Sauropods – the giant, long-necked dinosaurs like Brachiosaurus – were the largest land animals to ever walk the Earth. As you'd expect, that would put immense strain on the creatures' legs and feet. Now, researchers at Australia's University of Queensland (UQ) have studied sauropod feet and found that the animals may have had an unexpected gait – tiptoeing around like they were wearing high heels.

Given the large, flat feet sauropods had, it's easy to assume they just plodded along, but the new study shows they were a bit more graceful than that. The UQ researchers created physical and virtual replicas of the fossilized foot bones of a sauropod species known as Rhoetosaurus. They then put them through a vigorous series of tests to figure out the range of motion the bones would have had, and how well the animal would have been able to support its huge weight.

Somewhat surprisingly, a high-heeled gait seemed to be the best bet. Of course, the weight wouldn't have been focused on the toes – instead, it's likely that Rhoetosaurus had a big fleshy heel to support itself.

The fossilized foot bones of Rhoetosaurus (with a replica in place of the missing fifth toe) Jay P. Nair & Andréas Jannel

"Looking at the bones of the foot, it was clear that Rhoetosaurus walked with an elevated heel, raising the question: how was its foot able to support the immense mass of this animal, up to 40 tonnes?" says Andréas Jannel, corresponding author of the study. "Our research suggests that even though Rhoetosaurus stood on its tiptoes, the heel was cushioned by a fleshy pad. We see a similar thing in elephant feet, but this dinosaur was at least five times as heavy as an elephant, so the forces involved are much greater."

Since soft flesh doesn't fossilize, the scientists looked elsewhere for extra evidence of this heel pad. Being some of the heaviest animals of all time, sauropod footprints are fairly common around the world, and the team found that many of these left the suggestion of a fleshy heel pad.

A cross section of an elephant's foot (left), and an x-ray of a human foot (right). Below the bones in the elephant's foot – which has a remarkably similar skeletal structure to the human – is a fleshy pad like that sauropods might have had University of Queensland

Interestingly, this find could represent a key step in the evolution of sauropods. Hailing from the Middle Jurassic about 160 to 170 million years ago, Rhoetosaurus was a relatively early sauropod, and was considered small at a length of "only" 15 m (49 ft). Developing this fleshy pad could have opened the door for sauropods' size to blow out to the huge dinosaurs we saw later on.

"The addition of a cushioning pad that supports the raised heel appears to be a key innovation during the evolution of sauropods, and probably appeared in early members of the group some time during the Early to Middle Jurassic Periods," says Jarrel. "The advantages of a soft tissue pad may have helped facilitate the trend towards the enormous body sizes we see in these dinosaurs."

The research was published in the Journal of Morphology.

Source: University of Queensland