Size and Weight:
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) would dare to take a bite out of a full-grown Ankylosaurus. (See 10 Facts About Ankylosaurus and a gallery of ankylosaur pictures.)
Ankylosaurus is also 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!