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Pollination in the pre-flower-power era

Scorpionflies may have aided plant reproduction long before blossoms evolved

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3:26pm, November 5, 2009
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An obscure group of scorpionflies with specialized mouthparts may have pollinated ancient plants millions of years before flowers evolved, a new study suggests.

Fossils indicate that before flowers evolved about 130 million years ago, most plants with seeds were wind-pollinated. Yet the pollen grains of some plants that lived in the prefloral era were too big to be wind-dispersed, say Conrad Labandeira, a paleoentomologist at Smithsonian Institution’s National Museum of Natural History in Washington, D.C. Also, he notes, pollen receptors were hidden deep within some of those plants and wouldn’t have been readily exposed to windborne pollen.

Now, in the Nov. 6 Science, Labandeira and his colleagues propose that an ancient group of scorpionflies might be counted among the missing pollinators of such plants.

The researchers analyzed 21 specimens of scorpionflies representing 11 long-extinct species, with body lengths ranging from 3 to 28 millimeters. Most of these insects were preserved in rocks laid down as fine-grained sediments, but one had been preserved in amber, says Labandeira. The fossil record suggests that these creatures were rare but present in Eurasia throughout a 62-million-year interval that began around 164 million years ago, well before flowers evolved, and stretched into the early evolution of blooms.

All of these scorpionfly specimens have long, siphon-like mouthparts capable of sucking liquids — in one case, the proboscis is about one-third the length of the insect’s body. Because pollen grains could be too large to fit through the slim siphons, the researchers suggest that the pollen stuck to ridges or hairlike structures on the creatures’ mouthparts or face as they fed on nutrient-rich fluids produced by the plants. Then, the insects carried the pollen from plant to plant as they foraged, just as modern-day pollinators do.

Labandeira and his colleagues didn’t find any pollen on or around the fossil insects they analyzed. “That was really disappointing,” Labandeira notes. But, he adds, the pollen may have decomposed or otherwise not been preserved in the sediments for any number of reasons. On the other hand, the amber that entombed one well-preserved scorpionfly didn’t contain any pollen, either — which probably reflects a true absence of pollen in that case, possibly due to entrapment of the insect at a pollen-poor time of year.

Grains of pollen preserved with such specimens would be the missing piece of evidence to definitively link these scorpionflies to the pollination of ancient plants, says Jeff Ollerton, an evolutionary ecologist at the University of Northampton in England. But he’s not surprised that pollen hasn’t been found. “Evidence for species interactions rarely fossilizes,” he notes.

On the other hand, David A. Grimaldi, an entomologist at the American Museum of Natural History in New York City, is surprised. “Pollen is really durable, and you’d think that if these insects were collecting pollen, there would have been some to be found,” he says. Nevertheless, he adds, the team’s new research “is a nice, thorough study.”

Other fossils of these creatures may provide the answer someday, says Ollerton. “If the evidence is there to be found, someone will find it.” In the meantime, he says, the specialized mouthparts of these scorpionflies suggest that they were feeding on something contained quite deep within the plants.

“The Mesozoic Era was biotically richer and more complex than previously realized,” he comments in the Nov. 6 Science. The new findings “are making us think again about the whole context of how pollination evolved,” he says.

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