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 lizards") are a case in point: to avoid being lunched on, these herbivorous dinosaurs 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. (See a gallery of ankylosaur pictures.)
Although Ankylosaurus is by far the best-known of all the ankylosaurs, it was far from the most common (or even the most interesting). By the end of the Cretaceous period, ankylosaurs were among the last dinosaurs standing; hungry tyrannosaurs couldn't wipe them out, but the K/T Extinction did. In fact, 65 million years ago, some ankylosaurs had developed such impressive body armor--Euoplocephalus even had armored eyelids!--that they would have given an M-1 tank a run for its money.
Tough, knobby 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 possess much in the way of brain power. As with other types of herbivorous dinosaurs, such as sauropods and ornithopods, some species may have lived in herds, which would have afforded even more defense against predation. (By the way, the closest relatives of ankylosaurs were stegosaurs, both groups being classified as "thyreophoran" ("shield-bearing") dinosaurs.)
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 period. Two likely candidates are Sarcolestes, a middle Jurassic herbivore known only from a partial jawbone (this dinosaur received its name--Greek for "flesh thief"--before it had been identified as a plant eater) and Tianchisaurus. On much better footing is the late Jurassic Dracopelta, which measured only about three feet from head to tail but possessed the classic armored profile of later, bigger ankylosaurs, minus the clubbed tail.
Scientists 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 period; 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 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 spawned North America and Asia.
Late Cretaceous Ankylosaurs
During the late Cretaceous period, ankylosaurs reached the apex of their evolution. From 75 to 65 million years ago, some ankylosaur genera (most notably Ankylosaurus and Euoplocephalus) developed incredibly thick and elaborate armor, doubtless a result of the ecological pressures applied by bigger, stronger predators like Tyrannosaurus Rex. One can imagine that very few carnivorous dinosaurs 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 genera; just click on the links for more information.
Acanthopholis No, it's not a city in Greece.
Aletopelta The first ankylosaur known to have lived in Mexico.
Animantarx This "living fortress" was discovered in an unusual way.
Ankylosaurus The Cretaceous equivalent of a Sherman tank.
Anodontosaurus This "toothless lizard" actually had a full set of choppers.
Antarctopelta The first dinosaur fossil ever discovered in Antarctica.
Crichtonsaurus This dinosaur was named after the author of Jurassic Park.
Dracopelta This early ankylosaur was discovered in Portugal.
Dyoplosaurus This ankylosaur was once confused with Euoplocephalus.
Edmontonia This armored dinosaur never actually lived in Edmonton.
Euoplocephalus Even this ankylosaur's eyelids were armored.
Europelta This early nodosaur was recently discovered in Spain.
Gargoyleosaurus This "gargoyle lizard" was an ancestor of Ankylosaurus.
Gastonia This ankylosaur was probably on Utahraptor's lunch menu.
Gobisaurus An unusually large ankylosaur of central Asia.
Hoplitosaurus Named after the heavily armored soldiers of classical Greece.
Hungarosaurus The best-attested ankylosaur ever discovered in Europe.
Liaoningosaurus One of the smallest ankylosaurs in the fossil record.
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.
Oohkotokia Its name is Blackfoot for "large stone."
Palaeoscincus This "ancient skink" was actually an armored dinosaur.
Peloroplites This "monstrous Hoplite" was recently discovered in Utah.
Polacanthus An extremely spiky ankylosaur of the middle Cretaceous.
Propanoplosaurus This baby ankylosaur was recently discovered in Maryland.
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.
Struthiosaurus The smallest nodosaur yet discovered.
Talarurus This ankylosaur was discovered in the Gobi Desert.
Tarchia Its name means "brainy," but that may be an exaggeration.
Tianchisaurus This dinosaur's species name honors Jurassic Park.