The Ankylosaur's Tail-Club Wasn't Just Swinging at T. Rex

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

To ward off oversized predators, many herbivorous dinosaurs were biologically armed to the teeth. Some had skulls studded with horns, while others had spiked tails. 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 protruded into jagged spikes, and some trailed around a hammer-like tail club capable of delivering a bone-breaking blow.

Due to their seemingly indestructible nature, paleoartists and researchers have spent decades hypothetically pitting these plant-powered chariots against tyrannosaurs and other advanced carnivores. However, the Predators might not have been the only creatures absorbing their blows.

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

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

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

From the skull of the ankylosaurus and its club on 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 researcher at the Ontario Museum, of the gnarled mouth of Zuul, the terrorist dog from the movie “Ghostbusters.” In 2017, she and her colleagues named the new species Zuul crurivastator, or “Zuul, the shin destroyer.”

The rest of Zuul’s body remained imprisoned in stone for over a year as fossil preparers painstakingly chipped away at the rock. They eventually discovered fossilized skin studded with osteoderms. As they made their way to Zuul’s back, they discovered that some of the spikes along the animal’s hips were missing their spikes, and the bony sheaths wrapping these osteoderms had broken off and healed into blunt spikes.

Because the damaged plates were clustered around Zuul’s hips, Dr. Arbor and his colleagues began to wonder if they were defensive scars from a failed attack. Bipedal hunters like Gorgosaurus, a lanky 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 spike-covered hips, which were within striking distance of his club.

Instead, Dr. Arbor and his team concluded that the placement of the battered plates, along with the lack of bite marks, was consistent with a crack from another Zuul cue club. Because the damaged osteoderms were in different stages of healing, it’s likely that this ankylosaur suffered its fair share of beatings 76 million years ago.

The authors proposed that the injuries occurred during a fight between Zuul and his muscle brothers. Like the head-butting bighorn sheep or neck-swaying giraffes of today, competing ankylosaurs may have established their dominance by landing armour-breaking body blows with their clubs of tail.

This new evidence is essential for studying the behavior of these classic, yet enigmatic dinosaurs. “Ankylosaurs left no living descendants, so we don’t have living analogues for what ancient ankylosaurs did,” said Jordan Mallon, a paleontologist at the Canadian Museum of Nature in Ottawa, who has no not participated in the study. “This is the first instance where we’ve been able to piece together evidence to support the fact that these things were actually using their tail clubs to ritually crash into each other.”

And this practice may have led to the evolution of gnarlier tail clubs, much like the way modern elk use their elaborate antlers not only to fight each other, but also to impress future mates. “The reason they have a tail club is probably not motivated by predation, but rather for intraspecific combat,” Dr Arbor said. “It’s more sexual selection than natural selection.”

Although these clubs may have evolved to help ankylosaurs bump into each other, they were still capable of delivering a debilitating blow below a tyrannosaur’s knee. “The shin destroyer is still totally appropriate,” Dr. Arbor said.

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