Ankylosaurus is a case study in how far evolution will go to protect otherwise defenseless plant-eaters against toothy carnivores. This dinosaur was lined front to back with bony armored plates and threatening spikes, and had a mace-like club on the end of its tail for good measure. One imagines that very few T. Rexes (which also roamed North America during the late Cretaceous) would dare to take a bite out of a living Ankylosaurus.
This dinosaur is also notable for belonging to a sparsely populated infraorder of dinosaurs, the thyreophora ("shield bearers"), another example of which is Stegosaurus. It's interesting to speculate how this armored breed--and its sub-family, the ankylosaurs--would have evolved if the dinosaurs hadn't been rendered defunct by the K/T Extinction 65 million years ago!
