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Scientific Classification
Kingdom: Animalia
Phylum: Chordata
Class: Sauropsida
SuperOrder: Dinosauria
Order: Theropoda
SubOrder: Carnosauria
Family: Allosauridae
Genus: Allosaurus
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The Allosaurus was a large, carnivorous dinosaur of the Jurassic period. It is believed to have existed about 157 to 149 million years ago. This lies in the Kimmeridgian and Tithonian ages of the Jurassic. It was one the first theropods to be discovered by man.
The length of the Allosaurus was about 7 to 9 meters. This may seem small as compared to the larger theropods of the Cretaceous period, but was one of the largest dinosaurs of its time. Only the giant sauropods exceeded the Allosaurus in size and they were herbivores. The weight of the Allosaurus was about 3000 kilos.
The Allosaurus became popular immediately after it was discovered in the latter half of the nineteenth century. This can be attributed to the fact that ‘bone wars’ or races to discover newer species of dinosaurs were prevalent everywhere in the paleontology circles at that time.

The name 'allosaurus' is derived from two words 'allo' and 'saurus'. The word 'allo' means 'not similar' and has Latin origins. The word 'sauros' means 'lizard' in the Greek language. Thus, the word 'allosaurus' roughly translates to 'a dinosaur that is not similar'. This name was chosen as the Allosaurus was different as compared to the other known dinosaurs during the time of its discovery.
The species name Allosaurus fragilis was selected as it was assumed that theropods were very susceptible to fractures of bones. And such fractures did not heal easily. Hence the name 'fragilis', which means fragile, was assigned to the fossils. But later it was proved that this was not the case. Theropods were very hardy.

Discovery of fossils

  • The fossils of the Allosaurus were found by different individuals in different locations during the nineteenth century. The first sets of remains were discovered in Colorado in the Morrison Formation in Grasby. They were discovered by Ferdinand Hayden in 1869 and were handed over to O. C. Marsh.
  • In the years to come, many fossils were discovered in the Morrison Formation and were assigned to genera such as the Epanterias and the Creosaurus. These were later clubbed with the remains of the Allosaurus.
  • There was a skeleton discovered in the Como Bluff area of Wyoming in 1879, but was never studied until 1903. They were found by one of the associates of scientist E. D. Cope. They are currently attributed to the Allosaurus.

These results were seen due to the ‘bone wars’ initiated by Othniel Charles Marsh and Edward Drinker Cope. Many fossils were discovered in this period but were not analyzed adequately. This led to the formation of many redundant genera.

  • In the early 1900s, a plethora of fossils were discovered in the Cleveand-Lloyd Quarry in Utah. Most of these are ascribed to the Allosaurus.

The Allosaurus is one the best understood theropod dinosaur. This is due to the large number of fossils available. Many bone diseases were also noted on these bones osteopetrosis, osteoarthritis, ankylosis, etc were also seen on these fossils.
Such findings are rare for dinosaur remains.

The Allosaurus is classified currently under clade Carnosauria, family Allosauridae, sub family Allosaurinae.

  • The fossils were initially classified in genus Atrodemus, a term coined by Joseph Leidy.
  • They were later grouped under genus Allosaurus by Marsh.
  • This classification was not very popular, with most scientists using the family Megalosauridae to classify the Allosaurus.
  • Only when Megalosauridae was omitted in the 1970s, the name Allosauridae came to be used more frequently.

The other genera which were considered synonyms of the Allosaurus were later de-recognized by the international nomenclature agency.

Nature of the skull of the Allosaurus
The most distinguishing feature of the skull of the Allosaurus was the projection on the lacrimal bones. There were small crests seen on the nasal bones as well. A region for attachment of muscles was also seen on the occipital bone which formed the base of the skull.
The Allosaurus was known to shed its teeth and grow new ones very frequently. This was determined by observing regular missing teeth in the skull fossils on individual dinosaurs.

Physical characteristics

  • The Allosaurus was a large dinosaur, but moderate in size as compared to the Cretaceous theropods. It grew to an average length of 27 feet. A large 40 feet specimen was discovered in the 1990s but it had many bone defects. Thus, this specimen is considered an exception.
  • The weight of the Allosaurus is estimated at 2 to 4 tons. The bigger, heavier specimens of the Allosaurus are considered to belong to other genera.
  • Its head was smaller as compared to other theropods.
  • The Allosaurus had a short neck, which was muscular to help the dinosaur tear apart its prey.
  • The fore arms of the Allosaurus were shortened in length like other larger theropods.
  • The Allosaurus’s femur was much smaller than its tibia. This indicated that it could not run at high speeds
  • The tail of the Allosaurus was thick and offered balance while walking to the dinosaur.
  • Differences were seen in the pelvic regions of male and female dinosaurs, with females having hip bones adapted for egg laying.

Habits and habitat

  • The Allosaurus was a bipedal dinosaur, mirroring other advanced theropods.
  • It was obviously ground-dwelling and carnivorous. It is speculated that the Allosaurus hunted ornithischian dinosaurs such as the Stegosaurus.
  • The Allosaurus is depicted in many artistic interpretations as hunting in packs. There is no fossil evidence suggesting such social harmony among the Allosaurus species. No other theropods lived or hunted in groups and there is no proof that the Allosaurus did so.
  • Many paleontologists postulate that the Allosaurus could also have been a scavenger, deriving its nourishment from fallen sauropods.
  • The habitat of the Allosaurus comprised desert-like conditions with intermediate rainfall. Forest covers may have been present along rivers in the environment of the Allosaurus.

Related and coexisting species

The Allosaurus most likely coexisted with the Ceratosaurus and the Torvosaurus. It is also believed to have lived alongside the Stegosaurus.
The fossils of the Dryosaurus, Camposaurus, Apatosaurus and Camarasaurus are frequently found along with those of the Allosaurus, indicating they may have coexisted.
The Allosaurus is related to the Acrocanthosaurus and the Giganotosaurus.

Concluding notes
The Allosaurus was an important Jurassic theropod. It played a vital role in the Jurassic ecosystem, being challenged by very few species.
No other theropod dinosaur is as comprehensively defined as the Allosaurus. It has provided paleontologists with substantial data about the lives of the bigger theropods.

Just for fun we have a soundclip available for you to hear what a Allosaurus could've sounded like. Click to the Dinosaur Sounds area to hear it. Please note that the dinosaur sounds are only for entertainment and are not an actual fact.