John L. Cisne/Cornell University
Size measurements of mayfly wings and flight muscles indicate similar atmospheric densities in the Permian and Cretaceous periods and in today's atmosphere.
Millions of years before humans invented the barometer to measure atmospheric pressure, a primitive winged insect was experimentally measuring air's density and leaving barometer readings in the fossil record, according to a Cornell geologist.
Because that insect -- the common mayfly -- has persisted with little change since its appearance some 300 million years ago, scientists can use it to estimate the mass and composition of ancient atmospheres, John L. Cisne, professor of geological sciences, told the annual meeting of the American Geophysical Union in San Francisco Dec. 14.
"Mayflies work like little helicopters as they dance up and down in a mating swarm," Cisne said, explaining how measurements of a fossilized insect's body can reveal information about the air in which it once flew:
Citing evidence (plotted in the diagram) that the wing's relative size is practically the same in fossils from the Permian and Cretaceous periods and in modern forms, Cisne concludes: "The atmosphere's mass must have been practically the same for the last quarter-billion years, at least so far as mayflies can tell us."
Members of the insect order Ephemeroptera, with some 2,000 species, mayflies are found around the world in freshwater habitats. They are much beloved by fly fishermen, who know that a fresh hatch of mayflies means trout will be biting and deploy their artificial lures (called flies) accordingly. Maturing into adults after a year or more as water-dwelling larvae, mayflies perform a mating dance by beating their wings 20 to 30 times a second to rise in the air. They rest their muscles and "parachute" downward, then fly upward again and again in hopes of finding a reproductive partner.
Geologists have known from qualitative indications in rocks that Earth's atmosphere was, at certain times, quite different from today's. During the first half of the planet's history, for instance, the atmosphere contained almost no oxygen. Until now, however, the only way geologists could quantitatively gauge atmospheric pressure in the distant past was to measure the size of gas bubbles frozen into lava.
Without more exacting evidence, such as "barometer readings" the mayflies promise to provide, many questions have gone unanswered:
Cisne said he hopes that further study of mayflies and other fossil insects will provide more definitive answers to geologists' questions.
"Mayflies are particularly good subjects because their flight mechanism is relatively simple and easy to characterize and because they are a very ancient group," he said.
A scientist who studied arthropod anatomy before concentrating on geology, Cisne has long been interested in indicators of primeval conditions.
"I was looking out in the backyard one evening, watching the jitterbug dance of the mayflies," he says, "when it hit me: These remarkable little insects are experimentally measuring the density of the atmosphere. And they have been doing it for millions of years."
The idea didn't come to him while fly-fishing. Cisne doesn't fish.
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