The ankylosaurs (Ankylosauria) were one of the two divisions that made up the thyreophorans (the stegosaur was the other). Paleontologists divide the ankylosauria into two more families—the Nodosaurids and Ankylosaurids—based on differences in skulls, shoulder blades, and armor.

The ankylosaurs were medium-sized quadruped dinosaurs found in many parts of the world. In fact, the first dinosaur ever discovered in Antarctica was the ankylosaurian Antarctopelta; the fossils were recovered from Ross Island in 1986. Ankylosaurs were short-legged and squat, with long, wide bodies. Their heads ranged from long and narrow to wide with broad muzzles. All of these dinosaurs had bony plates of armor over their bodies, often with spines, spikes, or studs projecting outward. Some even had bony clubs on the ends of their tails. Armor consisted of plates of bone, known as scutes, embedded in the skin; some dinosaurs even had their heads covered with this armor. They were the “tanks” of the dinosaur world and were able to fight off almost any predator.

Unfortunately, very little is known about these dinosaurs, since so few complete fossils have been found. The ankylosaurs first appeared during the Early to Middle Jurassic periods, but they became most numerous and diverse during the Cretaceous period. Most of the ankylosaurs belonged to the Nodosaurid subgroup early in the Cretaceous period (although one genus, Sarcolestes, is from the Jurassic); the subgroup Ankylosaurid were more prevalent in the latter Cretaceous period, and were distinguished by their broad heads, spikes in the back of their skulls, and club-like tails.