The ankylosaurus tail club wasn’t just swung at T. Rex

To avoid large predators, many herbivorous dinosaurs were biologically armed to the teeth. Some had horned skulls, while others had spiked tails. But few matched the arsenal of ankylosaurs, a group of herbivores that reached a peak of diversity during the Cretaceous period. Most of the ankylosaur’s body was encased in bony plates that jutted out at jagged points, and some curled around a hammer-like tail club capable of delivering a bone-shattering blow.

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

In a study published on Wednesday at Royal Society Open Science, researchers analyzed the anatomy of one of the most complete ankylosaur skeletons in the world. They discovered several broken and healed armor plates clustered around the creature’s hips that bore no clear signs of disease or predation. Instead, the armor appeared to have been split open by another ankylosaur’s club.

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

The exquisitely preserved ankylosaurus skeleton, sporting a full suit of armor plates called osteoderms, was accidentally discovered in 2014 by commercial fossil hunters excavating a tyrannosaurus near Montana’s Judith River Formation. When the Royal Ontario Museum in Toronto acquired it, most of the creature’s skeleton was still buried in a 35,000-pound slab of sandstone, leaving only the skull and tail exposed.

From the skull of the ankylosaurus and its club at the end of a spiny tail, it was clear that the animal was a unique species. The dinosaur’s horn-encrusted head reminded Dr. Arbour, then a postdoctoral fellow at the Ontario Museum, the gnarled cup of Zuul, the horror dog from the movie “Ghostbusters”. In 2017, she and her colleagues named the new species Zuul crurivastator, or “Zuul the Cinnamon Destroyer.”

The rest of Zuul’s body remained trapped in the stone for more than a year while fossil preparers carefully carved away the rock. Eventually they discovered fossilized skin dotted with osteoderms. As they made their way to Zuul’s back, they discovered that some spikes along the animal’s hips had no spikes, and that the bony sheaths containing these osteoderms had broken off and healed into blunt points.

Because the damaged plates were clustered around Zuul’s hips, Dr. Arbor and her colleagues began to wonder if they were defensive scars from a failed attack. Bipedal hunters like Gorgosaurus, a scrawny cousin of Tyrannosaurus rex, would have attacked Zuul from above instead of crashing into its flank. And few places were as unappetizing as Zuul’s spiky ducks, which were within walking distance of his club.

Instead, Dr. Arbor and her team concluded that the placement of the battered plates, along with the absence of bite marks, were consistent with a tail club crack from another Zuul. Because the damaged osteoderms were in various stages of healing, this ankylosaur likely took its fair share of blows 76 million years ago.

The authors proposed that the lesions occurred during combat between Zuul and his muscular brothers. Like today’s sheep that touch their heads o giraffes swinging their neckscompeting ankylosaurs may have established dominance by landing armor-shattering body blows with their tail clubs.

The new evidence is essential to studying the behavior of these classic but enigmatic dinosaurs. “Ankylosaurs didn’t leave living descendants, so we don’t have living analogues to know 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 piece together some evidence to support that these things were actually using their tail clubs to bump into each other in a ritualistic way.”

And this practice may have fueled the evolution of more gnarled tail clubs, just as modern moose use their elaborate antlers not only to fight with each other, but also to impress potential mates. “The reason they have a club tail is probably not because of predation, but more because of intraspecific combat,” said Dr. tree “It’s more sexual selection than natural selection.”

While these clubs may have evolved to help ankylosaurs strike each other, they were still capable of delivering a debilitating blow below the knee of a tyrannosaurus. “The shin breaker is still pretty fit,” Dr. Arbor said.