Allosaurus fragilis

Last updated: October 10, 2022
Verified by: AZ Animals Staff
Image Credit Cheng Wei/

Differnt Lizard” or Allosaurus weighed around two tonnes that is almost equal to a car.

Allosaurus Scientific Classification

Scientific Name
Allosaurus fragilis

Read our Complete Guide to Classification of Animals.

Allosaurus Conservation Status

Allosaurus Facts

Stegosaurus, Brachiosaurus
Group Behavior
  • Social
Fun Fact
Differnt Lizard” or Allosaurus weighed around two tonnes that is almost equal to a car.
Biggest Threat
Most Distinctive Feature
Small horn and ridges on the skull
Gestation Period
6 months
Litter Size
10-20 eggs
Forests, Flood plainlands
Torvosaurus and Ceratosaurus
  • Group
Number Of Species

Allosaurus Physical Characteristics

  • Dark Brown
Skin Type
Top Speed
55 mph
28 years
4600 lbs (2086 kg)
16 ft
39 ft
Age of Sexual Maturity
15 years

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Description and Size

Allosaurus fragilis is a species whose Greek translation is “different lizard.” It is considered a main part of the Theropod group, meaning it preys on flesh. 

They are one of the most dominant fossils found around the world which is why paleontologists had a lot to study. Here are a few facts related to its physical attributes. 

  • Allosaurus weighed about 4600 lbs or 2.3 tons and it had an average length of approximately 12 meters.
  • The female Allosaurus (3.2 meters) was slightly shorter in height compared with the male Allosaurus (3.8 meters). 
  • The skull of the Allosaurus was quite large in comparison to its neck. In addition, it was elongated and carried ridges from the nose to the eyes. On top of the eyes, there were small curved horn-like structures. 
  • The teeth of Allosaurus were sharp, dagger-like, and five to ten centimeters long. But an interesting thing is that they were curved on the inside, helping them in hunting and holding down the prey. 
  • It had bigger hind limbs in comparison with its forelimbs but the difference is not that large. It was a bipedal organism with three fingers on each hand and three toes on each foot. But there was another short toe on the inner side of each of its feet.
  • It had quite a heavy tail and a relatively light build that helped it to maintain balance. 
  • The tail of the Allosaurus was quite large and pointy. It is made up to be half the length of the whole organism. 
  • Another interesting thing about this dinosaur is that it had vertebrae, unlike any other dinosaur. It had a series of concave bones that resembled an hourglass-type structure.  
A pack of allosauruses on the move through a snowy landscape.
A pack of Allosauruses on the move through a snowy landscape.

Daniel Eskridge/

Diet – What Did Allosaurus Eat?

As is seen through its physical attributes, Allosaurus is a carnivore. It had a fast-running nature that suggests it used to hunt and prey. Along with the legs, its claw-shaped fingers can easily be used to grasp the prey. Although it did not have a strong bite due to smaller teeth, the Allosaurus is predicted to have used its skull and ridges as a way to kill its hunt.

The diet of Allosaurus mainly consisted of large herbivores like Ornithopods, Sauropods, Stegosaurus, or Apatosaurus. Many studies have even suggested that it used to eat animal carcasses and was a scavenger. Now, there may be a debate on whether or not it hunts dead animals, but one thing that every study agrees on is that they ate meat.     

Habitat – When And Where It Lived?

Allosaurus existed in the late Jurrasic period and were quite prevalent till the Turonian period. Their fossils have been found from over 163 million years ago and even from over 89 million years ago. Most of the fossils are found in the USA and the first one was discovered in Colorado

They were mainly spread across North America, Europe, and Africa. Their habitat was researched to be near plain lands and streams. Some of them even lived in forests with rivers flowing between them. 


The Allosaurus mostly lived in groups and were rather sociable. They even hunted in the form of groups. But the studies have shown quite a disparity in their group behavior. Some suggest that they fought against each other while others seem to think that a group of Allosaurus hunted the same prey.

There are signs of injuries on their bones and fossils that seem to support this idea. However, thinking of carnivorous animals hunting the same prey seems quite out of the ordinary. Hence, it is very much possible that they depicted antagonistic and maybe even cannibalistic behavior. 

Threats And Predators

Many studies have shown that the Stegosaurus was a fierce threat to the Allosaurus. The Stegosaurus was a plant-eater and had a spiky tail. Many fossils exist that show injuries on one dinosaur due to the other. So, it can be believed that there was a raging battle between the two.  

Apart from that, the Torvosaurus and Ceratosaurus were known predators of Allosaurus. They were also carnivores but had similar or smaller sizes than the Allosaurus. But the battle was mostly on who gets to eat the prey.

However, one distinct quality of Allosaurus is its cannibalism. Being one of the biggest predators in the Late Jurassic era, they were a threat to themselves.

Discoveries and Fossils – Where It was Found

Allosaurus is one of the most abundant fossils found to date. Most of these fossils are found in the US but they were present in Europe and Africa too.

The first fossil of an Allosaurus was found in the year 1869 in Colorado. It was believed to be soft horse hoofs by locals but then was later studied to be the vertebrae of an Allosaurus. 

Many fragments were discovered later on by many scientists like its vertebrae, part of a rib, tooth, and even a toe bone. But the biggest discovery was made in the year 1945. A minimum of 73 dinosaurs were found in Utah and out of 73, 46 were at least the specimens of an Allosaurus. The fossils were of both juvenile and adult dinosaurs. 

To this day, it remains a mystery how such a large resource of fossils was found in one place. But there are some basic assumptions. 

Some scientists think there was a flood that carried the carcasses of the dead Allosaurus to one place while others think they got trapped in a mud slope. Even a drought could have brought them all into one place. So, it is only safe to assume and nothing concrete enough to make it a theory.  

After that, many fossils of an Allosaurus have been found in Portugal and Poland. But another great discovery was in the year 1991 in Wyoming

A fossil about 95% complete was found that contributed majorly to the Paleontological study. It was named “Big Al.” Then, another complete specimen was discovered in 1996 that was considered the biggest complete specimen to date. It was named “Big Al II.”

Extinction – When Did It Die Out?

Data shows that Allosaurus went extinct around 144 million years ago. The cause is uncertain but there are certain theories about the species’ termination. Some suggest that a drought had befallen the then era and a lot of dinosaurs along with Allosaurus died due to the cause. The biggest fossil found in Utah also supports this theory. But the latest fossil discovered, named Big Al II, was found to have pathological infections. Hence, it could be that there was an infection spreading but nothing can be confirmed.

Similar Animals to The Allosaurus

  • Tyrannosaurus rex (T-Rex) – a carnivorous dinosaur with sharp teeth, a long tail, small arms, and a heavier body. It resembled Allosaurus but they had two fingers on each of their hands rather than three for an Allosaurus. 
  • Ceratosaurus – another carnivore from the same era as that of an Allosaurus, this animal weighed two tons too. It had a long tail and similar anatomy. However, it had a distinctive horn and four fingers instead of three.   

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About the Author

Alan is a freelance writer and an avid traveler. He specializes in travel content. When he visits home he enjoys spending time with his family Rottie, Opie.

Allosaurus FAQs (Frequently Asked Questions) 

Is An Allosaurus Bigger Than A T-Rex?

No. The T-Rex was significantly bigger than an Allosaurus as it carried a height of 12-15 meters. In contrast, the Allosaurus had a height of 8-12 meters.

Was Allosaurus A Top Predator?

In its time, Allosaurus was one of the biggest predators. Having the most abundant fossils in the Morrison formation, also suggests that they were on the top.

Thank you for reading! Have some feedback for us? Contact the AZ Animals editorial team.

  1. Jurassic-pedia, Available here:
  2. Kidadl, Available here:
  3. Britannica, Available here:
  4. Animals, Available here:
  5. Wikipedia, Available here:
  6. Fossilera, Available here:

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