New research suggests that the tail clubs of giant armored dinosaurs known as ankylosaurs may have evolved to smack each other rather than scare away hungry predators. This is a complete departure from what was previously believed.
Prior to today’s publication in Biology Letters, most scientists thought of the dinosaur’s tail club, a sizable bony prominence composed of two oval knobs, primarily as a defense against predators. The team behind the new paper argues that’s not necessarily the case. To make their point, they focus on years of ankylosaur research, analysis of the fossil record, and data from an exceptionally well-preserved specimen with the name Zuul Crurivastator.
Zuul’s name actually encompasses this earlier idea. While “Zuul” refers to the creature in the original ghostbustersthe two Latin words that make up its species name are Crus (shinbone or shaft) and Vasator (Destroyer). Hence the destroyer of shins: a direct indication of where the dinosaur’s club might have struck approaching tyrannosaurs or other theropods.
But that name was given when only its skull and tail had been unearthed from the rock in which the fossil was encased. After years of expert work by the Royal Ontario Museum’s fossil taxidermists, Zuul’s entire back and flanks have been exposed, providing important clues as to where its tail club might be aiming.
target identification
The 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 Zuul’s home since 2016, two years after he was first discovered in Montana. She has spent years studying ankylosaurs, a species of dinosaur that appears in the fossil record from the Jurassic to the end of the Cretaceous. Some species of ankylosaurs have tail clubs, while others known as nodosaurs do not. This difference raises some questions about what these structures were used for.
“I think a natural follow-up to ‘Could they use their tail clubs as a weapon?’ is ‘who are they using this weapon against?’” Arbor explained. “And that’s when I really started thinking about it.
Already in 2009 she wrote a paper This suggested that ankylosaurs could use their tail clubs for intraspecific fights – fights with other ankylosaurs. This work focused on the potential effects of tail clubs when used as a weapon, particularly since the clubs come in a variety of shapes and sizes, and in some species were not even present until the animal was fully grown. When she measured available fossil tail clubs and estimated 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 when ankylosaurs used them for intraspecific combat, injuries to adult flanks were to be expected, since an ankylosaurid 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 with preservation of tissue 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 the entire back – most of the skin and all – intact.
“I floated this idea that we would expect damage on the flanks just based on how they might line up against each other,” Arbor told Ars. “And then a decade and a little bit later, we get this amazing skeleton of Zuul with damage right where we thought we could see. And that was pretty exciting!”
damage assessment
Zuul’s back and flanks are covered with various spines and bony structures called osteoderms. As predicted by Arbor, there is evidence of fractured and injured osteoderms on either side of the flanks, some of which appear to have healed.
“We also ran some basic statistics to show that the injuries are not randomly distributed on the body,” she continued. “They’re really just limited to the sides around the hips. This cannot simply be explained by coincidence. 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 that has a name like nodosaurs Borealopelta at the Royal Tyrrell Museum. The authors note that there are no comparable injuries in known nodosaurs, an important point. As mentioned before, nodosaurs don’t have tail clubs and thus couldn’t have used them against each other.
Equally important, the damage is not accompanied by evidence of predation. There are no bite marks, puncture wounds or tooth scrapes anywhere on Zuul’s body.
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