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Carnotaurus was likely one of the fastest dinosaurs of its size, capable of speeds of up to 56 kilometres per hour. (Lida Xing and Yi Liu/University of Alberta)

A horned dinosaur as long as an RV may have reached speeds above 55 kilometres an hour as it ran down its prey, estimates an Alberta researcher.

"This was a super-fast predator," said W. Scott Persons, the University of Alberta PhD student who uncovered carnotaurus's gifts as a sprinter.

"I would not be surprised if carnotaurus could push 30, 35 miles per hour (48-56 km/h), at least in the short burst."

Carnotaurus, whose scientific name means "meat-eating bull," was a predatory dinosaur that lived in South America during the Late Cretaceous period, about 71 million years ago.

It was less than eight metres long, or about half the size of a Tyrannosaurus rex, and had a horn over each eye.

"So, it has a very devilish appearance," said Persons in an interview with CBC Radio's Quirks & Quarks set to air Saturday.

His interest in the dinosaur was piqued when he caught a glimpse of its tailbones, which he described as "really, really weird."

Unlike most dinosaurs whose tailbones have structures called caudal ribs that stick out to either side, carnotaurus had caudal ribs that tilted upward and hooked over each other, stiffening the tail.

Persons hypothesized that the strange, upward-tilted shape of the caudal ribs might allow the dinosaur to fit a much larger muscle underneath compared to most dinosaurs.

A muscle found in that tail area in modern crocodiles and lizards, called the caudal femoralis, pulls the leg toward it when it contracts.

"If you want to understand how fast a dinosaur can run, you need to look at how big that muscle was because that gives you a sense of how much power it's packing," Persons said.

He created a computer model of the dinosaur's tailbone and then sculpted in the caudal femoralis muscle to fit underneath so he could estimate its mass.

"I was floored," he said. "It's a really big muscle."

He estimates that would have put carnotaurus among the fastest dinosaurs of its size.

Persons and his supervisor, Philip Currie, published their results in the journal PLoS One earlier in October.

A year ago, the pair published a similar study on tyrannosaurus rex that suggested it, too, was a fast sprinter.