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Dimetrodon, one of the most recognisable of the pre-dinosaur predators, is due a makeover. For more than a century, it has been depicted as a sluggish, belly-dragging beast with sprawling legs – but it might actually have held its legs in a more upright position and kept its stomach off the ground as it walked.

Often mistaken for a dinosaur, Dimetrodon actually belonged to a group called the pelycosaurs that were more closely related to mammals. It lived between , with some species from nose to tail. Its most iconic feature was a gigantic sail on its back, the function of which is .

as a sluggish-looking animal with legs sprawled out to each side of its body, resting its weight on an enormous belly – and even in the 21st century nothing much has changed.

“I was baffled as I was going through the literature how little this had been questioned,” says Caroline Abbott at the College of William & Mary in Williamsburg, Virginia. It’s particularly surprising given that the fossil trackways left by Dimetrodon seem to tell a different story. The relatively narrow distance between left and right sets of footprints suggest Dimetrodon did not have sprawling legs.

“That’s where the real head-scratcher is,” says Abbott. “The trackways are more narrow than you’d expect and in a lot of cases they lack belly dragging marks.”

Modern moves

With her colleague at the Smithsonian Institution in Washington DC, Abbott measured Dimetrodon bones, and looked at the configuration of the skeleton at the shoulder and the hip. She then compared this information with data collected from 11 living mammals – including the short-beaked echidna – and 12 living reptiles, including the Komodo dragon, the savannah monitor and the spectacled caiman.

When Abbott and Sues used software to run their data, they found that Dimetrodon seemed to most closely match the caiman, a crocodilian that can hold its legs vertically enough to raise its body off the ground – particularly when it runs. Dimetrodon might have held its body in a similar way.

“That’s the best I have right now,” says Abbott, who will present her findings at a in Seattle later this month. “But I’m hoping that as I broaden the modern analogues I look at, and do more complicated stats, I can pinpoint that a bit better.”

Sprawling logic

at the New Mexico Museum of Natural History and Science in Albuquerque agrees that Dimetrodon’s posture needs a reassessment. He , after he and his colleague Adrian Hunt studied Dimetrodon trackways.

“When we wrote that paper we were just throwing the gauntlet down and saying: look, the trackways are showing something really different than anybody has thought from the skeletons,” says Lucas. “But we didn’t try to resolve it.”

He says some palaeontologists did offer an explanation – that Dimetrodon thrashed its spine from side to side so much as it walked that it could leave narrow sets of footprints despite having sprawled legs. Abbott and Sues are suggesting a different solution, he says.

Lucas thinks the debate could be resolved by returning to the Dimetrodon trackways and using them to assess how fast the animals were moving. The “thrashing spine” idea assumes they were lumbering along slowly, whereas Abbott and Sues’s reconstruction would be more consistent with an animal moving at speed – some crocodiles can run at more than 10 miles per hour.

“If this is a pelycosaur moving quickly in a crocodilian-like fashion that would support [Abbott’s] argument,” says Lucas.