Middle Fork a world-class fossil bug collection

The Middle Fork of the Flathead River, renown for its whitewater rafting and scenic beauty, should also be known for its bugs. Fossilized ones, that is.

The Middle Fork is one of the best sites in the world for finding fossilized insects, according to Dale Greenwalt, a paleobiologist at the Smithsonian Institution’s National Museum of Natural History.

Greenwalt has been exploring the Middle Fork’s fossil collection for the past five years under a Forest Service permit. He’s working as a volunteer to discover and catalog insects that date back to when the region lay under a vast lake and temperatures were tropical.

The river has been eroding through the Kishenehn formation, revealing shales holding insect fossils — katydids, mosquitoes, wasps, cockroaches and damselflies.

“They’re arguably the best preserved fossilized insects in the world,” Greenwalt said during a talk in Glacier National Park last week.

Greenwalt gained 15 minutes of fossil fame last October when he determined that one fossilized mosquito still had blood in its belly. The story was written up in newspapers around the world.

The bugs are difficult to find and it takes a trained eye to see them. They’re harbored in shale just a millimeter or two thick, and the insects themselves are rarely longer than 5 millimeters — most are smaller than that.

Greenwalt said the insects don’t reveal themselves until the rock is wet, so he typically harvests a slice of shale, dunks it in water and then looks it over with a loupe.

The results are sometimes spectacular. Many of the fossils are so well preserved the individual veins on the wings and the hair on the legs are visible — even the colors are preserved in some cases.

The fossils provide scientists with clues as to what the climate was like eons ago. One cockroach sample was traced to a cockroach that, today, lives in the Caribbean. The fossils also provide clues about evolution, as the fossilized insects can be traced to modern ancestors.

“It provides us an opportunity to see what worked and what didn’t in evolution,” he said.

So far Greenwalt has gathered more than 15,000 individual fossilized insects. The goal is to one day create an online database.

In the meantime, he keeps exploring. He flies out each summer at his own expense and spends several weeks looking for fossils. This summer has proved challenging. A rock slide buried one site, and high water has made it difficult to access other sites.