When Giants Had Wings and 6 Legs
- When Giants Had Wings and 6 Legs
By HENRY FOUNTAIN
Published: February 3, 2004
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.
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