Prehistoric Birds Caused Smaller Insects

By Amir Khan on June 5, 2012 9:59 AM EDT

Giant Insect
An image of a 7.5-inch-long 300-million year old fossil wing from Stephanotypus schneideri, along with an illustration of the largest relatively recent insect wing for comparison, that of 12-million-year-old 2.6-inch-long Epiaeschna lucida wing. (Photo: Matthew Clapham)

Giant insects ruled the skies 300 million years ago, and if not for ancient birds, they would still today. The evolution of predatory birds brought the reign of gigantic insects to the end and caused them to shrink to their more familiar size we see today, according to a new study, published on Monday in the Proceedings of the National Academy of Sciences.

Prehistoric bugs grew to monstrous sizes. The griffinfly, an early ancestor of the dragonfly, was approximately the size of a crow and had a wingspan of over 2 feet. The ancient atmosphere, which contained 30 percent oxygen, compared to today's 21 percent, allowed the bugs to take in more energy per breath and power their bigger bodies.

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"When oxygen went up, insects got bigger. And when oxygen went down, they got smaller," said Matthew Clapham, study coauthor and  paleobiologist at the University of California, Santa Cruz, told National Geographic.

But around the time birds appeared, approximately 150 million years ago, something interesting happened, researchers said. Although oxygen levels continued to rise, insects stopped growing.

"Maximum insect size does track oxygen surprisingly well as it goes up and down for about 200 million years," Clapham said in a statement. "Then right around the end of the Jurassic and beginning of the Cretaceous period, about 150 million years ago, all of a sudden oxygen goes up but insect size goes down. And this coincides really strikingly with the evolution of birds."

As predatory birds appeared, insects needed maneuverability in order to prevent being eaten, which favored a smaller body size, according to study.

The maximum insect size changed again between 90 and 65 million years ago, according to the study, which can be attributed continued specialization of birds, the evolution of bats, and a mass extinction at the end of the Cretaceous.

"I suspect it's from the continuing specialization of birds," Clapham said in a statement. "The early birds were not very good at flying. But by the end of the Cretaceous, birds did look quite a lot like modern birds."

Clapham stressed that there have always been small insects and that the study looked at the maximum size they grew to at the time.

"There have always been small insects," he said. "Even in the Permian [300 million years ago] when you had these giant insects, there were lots with wings a couple of millimeters long. It's always a combination of ecological and environmental factors that determines body size, and there are plenty of ecological reasons why insects are small."

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