Allosaurus ruled the late Jurassic period as the top predator. It lived from 155 to 150 million years ago, nearly 90 million years before T. rex ever walked the planet. Reaching nearly 40-feet in length and 2 tons, allosaurus was a voracious hunter and could take down large prey such as stegosaurus and diplodocus. Allosaurus was built to kill with powerful arms, long ripping claws, huge legs and a muscular neck. Its skull was massive, deep and narrow. Packed with rows of serrated knife-like teeth, its jaw was well suited for rapid opening and closing. These features give paleontologists insights into the animal's lifestyle and behavior. Computer modeling supports the idea that allosaurus used its skull as a hatchet-like weapon to slam into the body of its prey and make slashing wounds. The bones of many victims have been found bearing marks from allosaurus teeth. When fossil hunters discovered allosaurus fossilized bones over 140 years ago, it gave birth to a global fascination with dinosaurs. Allosaurus quickly became the quintessential Jurassic carnivore. Paleontologists have since found the remains of many allosaurus specimens, especially in the Morrison Formation in the western USA. This large expanse of sedimentary rock is a rich fossil-hunting ground. With an abundance of fossils, allosaurus has become one of the most loved and famous dinosaurs.