End of big bug era

Flying predators probably gobbled up the biggest of the large flying insects 150 million years ago

a photo of a brightly colored dragonfly resting on a twig

Modern dragonflies (shown here in close-up) are much, much smaller than the flying insects that roamed the skies 300 million years ago.

André Karwath

About 300 million years ago, long before the first dinosaurs appeared, a different type of oversized critter inhabited Earth: giant insects. Scientists suspect bugs grew bigger then because the atmosphere contained more oxygen than it does now. For example: Wings of one ancient dragonfly measured almost as long, tip to tip, as a Little League baseball bat.

Alas, the giant insects didn’t last, and a modern dragonfly can fit comfortably inside a Wiffle ball. In a new study, researchers say the reign of mammoth insects ended when hungry, flying predators came along about 150 million years ago.

“That’s when birds evolved and started to become better at flying,” Matthew Clapham told Science News. Clapham, a paleontologist at the University of California, Santa Cruz, worked on the new study. “Even though oxygen continued to increase during that time, the insects got smaller.”

Paleontologists study prehistoric life. They often scour old rock, looking for signs of ancient life called fossils. A fossil may be a footprint or a skeleton or another trace of long-dead life; it’s an imprint in stone showing that some organism was once there. By determining the age of the rock, paleontologists can estimate how long ago the animal, plant or other type of living thing died.

a fossil of a dragonfly wing
The drawing at top left shows the wing of a modern dragonfly. The fossil shows the wing of an insect that lived 300 million years ago. Photo by Wolfgang Zessin

To investigate what happened to the giant beasties, Clapham and his colleague Jered Kerr, also at Santa Cruz, studied traces of prehistoric insects in more than 10,500 ancient fossils. They compared the size of insects with the amount of atmospheric oxygen at the time those insects had lived.

Bugs grew bigger when the air contained more oxygen, they found, and smaller in air with less oxygen. That observation is consistent with modern experiments showing the same effect: Insects grow larger when they live in an environment that has more oxygen.

But about 150 million years ago, something changed. Even though oxygen levels again started to rise, insects stayed small (and swattable). One possible explanation is that birds had evolved, or changed slowly over many generations, to become better hunters — and gobbled up the supersized bugs.

So next time you swat away a dragonfly, remember: 300 million years ago, it might have swatted you. And thank a bird.

Stephen Ornes lives in Nashville, Tenn., and his family has two rabbits, six chickens and a cat. He has written for Science News Explores since 2008 on topics including lightning, feral pigs, big bubbles and space junk.

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