Allosaurus fragilis had an average length of 8.5 meters (28 ft), with the largest definitive Allosaurus specimen (AMNH 680) estimated at 9.7 meters long (32 ft), and an estimated weight of 2.3 metric tons (2.5 short tons).Allosaurus was a typical large theropod, having a massive skull on a short neck, a long tail and reduced forelimbs. As with dinosaurs in general, weight estimates are debatable, and since 1980 have ranged between 1500 kilograms (3300 lb), 1000 to 4000 kilograms (2200 to 8800 lb), and 1010 kilograms (2230 lb) for modal adult weight (not maximum).
Several gigantic specimens have been attributed to Allosaurus, but may in fact belong to other genera. The closely related genus Saurophaganax (OMNH 1708) reached perhaps 10.9 meters (36 ft) in length, and its single species has sometimes been included in the genus Allosaurus as Allosaurus maximus, though recent studies support it as a separate genus. Another potential specimen of Allosaurus, once assigned to the genus Epanterias (AMNH 5767), may have measured 12.1 meters in length (40 ft). A more recent discovery is a partial skeleton from the Peterson Quarry in Morrison rocks of New Mexico; this large allosaurid may be another individual of Saurophaganx.
Skull of the Allosaurus fragilis skeleton mounted in the lobby of the San Diego Natural History Museum. The skull and teeth of Allosaurus were modestly proportioned for a theropod of its size. Paleontologist Gregory S. Paul gives a length of 845 millimeters (33.3 in) for a skull belonging to an individual he estimates at 7.9 meters long (26 ft). Each premaxilla (the bones that formed the tip of the snout), held five teeth with D-shaped cross-sections, and each maxilla (the main tooth-bearing bones in the upper jaw) had between fourteen and seventeen teeth; the number of teeth does not exactly correspond to the size of the bone. Each dentary (the tooth-bearing bone of the lower jaw) had between fourteen and seventeen teeth, with an average count of sixteen.
The skull had a pair of horns above and in front of the eyes. These horns were composed of extensions of the lacrimal bones,and varied in shape and size. There were also lower paired ridges running along the top edges of the nasal bones that led into the horns. The horns were probably covered in a keratin sheath and may have had a variety of functions, including acting as sunshades for the eye, being used for display, and being used in combat against other members of the same species (although they were fragile). There was a ridge along the back of the skull roof for muscle attachment, as is also seen in tyrannosaurids.
Allosaurus is a genus of large theropod dinosaur that lived 155 to 145 million years ago, in the late Jurassic period (Kimmeridgian to Tithonian). The name Allosaurus means "different lizard" and is derived from the Greek αλλος/allos ("different, strange") and σαυρος/sauros ("lizard"). The first remains that can definitely be ascribed to this genus were described in 1877 by Othniel Charles Marsh. As one of the first well-known theropod dinosaurs, it has long attracted attention outside of paleontological circles, and has been a lead dinosaur in several films and documentaries.
Allosaurus was a large bipedal predator with a large skull, equipped with dozens of large, sharp teeth. The preys of the Allosaurus were the Diplodocus or Stegosaurus.