Aerial competition trumped the advantage of extra atmospheric oxygen
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When atmospheric oxygen levels were higher about 300 million years ago, dragonflies sported 19-centimeter wings like the fossilized one shown here. By about 12 million years ago, long after the rise of birds, the insects’ wings measured a mere 6.7 centimeters in length (superimposed drawing).Wolfgang Zessin
A prehistoric prequel to Godzilla took place about 150 million years ago, when insects of monstrous size met their doom battling the ancestors of modern birds.
The epic struggle ended an era of insect growth spurts that coincided with upticks in the amount of oxygen in the air. Starting with the Cretaceous period, predators kept the sizes of insects down, researchers report online the week of June 4 in the Proceedings of the National Academy of Sciences.
“That’s when birds evolved and started to become better at flying,” says Matthew Clapham, a paleontologist at the University of California, Santa Cruz. “Even though oxygen continued to increase during that time, the insects got smaller.”
Oxygen’s ability to supersize insects has been repeatedly demonstrated in the lab. Beetles and dragonflies that breathe oxygen-enriched air tend to grow bigger. Some scientists argue that the gas places an upper ceiling on growth, but only for some kinds of insects. Others believe it to be a universal fuel that boosts the metabolism of any insect.
To work out oxygen’s effects on prehistoric insects, Clapham and Santa Cruz colleague Jered Karr examined more than 10,500 fossil specimens from the last 320 million years. The researchers compared changes in maximum size over time to changes in atmospheric oxygen, as reconstructed from ancient sediments.
“There’s been a lot of theories about body size and oxygen, but the problem has been not having enough fossils to actually test some these theories,” says Wilco Verberk, an ecologist who studies insect physiology at Plymouth University in England. “This is the most comprehensive dataset gathered to date.”
Small creepy-crawlies were ubiquitous throughout the record. But the biggest of the big insects enlarged when oxygen levels rose and shrank when oxygen levels fell. About 300 million years ago, dragonflies sported wings comparable in size to those of a modern heron. At the time, oxygen made up more than 30 percent of the atmosphere — compared with about 21 percent today.
The largest insects shrank as oxygen levels declined during the Triassic and Jurassic periods. But grasshopper-like critters called titanopterans still boasted wingspans of up to 36 centimeters. Then about 150 million years ago, giant insects disappeared even though oxygen levels had started to rise again.
One explanation for the insects’ failure to regain their former glory is that an avian revolution had begun. Creatures like Archaeopteryx gave way to birds with greater maneuverability. Upgraded body plans featured smaller tails, specialized wing bones and giant breastbones.
Clapham speculates that the feathered newcomers devoured the oversized bugs. But it’s also possible that both types of large predators fed on the same small insects, and birds simply proved to be the better hunters.