Ankylosaurus was a "bird-hipped" dinosaur from the heavily armoured group known as the ankylosaurs. Much of its skeleton was fused together to produce a heavily plated back. Its head had robust triangular horns in each corner. Its skull was nearly 1 metre long, very broad but very thick leaving little room for a brain.

Ankylosaurus was one of the larger armoured dinosaurs. It's club-like tail, the most impressive part of its body, was made of large clumps of bony tissue fused to the tail vertebrae and encased in tough reptilian skin. Its muscular tail was flexible and able to swing the club powerfully from side to side. Ankylosaurus was not built to reach upwards and grazed on low-lying plants.

It was named by Barnum Brown in 1908 and is known from three fairly complete specimens from Alberta in Canada, and Wyoming and the Hell Creek Formation in Montana, USA. Its fossil remains are restricted to rocks from the Cretaceous period. Ankylosaur trackways have been found near Sucre in Bolivia, South America.