The Ankylosaur’s Tail-Club hadn’t just swung at the T. Rex

To protect themselves from super-large 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 match the arsenal of ankylosaurs, a group of herbivores that peaked in diversity during the Cretaceous period. Most of the ankylosaurus’ body was encased in bony plates that jutted out into jagged tips, and some moved around a sledgehammer-like tail capable of delivering a bone-shattering blow.

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

In a study published Wednesday in Biology Letters, researchers analyzed the anatomy of one of the most complete ankylosaur skeletons in the world. They found several broken and healed armor plates concentrated around the creature’s thighs, which lacked clear signs of disease or predation. Instead, it looked like the armor had been split open by another ankylosaur club.

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

The exquisitely preserved skeleton of an ankylosaurus, which wears a full set of armor called osteoderms, was accidentally discovered in 2014 by commercial fossil hunters excavating a nearby tyrannosaurus in 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 its skull and tail exposed.

Based on the skull of the ankylosaurus and its club at the end of a barbed tail, it was clear that the animal was a unique species. The dinosaur’s horn-encrusted head reminded Dr. Arbor, then a postdoctoral fellow at the Ontario Museum, of the bumpy cup of Zool, the terror dog from the movie Ghostbusters. In 2017, she and her colleague named the new species Zuul crurivastator, or “Zuul, the shin destroyer.”

The rest of Zuul’s body remained trapped in the rock for over a year while fossil preparers painstakingly chipped away at the rock. Eventually, they found fossilized skin studded with osteoderms. As they made their way to the rear of Zool, they discovered that some spines along the animal’s thighs were missing their tips, and that the bony sheaths surrounding these osteoderms had broken off and grown into blunt points.

As the damaged plates clustered around Zool’s thighs, Dr. Arbor and her colleagues began to wonder if they were defensive marks from a failed attack. Bipedal hunters like Gorgosaurus, a long cousin of Tyrannosaurus rex, would have attacked the Zuul from above rather than crashing into its flank. And few spots were as unappetizing as Zuul’s spiked flanks, which were within striking distance of the club.

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

The authors suggest that the injuries occurred during battle between Zool and his muscle brothers. Similar to today’s bighorn sheep or swinging giraffescompeting ankylosaurs may have established dominance by delivering armor-piercing blows to the body with their tails.

The new evidence is essential for studying the behavior of these classic but enigmatic dinosaurs. “Ankylosaurs left no living descendants, so we don’t have living counterparts 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 gather some evidence to support that these creatures actually used their tails to bump into each other in a ritualistic way.”

And this practice may have led to the evolution of thicker tails, similar to how modern moose use their complex antlers not only to fight each other, but also to impress prospective mates. “The reason they have a tail is probably not because of predation, but rather because of intraspecific fighting,” Dr Arber said. “It’s more sexual selection than natural selection.”

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