New research indicates that the tail clubs on huge armored dinosaurs known as ankylosaurs may have evolved to whack each other rather than deter hungry predators. This is a complete shift from what was previously believed.
Prior to the paper published today in Biology Letters, most scientists looked upon the dinosaur’s tail club, a substantial bony protrusion composed 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, they focus on years of ankylosaur research, analysis of the fossil record, and data from an exceptionally well-preserved specimen named Zuul crurivastator.
Zuul’s name, in fact, embraces that previous idea. While "Zuul" references the creature in the original Ghostbusters, the two Latin words that make up its species name are crus (shin or shank) and vastator (destroyer). Hence, the destroyer of shins: a direct reference to where the dinosaur’s club may have struck approaching tyrannosaurs or other theropods.
But that name was given when only its skull and tail had been excavated from the rock where the fossil was encased. After years of skilled work by the fossil preparators at the Royal Ontario Museum, Zuul’s entire back and flanks are exposed, offering important clues as to what its tail club might target.
Target identification
Lead author Dr. Victoria Arbour is currently the curator of paleontology at the Royal British Columbia Museum, but she’s a former NSERC postdoctoral fellow at the Royal Ontario Museum in Toronto. That has been Zuul’s home since 2016, two years after its initial discovery in Montana. She has spent years studying ankylosaurs, a type of dinosaur that appears in the fossil record from the Jurassic through the end of the Cretaceous. Some species of ankylosaurs 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 from, ‘Could they use their tail clubs as a weapon?’ is ‘Who are they using that weapon against?’” Arbour explained. “And so that’s where I really started thinking about this."
Back in 2009, she authored a paper that suggested 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 as the clubs come in various shapes and sizes, and in some species, weren’t even present until the animal matured. Measuring available fossil tail clubs and estimating the force of the blows they could produce, she found that smaller clubs (approximately 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 were using them for intraspecific combat, one might expect to see injuries along adult flanks, as an ankylosaur tail can only swing so far.
It’s one thing to have an idea about an extinct animal, but it’s another to have evidence. Ankylosaur fossils are rare in general; dinosaurs with preservation of the tissues that would have been damaged in these fights are much rarer. So it’s astounding that Arbour could test her ideas, thanks to an animal with its entire back—most of its skin and all—intact.
“I put out this idea that we would expect to see damage on the flanks, just based on how they might line up against each other,” Arbour told Ars. “And then a decade and a bit later, we get this amazing skeleton of Zuul with damage right where we thought we might see it. And that was pretty exciting!”
Damage assessment
Zuul’s back and flanks are covered in various spikes and bony structures called osteoderms. Just as Arbour 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 sort of basic statistics to show that the injuries are not randomly distributed on the body,” she continued. “They really are just restricted to the sides in the areas around the hips. That can’t be explained just by random chance. It seems more likely that it’s [the result of] repeated behavior.”
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 aren’t any comparable injuries on known nodosaurs, a germane point. As mentioned previously, nodosaurs don’t have tail clubs and thus wouldn’t have been able to use them against each other.
Equally important, the damage isn’t accompanied by evidence of predation. No bite marks, puncture wounds, or tooth scratches are found anywhere on Zuul’s body.
Bringing the hurt
The team turned to the fossil record for further evidence. Did tail clubs increase in size to match the increased sizes of coexisting predators? In this regard, the evidence was inconclusive.
“We’re not seeing anything that we can quantify that suggests that predators are influencing the evolution of tail clubs. It doesn’t mean that they didn’t,” Arbour clarified, “but we just don’t have any evidence in the fossil record.”
But the fossil record does show that nodosaurs, which didn’t have tail clubs at all, coexisted with a number of carnivorous theropods. Other ankylosaurs lived among these predators for millions of years before their tail clubs evolved in these lineages.
The idea that ankylosaurs would fight each other is, arguably, a bit terrifying. These were not small animals. Zuul, for example, was approximately 6 meters (20 feet) long and weighed about 2.5 tons. Its tail is about 3 meters (10 feet) long and edged with spikes; the tail club's width is 36.8 centimeters (1.2 feet) of bone. That’s a powerfully painful weapon to hurl at an opponent. Even an armored one.
As Arbour pointed out, “One thing that I don’t think people really appreciate is that animals can really hurt each other.”
Arbour said that her “favorite analog for ankylosaurs is actually giraffes.” Male giraffes swing their 500-pound necks and their heads—topped with two horns called ‘ossicones’—into one another in combat. “It’s kind of like a tail club but on the other side of your body,” she offered. “Giraffes will really hurt each other doing that. They’ll leave little puncture marks on their sides, and they can break each other’s legs.”
To date, we don’t yet know how to determine whether an ankylosaur fossil is a female or a male, even in fossils as well-preserved as Zuul. So, could females have been the ones engaging in combat?
“I think it’s possible,” she said, though noting that “it’s a lot less common in animals today.”
Equal-opportunity clubbers?
Dr. Andrew Farke, who was not involved in this research, is the director of the Raymond M. Alf Museum of Paleontology at The Webb Schools in California. In 2014, he examined the possibility of intraspecific combat in a number of dinosaurs, including ankylosaurs.
“Quite a few animals today battle members of their own species (intraspecific combat), so I totally expect that ankylosaurs and other extinct dinosaurs did the same,” he said. “Short of a time machine, looking for patterns of injuries is the best evidence we have to reconstruct dinosaur versus dinosaur battles.”
“The pattern of damage on this animal is pretty intriguing, and I think the hypothesis that it’s from combat with another ankylosaur is quite plausible,” he said of today’s research. “Although there aren’t a ton of specimens out there that are this beautiful, a logical next step would be to see if other individuals have similar damage. Other ankylosaurs with tail clubs should definitely show common damage on their flanks (or elsewhere) if they were indeed doing battle against each other.”
Could ankylosaurs have used their weapon against predators as well? Yes, according to the team, but they argue that evidence suggests this wasn’t the club’s main or original function.
Co-author Dr. Lindsay Zanno is head of paleontology at North Carolina Museum of Natural Sciences and an associate research professor at NC State University. She says this new find expands our perspective on these creatures.
“Scientists and the public alike have taken a tyrannosaur-centric view of ankylosaurs,” Zanno said, referencing how the two are often depicted in battle. “If you believe the storybooks, [the clubs] only existed to beat off hungry predators. My hope is that this research helps people see ankylosaurs for the socially complex and behaviorally fascinating creatures they really were.”
Biology Letters, 2022. 10.1098/rsbl.2022.0404
