Published online 23 September 2010 | Nature | doi:10.1038/news.2010.488

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Fossils reveal the origin of the largest group of flowering plants.

The 47-million-year-old fossils are unlike any living member of the daisy family. Science / AAAS

The family that includes daisies, sunflowers and dandelions is the most widespread and diverse of all flowering plants, but its fossil record has remained scant. Now a team from Argentina has reported the first discovery of a large fossil that unmistakably hails from this group.

Complete with large flower heads, leaf-like structures and slender stems, the remains come from rocks in Patagonia that are 47.5 million years old, dating from the Middle Eocene. They were discovered by an amateur fossil-hunter in 2002, and are described by a team led by palaeobotanist Viviana Barreda, of the Argentine Museum of Natural Sciences in Buenos Aires, in the journal Science1 this week.

Genetic comparisons of living plants had suggested that the family, the Asteracaea, originated about 50 million years ago in southern South America2. But without fossils, molecular evidence is a shaky basis for determining a lineage's origin, says Alan Graham, a palaeobotanist at the Missouri Botanical Garden in St Louis.

"We've been guessing at this age, but we had no data to confirm it," says Vicki Funk, a botanist at the Smithsonian Institution in Washington DC, who uses DNA evidence to reconstruct evolutionary history. "Now we finally have hard evidence."

Blossom bounty

The flowers are several centimetres across. The researchers classified the plant remains as Asteraceae based on the grouping of flowers, the presence of hair-like projections among the flowers, and the overlapping arrangement of leaf-like structures at the base of the flower clusters.

The plant doesn't look much like any living member of its family. The authors suggest that its closest living relatives belong to a group called Mutisieae, which includes the popular ornamental daisy Gerbera. Funk says the fossils superficially resemble Brazilian members of a group called the Stifftieae.

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The team also unearthed spiny pollen grains typical of the family. Thick-walled pollen grains are tougher than flowers, and fossilize more easily. Asteraceae pollen has previously been found in South America, but in younger rocks, Barreda says.

Although scientists now know what early members of the family looked like, the reasons for the plants' success remains a mystery. The authors speculate that the Asteraceae, which today contains about 23,000 species and lives on every continent except Antarctica, arose on the supercontinent Gondwana before it split into South America, Africa and Australia.

The shape of the fossil flowers, and the tropical climate in which they lived, makes Barreda and her colleagues suspect they were pollinated by hummingbirds. They are now looking for the fossils that might confirm this hunch.