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Competition between ankylosaurs may be responsible for their iconic tail clubs.
While the club may still have had a role to play in fighting off predators, a new study suggests it would have been less important than its other role.
Ankylosaurs may have turned their tail clubs on each other, rather than predators, a new paper suggests.
The pattern of injuries on a well-preserved ankylosaur found in the USA adds evidence to support claims that ankylosaur tails may have evolved primarily to compete with each other in display or combat over resources and territory.
Dr Victoria Arbour, the lead author of the research, says, 'I've been interested in how ankylosaurs used their tail clubs for years and this is a really exciting new piece of the puzzle.'
'We know that ankylosaurs could use their tail clubs to deliver very strong blows to an opponent, but most people thought they were using their tail clubs to fight predators. Instead, ankylosaurs like Zuul crurivastator may have been fighting each other.'
Though the research helps to add evidence to support theories about ankylosaur competition, it doesn't deliver a killing blow to alternatives. A patchy fossil record means a number of the study's analyses were inconclusive, leaving open the possibility that it evolved to fend off predators.
The findings of the study were published in the journal Biology Letters.
Ankylosaurs are one of the armoured dinosaurs, a group which also includes animals such as Stegosaurus.
While stegosaurs often had a spiked structure on the end of their tail, ankylosaurs instead had a hard, bony structure known as a tail club. These structures are uncommon in nature, with only glyptodonts and a group of extinct turtles also known to have evolved them.
This club is generally thought to have been swung at the leg bones of predatory dinosaurs to fend them off. While clubs of any size would have caused injuries, only the largest clubs would have been able to break bones.
Species such as Anodontosaurus are estimated to have been able to generate forces sufficient to snap living bone, but would have found their tail harder to wield. Meanwhile, ankylosaurs with smaller tail clubs would have been more able to strike quickly even if they were less likely to break bones.
In general, these clubs have been thought of as purely defensive weapons, as the herbivorous ankylosaurs would not have needed to actively attack other animals. Alternative suggestions, such as the tail acting as a decoy head to draw predators away from the neck, have also been dismissed over the years.
But one theory that hasn't gone away is that the tail club was used in competition between individuals of the same species. Modern giraffes, for instance, use their long necks to strike each other in contests between individuals, and it is possible that ankylosaurs may have done something similar with their tails.
While it is difficult to directly test this theory, the study of well-preserved specimens in recent years is adding more evidence to support it.
The specimen of Zuul crurivastator was uncovered in 2014 in the Judith River Formation of the USA. It was later purchased by the Royal Ontario Museum, Canada, and two of its researchers described the specimen in 2017.
At the time, only the tail and head had been fully excavated, with the specimen's distinctive horns leading the researchers to name its genus after the pop culture character Zuul, a monster from the film Ghostbusters, while its tail club gives the animal its specific name, which translates as 'destroyer of shins'.
Since then, more of the body has been revealed from within the rock, revealing preserved skin and bony armour along its back and sides. Some of the bony plates around the hips are missing parts of their spikes, and appear to have healed in a more blunt shape.
Co-author Dr David Evans says, 'The fact that the skin and armour are preserved in place is like a snapshot of how Zuul looked when it was alive. The injuries it sustained during its lifetime tell us about how it may have behaved and interacted with other animals in its ancient environment.'
The researchers analysed the position and shape of the healed wounds in Zuul to assess whether predation or competition was a better explanation for the observed injuries.
The researchers concluded that the pattern of injuries makes it unlikely that they were caused by predator attacks, which are more likely to be spread randomly across the body. Instead, they were significantly more likely to occur on the sides of the animal, with roughly equal levels of damage on both sides.
Similar injuries have also been found around the pelvis of the Mongolian ankylosaur Tarchia tumanovae, which also has a number of wounds on its tail.
The scientists have interpreted damage to Zuul's sides as evidence of combat, suggesting that ankylosaurs would have competed with each other by striking the side of each other's body with their tail clubs. The lack of injuries on the top of the body supports this, as the ability of the dinosaurs to move their tails up and down is thought to have been limited.
These fights may have been over territory, resources or social dominance, but the difficulty in demonstrating behaviours in extinct species means it's very unlikely for a cause to ever be conclusively proven.
It could also have been a fight over mates, but as scientists can't yet reliably sex a dinosaur the role of sexual selection is near impossible to show.
While wider knobs on the tail clubs may have inflicted more damage on a predator than thinner ones and would be expected to grow bigger if larger predators were around, the researchers attempted to make this comparison but were hampered by a lack of fossil evidence.
'We didn't see any correlation between the origins or size of tail clubs as predators got bigger, but it's also important to note that the fossil record of ankylosaurs is really patchy,' Victoria explains. 'As a result, we have a fairly small sample size to test these ideas with.'
'Right now, there's no evidence in the fossil record to suggest that predation played a major role in the origin of ankylosaur tail clubs, but we can't rule it out completely. Overall, we think we have better evidence that it arose through sexual selection to use in intraspecific combat.'
Though the competition theory appears to be gaining the upper hand in this fight between opposing theories, only more evidence will allow one side or the other to land the knockout blow.