Rocks yield clues to flower origins.

Organic chemicals trapped in rocks for more than 240 million years may now provide a clue to a biological riddle that has stood since the 19th century: Where did flowers come from?

Flowering plants suddenly appeared in the fossil record about 130 million years ago. The absence of progenitors in that record posed a puzzle that Charles Darwin referred to as "an abominable mystery." Now, geochemist J. Michael Moldowan of Stanford University and his coworkers say they've found a chemical signature of flowering plants in ancient fossil-bearing rocks.

Many modern-day flowering plants produce a family of defensive compounds called oleanenes, which work against insects, fungi, and various microbes, says Moldowan. Other seed-bearing plants with more ancient lineages, such as pines and ginkgoes, don't generate these chemicals. When oleanene-producing plants fossilize, the chemicals are transformed into a related substance called oleanane. The compound is common in sediments containing flower-bearing fossils.

Moldowan and his colleagues, who presented their findings on April 2 in San Diego at the spring meeting of the American Chemical Society, found that oleanane was absent from many ancient sediments. However, the chemical was present in oily sediments that contained fossils of gigantopterids, an extinct type of shrub that evolved more than 245 million years ago.

The researchers say the presence of oleanane in the sediments that contain gigantopterids probably indicates that the ancient plants were some of the first to produce oleanenes. This is a sign that they were among the earliest long-lost relatives of flowering plants, says Moldowan.