Ankylosaurid dinosaurs were heavily armored herbivores with tails modified into club-like weapons. These tail clubs have widely been considered defensive adaptations wielded against predatory dinosaurs. In a new paper, published in the December 2022 issue of the journal Biology Letters, paleontologists argue instead that ankylosaurid tail clubs were sexually selected structures used primarily for intraspecific combat.
Zuul crurivastator in battle. Image credit: Henry Sharpe.
In their research, Dr. Victoria Arbour, a paleontologist with the Royal BC Museum and the University of Victoria, and her colleagues focused on Zuul crurivastator, a herbivorous ankylosaurine dinosaur that lived 76 million years ago during the Cretaceous period.
The dinosaur’s fossilized remains were discovered in the Judith River Formation in Montana in 2014.
After years of work, its body was revealed to have preserved most of the skin and bony armor across the entire back and flanks, giving a remarkable view of what the dinosaur looked like in life.
The body was covered in bony plates of different shapes and sizes and the ones along its sides were particularly large and spiky.
Interestingly, the paleontologists noticed that a number of spikes near the hips on both sides of the body are missing their tips and the bone and horny sheath has healed into a blunter shape.
The pattern of these injuries is more consistent with being the result of some form of ritualized combat, or jousting with their tail clubs, and probably weren’t caused by an attacking predator like a tyrannosaur because of where they are located on the body.
“I’ve been interested in how ankylosaurs used their tail clubs for years and this is a really exciting new piece of the puzzle,” Dr. Arbour said.
“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.”
Zuul crurivastator’s tail is about 3 m (10 feet) long with sharp spikes running along its sides.
The back half of the tail was stiff and the tip was encased in huge bony blobs, creating a formidable sledgehammer-like weapon.
Dr. Arbour and co-authors don’t refute the idea that tail clubs could be used in self-defense against predators, but they show that tail clubs would also have functioned for within-species combat — a factor that more likely drove their evolution.
“The fact that the skin and armour are preserved in place is like a snapshot of how Zuul crurivastator looked when it was alive,” said Dr. David Evans, Temerty chair and curator of vertebrate palaeontology at the Royal Ontario Museum.
“And the injuries Zuul crurivastator sustained during its lifetime tell us about how it may have behaved and interacted with other animals in its ancient environment.”
_____
Victoria M. Arbour et al. 2022. Palaeopathological evidence for intraspecific combat in ankylosaurid dinosaurs. Biol. Lett 18 (12): 20220404; doi: 10.1098/rsbl.2022.0404
