limb press

University of Alberta doctoral candidate Scott Persons measured the limb length for 53 different species of carnivorous dinosaurs to create an equation to score how much each was built for speed. (Scott Persons)

A new study published out of the University of Alberta has found that carnivorous dinosaurs evolved to become faster and faster over time — and the proof is in the leg length.

As part of his doctoral research, lead author Scott Persons travelled the world measuring the limb lengths for 53 different species of carnivorous bipedal dinosaurs, including the Velociraptor, Allosaurus and Tyrannosaurus rex.

In particular, Persons looked at the length of the dinosaurs' legs below the knee. As a rule, he said, the longer the lower leg is in comparison to the upper leg, the faster the animal is.

Modern-day cheetahs have proportionately longer legs than hyenas. That relationship is mirrored in the animals' speeds, Persons said.

"That's true for modern carnivores, and must have been true for dinosaurs," Persons said in a statement released Wednesday.

Looking at each fossil carnivore's "cursorial limb proportion score" — in other words, a measure of how much each was built for speed — Persons determined that they evolved to be faster and faster over time.

Dinosaur Limb Length

By measuring the lower limb length from 53 different fossilized dinosaur species, which ranged from chicken-sized to the length of a school bus, Persons hypothesizes bipedal carnivores evolved to become faster and faster over time. (Scott Persons)

And leading the pack: the Nanotyrannus, a five-metre tall dinosaur similar in appearance to a young Tyrannosaurus. In fact, the resemblance between the two is so close that it wasn't until leg lengths were compared that it was conclusively determined they were not, in fact, the same species.

"In terms of Cretaceous ecology, T. rex was the lion and Nanotyrannus was the cheetah," Persons said of his findings.

"As far as I'm concerned, it was the scariest dinosaur. Sure, it might take it four to five bites to eat you, while T. rex could do it in just one or two. But eaten is eaten—and no dinosaur was better adapted to chase you down."

That burst in speed comes at a cost, however, as longer legs are less-suited to carrying heavy weights and are weaker.

"Speed determines what prey you can catch, how you hunt it, and the sort of environment that you are most successful in," Persons said.

"Over evolution, you have these two conflicting forces: the need for speed and the need for weight support."