The Long Legs Of Tyrannosaurus Rex Were Made For Walking, Not Running

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Rachael Funnell 14 May 2020, 19:54

A new study published in the journal PLOS One suggests that the long legs of dinosaurs such as the Tyrannosaurus rex might not have been for speed but longevity, allowing them to travel long distances without using up too much energy. The discovery paints a new picture of the ancient predators, whose legs were long thought to have made them great runners.

"The assumption tends to be that animals with adaptations for running, such as long legs, are adapted for a higher maximum speed, but this paper shows that there's more to running than top speed," said Thomas Holtz, principal lecturer in the UMD Department of Geology, in a statement. "When you're a bigger animal, those adaptations may also be for endurance and efficiency. It may be about being a marathoner rather than a sprinter."

Holtz and his colleagues came to the realization following an analysis of limb proportions, size ratio, body mass, and gait. They used this data to estimate the top speeds of more than 70 species of theropods, the group of dinosaurs that is characterized by hollow bones and three-toed limbs. All known theropods were bipedal, meaning they walked on two legs with reduced front legs, as seen in T. rex.

Their bipedal locomotion was thought to be the key to their success, but the study adds more to the story. The analyses revealed that longer legs were good for running in the small and medium-sized dinosaurs, but for species above 998 kilograms (2,200 pounds), longer legs likely had no benefit for speed. This meant that big dinosaurs such as T. rex were likely no faster than their stubby-legged neighbors, but they could move more efficiently.

Among small theropods, long hind limbs gave the advantage of speed, but among the giants, long hind limbs enabled more efficient locomotion. T. Holtz, University of Maryland

The researchers calculated how much energy would be expended when walking for dinosaurs with differing leg lengths and found that this was where the largest species came out ahead. Giant species such as T. rex would have used less energy when walking, meaning they could go for miles and miles in search of food without tiring.

"That's actually a very beneficial savings, because predators tend to spend a great deal of their time foraging, searching for prey," Holtz said. "If you are burning less fuel during the foraging part of the day, that's an energy savings that dinosaurs with shorter leg forms didn't get."

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