Ankylosaurus (ăn'kələsôr`əs), [Gr.,=crooked lizard], genus of heavily armored, herbivorous quadripedal dinosaurs, the best known of which is the species A. magniventris, the largest and heaviest of the ankylosaurs. Sturdy and built low to the ground, the largest ankylosaurs are estimated to have been 33 ft (10 m) from head to tail. Ankylosaurus was covered by tough, rounded bony plates that were keeled, like the bottom of an overturned boat. Bony plates were also embedded in the upper eyelids. The dinosaur was characterized by a large club of fused plates that it carried aloft at the end of the tail. The vertebrae of the end of the tail were fused to support the club, which is thought to have been used in defense. Like other ornithischian dinosaurs, ankylosaurus was beaked. It also had small teeth toward the back of the jaws and an intricate system of nasal sinuses, the purpose of which is unclear. Ankylosaurus flourished in the late Cretaceous period. Specimens have been found in Montana, Wyoming, and in the Red Deer River area of Alberta, Canada.