Ankylosaur’s tail club wasn’t just swung at T. Rex

To ward off massive predators, many biologically herbivorous dinosaurs were armed to the teeth. Some had skulls studded with horns, while others had tails riddled with spikes. But few match the arsenal of the ankylosaurs, a group of herbivores that peaked in diversity during the Cretaceous period. Most of Ankylosaurus’ body was encased in bony plates hanging in jagged points, some of which were pulled around a sledgehammer-like tail stick capable of delivering a bone-crushing blow.

Because of their seemingly indestructible nature, paleontologists and researchers hypothetically spent decades pitting these plant-powered tanks against tyrannosaurs and other carnivores. However, predators may not be the only creatures that take a beating.

In a study published Wednesday in Royal Society for Open Science, researchers analyzed the anatomy of one of the most complete ankylosaurus skeletons in the world. They discovered several fractured, healed carapace plates centered around the creature’s hips that lacked any obvious signs of disease or predation. Instead, the shield appeared to have been splintered by the club of another Ankylosaurus.

“Injuries are right where you would expect two Ankylosaurus species to break things,” said Victoria Arbor, a paleontologist at the Royal British Museum in British Columbia and an author of the study.

The exquisitely preserved skeleton of an ankylosaur, containing a full set of armor plates called osteoderms, was accidentally discovered in 2014 by commercial fossil hunters excavating a nearby Tyrannosaurus in the Judith River Formation in Montana. When the Royal Ontario Museum in Toronto acquired it, the bulk of the creature’s skeleton was still buried in a 35,000-pound slab of sandstone, leaving only its skull and tail.

Based on the skull of Ankylosaurus and its clubs at the end of a spiky tail, it was clear that the animal was a unique species. The horned dinosaur head reminded Dr. Arbour, then a postdoctoral researcher at the Ontario Museum, of the evil mug of Zuul, the horror dog from the movie “Ghostbusters”. In 2017, she and her colleagues christened the new species Zuul crurivastator, or “Zuul, leg destroyer.”

The rest of Zul’s body lay trapped in stone for over a year as fossil preparers painstakingly cut through the rock. They eventually discovered fossilized skin speckled with osteoderms. As they made their way towards Zul’s nape, they discovered that some of the spikes along the animal’s hips were missing their tips and that the bony sheaths encasing these osteoderms had broken off and healed into sharp points.

As the damaged plates were bunched up around Zul’s hips, Dr. Arbor and her colleagues began to wonder if they were defensive scars from a botched attack. Bipedal hunters like Gorgosaurus, a slender cousin of Tyrannosaurus rex, would attack Zuul from above rather than smash its wing. And few locations were as unattractive as the steel-encrusted Zoll, which was within walking distance of her clubhouse.

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

The authors suggested that the injuries occurred during fighting Between Zul and her mighty brothers. Like the big horned bighorn sheep or the head butting of today Giraffes swinging their necksRival ankylosaurids may have established their dominance by landing armor-destroying body strikes with the sticks of their tail.

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 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 muster some evidence to support that these things were actually using their tail clubs to crash into each other in a ritualistic way.”

This practice may have prompted the evolution of the more club-like tail, much like the way modern elk use their elaborate antlers not only to squabble with each other but also to impress potential mates. “Maybe the reason they have a stick tail is not from predation, but from indeterminate fighting,” said Dr. Arbor. “It’s more sexual selection than natural selection.”

While these clubs may have evolved to help ankylosaurs beat each other, they were still capable of delivering a devastating blow just below the knee of a tyrannosaurus. “The shin destroyer still fits perfectly,” said Dr. Arbour.