A stegosaur is a type of dinosaur that walked on four legs and had a double row of large, bony plates running down its back, plus a tail armed with long spikes. The most famous stegosaur is Stegosaurus, whose name means “roofed lizard” because scientists first thought its plates lay flat like roof tiles (they actually stood upright).

These plant-eating dinosaurs lived during the Jurassic Period, about 150 million years ago. A full-grown stegosaur could be as long as a school bus but had a brain only about the size of a walnut. Despite their small brains, stegosaurs thrived for millions of years, using their spiked tails to defend themselves against predators like Allosaurus.

Scientists still debate what the back plates were for. They might have helped regulate body temperature, attracted mates, or made the stegosaur look bigger and more intimidating to predators. The plates were covered in skin and filled with blood vessels, not solid armor. Some stegosaurs also had small bony studs called osteoderms protecting their throats and hips, adding extra defense against attackers.