When Giants had Wings and six Legs
By HENRY FOUNTAIN
Published: February 3, 2004
There was a time when giants roamed the Earth.
No, not those giants, the dinosaurs that stomped and slogged their way through the Mesozoic Era. These giants crawled and crept, slithered and scurried, burrowed, slinked, skittered and, above all, flitted and fluttered millions of years before the dinosaurs arrived.
They were the giant arthropods of the Carboniferous.
There were extra-large mayflies, supersized scorpions and spiders the size of a healthy spider plant. There was an array of giant flightless insects, and a five-foot-long millipede-like creature, Arthropleura, that resembled a tire tread rolled out flat.
But perhaps the most remarkable of all were the giant dragonflies, Meganeuropsis permiana and its cousins, with wingspans that reached two and a half feet. They were the largest insects that ever lived.
These large species thrived about 300 million years ago, when much of the land was lush and tropical and there was an explosion of vascular plants (which later formed coal, which is why the period is called the Carboniferous). But the giant species were gone by the middle to late Permian, some 50 million years later.
Scientists have long suspected that atmospheric oxygen played a central role in both the rise and fall of these organisms. Recent research on the ancient climate by Dr. Robert A. Berner, a Yale geologist, and others reinforces the idea of a rise in oxygen concentration - to about 35 percent, compared with 21 percent now - during the Carboniferous. Because of the way many arthropods get their oxygen, directly through tiny air tubes that branch through their tissues rather than indirectly through blood, higher levels of the gas might have allowed bigger bugs to evolve.
But there are other possibilities - a lack of predators, for example. Fundamentally, no one is certain why there were giants.
"It's been out there in the literature for a long time without a causal mechanism,"
said Dr. Robert Dudley, a professor at the University of California at Berkeley who has studied the effects of elevated oxygen pressures on modern insects.
"This is a very imperfect science. There's a very fragmented paleontological record."
Dr. Jon F. Harrison, a professor at Arizona State who has performed similar studies, said,
"It's still in the realm of speculation."
While there has been much interesting research, he added,
"it doesn't prove anything yet."
Some scientists argue that these large species may have been nothing out of the ordinary, that, in effect, they may not have been giants at all.
Dr. David Grimaldi, a curator in the division of invertebrate zoology at the American Museum of Natural History and co-author of a forthcoming book on the evolution of insects, noted that most Carboniferous insects were of very similar size to those found today. But the fossil record tends to be biased toward larger specimens for the simple reason that they are easier to find.
Though about a million insect species now exist, Dr. Grimaldi added, over about 75 million years of the Carboniferous, as species came and went, there were bound to be many more. So the largest species may simply represent the upper range of a far more diverse population.
"If you increase the sampling over millions of years, to some extent you are bound to encounter some giants,"
Dr. Grimaldi said.
Still, the idea that there were bugs larger than anything to be found today captures the imagination, particularly the idea of a dragonfly with wings as wide as some hawks' (though much less substantial), plucking smaller prey out of the air as modern dragonflies do.
For a long time, scientists believed that an insect of that size must have been able only to glide, but most now believe that the giant dragonflies actually flew.
"It's pretty obvious that they were active fliers,"
said Dr. Roy J. Beckemeyer, a retired aeronautical engineer in Wichita, Kan., who has studied modern and fossil dragonflies for a decade. Dr. Beckemeyer says he is fortunate to live where he does because many of the best fossil insect specimens come from deposits along ancient bays in what are now Kansas and Oklahoma.
One of his specialties in aeronautics was wing flutter, the relationship between bending and twisting that in the worst of circumstances can cause an airplane's wings to fall off. Modern dragonflies, he said, bend and twist their wings, giving them both loft and propulsion.
There are similarities in the corrugated structure of ancient and modern dragonfly wings, Dr. Beckemeyer said, though in modern species the twisting occurs in the outer half of the wing.
"In ancient dragonflies, it appears there was a more gradual twisting over the whole length,"
he said.
"It's likely that they didn't fly as fast."
Even slow flight for an insect that big, however, requires much muscular activity, which creates heat. Dr. Michael L. May, an entomologist at Rutgers, was the first to show that the ancient dragonflies must have had some way to dissipate the extra heat.
"If they didn't, they would have cooked themselves,"
Dr. May said.
Modern dragonflies, like other insects, pump a fluid called hemolymph throughout their bodies. When they get too hot, they can increase the flow of hemolymph to the abdomen, taking the heat away from the tissues in much the same way a car's cooling system carries heat from the engine. The abdomen, which is long and skinny, can dissipate the extra heat through convection.
Although there is no direct evidence, Dr. May said it was possible that ancient dragonflies had a similar system, enabling them to fly for longer periods without overheating.
The lack of evidence - with fossils, generally only the skeletal tissues are preserved - presents problems in figuring out just how these large species were able to exist. But more certainty surrounds the way the oxygen content of the prehistoric atmosphere changed over millions of years.
One way Dr. Berner and his colleagues study oxygen levels is by looking at a different element, carbon, in ancient sedimentary rocks.
"The guiding light in all this is the burial of carbon,"
Dr. Berner said.
Photosynthesis takes carbon dioxide out of the atmosphere and converts it to oxygen, which is released, and organic matter, which is incorporated in the plant. Plants die and decay and are buried, Dr. Berner said,
"and for every carbon you bury, you leave an oxygen behind."
So during the Carboniferous, as plants spread on land, there was less carbon dioxide and more oxygen. Dr. Berner calculated that oxygen concentration reached its peak of about 35 percent 300 million years ago. It declined abruptly at the end of the Permian, about 250 million years ago, a time of the greatest mass extinction in the planet's history. (The cause of the decline, and of the extinctions, is a subject of much debate.)
To determine whether all that extra oxygen could have led to giant dragonflies and the like, Dr. Harrison, Dr. Dudley and others turn to modern insects.
Insects "breathe" through holes in their bodies, called spiracles, which are attached to hollow tubes, or tracheas. The tracheas branch into smaller and smaller tubes, and the oxygen diffuses through them, nourishing the extreme reaches of the insect's body.
At current oxygen levels, there is an overall length limit of these tracheal tubes; beyond that, the oxygen level is inadequate. This effectively limits insect size.
One approach, Dr. Harrison said, involves determining whether it is harder for larger insects to get oxygen. If this is true, he added, higher oxygen levels are a benefit to them, and it can be argued that larger insects have had an evolutionary advantage in a high-oxygen atmosphere.
But Dr. Harrison said most of his experiments with grasshoppers and dragonflies do not really support the idea that raising the oxygen level makes a difference.
"You've got all the oxygen you need already,"
he said.
For one thing, he noted, larger insects do not breathe through passive diffusion only. There is some pumping that creates pressure differentials that cause air to actually flow through the tubes, reaching farther than by diffusion alone.
Other research, however, has shown that there is some effect of greater oxygen concentration on the size of an organism. Studies of marine invertebrates, for example, have found a correlation between larger species and colder, more oxygen-rich waters: the more oxygen in the water, essentially, the bigger the creatures get.
Dr. Dudley and others have conducted experiments raising fruit flies and other insects in oxygen-rich environments. Some have shown size increases; others have not.
Dr. Dudley has focused on pressure because, in addition to having a higher concentration of oxygen, the Carboniferous atmosphere would have had much more of the gas.
"Plants were pumping oxygen into the atmosphere,"
he said. The amount of nitrogen would have been undiminished, so overall pressure would have risen.
Though the results have yet to be published, his experiments with fruit flies raised under elevated pressures show a 20 percent increase in body mass over five generations.
But why would more oxygen make for bigger insects?
One idea, Dr. Harrison said, is that oxygen may be a trigger for molting. Before they shed their skin, Dr. Harrison said, invertebrates generally double their weight. During this period, their spiracles and tracheas are of pre-molting size, but they could use much more oxygen to grow. So the ancient atmosphere would have provided more oxygen during molting, enabling greater growth.
"That might be a mechanism that would explain this,"
he said.
Or it may not. For so much remains unknown about these giants. There may be plenty to suggest that oxygen played a role in their evolution, Dr. Dudley said,
"but it's real difficult to take it one step further."
