A new find suggests that the clubs on Ankylosaurus’ tail were for hitting each other

A new find suggests that the clubs on Ankylosaurus’ tail were for hitting each other

Zoom in / The tails of the ankylosaur species seem to have been used for striking each other, not for predators.

Henry Sharp

New research suggests that the tails of huge armored dinosaurs known as ankylosaurs may have evolved to strike each other rather than deter hungry predators. This is a complete change from what was previously thought.

Before the paper published today in Biology Letters, most scientists viewed the dinosaur’s tail, a substantial bony protrusion consisting of two oval projections, mainly as a defense against predation. The team behind the new paper argues that this is not necessarily the case. To make their case, they draw on years of ankylosaur research, fossil analysis and data from an exceptionally well-preserved specimen named The blood curdling roar.

Zuul’s name actually encapsulates this previous idea. While “Zuul” refers to the creature in the original Ghostbustersthe two Latin words making up the name of its species are crus (shin or lower leg) and the one in charge (destroyer). Hence the shin-buster: a direct reference to where the dinosaur club might have struck the approaching tyrannosaurs or other theropods.

But this name was given when only its skull and tail were excavated from the rock where the fossil was encased. After years of skilled work by fossil preparators at the Royal Ontario Museum, Zool’s entire back and flanks have been uncovered, offering important clues as to what its tail might be aimed at.

Target identification

Lead author Dr. Victoria Arbor is currently curator of paleontology at the Royal British Columbia Museum, but she is a former NSERC postdoctoral fellow at the Royal Ontario Museum in Toronto. This has been Zool’s home since 2016, two years after it originally opened in Montana. She has spent years studying ankylosaurs, a type of dinosaur that appears in the fossil record from the Jurassic to the end of the Cretaceous. Some species of ankylosaurs had tails, while others, known as nodosaurs, did not. This difference raises some questions about what these structures were used for.

“I think a natural follow-up question from ‘Can they use their tails as a weapon?’ is ‘Who are they using this weapon against?'” Arber explained. “So that’s where I really started thinking about it.

Back in 2009, she was the author of a paper this suggests that ankylosaurs may have used their tails for intraspecific fights—fights with other ankylosaurs. This work focuses on the potential impact of tails when used as weapons, particularly as clubs come in a variety of shapes and sizes, and in some species are not even present until the animal is mature. By measuring the available fossil tail clubs and estimating the force of the blows they could produce, she found that the smaller clubs (roughly 200 millimeters or half a foot) were too small to be used as defense against predators.

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The blood curdling roarthe shin bump.

Royal Ontario Museum

She recommended further research, noting that if ankylosaurs used them for intraspecific fighting, one would expect to see injuries on the adults’ flanks, since an ankylosaur’s tail could only swing so far.

It’s one thing to have an idea about a missing animal, it’s another to have evidence. Ankylosaurus fossils are rare in general; dinosaurs with tissue preservation that would have been damaged in these battles are much rarer. So it’s amazing that Arbor can test his ideas thanks to an animal with its entire back — most of its skin and all — intact.

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

Damage assessment

Zool’s back and flanks are covered with various spines and bony structures called osteoderms. Just as Arber 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 basic statistics to show that injuries are not randomly distributed across the body,” she continued. “They’re really only limited to the sides in the thigh areas. This cannot be explained by random chance alone. That seems more likely to be the case [the result of] repetitive behavior.’

Damaged but partially healed spike on Zuul's side.

Damaged but partially healed spike on Zuul’s side.

Royal Ontario Museum

There are only a few well-preserved ankylosaurs, including at least one named nodosaur From the boreal forest in the Royal Tyrrell Museum. The authors note that there are no comparable injuries in known nodosaurs, which is a pertinent point. As mentioned earlier, nodosaurs did not have tails and therefore could not have used them against each other.

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

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