Dinosaurs | Egypt | Mesopotamia |Human Ancestors

Allosaurus fragilis

During the Late Jurassic, the arch predator of North America was Allosaurus fragilis. Later, during the Cretaceous Period, T. rex would have this honor. Allosaurus grew to lengths of over 10 meters (about 30 ft.), and weighed as much as 1.5 tons. Because allosaurs have been found in groups that probably died together, paleontologists, think that they may have traveled in packs. Scientists still debate over whether large carnosaurs like Allosaurus were scavengers or active predators. There seems to be evidence to support both sides. Fossil remains have been found in the western United States and in Eastern Africa. Other members of the Allosaur family have been excavated in China and Russia..

Unlike T. rex, Allosaurus fragilis had long, strong arms and hands with three claws. The largest claw (up to 6 inches) was on the inside finger and they got progressively smaller as you move to the middle digit (4 inches) and then to the smallest outside one (3 inches).

Discovery
Allosaur fossils date from the Middle Jurassic until the beginning of the Cretaceous. While the first discovery was in 1869, the richest find was the Cleveland-Lloyd quarry in Utah. Since 1927 the bones of over 44 different Allosaurs have been found in this one spot. This quarry has revealed mostly allosaurs, but the fossil remains of stegosaurs, ankylosaurs, sauropods and other theropods have also been excavated. Obviously, this quarry was some sort of predator "trap", but it still remains unclear why so many allosaurs died and were buried together here.
Taxonomy

Allosaurus, like its predatory cousins T. rex and Deinonychus, is a theropod. Its skeleton shows all the distinguishing characteristics of this group including clawed hands, hollow bones, and a bipedal stance. Most theropod dinosaurs fall into three subgroups: the more primitive ceratosaurs; the large carnosaurs; and the smaller, bird-like coelurosaurs. Allosaurs are carnosaurs and within the carnosaur group have their own family. Two of the features that characterize allosaurs are noticeable on the skull you hold. First, the end of the snout is evenly rounded across both maxilla and mandible. With other theropods the top and bottom jaws are rounded differently creating less symmetry. Second, notice how far back into the mouth the top teeth extend. The last tooth is almost under the orbit. If you look at T. rex, you'll see the teeth are all in the front half of the face.

Anatomy
When you study the skull of Allosaurus fragilis, several things stand out. The teeth are always noticed. They were large, sharp, and grew up to 10 cm in length (4"). The upper and lower teeth did not touch when the mouth was closed. Rather, they passed near each other like blades of scissors, cutting hunks of flesh out of their prey. Because of this configuration, Allosaurus could not chew it's food; it had to swallow chunks of meat whole. All its teeth had serrations on both front and back edges. It is thought that these serrations acted to increase the cutting ability of the tooth. Like all dinosaurs, its teeth were deciduous--constantly forming, erupting and falling out during the lifetime of the animal. As one tooth was being used, another was forming to take its place, as in modern sharks.

Unlike the heavily-built, massive-boned T. rex skull, Allosaurus' had an more open, strut-like construction that combined both strength and light weight. The head connected to an "S"-shaped, flexible neck that carried the head high and over the shoulder. One curious feature of the Allosaur skull is the bony protuberance over each eye (formed by the Lacrymal bone). Paleontologists don't know their purpose, but have guessed that they may have been attachment points for some elaborate head ornamentation.

[ Dinosauria -> Saurischia -> Theropoda -> Tetanurae -> Carnosauria -> Allosauridae ]

[ Dinosauria Main Page | InHandMuseum ]