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Ankylosaurus is a case study in how far evolution will go to protect otherwise defenseless plant-eaters against toothy carnivores. This famous 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 that it could swing to deter curious predators. One imagines that very few Tyrannosaurus Rex adults (which also roamed North America during the late Cretaceous period) would have dared to take a bite out of a full-grown Ankylosaurus. (See 10 Facts About Ankylosaurus and a gallery of ankylosaur pictures.)
You can imagine how a heavily armored, tail-swinging, small-brained Ankylosaurus managed to survive in its dangerous ecosystem. Most likely, this dinosaur simply lay flat on its stomach when it was threatened by tyrannosaurs or raptors--whose only hope of besting the Ankylosaurus would have been to dislodge its five-ton bulk (all whilst steering clear of its wildly flailing tail) and then digging their teeth and claws into its soft underbelly. Clearly, Ankylosaurus had no hope of outrunning its speedier antagonists; given its size and bulk, this was probably one of the slowest dinosaurs that ever lived.
Ankylosaurus is notable for belonging to a sparsely populated infraorder of dinosaurs, the thyreophora ("shield bearers"), another famous example of which is Stegosaurus. It's interesting to speculate how this armored breed--and its sub-family, the ankylosaurs--would have continued to evolve if the dinosaurs hadn't been rendered defunct by the K/T Extinction 65 million years ago; as far as we can tell from the fossil evidence, Ankylosaurus and its armored relatives were some of the last dinosaurs standing before the entire breed went kaput from cold and starvation.