Both the Allosaurus and the Tyrannosaurus rex were bipedal, carnivorous theropods, but the Tyrannosaurus rex was much larger and heavier. An Allosaurus reached 30 feet and weigh around 3.5 tons, while a Tyrannosaurus reached 45 feet and weighed 7.5 tons. The Allosaurus existed around 150 to 155 million years ago, during the late Jurassic period. The Tyrannosaurus lived around 67 to 65 million years ago, during the upper Cretaceous period.
The Tyrannosaurus had extremely short arms and two fingers, while the Allosaurus's forelimbs were longer and had three fingers. The Tyrannosaurus had a wider skull with more powerful jaw muscles for crushing through solid bone, while the Allosaurus tore apart flesh with its narrower jaws. The Allosaurus was a better hunter due to its superior speed and mobility. The name Allosaurus means “strange lizard,” and Tyrannosaurus means “king lizard."
The Allosaurus died out at the end of the Jurassic period during an era of mass extinctions. The Cretaceous era followed the Jurassic era, and large dinosaurs became common again. The Tyrannosaurus rex evolved from earlier, smaller species of theropods and eventually became one of the most powerful predators in history. It was among the last species of dinosaurs to exist before final, mass dinosaur extinction.