The new species is a microsaur – small, lizard-like animals that roamed the Earth more than 250 million years ago, well before proper dinosaurs made their appearance.
The find sheds important light on the evolution of different animal groups, including amphibians and reptiles, scientists wrote in the journal Royal Society Open Science.
Microsaurs lived during the Carboniferous period, when the forebears of modern mammals and reptiles, called amniotes, first appeared.
“Many details of that transition aren’t well known,” study co-author Arjan Mann, a post-doctoral research fellow at the Smithsonian Institution, said.
“Microsaurs have recently become important in understanding the origins of amniotes. Many of these microsaurs have been thought to be either ancestors of amphibians or reptiles.”
Encased in a bog in what is today the central US, the specimen’s serpent-like body measures about 5cm. The animals had four short, plump legs.
In deference to its tiny size, researchers dubbed the new species “Joermungandr bolti”, after a giant sea serpent from Norse mythology who did battle with Thor.
Scientists were astonished to discover the fossil also contained the animal’s skin.
“Areas of the skin had only been known from fragmentary fossils before,” said Mann. “That’s very rare for these fossils. It’s very rare for anything 300 million years old to have skin with it.”
‘Head-first burrower’
Mann said the research suggests not only that microsaurs might be early relatives of reptiles, but also that the ability to burrow may have played a bigger role in the origin of amniotes than originally thought.
The researchers used a highly sensitive imaging technique called scanning electron microscopy (SEM) to get an up-close look at the nearly perfect fossil. They discovered a pattern of ridges similar to those found on the scales of modern reptiles that dig into the ground.
Along with other features like a robust skull and elongated body, the scale shape led researchers to hypothesise that Joermungandr burrowed as well.
“It would probably have been a head-first burrower, using its head to smack itself into the soil,” said Mann.
“Its limbs were probably not very functional. It may have used them to stabilise itself as it was wobbling around.
“But its primary mode of movement would have been sidewinding like a snake.”
The SEM imaging technique is now being applied to many other ancient fossils, he added.