Fossil pond plants bear tattoo of K-T crash.

The ancient remains of a humble lily pond in eastern Wyoming provide a blow-by-blow record of the cataclysm that wiped out a large fraction of existing species 65 million years ago, according to an expert on fossil plants.

Botanical evidence from the pond suggests a meteorite walloped Earth around early June, sending the northern hemishphere into an out-of-season freeze, says Jack A. Wolfe of the U.S. Geological Survey (USGS) in Denver. The climate quickly warmed, and then, within months, a second, smaller meteorite struck, he reports in the Aug. 1 Nature.

Such a detailed description of the long-ago event has sparked criticism from researchers who believe the story may contain more fantasy than fact.

Wolfe constructed his scenario after studying the plant fossils within a layered sequence of rocks at Teapot Dome, a site dating to the Cretaceous-Tertiary (K-T) boundary - the time some 65 million years ago went the last remaining dinosaurs and a number of other animal and plant species died off. Many researchers blame the mass extinction on climate disruptions caused by one or more meteorite impacts.

At Teapot Dome, Wolfe found water-lily and lotus leaves bearing distinctive irregular folds that he believes document the chilly aftermath of the impact. He suggests the crash lofted enough light-blocking debris into the atmosphere to freeze the pond, causing ice to develop inside the lily and lotus leaves. Because water expands as it freezes, ice inside the leaves would have buckled their outer cuticle. To test that hypothesis, he produced the same pattern of folds by freezing modern lily and lotus leaves.

Researchers have discussed possible climatic implications of a huge meteorite strike at the K-T boundary for a decade. But until now, none had found proof of a post-impact cooling says Wolfe. "This is the first physical evidence of actual freezing from an impact winter," he says.

Wolfe thinks the pond fossils contain enough information to pinpoint the approximate month of the impact. The Teapot Dome rocks contain seeds from the water lilies, but none from the lotuses, indicating that only the lilies had bloomed and produced fruit by the time they froze. Botanical evidence from eastern Wyoming suggests that around the time of the K-T boundary, lotuses bloomed in late June, Wolfe says, indicating the impact occurred earlier, near the beginning of June.

In previous work, Wolfe determined that a moist, superwarm period followed the impact winter. Teapot Dome rocks support that idea because fern spores lie atop the debris layer from the first impact. A debris layer higher up suggests a second, smaller impact occurred after that warm period started, only months after the first strike, he says.

Recent reports have focused attention on the Caribbean as the possible site of a K-T crash (SN: 11/17/90, p.319), and Wolfe proposes this could have produced the lower impact layer at Teapot Dome. The second layer could have come from a strike much closer to the pond. One candidate is the Manson, Iowa, crater - a structure researchers have dated to the time of the K-T boundary.

Like almost all research on the K-T boundary, Wolfe's work has excited considerable controversy. Many scientists, including some at his own office, think he has speculated too much.

"I think he's extrapolated an awful lot," says pollen expert Douglas J. Nichols, also with the USGS in Denver. "I predict that the scientific community will not accept this story; some people may even laugh at it." Geologist Bruce F. Bohor from the USGS says Wolfe's story does not fit with other K-T boundary evidence from around the world.

Wolfe contends, however, that most geologists and paleobiologists think about events on a geologic time scale and are not accustomed to considering what happens on a scale of months or weeks.

James A. Doyle, a paleobotanist at the University of California, Davis, agrees about that. While not totally convinced by the scenario, he says that "[Wolfe's] observations and story form a nice consistent whole. I don't see any major flaws in his hypothesis."