An Ankylosaurus probably couldn't kill a Tyrannosaurus rex with the club at the end of its tail, but new research suggests the bony bludgeon could definitely break its ankles.
To estimate just how hard Ankylosaurus could hit with its tail club, Canadian researchers examined CT scans of several fossilized tails from dinos of different sizes. Combining the imaging data with measurements of the dinosaur's backbone, they determined the Ankylosaurus could swing its tail in a 100 degree lateral arc, and that larger clubs could generate forces strong enough to crush bone.
"The small ones would be like hitting something with a bowling ball," said dinosaur researcher Victoria Arbour of the University of Alberta in Canada, author of the study, published Tuesday in PLoS ONE. "And you really wouldn't want to be around when the bigger ones are swinging."
Scientists have long speculated that heavily armored ankylosaurids used their tail clubs to fend off other dinosaurs, but until now, no one had studied whether the club was a biologically feasible weapon. Using CT data and three-dimensional computer modeling programs, Arbour calculated the volume, mass and impact speed of both small and large tail clubs from dinosaurs in the Ankylosaurus family.
Researchers think the tail club, composed of tightly interlocking vertebrae with a large bony ball on the end, probably had limited vertical range of motion but could swing freely from side to side. Arbour concluded that the biggest clubs could generate between 364 and 718 megapascals of impact stress — plenty strong enough to crush bone.
"Twisting or cutting across a bone, it takes about 100 megapascals to break it," Arbour said. "I found out, given a certain area, the smallest tail clubs couldn’t break bone but the largest clubs could."
Because tail clubs are often discovered separately from the rest of a dino skeleton, it's hard to know who carried a large tail club and who had a small one. But researchers think the smaller tail clubs probably came from young dinosaurs who hadn't reached sexual maturity.
"Juvenile ankylosaurids actually don’t have a knob at the end of their tail," Arbour said. "They're born without it, and then as they grow, their armor develops slowly from the head toward the tail. It's possible that they only develop tail clubs at sexual maturity or at the very least when they are very large."
Since baby dinosaurs were most at risk for predator attacks, it seems odd that only adults would have tail clubs that worked as effective weapons. Arbour thinks it might point to another use for the bony ball on the end of the dino's tail. "It’s important to keep in mind that structures can often have more than one function," she said. "If you look at modern animals like giraffes, when the males are fighting they're actually clubbing each other with their necks and heads. I kind of wonder if ankylosaurids were doing the same thing but using their tails instead."
To find out, Arbour hopes to conduct future studies looking for rib fractures among adult ankylosaurids. If they've got more broken ribs than your average dinosaur, it might mean that they were bludgeoning each other to compete for mates. Unfortunately, we may never know for sure how the dinosaurs used their tail clubs — or exactly how hard they could swing.
"Biomechanical studies such as this do offer approximations, which can help us understand structures we see in dinosaurs," paleontologist Kenneth Carpenter of the Denver Museum of Nature and Science, who wasn't involved in the study, wrote in an e-mail. "However, all such studies are really 'best' guesses and a different approach will undoubtedly produce different results.... without a time machine we will never really know the true force of impact or even how close we are."
Image 1: Artist's rendering of two ankylosaurids fighting a Tyrannosaurus rex. Matthew Finbow/University of Alberta*. Image 2: Victoria Arbour posing with the smallest and largest of the ankylosaurid tail clubs she used in her study. Photo by Robin Sissons/University of Alberta*. *Image 3: CT scan and 3-D rendering of a tail club/*PLOS.
Citation: "Estimating Impact Forces of Tail Club Strikes by Ankylosaurid Dinosaurs." By Victoria Megan Arbour. Public Library of Science ONE*, August 25, 2009.*
