Fossils found of dinosaurs that filtered water for food

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Fossils found of dinosaurs that filtered water for food
Beak illustrates how animals adapt to new environment

David Perlman, Chronicle Science Editor Thursday, August 30, 2001


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Fossil hunters digging in the Gobi Desert and Canada have found evidence for birdlike dinosaurs whose toothless beaks were structured very much like those of modern dabbling ducks that filter-feed their nutrients from water.

The dinosaurs provide a striking example of how creatures evolve to adapt to new environments, the scientists say, because they differ so remarkably from their close relatives that lived millions of years earlier and hunted across the land armed with claws and teeth.

The large, long-necked beasts are known as ornithomimids, and they lived about 75 million years ago. Half in fun, the scientists liken the particular branch of ornithomimids they examined to ostriches because of their long legs and necks.

Some 50 million years earlier their more primitive close relatives were sharp-toothed, swift-racing predators and were related, in turn, to the ferocious velociraptors known now to every moviegoer who has seen the Jurassic Park films.

Through evolution, the duck-like ornithomimids must have lost their sharp teeth and instead evolved beaks with soft, comb-like structures that proved just right for living in an ecological niche of quiet ponds and streams.

There, the scientists say, the animals could dip their duck-like beaks into the sediment-filled water and strain out a nutritious diet of plants mixed with organisms like tiny snails and brine shrimps.

The researchers discovered the imprint of soft tissues inside the beaks of the fossil remains, the kind of tissues that are rarely preserved during the long millennia that pass as they turn to stone from flesh and bone.

It is the evidence of those soft tissues in two distinct species that has revealed the evolutionary transformation in the animals and allowed the researchers to speculate about their lifestyle, said Mark A. Norell of the American Museum of Natural History in New York, who led the research team.

He and his colleagues, Peter J. Makovicky of the Field Museum in Chicago and Philip J. Currie of Canada's Royal Tyrell Museum of Paleontology in Drumheller, Alberta, are reporting on their research today in the journal Nature.

Makovicky found one of the fossil creatures, called Gallimimus bullatus, during an expedition to the harsh and arid Gobi Desert in southern Mongolia, while Currie discovered the other, called Ornithomimus edmontonicus, in Alberta's Dinosaur Provincial Park.

The creatures, varying in size from 5 feet as juveniles to 15 feet as adults, had toothless beaks that were probably made of keratin, the same material that forms the beaks of all modern birds, as well as the hair and nails of humans and other animals, the scientists said.

"Those filter-feeders are a pretty neat example of evolutionary convergence, " Norell said. The term applies to the pattern of change in which two unrelated species independently evolve similar traits adapted to similar ecological and environmental conditions.

In the region of Mongolia where Makovicky found the desert fossil, the arid land was far different millions of years ago, Norell said. Evidence there shows that it must have been a pleasant area -- cooler and well forested, and dotted with streams and lakes.

E-mail David Perlman at dperlman@sfchronicle.com.