, which was later shown on display as AMNH 5753 in a pose scavenging an Apatosaurus, as painted by Charles R. Knight.

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

Description

Allosaurus was a typical large theropod. It had a massive skull on a short neck, a long tail that sloped slightly, and small front legs. The best-known species, A. fragilis, has been estimated at 7–9 m (23–30 ft) long and 1.7–2.7 t (1.9–3.0 short tons) in weight. In his 1976 monograph on Allosaurus, James H. Madsen described bones that he thought showed a maximum length of 12 to 13 m (39 to 43 ft). Weight estimates are debated. Since 1980 they have ranged from 1 to 4 t (1.1 to 4.4 short tons) for a typical adult. John Foster, an expert on the Morrison Formation, thinks 1 t (1.1 short tons) is reasonable for large adults of A. fragilis, while 700 kg (1,500 lb) is closer for average individuals. A young specimen nicknamed "Big Al" is now named Allosaurus jimmadseni, and it is estimated to have been 7.6 metres (25 ft) long. A. europaeus has been measured at up to 6–8 m (20–26 ft) long and 1 t (1.1 short tons) in weight. Some gigantic specimens may belong to other kinds. The Saurophaganax (OMNH 1708) was estimated at about 10.5 m (34 ft) long, and it is sometimes placed in Allosaurus as A. maximus. A 2024 study linked other remains from Allosauridae to a new species, Allosaurus anax. Another possible specimen, once called Epanterias (AMNH 5767), may have been 12.1 m (40 ft) long. A large partial skeleton from the Peterson Quarry in New Mexico has also been found.

David K. Smith found that Allosaurus fossils 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 bone shapes were the same. Later work from Dinosaur National Monument and other sites found no good reason for separate species based on skeleton differences. Kenneth Carpenter found wide variation between skulls and questioned earlier species 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 an individual he estimates at 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 extend above and in front of the eyes to form a pair of horns that varied in shape and size. The horns were probably covered in keratin and may have shaded the eyes, been used for display, or been used in fights, although they were fragile. Inside the lacrimal bones were depressions that may have held salt glands. The roof of the braincase was thin, perhaps to help control the temperature of the brain (thermoregulation). Allosaurus had nine vertebrae in the neck, 14 in the back, and five in the sacrum that supports the hips. James Madsen estimated about 50 tail vertebrae, while Gregory S. Paul suggested 45 or fewer. Hollow spaces in the neck and front back vertebrae are thought to have held air sacs used in respiration, as in birds. The forelimbs are short compared with the hindlimbs, with three fingers per hand tipped with large, curved, pointed claws. The wrist has a version of the semilunate carpal also found in maniraptorans. Skin impressions show small scales on one juvenile, and larger scales on another specimen 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, which Marsh named in 1878, one year after he described A. fragilis. At first, this family contained only Allosaurus. Marsh named two more allosaurids in 1879, Creosaurus and Labrosaurus, but these were later found to be the same as Allosaurus.^{: 10} In 1890, Karl Alfred von Zittel placed Allosaurus in the Megalosauridae family, while Marsh placed it in Dryptosauridae in 1895 and 1896. Most studies followed Zittel until Madsen, in 1976, argued that Allosauridae should be kept as a separate family, because Allosaurus differed from Megalosaurus in many ways.^{: 213 : 10} Some studies in the 1980s and 1990s suggested that Allosaurus was more closely related to the Tyrannosauridae than to most other theropods, but this idea was rejected. Tyrannosaurids are now placed in a separate branch of theropods, the Coelurosauria. Other genera, such as Piatnitzkysaurus or Acrocanthosaurus, have been placed in Allosauridae by some scientists. However, two later reviews from 2004 and 2012 kept only Allosaurus and Saurophaganax in the family. In 2024, Andy Danison and colleagues showed that Saurophaganax was not a separate allosaurid, because it was 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, which first included any large theropod that was not a ceratosaur or a tyrannosaurid. Carnosauria is still sometimes used, either to include Allosauroidea and Megalosauroidea together, or as a close match for Allosauroidea. In a 1988 popular book, Gregory S. Paul used the subfamily Allosaurinae to group Allosaurus and Chilantaisaurus. This group has not been widely used. The cladogram shown below comes from a 2015 study by Christophe Hendrickx and colleagues:

Paleobiology

Scientists have learned about 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, which is 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, which is found in the medulla of the bones of egg-laying female birds, where it supplies calcium for shells. Some scientists used this to suggest the animal was a female that had reached adulthood, 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 in size without changing in proportion.

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 dead, which is called scavenging. Robert T. Bakker compared Allosaurus to Cenozoic saber-toothed mammals and suggested its short teeth formed a saw-like edge for slashing prey. A finite element analysis of its skull found it was strong and could bite with force between 805 and 8,724 N, and 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 deal with prey. Its skull limited its binocular vision to about 20 degrees, similar to crocodilians, and 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 multiple 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 diseases 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 study in 2001 found stress fractures in hand and foot bones more often than in some related dinosaurs, suggesting these injuries came from struggling prey rather than walking. The famous "Big Al" specimen has 19 injuries, including a 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 fracture in its pelvis 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, and 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), though these specimens are no longer assigned to the genus. Most Morrison specimens are not yet assigned to a particular species. Allosaurus is the most common theropod in the Morrison Formation, making up 70 to 75% of theropod specimens.

The Morrison Formation is thought to have been a semiarid area with distinct wet and dry seasons and flat floodplains. Plants included river-lining forests of conifers, tree ferns, and ferns (gallery forests), and 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 are thought to have been similar to the Morrison but with a stronger marine influence. Many of their dinosaurs are the same genera as Morrison ones, or have close counterparts 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 that ate Camarasaurus, Camptosaurus, and Diplodocus, though it is unclear whether it hunted or scavenged the sauropods. It mostly ate flesh 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 appear to have had different ecological niches. Allosaurus was more compact, with longer legs, and seems to have preferred dry floodplains. Ceratosaurus and Torvosaurus may have stayed closer to waterways and had lower, thinner bodies that helped them move through forests and underbrush. 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 carnivore, A. fragilis. The bones are disarticulated (no longer connected to each other) and well mixed. 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 which can be linked to Allosaurus and Ceratosaurus.