Its fossils have been found in geological formations dating to the very end of the Cretaceous Period, about 68-66 million years ago, in western North America, making it among the last of the non-avian dinosaurs.

Ankylosaurus means "fused lizard" in Greek. It was given that name because bones in its skull and other parts of its body were fused, making this dinosaur extremely rugged.

It had a massive tail club that seems to have been an active defensive weapon, capable of producing enough force to break the bones of an assailant.

Ankylosaurus has been likened by many paleontologists to a tank, not only due to the thick armor that shielded its body from attacking carnivores, but also due to it's size. Possibly the largest-known ankylosaurid, it is estimated to have been between 6 and 8 meters (20 and 26 ft) long and to have weighed between 4.8 and 8 metric tons (5.3 and 8.8 short tons).

As imposing as it was, Ankylosaurus was powered by an unusually small brain--which was about the same walnut-like size as that of its close cousin Stegosaurus, long considered to be the most dim-witted of all the dinosaurs.

Like other ornithischians, Ankylosaurus was herbivorous. Its wide muzzle was adapted for non-selective low-browse cropping, and it probably fed on abundant ferns and low-growing shrubs. It may also have dug in the ground for roots and tubers.

Osteoderms are dermal bone structures that support the upper layer of skin and serve as protection against the elements in a large variety of extinct and extant organisms, especially reptiles. In the case of Ankylosaurus, the osteoderms ranged from 1 centimeter (1/2 in) in diameter to 35.5 cm (14 in) in length, and varied in shape.

Assuming it was endothermic, Ankylosaurus would have eaten 60 kilograms (130 pounds) of ferns per day, similar to the amount of dry vegetation a large elephant would consume.

The short, stumpy legs of Ankylosaurus, combined with its massive, low-slung torso, point to a reptile that was only capable of "running" about 6 miles per hour (9.5 km/h), which equates to a light jog in humans--probably not fast enough to outrun a large carnivorous predator like Tyrannosaurus rex.

Unlike other ankylosaurs, Ankylosaurus had nostrils that faced sideways rather than towards the front. The widely separated nostrils may have allowed for stereo-olfaction (where each nostril senses smells from different directions), as has been proposed for the modern moose.