Taxonomy of Allosaurus

The genus Allosaurus is a group of dinosaurs that scientists have studied for many years. It was first described in 1877 by Othniel Charles Marsh using a partial fossil found in Colorado. For a long time, these dinosaurs were called Antrodemus, but in 1976, the name Allosaurus was brought back into use.

Today, scientists recognize several possible species of Allosaurus. The most well-known is A. fragilis. Others include A. europaeus, found in Portugal; A. jimmadseni, described in 2020; and A. anax, described in 2024. These fossils help us learn more about how these dinosaurs lived and changed over millions of years ago.

The study of Allosaurus helps scientists understand the world of dinosaurs better. By looking at fossils, researchers can find important details about their lives, habits, and place in the history of life on Earth.

The genus Allosaurus

Allosaurus is one of the most famous dinosaurs, both among scientists and the general public. It is the most common theropod found in the Morrison Formation, an important area for dinosaur fossils in the western part of the United States. It makes up most of the theropod fossils found there, with some fossils also found in Portugal.

Allosaurus was first discovered during a time called the Bone Wars, when two scientists, Othniel Charles Marsh and Edward Drinker Cope, raced to find new fossils. In 1877, Marsh named the dinosaur Allosaurus fragilis based on a partial fossil. The name Allosaurus means "different lizard" in Greek, because Marsh thought its bones looked unique. For many years, the dinosaur was also known by another name, Antrodemus valens, but in 1976, a scientist named James Henry Madsen brought back the original name, Allosaurus. This decision was supported by new fossils found in places like the Cleveland-Lloyd Dinosaur Quarry and Dinosaur National Monument.

The exact set of body features used to tell Allosaurus apart from other dinosaurs changes between studies. One common feature is a large horn made by the lacrimal bone above and in front of the eye. The lower jaw has a bone called the antarticular, which is special to Allosaurus. Another feature is found in the pelvis, where the lower end of the ischium looks suboval when viewed from the side.

Main article: Allosauridae

Allosaurus is the main member of several larger groups: Allosaurinae, Allosauridae, Allosauria, and Allosauroidea. The subfamily Allosaurinae was first used by Gregory S. Paul in a 1988 book and included Allosaurus and Chilantaisaurus, but it is not used much. Allosauridae is a family that Marsh named in 1878, one year after describing A. fragilis. Originally, Allosauridae only included Allosaurus. Though more dinosaurs were added later, recent reviews say the family includes only Allosaurus and Saurophaganax. Allosauria is a larger group that usually includes Allosauridae and Carcharodontosauria. Allosauroidea is an even larger group that also includes Metriacanthosauridae.

Currently recognized species

Allosaurus fragilis

The species A. fragilis was named in 1877 by Othniel Charles Marsh. The name fragilis means "fragile" in Latin. Marsh used an incomplete fossil to describe this dinosaur. In 2023, scientists chose a more complete fossil found in the same place to be the main example for this species.

Allosaurus europaeus

In 2006, scientists Octávio Mateus and colleagues named A. europaeus using fossils from Portugal. They used a partial skull and some neck bones. Later studies have questioned if this is a separate species or just a variation of A. fragilis. In 2025, new studies suggested differences in skull and neck features, but these findings are still being reviewed.

Allosaurus jimmadseni

The name A. jimmadseni was first mentioned in 2000 but was officially described in 2020 by Daniel Chure and Mark Loewen. It honors James Madsen for his work on dinosaur fossils. The main fossil for this species is nearly complete, and several other fossils were linked to it. Scientists identified this species by certain skull and cheekbone features. In 2024, studies showed that A. fragilis and A. jimmadseni lived at the same time but in different areas.

Allosaurus anax

In 2024, scientists Andy Danison and colleagues described A. anax using fossils that were previously part of another dinosaur named Saurophaganax. They found that these fossils actually belonged to a new species of Allosaurus. The name anax means "king" in Greek. This species is known from larger fossils than other Allosaurus species and has unique features in its skull and spine bones.

Previously assigned species and synonyms

Poikilopleuron valens (Antrodemus)

In 1869, a scientist named Ferdinand Vandeveer Hayden found a fossil in Colorado. He sent it to another scientist, Joseph Leidy, who thought it was part of a dinosaur’s tail. At first, Leidy called it Poikilopleuron valens, but later changed its name to Antrodemus valens. For many years, scientists used the name Antrodemus, but in 1976, another scientist named Madsen decided that Allosaurus was the better name. Antrodemus is now considered an unclear name, and Allosaurus valens is a way to refer to Antrodemus valens.

Laelaps trihedrodon

Laelaps trihedrodon was named by a scientist named Cope in 1877. He found a jaw with teeth in Colorado. Later, the jaw was lost, but some teeth remained. In 2001, a scientist named Chure thought these teeth might belong to Allosaurus.

Labrosaurus

The name Labrosaurus was given by Marsh in 1879. It means “greedy lizard.” Over time, scientists decided that Labrosaurus was actually the same as Allosaurus. Some other names like L. ferox, L. sulcatus, L. stechowi, and L. meriani were also suggested, but they might belong to Allosaurus or another dinosaur called Ceratosaurus.

Allosaurus lucaris

The name Allosaurus lucaris was given by Marsh in 1878. It was found in Colorado or Wyoming. Later, scientists often thought it was the same as Allosaurus fragilis. Some scientists in 2010 thought it might be a different genus, but they also said the fossils weren’t clear enough to be certain.

Labrosaurus ferox

Labrosaurus ferox was named in 1884 by Marsh. It was found in the same place as the first Allosaurus fossils. The jaw looked unusual, and some scientists thought it might have been injured. Others thought it might just be a strange shape or a mistake in how the fossil was put together. It could be a specimen of A. fragilis or something else altogether._

Epanterias amplexus

Epanterias amplexus was named by Cope in 1878. The name means “buttressed vertebrae” and “embracing.” The fossils were found in Colorado. Some scientists thought it was related to Camarasaurus, but later, others thought it was a large theropod dinosaur. In 2010, scientists said the fossils weren’t clear enough to be certain, and listed it as an unclear name._

Creosaurus atrox

Creosaurus atrox was described by Marsh in 1878. The name means “flesh lizard.” It was found in Wyoming. Marsh thought it was related to another dinosaur called Dryptosaurus. Later scientists had trouble telling Creosaurus apart from Allosaurus, and some even thought it was the same. In 2010, some scientists said it was an unclear name._

Camptonotus amplus

In 1879, Marsh named a dinosaur Camptonotus, which means “bent lizard.” He found a foot bone in Wyoming and thought it belonged to a very large dinosaur. Later, scientists thought the foot might actually belong to an allosaurid dinosaur. In 2025, scientists decided it was an unclear name._

Allosaurus medius

A. medius was named by Marsh in 1888. It was found in Maryland. Later, scientists moved the fossils to other names, and A. medius is now considered an unclear name._

Allosaurus sibiricus

Allosaurus sibiricus was described in 1914. It was found in Russia. Later, scientists decided it was an unclear name and could not be identified beyond being a theropod dinosaur._

Allosaurus tendagurensis

A. tendagurensis was named in 1925. It was found in Tanzania. Scientists later decided it was not very similar to Allosaurus and could not be clearly identified._

Saurophaganax maximus

The fossils that would later become known as Saurophaganax were discovered in Oklahoma. In 1995, a scientist named Chure described the taxon and renamed it Saurophaganax. In 2024, scientists suggested some of the fossils might have been from a sauropod, while the rest were confirmed as those of a theropod and assigned to a new species of Allosaurus, A. anax.

Allosaurus lucasi

Allosaurus lucasi was named in 2014. It was found in Colorado. The scientist who named it said it was different from A. fragilis, but later scientists did not accept this name and considered it an unclear name._

Typographical errors and nomina nuda

Some early names for the dinosaur Allosaurus were mistakes. For example, “Allosaurus agilis” from 1912 was really meant to be A. fragilis. Another mistake was “Allosaurus ferox,” which the scientist Othniel Charles Marsh wrote by accident in 1896, also meaning A. fragilis. Similarly, “Labrosaurus fragilis” from 1896 was a mistake for Labrosaurus ferox.

A name called “Labrosaurus huene” was used in 1956 and 1958 for a tooth found in Sichuan, China, but it was not properly described. “Allosaurus whitei,” created in 1995, is considered another name for A. fragilis.

Other misassigned specimens

Scientists studied many old dinosaur pieces from Europe and Asia. They thought these pieces might belong to a dinosaur called Allosaurus. But in 2025, they found that only a few pieces from Portugal and some teeth from Germany really belong to Allosaurus. Other teeth and dinosaur tracks from Germany, France, and England could not be matched to any specific dinosaur group.

One set of dinosaur bones from Japan was once thought to be Allosaurus. But in 2025, scientists decided it was actually a different dinosaur called Segnosaurus. Some teeth from Siberia were also thought to be Allosaurus, but in 2012, scientists said they could not tell for sure what kind of dinosaur they came from.

An ankle bone found in Australia was once thought to be from Allosaurus. But later studies suggested it might belong to a different kind of dinosaur, maybe similar to Fukuiraptor or Australovenator, or perhaps an abelisaur.