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The discovery of some fossils along a road construction site here has opened a wider window on the natural history of the nation's first national park.

The finds last week also have obstructed progress on an eight-year project to rebuild the park road between Fishing Bridge and Yellowstone's east entrance.Work has stopped on the segment of road where construction workers found fossilized leaves last Monday, and for now, crews have diverted equipment to other portions of the road. A delay of more than a week could seriously affect the project, said Eleanor Williams, a Yellowstone landscape architect.

"Right now, we have to determine how significant these deposits are," Williams said last week while touring the area with fossil experts from the Smithsonian Institution and Georgia State University.

Authorities did not consult paleontologists when planning the $35 million road reconstruction that began this spring.

"Lots of people went up and down this road looking for things to watch out for, but they didn't have any idea this was here," Williams said. Fossil forests are nothing new in Yellowstone; the park holds groves of ancient trees that turned to stone after they were buried in volcanic debris.

There are plant remains along several miles of the east entrance road, which is the main route into Yellowstone from Cody, Wyo. Bull-dozers have knocked loose only a few fossils, and in only one spot.

"What's great is that someone noticed this before it was too late," said Scott Wing, a Smithsonian research curator.

Complete leaves from sycamore-like trees fell away when a bulldozer hit the fossil deposit Monday. Although such plants were common during the Eocene epoch, about 40 million to 50 million years ago, it is rare to find leaves preserved in their entirety, Wing said.

Rocks knocked loose during the construction also included a fossilized fragment of a palm frond, the first evidence that palms survived in Yellowstone's ancient forests when the climate was tropical.

"This flora tells an interesting story of climate change in the region," Wing said. "In these rocks we see a time when the global climate was much different than it is now. You don't see many palm trees growing in Yellowstone these days."