A new finding suggests that ankylosaurs’ tail clubs were meant for bashing each other

New research indicates that the tails of the massive armored dinosaurs known as ankylosaurs may have evolved to slam into each other rather than deter hungry predators. This is a complete change from what was previously believed.

Before the paper published today in Biology Letters, most scientists considered the dinosaur’s tail, a substantial bony protrusion made up of two oval-shaped protrusions, primarily as a defense against predation. The team behind the new document argues that this is not necessarily the case. To make their case, they turn to years of ankylosaur research, analysis of the fossil record, and data from an exceptionally well-preserved specimen named Zuul Crurivastator.

The Zuul name, in fact, embraces that earlier idea. While “Zuul” references the creature in the original Ghostbustersthe two Latin words that make up the name of its species are cru (shin or leg) and devastating (destroyer). Hence, the shin buster: a direct reference to where the dinosaur’s club might have hit approaching tyrannosaurs or other theropods.

But that name was given to it when only its skull and tail had been excavated from the rock where the fossil was encased. After years of skillful work by fossil preparers at the Royal Ontario Museum, Zuul’s entire back and flanks are exposed, offering important clues as to what his tail club might be pointing at.

Target identification

Lead author Dr. Victoria Arbor is currently Curator of Paleontology at the Royal British Columbia Museum, but was a former NSERC Postdoctoral Fellow at the Royal Ontario Museum in Toronto. That has been Zuul’s home since 2016, two years after his initial discovery in Montana. She has spent years studying ankylosaurs, a type of dinosaur that appears in the fossil record from the Jurassic to the late Cretaceous. Some species of ankylosaur have clubs on their tails, while others, known as nodosaurs, do not. That difference raises some questions about what these structures were used for.

“I think a natural follow-up question of, ‘Could you use your tail clubs as a weapon?’ it’s ‘Who are they using that weapon against?’” Arbor explained. “And that’s where I really started thinking about this.

In 2009, he wrote a paper that suggested that ankylosaurs might use their tail clubs for intraspecific combat: fights with other ankylosaurs. That work focused on the potential impact of tail clubs when used as a weapon, especially since clubs come in various shapes and sizes, and in some species were not even present until the animal was mature. By measuring the available fossil-tailed clubs and estimating the force of the blows they could deliver, he discovered that the smallest clubs (approximately 200 millimeters or half a foot) were too small to be used as a defense against predators.

Zuul Crurivastatorthe pimple thumper.

Royal Ontario Museum

He recommended further research, noting that if ankylosaurs were using them for intraspecific combat, one might expect to see lesions along the flanks of adults, since an ankylosaur’s tail can only swing so far.

It is one thing to have an idea about an extinct animal, but another to have proof. Ankylosaur fossils are rare overall; tissue-sparing dinosaurs that would have been damaged in these fights are much rarer. So it’s amazing that Arbor could test his ideas thanks to an animal with all of its back, most of its skin and all, intact.

“I put forward the idea that we would expect to see damage to the flanks, just based on how they might line up with each other,” Arbor told Ars. “And then a decade and a little later, we have this amazing Zuul skeleton with damage right where we thought we might see it. And that was very exciting!”

damage assessment

Zuul’s back and flanks are covered in various spines and bony structures called osteoderms. Just as Arbor predicted, there is evidence of broken and injured osteoderms on both sides of the flanks, some of which appear to have healed.

“We also did some kind of basic statistics to show that lesions are not randomly distributed in the body,” he continued. “They’re really restricted to the sides in the areas around the hips. That cannot be explained by chance alone. It seems more likely to be [the result of] repeated behaviour”.

There are only a handful of well-preserved ankylosaurs, including at least one of a nodosaur named borealopelta at the Royal Tyrrell Museum. The authors note that there are no comparable lesions in known nodosaurs, a pertinent point. As mentioned above, nodosaurs do not have tail clubs and therefore would not have been able to use them on each other.

Equally important, the damage is not accompanied by evidence of predation. No bite marks, puncture wounds, or tooth scratches are found anywhere on Zuul’s body.