New research suggests that tail claws in huge armored dinosaurs known as ankylosaurs may have evolved to hit each other rather than deter hungry predators. This is a complete change from what was previously believed.
Prior to the paper published today in Biology Letters, most scientists looked at the dinosaur’s tailbone, a prominent bony prominence consisting of two oval-shaped knobs, primarily as a defense against predation. The team behind the new paper argues that this isn’t necessarily the case. To make their claims, they focus on years of ankylosaur research, analysis of the fossil record, and data from an exceptionally well-preserved specimen. Zuul crurivastator.
Actually, Zuul’s name embraces the previous idea. While “Zuul” refers to the creature in the original Ghost Hunterstwo Latin words that make up the species name grumpy (shin or handle) and extender (destroyer). Hence, the destroyer of the shins: a direct reference to where the dinosaur’s stick might have struck approaching tyrannosaurs or other theropods.
However, this name was given when only the skull and tail were removed from the rock on which the fossil was covered. After years of resourceful work by fossil preparers at the Royal Ontario Museum, Zuul’s entire back and flanks have been revealed, providing important clues as to what the tailbone might have targeted.
Lead author Dr. Victoria Arbor is currently the Curator of Paleontology at the Royal British Columbia Museum, but is a former NSERC postdoctoral fellow at the Royal Ontario Museum in Toronto. It has been Zuul’s home since 2016, two years after his initial discovery in Montana. He spent years studying ankylosaurs, a type of dinosaur that appears 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. This difference raises some questions about what these structures are used for.
“‘Can they use their tails as weapons?’ “Who are they using this weapon against?” Arbor explained. “That’s where I really started to think about it.
In 2009, he wrote a paper suggesting that ankylosaurs might have used their tail sticks for interspecies combat with other ankylosaurs. This study focused on the potential effect of tail sticks when used as a weapon, especially since sticks come in a variety of shapes and sizes, and in some species they are not even present until the animal matures. By measuring existing fossil tail sticks and estimating the strength of the blows they could produce, he found that the smaller sticks (about 200 millimeters or half a foot) were too small to be used as a defense against predators.
He suggested further research, noting that if ankylosaurs were using them for interspecies combat, one could expect to see injuries on their adult flanks, as an ankylosaur’s tail could only ever wag.
Having an opinion about an extinct animal is one thing, having evidence is another. Ankylosaur fossils are generally rare; Dinosaurs that protect tissue to be damaged in these fights are much rarer. It’s amazing, therefore, that the Arbor was able to put his ideas to the test thanks to an animal with all of its back – most and all of its skin – intact.
“I came up with the idea that depending on how they rank up against each other, we would expect to take damage on the flanks,” Arbor told Ars. “And then ten years and a little later, we found this wonderful skeleton of Zuul damaged right where we thought we could see it. And that was pretty exciting!”
Zuul’s back and sides are covered with various spikes and bony structures called osteoderm. Just as Arbor predicted, there is evidence of broken and injured osteoderms on either side of the wings, some of which appear to have healed.
“We also did some basic statistics to show that the scars are not randomly distributed throughout the body,” he continued. “They’re really only limited to the sides in the areas around the hips. This cannot be explained by mere random chance. seems more likely [the result of] repetitive behavior.”
There are only a handful of well-preserved ankylosaurs, including at least one of a nodosaur named Borealopelta in the Royal Tyrrell Museum. The authors note that there were no comparable injuries in known nodosaurs, an important point. As previously mentioned, nodosaurs did not have tails and therefore could not use them against each other.
Equally important, the damage is not accompanied by evidence of hunting. No bite marks, puncture wounds, or bruises were found anywhere on Zuul’s body.