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Allosaurus: Facts About the 'Different Lizard'

Allosaurus was among the earliest dinosaur discoveries, and fossils are plentiful, making it a darling of paleontologists as well as amateur dinosaur observers.

This large theropod lived 155 million to 150 million years ago during the late Jurassic Period. More than 10,000 bones from about 46 specimens have been unearthed in the Cleveland-Lloyd Quarry in Utah, making the Allosaurus the most commonly discovered fossil in Utah. It is also Utah's state fossil. Allosaurus fossils have also been discovered in Thailand as well as in Portugal and other parts of Europe. Possible Allosaurus fossils have been discovered in Africa and Australia.

Artwork by Scott Hartman reveals the bone structure of Allosaurus
Allosaurus Skeletal Reconstruction
Credit: © Scott Hartman / All rights reserved

Seven species of Allosaurus have been identified, Allosaurus fragilis being the best-known. There is some debate about whether there are more species or whether the specimens in question belong to other genera.

The name Allosaurus is derived from the Greek allos ("different" or "other") and sauros ("lizard"). One feature that makes this dinosaur "different" is its unusual vertebrae. It had nine vertebrae in its neck, 14 in the back and five supporting its hips. There were hollow spaces in the neck and anterior back vertebrae. Such spaces, which are also found in modern birds, are believed to have contained air sacs for respiration. The rib cage was broad, giving it a barrel chest. Allosaurus had gastralia, or belly ribs.

Allosaurus was a massive dinosaur, weighing about 3 tons (2,721 kilograms) and averaging 28 feet (8.5 meters) in length and about 16.5 feet (5 meters) tall. While not as large as Tyrannosaurus rex or Giganotosaurus, Allosaurus is typically ranked fifth among the biggest predatory dinosaurs.

At 3 feet (90 centimeters) long, Allosaurus' skull was modestly proportioned to its body and sat on a relatively short neck. Another distinguishing feature was a pair of bony ridges above and in front of its eyes that resembled horns. These horns varied by animal in shape and size. There were also ridges running along the top edges of the nasal bones that led into the horns.

Based on the shape of its skull and placement of its eye sockets, paleontologists do not believe that Allosaurus had limited sight.

Each lower jaw held 14 and 17 teeth and the teeth became shorter, narrower and more curved toward the back of the skull, designed to strip meat off of a carcass. It was not always an efficient eater, and other dinosaurs would feast on its leftovers.

Its large teeth with saw-like edges were dislodged easily and replaced continually throughout the dinosaur's life. Like many carnivorous dinosaurs, the Allosaurus' jaws expanded up and wide, enabling it to get large chunks of food in its mouth.

Allosaurus' massive body was supported by two powerful hind limbs and a large tail. Each foot had three weight-bearing toes and an inner dewclaw. Scientists believe that it could travel up to about 20 mph (32.2 kph), and if it went any faster it would lose its balance and its arms would not break its fall and would be crushed by its weight. However, because it was so large and was a fierce predator, speed was not an issue.

What did Allosaurus eat?

Allosaurus feasted mostly on large herbivore dinosaurs and often tangled with Stegosaurus. Paleontologists have unearthed an Allosaurus vertebra with a puncture wound that matches the shape of a Stegosaurus tail spike and a Stegosaurus neck bone bearing a bite mark that matches those of an Allosaurus.

It was capable of killing could kill medium-sized sauropods or large sauropods, such as Apatosaurus, that were sick or injured. It was a fierce and aggressive predator, as indicated by the tooth marks discovered on the vertebrae of an Apatosaurus. There has been speculation that Allosaurus hunted in packs, but the majority option has swayed toward believing that they worked their prey alone.

Paleontologists believe Allosaurus attained its full adult size by age 15 and typically lived to 25, barring disease or injury, because it did not have any real match in terms of predators. Because there is such a wealth of Allosaurus fossils at various stages of development, there is evidence that baby Allosauruses might have survived on insects such as dragonflies, centipedes and other small animals until the age of 2, when they graduated to eating small dinosaurs.

Fossil finds

The first Allosaurus was discovered in 1877 in Colorado and named by Othniel Charles Marsh. This specimen offered just a few fragments of the dinosaur and Marsh went on to name other specimens, which, with time, were discovered to also be part of the Allosaurus, causing quite a bit of confusion in the early going of documenting this dinosaur.

A more complete skeleton was discovered in 1879 by H.F. Hubbell but was never unpacked. In 1908, after his death, the specimen was finally examined and turned out to be one of the most complete theropod skeletons unearthed to date.

A fossil that was 95 percent intact and dubbed "Big Al" was excavated near Shell, Wyo., by a joint Museum of the Rockies and University of Wyoming Geological Museum team. In 1996, the same team discovered a second Allosaurus that is the best preserved skeleton of its kind to date and named it named "Big Al Two."

In 2007, a group of palaeontologists working in northeast Thailand close to the village of Ban Saphan Hin unearthed the remains of an Allosaurus.

Learn about the huge meat-eating dinosaur Allosaurus.
Learn about the huge meat-eating dinosaur Allosaurus.
Credit: Ross Toro, Livescience contributor

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