U.S. researchers say they have discovered what appears to be the oldest imprint of a prehistoric insect, made while the dragonfly-like creature was still alive.

The imprint found at a rocky outcrop near a large shopping centre in North Attleboro, Massachusetts, is believed to have been made by an insect about eight centimetres long as it stood on mud some 312 million years ago.

"It's not a dragonfly, but picture a dragonfly-like body. We're looking at something related, maybe a mayfly. They have the same body plan," says the discoverer, Richard Knecht, a geology student at Tufts University in Massachusetts.

This undated handout image shows what U.S. researchers say appears to be the oldest imprint of a prehistoric insect, made while the dragonfly-like creature was still alive. The fossilized remains were uncovered at a rocky outcrop near a large shopping center in North Attleboro, Mass., and is believed to have been made by an insect as it stood on mud some 312 million years ago. (Jacob Benner-Tufts University/Reuters)The fossilized remains of a wing that may have belonged to the same species were uncovered two weeks ago.

The imprint of the insect shows the thorax and abdomen, along with six legs, two of which may have moved slightly to create drag marks that hardened into burgundy-coloured stone.

"It's unusual to see a flying insect make such a deep impression in this muddy sediment," says Tufts paleontologist Professor Jake Brenner.

"We don't have many good body fossils from this time period with these early flying insects. The level of detail is really unseen in continental deposits."

At the time the impression was made, insects and amphibians were common. Reptiles, the ancestors of the dinosaurs, were just beginning to come to prominence.

Many of the fossilized remains of insects show only individual body parts, such as a wing left behind when a predator ate the rest. The impression of an intact insect "fills in the blanks of what we imagined," says Knecht.

The rock that contained the fossil broke off in Knecht's hands as he was trying to rediscover an outcrop that had yielded four-digit footprints of an ancient amphibian the day before.

"It was a really lucky find," he says.