When the bones of Allosaurus were officially described in 1877, they were quite different from any other dinosaur fossils known at the time; hence the name Allosaurus (="other lizard"). A vast majority of what we know for certain about this animal has been learned from its skeletal remains.
The Skull-
The skull of Allosaurus was, by carnosaur standards, quite small in relation to its body. As with all members of the allosauridae family, the skull was adorned with prominent lacrimal horns and crests in front of each eye. One of the most noticeable features of the skull is the large window-like anorbital fenestra. The purpose of these wide openings was to lighten skull without sacrificing much in the way of strength andmay gave increased the area for jaw muscle attachment. Actually, a recent study indicates the skull of Allosaurus seems to have been able to withstand incredible amounts of force, to the point that it was seemingly over-engineered. For further explanation of this see "feeding and hunting."
Allosaurus' jaws were extremely flexible. They possessed extra hinges that would allow the jaw to expand laterally, in order to bite of much larger chunks of flesh than would have been possible otherwise. Along the rim of allosaur jaws is a slightly raised edged that is marked with small grooves and pits that, in life, held blood vessels and nerves. This is where the lips would have been.
Teeth-
The teeth of Allosaurus were similar to steak-knives in form and function. The teeth were laterally flattened and serrated, 2-4 inches in length. Much like many modern-day reptiles, an Allosaurus would constantly shed and replaced its teeth throughout its lifetime.
Neck-
The neck of Allosaurus was composed of nine cervical vertabrae. It exhibited the classic "S" curve found in birds and most other theropods. Large ribs were attached to the cervical (neck) vertabrae. These ribs protected some of the more vulnerable parts of the neck and at the same time served as an attachment point for muscles. Allosaurus' powerfully muscled neck was been of great assistance in rending flesh from its victims.
The cervical vertabrae, as well as the first five dorsal (back) vertabrae, possessed chambers that are very similar to hollows located in the vertabrae of birds. In birds these chambers house a complex system of air-sacs that siginificantly increase the efficiency of the respiratory system beyond that of any mammal's. It is highly probable that Allosaurus had the benefit of an air-sac system as well.
Forelimbs-
The arms of Allosaurus were quite short, but they were powerful. They terminated in murderous talons that, on the first digits(thumbs), could be up to 10 inches in length. Allosaurus almost certainly employed its arms in dismembering prey and and carrion alike.
Furcula-
A very birdlike feature on Allosaurus is the presence of a furcula (wish-bone). When this bone was first found, it was thought to be one of the gastralia (belly ribs). The presence of this bone in Allosaurus and numerous other theropods atests to their kinship to modern birds.
Hindlimbs-
The legs of Allosaurus were long and powerfully muscled. The claws on the hindlimbs were surprisingly blunt and hooflike. From this it can be inferred that Allosaurus was capable of running, kicking, and probably even jumping, but it is highly unlikely that it tore open its prey with its feet.
Tail-
No complete Allosaurus tail has yet been found, but it is estimated that tail comprised more than half of the animal's total length. The main purpose of the Allosaurus tail was to serve as a counter-balance for its forward leaning body.
