Given the ferocious dinosaurs that roamed the planet during the Jurassic and Cretaceous periods–toothy beasts like Allosaurus, Utahraptor and T. rex–it would be surprising if some plant-eaters 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 species had dangerous clubs on the ends of their long tails that they swung at approaching carnivores.
Although Ankylosaurus is by far the best-known of all the ankylosaurs, it was far from the most common (or even the most interesting, if the truth be told). By the end of the Cretaceous period, ankylosaurs were among the last dinosaurs standing; hungry tyrannosaurs couldn’t wipe them off the face of the earth, 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 ankylosaurs apart (though it was certainly the most noticeable). As a rule, these dinosaurs were stocky, low-slung, short-legged, and probably extremely 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 family of armored dinosaurs closely related to, and sometimes categorized under, the ankylosaurs) flourished in the mid-Cretaceous period; these 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 on earth.
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, Gondwana and Laurasia, that later 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, an extreme example of sexual selection. 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.