That's because the bodies of flying insects are usually not preserved due to their softer, fragile nature, he said. Scientists more often find only the remains of wings, which are not digested easily by predators.
The 3-inch (7.6-centimeter) insect that made this imprint likely stayed in the mud long enough to move its legs before flying off, leaving a near-perfect impression, Knecht said in 2008.
A sketch of the fossil imprint shows the directions that the insect's appendages moved (green arrows) while it was trapped in the soft mud, part of an ancient freshwater habitat in what is now Massachusetts.
Though insects evolved powered flight perhaps 90 million years before vertebrates, early fossil evidence of flight has been very poor—until now, according to the study. (See a prehistoric time line.)
For instance, the fossil may offer clues to how the ancient insect flew. One theory is that the insect skimmed above the ground before "docking" at the edge of a shallow pool and resting its weight in the mud, leaving the imprint. Another idea is that the insect may have glided or flown from above and directly plopped into the mud.
Although both hypotheses are possible, skimming may be less likely, the study authors say. That's because there's no evidence that early flying insects were aquatic—or had the associated modern behavior of skimming on the water's surface.
Illustration courtesy Richard Knecht, Tufts University
A male mayfly in the Ecdyonurus genus clings to a stem in an undated picture.
"If you look at a modern photograph of a mayfly, it would be the closest to the fossil one," Knecht told National Geographic News by email this week.
Knecht noted that mayflies usually sit with most of their abdomens touching the ground. "It is this behavior that has helped to create such an anatomically informative and complete impression," he said.