New research indicates that the tail maces on massive armored dinosaurs known as ankylosaurs may have evolved to swat each other rather than scare off hungry predators. This is a complete shift from what was previously assumed.
Prior to the paper published today in Biology Letters, most scientists looked to the dinosaur’s tail club, a substantial bony protrusion consisting of two oval-shaped knobs, primarily as a defense against predation. The team behind the new paper argues that this is not necessarily the case. To make their case stronger, they turn to years of ankylosaur research, analysis of the fossil record, and data from an exceptionally well-preserved specimen called Zuul crurivastator.
Zuul’s name actually embraces that earlier idea. While “Zul” refers to the creature in the original Ghostbustersthe two Latin words that make up the species name are crust (shin or shank) and retainer (destroyer). Hence the destroyer of shins: a direct reference to where the dinosaur’s bat might have hit approaching tyrannosaurs or other theropods.
But that name was given when only the skull and tail had been excavated from the rock that contained the fossil. After years of skillful work by the fossil preparators at the Royal Ontario Museum, Zuul’s entire back and flanks have been exposed, providing important clues as to what his tail club might be aiming for.
Lead author Dr. Victoria Arbor is currently the curator of paleontology at the Royal British Columbia Museum, but she is a former NSERC postdoctoral fellow at the Royal Ontario Museum in Toronto. That’s been Zuul’s home since 2016, two years after the initial discovery in Montana. She has spent years studying ankylosaurs, a type of dinosaur found in the fossil record from the Jurassic to the end of the Cretaceous. Some ankylosaur species have tail clubs, 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 is of, ‘Could they use their tail clubs as a weapon?’ is ‘who are they using that gun against?’” Arbor explained. “And so I really started to think about that.
In 2009, she wrote a paper suggesting that ankylosaurs might use their tail maces for intraspecific combat — battles with other ankylosaurs. That work focused on the potential impact of tail clubs when used as a weapon, especially since the clubs come in a variety of shapes and sizes, not even present until the animal reached adulthood in some species. By measuring available fossil tail clubs and estimating the power of the blows they could produce, she found that smaller clubs (about 200 millimeters or half a foot) were too small to be used as a defense against predators.
She recommended further research, noting that if ankylosaurs used them for intraspecific combat, one might expect to see injuries along adult flanks, since an ankylosaur’s tail can only swing so far.
It’s one thing to have an idea about an extinct animal, but it’s another to have proof. Ankylosaur fossils are generally rare; dinosaurs retaining the tissues that would have been damaged in these battles are much rarer. So it’s amazing that Arbor was able to test her ideas thanks to an animal with its entire back — most of its skin and all — intact.
“I put forward the idea that we should expect damage to the flanks just based on how they might stack against each other,” Arbor told Ars. “And then, a decade and a little later, we get this amazing skeleton of Zuul with damage right where we thought we’d see it. And that was quite exciting!”
Zuul’s back and flanks are covered in various spines and bony structures called osteoderms. As Arbor predicted, there is evidence of fractured and injured osteoderms on both sides of the flanks, some of which appear to have healed.
“We also did some kind of basic stats to show that the injuries aren’t randomly distributed across the body,” she continued. “They are really only confined to the sides in the areas around the hips. That can’t be explained by random chance. It seems more likely that it is [the result of] repeated behavior.”
There are only a handful of well-preserved ankylosaurs, including at least one of a nodosaurus named Borealopelta in the Royal Tyrrell Museum. The authors note that there are no similar injuries in known nodosaurs, a pertinent point. As mentioned earlier, nodosaurs don’t have tail maces and so wouldn’t be able to use them against each other.
Equally important, the damage is not accompanied by evidence of predation. There are no bite marks, stab wounds or tooth scratches anywhere on Zuul’s body.