Stories in Stone Read From Ancient Leaves
A Smithsonian scientist studies the relationship between Eocene insects and the plants they ate
As leaves go, this one certainly does not look like a keeper. The alder leaf is full of holes caused by a marauding insect and is not the sort you would collect and press between the pages of a book, even if it weren't stone — a 48-million-year-old fossil. But it is precisely the ugly, insect-munched holes in the object at hand that fascinate Conrad Labandeira, of the Smithsonian's National Museum of Natural History.
Labandeira is curator of fossil arthropods, which include fossil insects, and he studies insect variety, distribution and activity going back millions of years in the fossil record. Labandeira's brand of science often requires him to collect not only the good specimens, but the bad and, in the case of insect damage, the ugly. In fact, sometimes they're the best.
One of the key places to go to collect insect-nibbled samples of the fossil record is the small town of Republic, east of the Cascade Mountains in Washington State. "All major types of herbivory appear to be present at Republic," says Labandeira, speaking broadly of plant eating. "The fossils go about as far back as we can go in the fossil record and still record insect damage on the standard, or closely related, genera of plants that occur today."
Republic dates back to the Eocene, the epoch when the earliest relatives of many of today's familiar plants took root. What's more, Republic was a crossroads between highlands and lowlands, warm climate and cool, north and south. Plants thrived together there that have not been found together anywhere else. It was also where many modern plant lineages that attract predatory insects made their earliest appearances, placing the Republic flora among the oldest in which modern plant-insect associations can be traced in the fossil record.
That is why, one recent fall, Labandeira and Kirk Johnson, curator of paleontology at the Denver Museum of Natural History, headed out to Republic, joined by Wesley Wehr, affiliate curator of paleobotany at the Burke Museum of Natural History and Culture in Seattle. Together they collected samples from the more than 300 types of leaves, cones, fruits, seeds, flowers and insects to be found there.
Tlick-tlick-tlick, tlick-tlick-tlick. Beneath a candy-striped canopy, more than a dozen people with hammers and chisels work Boot Hill, an unassuming pile of rock and dirt that lies at the north end of Republic. They are "Leaf Whackers," volunteers from Johnson's museum. With others from the Burke, they split open slabs of outcrop as quickly as a shale-prying colleague can deliver them, the layers dried and loosened with a weed burner that looks like a flamethrower. This is the first large-scale, systematic dig at Republic.
Johnson's eyes are locked on a just-delivered rock. "Look at the beautiful angiosperms. That's the fun stuff, this forest coming up out of the ground, hearing the oohs and aahs." He passes the fossil to two Leaf Whackers at a nearby picnic table, who catalogue it on the spot for the Denver Museum's collection. Johnson describes this as the "blitzkrieg method — we bore into the hill, and we get years of data in one week. Bit by bit the various fossils check in. To get this many samples would take one person two years." By the time the five-day dig is up, they will have chipped out and logged more than 1,500 fossils, far more than the minimum number of samples required for an unbiased reconstruction of the Republic flora, the plants represented pretty much in the proportions in which they lived.
Over the past two decades, Wehr has brought legions of other paleontologists to Republic — and taken pieces of Republic to them. Among the more than 3,000 Republic specimens Wehr and his assistants have gathered for museum collections are the first known occurrences of fir, apple, blackberry, holly, horse chestnut and other plants with living relatives.
None of this interest in Republic would have been piqued had not Wehr and Johnson come over the mountains from Seattle in 1977, when Johnson was only 17, to see what they could find. The route they followed from Seattle is an imitation of how geological events literally made Republic what it was. Like many visitors, they came to the town by way of the North Cascades Highway. East of the mountains, the first town Johnson and Wehr hit was Winthrop. One hundred million years ago, Winthrop would have been on the east coast of a big island, the North Cascades subcontinent, which was banging into North America. The land Wehr and Johnson were headed for, the Okanogan subcontinent, where Republic roosts today, had already merged into North America.
As the North Cascades and Okanogan subcontinents collided, what was left of the ocean floor between them was pinched against and beneath the Okanogan. The pressure caused the Okanogan to bulge, raising the elevation perhaps as high as 4,500 feet above sea level.
Volcanoes form when continents collide. Some of them near Republic erupted, raining ash on the lake, burying and preserving leaves and anything else around in its mud. The ash stamped a date on the lake bed, placing it in the mid-Eocene, about 48 million years ago, a time in earth's history when the greenhouse-gas carbon dioxide was high. There were no land masses to block an equatorial ocean current that warmed the planet. There was no polar ice, and life flourished — all over. Alligators and turtles swam in streams flowing into the Arctic Ocean, and temperate forests stretched above the Arctic Circle and across the Northern Hemisphere.
Plants that did well at higher elevations — pine, spruce, hemlock, golden larch, cedar and other conifers — thrived at Republic beside maple, sycamore and other broadleaf trees that stretched across the Northern Hemisphere. Add to the understory more than 30 members of the rose family, clematis, hydrangea, grape and other plants, and the panorama begins to get crowded.
Wehr and Johnson didn't expect to find much there. Republic was little more than a footnote in the annals of Washington geology; only a dozen or so specimens worth mentioning had been found early in the century. "I hit a rock with my foot," Johnson says, "and boom, Metasequoia" — dawn redwood.
Republic today is special in another way: the Boot Hill site is open to the fossil-hunting public. This is particularly unusual in an age in which most ancient natural treasures are protected. From May through October, anybody who ventures here may get a permit and rent a gunnysack with a hammer and chisel from the Stonerose Interpretative Center, a museum and gift shop a few blocks from Boot Hill. Like trout fishermen, fossil hunters here have a limit of three keepers a day.
This outcrop greeted more than 8,000 visitors in 1997, one of the Stonerose Center's busiest years since it opened in 1987. The locals are proud of Republic's reputation as a garden of stone. Local companies lend earthmoving and other equipment to visiting scientists. The Republic News-Miner carries front-page stories about Stonerose events.
At the site, Stonerose curator Lisa Barksdale checks fossil-hunters' finds to ensure that something new and of scientific value doesn't shuffle off to Buffalo. Usually, people are happy to turn over an old leaf. Once, the National Museum of Natural History put out an urgent call for an Eocene beech leaf — it needed one for an exhibit. The next day, two boys rented hammers and returned to the interpretive center with one in hand. They were thrilled that their rock was going to the Smithsonian.
Sometimes, though, it can get ugly. A gold-rush mentality sets in, Barksdale says. "Once a lady screamed at me, ‘You confiscated my fossil!'"
The Republic dig was just the beginning for Conrad Labandeira. After Johnson definitively IDed the week's haul, the fossils, along with Republic fossils from other digs, were shipped to Labandeira's lab at the Smithsonian to be photographed so that insect leaf-damage could be computerized and measured. Labandeira's pre-analysis educated guess: that the fossils' insect damage would claim between 3 and 5 percent of the plant material, toward the high end of what he's seen in the fossil record. (It actually turns out to be only 3 percent, not counting conifers, which tend to be bug resistant.) Should you be keeping score at home, the insects are winning. Today they claim 7.5 to 12 percent of our foliage.
Along with tallying the damage, Labandeira says, "there's a qualitative element, too — a question of who's doing what to whom." Paleoentomologists associate different damage with a variety of strategies that may change as plants and insects coevolve. Based on what the Leaf Whackers have collected so far, and on Labandeira's inspection of the Burke and Stonerose collections, which are also part of his study, "much of the damage is host-specific."
Some damage is obvious and familiar to anybody who has ever looked casually at leaves and noticed that some look like they have had holes punched in them, or has seen a tree denuded of its foliage by caterpillars. This is external feeding, "by far the most abundant feeding strategy in the fossil record," says Labandeira.
Then, in keeping with Republic's gold-rush past, there are the internal feeders, or leaf-miners. As fly, beetle, moth and other tiny larvae eat their way through a leaf's innards, they leave typical patterns that look like minuscule mole paths. Some species like to chew along the middle vein of a leaf; others will meander along the margin and leave serpentine trails.
Damage also can be quite literally galling. A gall is a tumorlike nodule made by the plant in response to infestation. A culprit at Republic appears to be the gall midge. "It's an arms race," Labandeira says. "Plants make noxious compounds that increase in intensity; the insect might be slowed down, but it adapts. In the case of the gall, the plant is co-opted — it secretes compounds that promote anomalous tissue growth. The insect lives in the gall and lays its eggs right in the plant tissue.
"The salient Republic issues are: Can any of this damage be matched taxonomically with damage on living relatives of the Eocene plant hosts? If so, what is the biological meaning of coevolutionary stasis for 48 million years? Are there good examples of host-shifts? If so, are the modern hosts taxonomically or otherwise chemically similar to the Republic Eocene hosts? How does any of the data support or refute any of the theories regarding how insects and plants coevolve in time?"
Labandeira worries that paleobiologists may not be sampling much of the Eocene herbaceous vegetation. Herbaceous vegetation is at the receiving end of much of the herbivory in modern communities. "What's more, we have only several data points from the fossil record from different vegetational settings; most likely we are not seeing anything of a robust signal in terms of the overall Phanerozoic trend dating back 310 million years."
This is Labandeira's way of saying he can't tell from Republic specimens whether insects are consuming more leafy matter now than they did 48 million years ago. Perhaps they are, but the available evidence could also mean that an unknown portion of the insect-damaged leaves that could decide the question one way or the other simply was not fossilized. What we do know for certain is Eocene insects at Republic were choosing the plants they consumed, and that the variety of damage qualitatively matches that of today. Republic seems to have been a modern but less intense world.
In the end, Labandeira and his fellow geological time travelers are like leaf beetles attempting to chew off more of the fossil record. Every once in a while, they connect with a Republic and, with a few more precious bug-damaged leaf assemblages, come a little closer to figuring out how the world we know got to be this way.