Giant Insects Ruled The Sky Until Evolution Of Birds Kicked In
June 5, 2012

Giant Insects Ruled The Sky Until Evolution Of Birds Kicked In

Giant insects in ancient days use to be kings of the sky, until the evolution of birds about 150 million years ago.

Scientists at the University of California, Santa Cruz found that despite rising oxygen levels, insects eventually lost their grip as the dominate species in the sky.

During the late Carboniferous and early Permian periods the skies were littered with 28-inch dragonfly-like insects and other ancient species that were too big to whack with a fly swatter.

The authors of the paper published in the online early edition of the Proceedings of the National Academy of Sciences found that the insects shrank in size not only as the oxygen levels increased, but as the evolution of birds began right around the end of the Jurassic and the beginning of the Cretaceous period.

They believe that as predatory birds began to take to the skies, insects had a need for smaller bodies for better maneuverability.

Matthew Clapham, an assistant professor of Earth and planetary sciences at UC Santa Cruz, said getting the data was not a simple task.

The researchers had to compile the dataset of over 10,500 fossil insect wing lengths from an extensive review of publications on fossil insects.

The team relied on the widely used "Geocarbsulf" model developed by Yale geologist Robert Berner.  They also repeated the analysis by using a different model, and still got similar results.

The study helps provide support for an effect on insect size from pterosaurs, which are flying reptiles that evolved in the late Triassic about 230 million years ago.

About a 20-million-year gap in insect fossil records made it hard for the team to tell exactly when insect size changed, and a drop in oxygen levels around the same time also complicated the research.

Another change in insect size occurred between 90 and 65 million years ago, but a lack of fossil records makes it hard to track the decrease in insect sizes during this period as well.

"I suspect it's from the continuing specialization of birds," Clapham said in a press release. "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."

According to Clapham, the study focused on changes in the maximum size of insects over time.

Average insect size would be more difficult to determine because larger insects are more likely to be preserved and discovered than smaller insects.

"There have always been small insects," Clapham said. "Even in the Permian 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."