Jurassic dinosaurs. Computer artwork of an Allosaurus (right) confronting a grazing Stegosaurus (left) in a Jurassic redwood forest. Allosaurs were large carnivorous reptiles that lived during the late Jurassic period (155 to 145 million years ago). They were bipedal (two-legged) predators that averaged a height of 8.5 metres. Stegosaurus, which could weigh as much as 5 tonnes, were herbivores that wielded powerful tails tipped with 3-foot spikes.