Ankylosaurus

Ankylosaurus (meaning "fused lizard") is a genus of ankylosaurid dinosaur. Fossils of Ankylosaurus have been found in geologic formations dating to the very end of the Cretaceous Period, between about 68–66 million years ago, in western North America. Ankylosaurus was named by Barnum Brown in 1908, and the only species classified in the genus is A. magniventris. A handful of specimens have been excavated to date, but a complete skeleton has not been discovered. Though other ankylosaurs are represented by more extensive fossil material, Ankylosaurus is often considered the archetypal member of the group.

Ankylosaurus is the largest known ankylosaurid, and measured up to 6.25 m (20.5 feet) in length, 1.7 m (5.6 feet) in height, and weighed 6 tonnes (13,000 lb). It was a quadrupedal animal, with a broad, robust body. It had a long, low skull, with two horns pointing backwards from the back of the head, and two horns below these that pointed backwards and down. The front part of the jaws were covered in a beak, with rows of small, leaf-shaped teeth further behind it. It was covered in armor plates, or osteoderms, with a bony half ring covering part of its neck and shoulders, and had a large club on the end of its tail. Bones in the skull and other parts of the body were fused, increasing their strength.

Ankylosaurus is a member of the subfamily Ankylosaurinae, and its closest relatives appear to be Euoplocephalus and Anodontosaurus. Ankylosaurus is thought to have been a slow moving animal, able to make quick movements when necessary. Its broad muzzle indicates it was a non-selective browser. Sinuses and nasal chambers in the snout may have been for heat and water balance or played a role in vocalization. The tail club is thought to have been used in defense against predators or in intraspecific combat. Ankylosaurus has been found in the Hell Creek, Lance, and Scollard formations, but appears to have been rare in its environment. Although it lived alongside another ankylosaur, Edmontonia, their ranges and ecological niches do not appear to have overlapped, and Ankylosaurus may have inhabited upland areas. Ankylosaurus lived alongside dinosaurs such as Tyrannosaurus, Triceratops, and Edmontosaurus, making it among the last of the non-avian dinosaurus.