Earth scientists assemble atop an ancient rift

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Author: Richard A. Kerr
Date: Nov. 13, 1992
From: Science(Vol. 258, Issue 5085)
Publisher: American Association for the Advancement of Science
Document Type: Article
Length: 2,373 words

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A Slow Start for the Flowering Plants?

Flower power was quick to win the world, most paleobotanists think. After the flowering plants began rapidly evolving into an array of new species about 100 million years ago, their energy-efficient life cycles and ability to attract wide-ranging insect pollinators presumably gave them such a marked advantage that they crowded out the once abundant ferns and gymnosperms--the conifers, ginkgoes, and palm-like cycads so often seen as the backdrop for dinosaurs. By 80 million years ago, in the conventional view, the ferns and gymnosperms had been pushed off center stage by the flowering angiosperms. But a bonanza of fossil plants recently found in Wyoming suggests that the angiosperms may not have been so quick to triumph.

The conventional view is based on fossil deposits that reveal the diversity of plant species but may not give a clear picture of each species' abundance. Now paleobotanist Scott Wing of the Smithsonian Institution has found the remains of a 72-million-year-old fern meadow dotted with palms--the first reported site where a head count could be made of fossil plants still rooted to the spot where they grew. The results of the census, presented at the meeting, show that flowering plants were indeed there in all their splendid diversity, but individual angiosperms were few and far between. It was the ferns that reigned in this field. If Wing's site is typical--and some paleobotanists aren't sure it is--the angiosperms' triumph must have been delayed until some external factor tipped the evolutionary scales.

Discovering this snapshot of an ancient landscape took two bits of luck. The first fortuitous event came 72 million years ago in what is now Wyoming, when volcanic ash sifted down on an open, fern-covered field. Only a few centimeters fell from the sky, but then a slurry of ash flowed across the field from higher ground and encased most of the plants to a depth of 30 centimeters. But even that would not have been enough to entomb the plants for good; what ultimately preserved them was the blocking of a stream that drained the wet, subtropical meadow, gently flooding it with water carrying another 4 or 5 meters of ash.

The second piece of luck came in 1990, when Wing happened to be driving along a Wyoming back road on Big Cedar Ridge, watching the roadside with a geologist's eye. Pulling off to see if a 5-meter-thick layer of volcanic ash-turned-to-clay might be dated isotopically, he dug into the base of the volcanic layer and "plants just started to fly out of the [rock]. There were plants anywhere you wanted to dig. It's the best-ever deposit for plants."

In order to date the site and analyze the flora, Wing enlisted geochronologist Carl Swisher of the Institute of Human Origins in Berkeley and paleobotanists Leo Hickey of Yale University and Robyn Burnham of the University of Michigan. In the completed census, the angiosperms lived up to expectations only in their diversity--not their numbers. Of the 146 species...

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Gale Document Number: GALE|A13217744