About Euoplocephalus
Probably the most evolved, or "derived," of all the ankylosaurs, or armored dinosaurs, Euoplocephalus was the Cretaceous equivalent of the Batmobile: this dinosaur's back, head and sides were completely armored, even its eyelids, and it wielded a prominent club on the end of its tail.
One can imagine that the apex predators of late Cretaceous North America (such as Tyrannosaurus Rex) went after easier prey, since the only way to kill and eat a full-grown Euoplocephalus would be to somehow flip it onto its back and dig into its soft belly--a process that might entail a few cuts and bruises, not to mention the occasional loss of limb.
Although its close cousin Ankylosaurus gets all the press, Euoplocephalus is the best-known ankylosaur among paleontologists, thanks to the discovery of over 40 more-or-less complete fossil specimens (including about 15 intact skulls) in the American west.