Prehistoric Wiki
Allosaurus as it appeared in Walking with Dinosaurs.





















Allosaurus was a genus of large theropod dinosaur native to North America in the Jurassic period. It is the type genus of the infraorder Carnosauria, the superfamily Allosauroidea, and the family Allosauridae. In other words, Allosaurus is the standard by which other carnosaurs are defined. Its closest relative was probably Antrodemus; many of the genera in the family Allosauridae are thought to be possible synonyms of Allosaurus, the most notable of which being Epanterias, which seems nearly identical to Allosaurus, albeit slightly larger.

Allosaurus, along with the unrelated Tyrannosaurus, is one of the best-known theropods. It has been featured in many books and films, and is probably the second-most popular theropod among the general public, after Tyrannosaurus.


A complete Allosaurus fossil on display.

Allosaurus was one of the first large theropods to be discovered, before Tyrannosaurus and Spinosaurus, but after Megalosaurus. The discovery and early study of Allosaurus is complicated by the 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 valens. He later decided it deserved its own genus, Antrodemus.

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. Allosaurus comes from the Greek allos/αλλος, meaning "strange" or "different" and saurus/σαυρος, meaning "lizard" or "reptile". It was named 'different lizard' because its vertebrae were different from those of other dinosaurs known at the time of its discovery. 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. 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 and Labrosaurus, and Cope's Epanterias.


Allosaurus was a large theropod by any standards. Despite being often compared in the media to Tyrannosaurus, it is only distantly related to it and bears only superficial simmilarities to it. Allosaurus had a lightly built skull, with a long jaw and very sharp and heavily serrated teeth. Its jaw joint was not very strong, and its skull was unfused. Combined with the reduction in jaw muscles and increase in neck muscles, Allosaurus would have a jaw gape of 1.75m when its skull measured 1.25m long. This gape would not only allow it to tear large chunks of flesh from prey, but also use its upper jaw as a hatchet weapon. Its arms, while short relative to the hindlimbs, were very long for a theropod. Although no particulary detailed studies have been done on it's arm mechanics, fellow allosauroid Acrocanthosaurus had very strong arms capable of withstanding even struggling prey. So it is possible Allosaurus used its large arms and hook like claws to hold prey while it hacked away with its upper jaw. Allosaurus was a slender theropod, comparable to a spinosaurid of equal length.


Size comparison of large theropod dinosaurs, Allosaurus in magenta.

Allosaurus was a mid to large sized theropod dinosaur. The average sized Allosaurus was around 8.5m long. The largest definitive Allosaurus specimen is 9.7m long. Two other jurassic alosauroids may represent large Allosaurus specimens. The first of which, Saurophaganax measured around 11m long. But it is generally thought to be its own genus now, with some people even classing it as a carcharodontosaurid rather than an allosaurid. The second of which, Epanterias is only really distinquishable from Allosaurus in its size, making it likely it is an Allosaurus. Epanterias measured around 12m long, and would have weighed in at 4 tonnes or more.


Allosaurus lived in the western United States, around 155-145 million years ago. This indicates a mixed, forest and plains habitat, likely very humid and wet, but also very hot. Allosaurus was a large carnivore in a world of large animals, mainly his prey.

Allosaurus was designed to open its mouth very wide, at a right angle. An Allosaurus with a skull 1.25m long could open its mouth to a full 1.75m; the same length as the skulls of the huge theropods Carcharodontosaurus and Spinosaurus. However, due to this, it had a relatively small biteforce, probably only as strong as a Lion's bite. But its wide gape, combined with its thin and heavily serrated teeth would make for a specialised sauropod killer.

Allosaurus may have also lived in the more tropical Africa, where it hunted Barosaurus and Dryosaurus, and competed with other theropods like Elaphrosaurus (although based on Limusaurus, Elaphrosaurus was probaly an omnivore, if not a herbivore).


  • A. fragilis, the type species, is the smallest species, averaging 8-9 m long. The largest individual discovered as of yet was approximately 10 m long.
  • ?A. maximus may be a species of Allosaurus. Slightly larger than Allosaurus fragilis, "Allosaurus" maximus seems slightly different in build and is now thought to belong to its own genus, Saurophaganax. This was a large theropod, at up to 11 metres in length.
  • A. amplexus is the largest species discovered to date. It is also known as Epanterias amplexus but as it is almost exactly the same as Allosaurus it is likely a larger species, reaching 12 metres in length. There is some debate as to whether it is simply a large specimen of A. fragilis.