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

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

To fend off super-sized predators, many herbivorous dinosaurs were biologically armed to the teeth. Some had skulls with horns on them, while others had tails full of thorns. But few could 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 protruding into jagged spikes, and some lugged around a sledgehammer-like tail club that could deliver a bone-breaking blow.

Due to their seemingly indestructible nature, paleoartists and researchers alike have hypothetically spent decades pitting these plant-powered tanks against tyrannosaurs and other top carnivores. However, predators may not have been the only creatures to absorb their beatings.

This is according to a study published Wednesday in the Biology Letters, researchers analyzed the anatomy of one of the world’s most complete ankylosaurus skeletons. They discovered several broken and healed armor plates around the creature’s hips that showed no obvious signs of disease or predation. Instead, the armor appeared to have been shattered by another club from an ankylosaurus.

“The injuries are exactly where you would 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 beautifully preserved ankylosaur skeleton, which wears a full suit of armor plates called osteoderms, was accidentally unearthed 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 sandstone slab, leaving only the skull and tail exposed.

Based on the skull of the ankylosaurus and its club at the end of a spiky tail, it was clear that the animal was a unique species. The dinosaur’s head encrusted with horns made Dr. Arbour, then a postdoctoral fellow at the Museum of Ontario, remembered the gnarled mug of Zuul, the terror dog from the movie ‘Ghostbusters’. In 2017, she and her colleague named the new species Zuul crurivastator, or “Zuul, the destroyer of shins.”

The rest of Zuul’s body remained trapped in the rock for more than a year while fossil preparators painstakingly carved away the rock. They eventually discovered fossilized skin studded with osteoderms. As they made their way to Zuul’s rear, they found that some spines along the animal’s hips were missing their tips and that the bony sheaths encasing these osteoderms had broken and healed to blunt points.

Since the damaged plates were around Zuul’s hips, Dr. Arbor and her colleagues wonder if they were defense scars from a failed attack. Bipedal hunters like Gorgosaurus, a lanky cousin of Tyrannosaurus rex, would have attacked Zuul from above rather than slamming into its flank. And few places were as distasteful as Zuul’s spiked hips, which were within striking distance of his mace.

Instead, Dr. Arbor and her team found the placement of the battered plates, along with the absence of bite marks, consistent with a crack from another Zuul’s tail club. Since the damaged osteoderms were in various stages of healing, it’s likely that this ankylosaur took quite a beating 76 million years ago.

The authors suggested that the injuries occurred during fight between Zuul and his muscular brethren. Like the head-butting bighorn sheep of today or neck-swinging giraffescompeting ankylosaurs may have gained dominance by landing armor-crushing body blows with their tail maces.

The new evidence is essential for studying the behavior of these classic yet puzzling dinosaurs. “Ankylosaurs left no living descendants, so we don’t have living analogs 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 any evidence to support that these things actually used their tail clubs to ritually bump into each other.”

And this practice might have spurred the evolution of gnarled tail clubs, just as modern moose use their elaborate antlers not only to wrestle with each other, but also to impress prospective mates. “The reason they have a tail club probably isn’t driven by predation, but more for intraspecific combat,” said Dr. Arbor. “It’s more sexual selection than natural selection.”

While these clubs may have evolved to help ankylosaurs pound each other, they were still capable of delivering a grueling blow below the knee of a tyrannosaurus. “The shin destroyer is still very appropriate,” said Dr. Arbor.



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