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NORTH ATTLEBORO - Geologists are buzzing about a fossil found in North Attleboro that researchers say may be the oldest whole-body impression of a prehistoric insect ever found.

Richard J. Knecht, a Tufts geology major and Jake Benner, a paleontologist and senior lecturer in the university's geology department, found the fossil in a shale and sandstone outcropping in a wooded field behind one of the town's strip malls.

The fossil is an impression left 300 million years ago by a flying insect from the Carboniferous Period - possibly an ancient relative of today's mayflies. It is believed to be the oldest preserved full-body impression of an insect ever discovered.

"This fossil in particular is a trace fossil," Knecht explained. "That means that it's not a fossil of the actual body of the insect, it's a record of the insect's behavior left in the rock.

"What we're looking at is a relative of the mayfly, so a proto mayfly. And what makes it distinctive is that it flew down, landed and left an impression of its body in the mud and then flew away." Knecht described the 3-inch long impression on the flat rock as the body and legs of a large insect.

"Imagine how an impression left by a dragonfly landing in the mud would look," he said.

Knecht decided to hunt for fossils in North Attleboro after reading an obscure reference to the location in a master's thesis written by a student almost 80 years ago.

"I came across a thesis that was written in 1929 and published in 1930 that mentioned fossils in the area," he said. "It's the only mention of fossils within this group of rocks that I could find, and it had a bad picture of it."

Using a hammer and chisel, Knecht and Benner found several fossils in the area, including tracks left by ancient lizards and reptiles, but they weren't aware of the significance of the insect fossil until they returned to the lab.

"I took a lot of fossils that day, and it wasn't until I got back to the lab and cleaned it off - it was filled with dirt - that I said, 'Oh my God,'" Knecht said. "If you think about it, imagine someone asking you today to get the impression of a dragonfly in the mud.

"You'd have to follow the dragonfly all day to see where the dragonfly set down to leave a perfect impression in the perfect conditions. You could be out there all day and never find one. That's what makes this fossil record really rather special."

The fossil will be used not only to study the type of insect that left it, but also to provide details about the world in which it lived.

"Most of the fossils we have of insects are only of the wings," Knecht said. "This fossil doesn't show the wings, but we can see the rest of the body plan. So, we'll be able to compare it with insects today and plug in the evolutionary clues to tie in it with the rest of the ecology of this region."

Knecht and Benner presented their insect fossil find last month at the Second International Congress on Ichnology, in Krakow, Poland. The team will present other trace fossils from the site, including tracks of amphibians and precursors to reptiles, later this month at the Annual Meeting of the Geological Society of America in Houston.