The Jurassic Period ran from 201 to 143 million years ago. The climate was much wetter than the dry Permian and Triassic periods that came before. Dense vegetation grew rapidly in the swampy lowlands. It provided a rich source of food for all plant-eating dinosaurs. New ones evolved to eat this abundant food. They included the giant, long-necked sauropods and the stegosaurs, dinosaurs with plates running down their backs. Larger, more powerful meat-eaters, called theropods, evolved at the same time.

Gastroliths

Sauropods could not chew their food properly, so they swallowed small stones. They rattled around in the dinosaurs’ stomachs and helped grind up the tough plant food there. These stones are called gastroliths. Some modern animals, such crocodiles, many birds and seals, swallow gastroliths.

Stegosaurs

To defend themselves against big predators, some plant-eaters developed weapons of their own. Stegosaurus, a 9-metre-long (30-foot), slow-moving dinosaur from North America, had a double row of diamond-shaped bony plates running the length of its back. It also had several long spines at the end of its tail. When threatened, it could swing its tail into the head or body of an attacker.

Allosaurus

The 12-metre-long (36-ft) meat-eating Allosaurus from North America preyed on these plant-eating dinosaurs during Jurassic times. Any prey trapped in its backwards-curving teeth would have found it hard to wriggle free.