Boundaries
by Jack Nisbet

Jack Nisbet is the author of The Mapmaker's Eye, Sources of the River, Purple Flat Top, Singing Grass, Burning Sage and Visible Bones. His newest, The Collector, won a 2010 PNBA Award.

One on One
July 2010

The first time anyone goes to the Stonerose fossil site in Republic, they grab a slab of 50-million year old Eocene shale, crack it open with a hammer, and stare in amazement at the figures embossed on each freshly-exposed surface. They touch the blackened compressed images of what a Stonerose veteran patiently explains to be sticks or charcoal fragments. The newcomer tosses that aside, cracks open a second sample, and looks again. This time, or the next, or the next, something new leaps out of the stone. One viewer's eye might catch the regular geometry of a tiny seed or the line of a needle from a coniferous tree; another's brain might register the clear veins of a familiar leaf or a delicate insect wing. The search is on, and anyone who has ever dragged a kid kicking and screaming from the Republic rubble at sunset on a hot summer day knows that there is some real emotional attraction about seeing such instant photographs of the deep past.

But what happens after that initial thrill wears off? To scientists interested in relationships between living things, each discarded fragment of Stonerose material has something to say about both the past and the present. In the late 1990s, scientists from the Denver Museum of Natural History and the University of Washington's Burke Museum, aided by a group of dedicated Stonerose fans, carried out what is called a ³stratigraphic² dig on the Boot Hill site. After carefully laying out a square and cleaning off the surface, the crew systematically dug through a vertical section of the fossil-bearing material. Every slab was split and examined; any fossil larger than 1 centimeter was catalogued and correlated to its depth along the vertical shaft. These fossils slowly reassembled a lake's shoreline over the astonishing span of 2 million years, and paleobotanists from all over the world have examined the results to add their own small tiles of data to the grand mosaic.

Entomologist Conrad Labandeira works with the Museum of Natural History at the Smithsonian Institution in Washington D.C. Although relatively few species of whole insects appear among the Stonerose fossils, Labandeira and others realized that, just as with any flora examined today, a large percentage of the leaves and needles in the Stonerose collection show insect damage of various kinds. Such evidence, cataloged and related to the kinds of insect work seen on modern plants, could provide clues about age-old relationships.

Insects and plants have shared something like 400 million years of time together on planet Earth, and during that span have developed associations that range from destructive (think of moth larvae chewing the broccoli crowns in your garden) to beneficial (think of bees pollinating the bean flowers two rows down). In his preliminary paper on what might prove to be an endless research project, Labanderia cataloged over 40 separate associations between insects and plants from the Stonerose samples. The first and most obvious category focused on bites along leaf margins, and one fossil that showed such chewing damage was a fan-shaped leaf from a ginkgo tree.

Ah, the legendary ginkgo. Although it can't survive quite as far north as our part of the Columbia country, the ginkgo is an ancient and attractive species often planted as an ornamental in North America. What garderners reckon as the oldest ginkgo on the continent, planted before the Revolutionary War in Philadelphia, is still growing strong, but the ginkgo story spans far more time than mere centuries.

Ginko leaf fossils date back to the Permian period, something like 270 million years ago. Many different ginkgo species occur with fossil dinosaur bones all around the world. Only a few of these ginkgos survived until the Eocene, and the trees represent only a tiny fraction of Stonerose fossils. But they must have continued to survive, because ginkgos leaf fossils appear in the Latah clays around Spokane, which preserves forests that lived about 15 million years before the present. ginkgos finally disappear from the North American fossil record around 7 million years ago, and from European sources about 2.5 million years before the present. The first scientists to study the tree were certain that the ginkgo existed only as a long-gone relic of the dinosaur era.

Then in 1691, a German botanist discovered ginkgo trees growing around a monastery in Japan. Ensuing centuries of detective work have developed the theory that a few wild populations of single species, ginkgo bilabo, must have somehow survived the ages in southwest China. Buddhist monks recognized the wild tree's many medicinal properties, and as early as 1100 A.D. began to cultivate ginkgos around their dwellings. From there it was only natural for round ginkgo seeds to cross the water to Japan, and maintain their amazing run as a living tree.

But of course there's more to the story than that. The single ginkgo leaf fossil found at Stonerose shows insects bites that take the form of exceptionally precise circles and arcs, cut to a very specific size. The marks are characteristic of Megachilidae, a family of leafcutter bees.

The Megachilidae are solitary bees that lack the kind of complex social organization we associate with honeybees. They are a common, diverse group that ranges around the globe, and many of the species in our region serve as important pollinators of native plants. Leafcutters are smaller than bumblebees, and show metallic black or bluish colors that radiate in the sunlight. They carry their pollen on stiff hairs protruding from the underside of their bellies rather than on their hind legs like most of their cousins. Leafcutters do not sting unless handled, and even then carry only a mild jolt.

They also bite: leafcutter bees come equipped with strong sawblade-shaped jaws that they use to snip large discs from the leaves of certain plants. Oval discs are put to use lining burrows in punky wood; the perfectly round ones make cell partitions to separate their eggs.

Unlike the mandibles of carpenter bees and carpenter ants, leafcutter bee jaws are not strong enough to damage healthy lumber; in fact, they seem to prefer leaves of a certain texture and thickness. Roses are one of their favorites. If you have ever wandered through a rose garden wondering why some of the leaves show precise, dime-sized holes, you are bearing witness to the art of leafcutter bees.

So one Stonerose fossil proves that 50 million years ago, at least one species of leafcutter found ginkgo leaves to its liking. But here's the rub: there are few current accounts of modern ginkgo trees with these distinct circular cuts, and none at all from the fossil record. At least for now, that single Stonerose ginkgo leaf represents the only known fossil example of a leafcutter bee working on the species. Conrad Labandeira figures that some species of leafcutter bee co-evolved with an Eocene ginkgo in our part of North America, made use of its luscious green leaves for some undetermined amount of time, and then went extinct. The ginkgo survived.

One leaf, marked by the scissoring jaws of a single artistic bee. Perhaps what Stonerose really tells us is that the story of the North Columbia country will continue to unfold - in the past, during our own brief residence, and far into the future. And it will do so in ways that we could never imagine.



Thanks to Jan Hartford for help with this article.

To find out more about the Stonerose Interpretive Center, go to www.stonerose.com.

Conrad Labandeira's paper on Stonerose insects appeared in Rocky Mountain Geology vol. 37 no. 1 (June 2002).

Illustration by Emily Nisbet

Jack Nisbet Homepage