Researchers from China and the United States have found fossil remains of what appears to be the oldest and most primitive flowering plant ever discovered, an aquatic progenitor of today's waterlilies that lived in northeastern China at least 125 million years ago.

The delicate imprints are in excellent condition on a slab of stone more than two feet long. They offer the best clues yet to how plants made one of the more extraordinary leaps in evolutionary history: the transition from primitive spores and seeds, such as those still used by ferns and pines, to the more sophisticated use of flowers and fruiting bodies.

That transition marked the beginning of a co-evolutionary pas de deux involving flowers and insect pollinators, which led to an eruption of new plant species and ultimately helped carpet the planet with today's bouquet of floral diversity.

The newly found fossil offers the first pictorial representation of how nature engineered that seminal advance, which Charles Darwin called an "abominable mystery."

It is forcing scientists to change several conceptions about the origins of flowering plants, or angiosperms -- a taxonomic group that includes not only plants commonly recognizable as flowers but also many of the crops upon which humans and other animals depend.

"I really think this is the most significant fossil angiosperm ever discovered," said William Crepet, chairman of plant biology at Cornell University.

On the basis of previous fossil findings, many scientists had thought that the earliest flowering plants had woody stems, lived on land and had separate male and female flowers.

By contrast, Archaefructus, as the new fossil plant has been named, had softer, herbaceous stems, apparently lived in water (it seems too top-heavy to have supported itself in the air, and the slab bore fish fossils), and had flowers with both male and female parts, though at some distance from each other.

Perhaps most counterintuitive, the flower had no petals, which are widely perceived today as a hallmark of flowerdom.

"It's a pre-flowering flowering plant," said David Dilcher, a University of Florida paleobotanist and co-author of the new report in today's issue of the journal Science.

The plant was undoubtedly a flower by scientific standards because the seeds developed in fleshy ovaries, which offered a level of protection unavailable to the "naked" seeds made by more ancient plants.

But the sophisticated idea of having male and female parts all wrapped up in a ring of insect-attracting petals had not yet occurred in the early Cretaceous period, when Archaefructus took root in the mud of shallow ponds and extended those first tentative flowers above the surface.

Instead, the plant's thin, upreaching stems bore female flower parts on their tips, and the pollen-producing male organs sprouted from the same stems a few inches below. Pollen was probably transported upward largely by the wind, Dilcher said.

Chemical dating indicates the fossil is 125 million to 144 million years old. The team used a novel approach involving DNA analyses of living plants and structural comparisons of those plants against the fossil image to find Archaefructus's place in evolutionary history.

The fossil of the plant Archaefructus is at least 125 million years old.