Euoplocephalus looks like a dinosaur you wouldn’t want to mess with. From the herbivore’s bumpy snout to the end of its club-shaped tail, this dinosaur was positively sheathed in armor. Even its eyelids were shielded. Little wonder, then, that paleontologists and artists just can’t help imagining this living tank and other ankylosaurs using their spikes and tails to defend themselves from all manner of attackers. But did they really?
Ankylosaur expert and North Carolina Museum of Natural Sciences paleontologist Victoria Arbour has been cataloging artists’ renditions of ankylosaur battles to see how people imagine these creatures. “Since January, I’ve been pestering people on Twitter to send me examples of ankylosaurs fighting things—what I’ve dubbed #AnkylosaurFightClub—in any kind of media, like books, videos, posters, etc.” Arbour says. So far, she’s collected 90 entries.
Most of the time, Arbour found, the imagined arch-nemesis of the ankylosaur is Tyrannosaurus or one of its toothy relatives. The rest of the face-offs featured ankylosaurs battling against all manner of creatures—from raptors and other horned dinosaurs to people and even Godzilla—with very few illustrations at all showing armored dinosaurs fighting each other.
Of course, ankylosaurs never fought people, robots or radioactive monsters in real life. But what about fighting each other? These ponderous dinosaurs shuffled their armored selves around the Mesozoic world for over 100 million years. It’s not difficult to imagine them using their spikes and tails for defense, but how do we know that they were used in combat?
We don’t, says Arbour. Osteoderms—the specialized bones that make up body armor—have a variety of functions in living animals. Osteoderms help protect armadillos, for instance, but they also help crocodylians regulate their body temperature and act as calcium storage for laying eggs, Arbour points out. Showing off is another option: “The flamboyantly spiky osteoderms present in a lot of ankylosaurs might have been useful for intraspecific signaling, such as sexual or threat displays,” she says.
In all Arbour’s searching, though, she hasn’t found any definitive evidence of ankylosaurs fighting other dinosaur species or each other. Given their formidable weaponry, this seems strange. Some ankylosaurs—like Euoplocephalus and Ankylosaurus itself—evolved hefty tail clubs at the end of stiffened, bat-like tails. And these tails, Arbour concluded in a pair of studies published in 2009, could pack quite a wallop. While a pro baseball player can swing a bat with a force of 13 Newtons per second, Arbour says, “an ankylosaurid tail club would impact with an impulse of up to 4,800 Newtons per second!”
Yet the dinosaur fossils humans have discovered show little evidence of injuries consistent with such impacts. “I took a look at ankylosaur tail and pelvis pathologies back in a 2011 paper because I was hoping to be able to find some direct evidence of predator-prey combat,” Arbour says. “I found a lot of pathologies in ankylosaur tails and butts”—but these were from abnormalities in bone growth and disease. None of them could be definitively attributed to fighting.
The answer could be that we’ve bestowed ankylosaurs with a more belligerent reputation than they truly deserve. Ornate armor might have had more to do with communication than combat. “Many animals today with ostentatious weapons and ornamentation use those structures as a signal of fitness,” Arbour says, “things like the antlers of deer and the tusks of an elephant come to mind.”
This doesn’t mean that ankylosaurs never tussled, but rather that defense might not have been the sole or even main driver of their evolution. Especially if it played a part in mating displays, ankylosaur armor might have been about making love, not war.