Allosaurus, meaning “different lizard”, is a genus of theropod dinosaur from the Kimmeridgian/Tithonian age of the Late Jurassic Period (155 to 145 million years ago). It was first described in 1877 by Othniel Charles Marsh. The first recovered specimen came, most likely, from the Morrison Formation near Granby, Colorado. It was discovered in 1869 by Ferdinand Vandiveer Hayden. The original finding was identified by locals as “petrified horse hoofs”. It was initially placed in the genus Poekilopleuron and then later given its own genus Antrodemus. This name held its place for more than fifty years until it finally became known as Allosaurus. There are numerous potential species of Allosaurus, but only seven are well documented. A. amplexus, A. atrox, A. fragilis, A. jimmadseni, and A. maximus are all known from the same formation and spread across Colorado, Montana, New Mexico, Oklahoma, South Dakota, and Wyoming.
Allosaurus was a large bipedal predator that was about 28 feet in length, but could have been as much as 39 feet in length. Its hind limbs were large and powerful, and its forelimbs were smaller with only three fingers. It had a long, heavy tail that provided balance. It may have weighed between 2200 and 5000 pounds. However, some studies show that its weight may have only been around 1500 pounds up to as much as 8800 pounds. The overall weight could be different for each species within the genera too. This dinosaur had a pair of horns above and in front of the eyes. These horns came in various shapes and sizes. There were also paired ridges running along the nasal bones. The purpose of these ridges is unclear but may have been used for shading the eyes from the sun, used as a display, or used for intrapsecies combat.
Allosaurus was the top predator of its time. It most likely preyed on large herbivores, but may have also taken down other predatory dinosaurs. Ornithopods, stegosaurids, and sauropods would have been fair game. Some paleontologists believed there may have been a social order with Allosaurus and they may have hunted in packs. Others believe differently. Individuals may have only existed together when possibly feeding on the same carcass. Social behavior may have been extended only to parental care. Some evidence shows that food may have been brought to the nesting lair for the young to eat until they were grown. Besides this, there is little evidence of gregarious behavior in these dinosaurs, as well as other theropods. Although it is possible these dinosaurs may have hunted in packs, it is likely their aggressive nature would have devastating effects on a social structure.