The Ankylosaur’s Club-Tail Wasn’t Only Swinging at T. Rex

To ward off supersized predators, many herbivorous dinosaurs were biologically armed to the teeth. Some had skulls studded with horns, while others had tails bristling with spikes. But few matched the arsenal of ankylosaurs, a group of herbivores that peaked in diversity during the Cretaceous period. Most of the ankylosaur’s body was encased in bony plates that jutted out into jagged points, and some lugged around a sledgehammer-like tail club capable of delivering a bone-cracking blow.

Because of their seemingly indestructible nature, paleoartists and researchers alike have spent decades hypothetically pitting these plant-powered tanks against tyrannosaurs and other apex carnivores. However, predators may not have been the only creatures absorbing their batterings.

In a study published Wednesday in the Royal Society Open Science, researchers analyzed the anatomy of one of the world’s most complete ankylosaur skeletons. They discovered several broken and healed armor plates concentrated around the creature’s hips that lacked any clear signs of disease or predation. Instead, the armor appeared to have been splintered by another ankylosaur’s club.

“The injuries are right where you’d expect two battling ankylosaurs would break things,” said Victoria Arbour, a paleontologist at the Royal BC Museum in British Columbia and an author of the study.

The exquisitely preserved ankylosaur skeleton, which sports a full suit of armor plates called osteoderms, was accidentally unearthed in 2014 by commercial fossil hunters excavating a nearby tyrannosaur in Montana’s Judith River Formation. When the Royal Ontario Museum in Toronto acquired it, the bulk of the creature’s skeleton was still entombed in a 35,000-pound slab of sandstone, leaving just its skull and tail free.

Based on the ankylosaur’s skull and its club at the end of a thorny tail, it was clear the animal was a unique species. The dinosaur’s horn-encrusted head reminded Dr. Arbour, then a postdoctoral researcher at the Ontario museum, of the gnarly mug of Zuul, the terror dog from the film “Ghostbusters.” In 2017, she and her colleagues christened the new species Zuul crurivastator, or “Zuul, the destroyer of shins.”

The rest of Zuul’s body remained trapped in the stone for more than a year as fossil preparators painstakingly chipped away at the rock. They eventually uncovered fossilized skin dotted with osteoderms. As they worked their way toward Zuul’s backside, they discovered that some spikes along the animal’s hips were missing their tips and that the bony sheaths encasing these osteoderms had broken and healed into blunt points.

Because the damaged plates were clustered around Zuul’s hips, Dr. Arbour and her colleagues began to question whether they were defensive scars from a failed attack. Bipedal hunters like Gorgosaurus, a lanky cousin of the Tyrannosaurus rex, would have attacked Zuul from above instead of smashing into its flank. And few spots were as unappetizing as Zuul’s spike-covered haunches, which were within striking distance of its club.

Instead, Dr. Arbour and her team concluded that the placement of the battered plates, along with an absence of bite marks, were consistent with a crack from another Zuul’s tail club. Because the damaged osteoderms were at different stages of healing, it was likely that this ankylosaur took its fair share of thumpings 76 million years ago.

The authors proposed that the injuries occurred during combat between Zuul and its brawny brethren. Like today’s head-butting bighorn sheep or neck-swinging giraffes, competing ankylosaurs may have established dominance by landing armor-shattering body blows with their tail clubs.

The new evidence is essential for studying the behavior of these classic, yet enigmatic, dinosaurs. “Ankylosaurs left no living descendants, so we have no living analogues to learn what ancient ankylosaurs did,” said Jordan Mallon, a paleontologist at the Canadian Museum of Nature in Ottawa, who was not involved in the study. “This is the first example where we’ve been able to marshal some evidence to support that these things were actually using their tail clubs to smash into one another in a ritualistic way.”

And this practice might have driven the evolution of gnarlier tail clubs, much like how modern elk employ their elaborate antlers not only to tussle with one another but also to impress prospective mates. “The reason that they have a tail club is probably not driven by predation, but more for intraspecific combat,” Dr. Arbour said. “It’s more sexual selection than natural selection.”

While these clubs may have evolved to help ankylosaurs bash each other, they were still capable of delivering a debilitating blow below a tyrannosaur’s knee. “The destroyer of shins is still quite apt,” Dr. Arbour said.