Given the ferocious predators that roamed the planet during the Jurassic and Cretaceous periods--toothy beasts like Allosaurus, Utahraptor and T. Rex--it would be surprising if some herbivores didn't evolve elaborate defenses. The ankylosaurs (Greek for "fused" or "rigid" lizards) are a case in point: to avoid being lunched on, these plant-eaters developed tough, scaly body armor, as well as spikes and bony plates, and some had dangerous clubs on the ends of their long tails that they swung at approaching carnivores.
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Although Ankylosaurus is the best-known of all the armored dinosaurs, it was far from the most common (or even the most interesting). By the end of the Cretaceous, ankylosaurs were among the last dinosaurs standing; hungry tyrannosaurs couldn't wipe them out, but the K/T Extinction did. Sixty-five million years ago, some ankylosaurs had developed such impressive body armor--Euoplocephalus, for example, even had armored eyelids!--that they would give an M-1 tank a run for its money.
Armor wasn't the only feature that set the ankylosaurs apart (though it was certainly the most noticeable). As a rule, these dinosaurs were stocky, low-slung, short-legged, and probably very slow quadrupeds that spent their days grazing on low-lying vegetation and didn't have much in the way of brain power. As with other herbivorous dinosaurs, some genuses may have lived in herds, which would have afforded even more defense against predation.
Ankylosaur Evolution
Although the evidence is spotty, paleontologists believe that the first identifiable ankylosaurs--or, rather, the dinosaurs that subsequently evolved into ankylosaurs--arose in the early Jurassic. One likely candidate is Sarcolestes, a middle Jurassic herbivore known only from a partial jawbone (this dinosaur received its name--which is Greek for "flesh thief"--before it had been identified as a plant eater).
Researchers are on much firmer ground with later discoveries. The nodosaurs (a group closely related to, and sometimes categorized under, the ankylosaurs) flourished in the mid-Cretaceous; these armored dinosaurs were characterized by their long, narrow heads, small brains, and lack of tail clubs. The most well-known nodosaurs included Nodosaurus, Sauropelta and Edmontonia, the last being especially common in North America.
One other notable fact about ankylosaur evolution is that these creatures lived just about everywhere. The first dinosaur ever discovered in Antarctica--named, appropriately enough, Antarctopelta--was an ankylosaur, as was the Australian Minmi, which possessed one of the smallest brain-to-body ratios of any dinosaur (a nice way of saying that it was very, very dumb). Most ankylosaurs and nodosaurs, though, lived on the land masses that later became modern North America and Asia.
Later Ankylosaurs
In the late Cretaceous, the ankylosaurs reached the apex of their evolution. From 75 to 65 million years ago, some ankylosaur genuses (most notably Ankylosaurus and Euoplocephalus) developed incredibly thick and elaborate armor, doubtless as a result of the ecological pressures applied by bigger, stronger predators. One can imagine that very few carnivores would dare to attack a full-grown ankylosaur, since the only way to kill it would be to flip it onto it back and bite its soft underbelly.
Still, not all paleontologists agree that the armor of ankylosaurs (and nodosaurs) had a strictly defensive function. It's possible that some ankylosaurs used their spikes and clubs to establish dominance in the herd, or to joust with other males for the right to mate with females. This is probably not an either/or argument, though: since evolution works along multiple paths, it's likely that ankylosaurs evolved their armor for defensive, display and mating purposes all at the same time.
Here's a list of the most notable ankylosaur and nodosaur genuses; you can click on the links for more information.
Acanthopholis No, it's not a city in Greece.
Ankylosaurus The Cretaceous equivalent of a Sherman tank.
Antarctopelta The first dinosaur fossil ever discovered in Antarctica.
Crichtonsaurus This dinosaur was named after the author of Jurassic Park.
Edmontonia This armored dinosaur never actually lived in Edmonton.
Euoplocephalus Even this ankylosaur's eyelids were armored.
Gargoyleosaurus This "gargoyle lizard" was an ancestor of Ankylosaurus.
Gastonia This ankylosaur was probably on Utahraptor's lunch menu.
Minmi An early (and very dumb) ankylosaur from Australia.
Nodocephalosaurus This armored dinosaur has been reconstructed from a single skull.
Nodosaurus One of the first armored dinosaurs ever discovered in North America.
Polacanthus An extremely spiky ankylosaur of the middle Cretaceous.
Saichania This ankylosaur's name is Chinese for "beautiful."
Sarcolestes The most likely ancestor of the ankylosaurs.
Sauropelta This ankylosaur's armor helped keep raptors at bay.
Scelidosaurus Among the earliest of all the armored dinosaurs.
Tarchia Its name means "brainy," but that may be an exaggeration.
