New fossil sheds light on dinosaurs' diet.

Vestiges of soft tissue preserved in a 70-million-year-old Mongolian fossil suggest that some dinosaurs strained small bits of food from the water and mud of streams and ponds, just as modern ducks, geese, and flamingos do.

The remnants of a comblike plate appear inside the beak on the fossil's upper and lower jaw. Individual strands of material, about 5.6 millimeters long, sit about 0.5 mm apart. This type of structure, never before seen on a dinosaur, suggests that the ancient animals had a wider variety of feeding strategies than previously recognized, says Peter J. Makovicky, a vertebrate paleontologist at the Field Museum in Chicago. He and his colleagues describe their find in the Aug. 30 NATURE.

Makovicky discovered the almost-complete fossil in the Gobi Desert last summer. The ancient bones belong to Gallimimus bullatus, a species of bipedal dinosaurs in the group ornithomimids, or bird mimics. Ornithomimids had long, flexible necks, small heads, and prominent beaks. They looked something like ostriches with long tails.

Gallimimus' forelimbs probably couldn't grasp well, but the dinosaurs' long legs and sleek build suggest they were fast runners. Adults were about 2.1 meters tall and weighed about 320 kilograms.

Primitive ornithomimids, which appeared about 130 million years ago, had teeth, says Makovicky. All later members of the group, including Gallimimus, sported toothless beaks. Because fossils indicate that the animals had weak jaw muscles, paleontologists previously suspected that these later ornithomimids pursued small prey or ate eggs. However, the newly discovered sieve-like structure suggests that Gallimimus should be crowned as the all-time largest known terrestrial filter feeder.

Such a feeding strategy would help explain why paleontologists have found most ornithomimid remains in sediments deposited in lakes, rivers, and other wet environments, Makovicky notes. Modern birds with similar filters in their beaks glean plant material and small crustaceans from shallow water or mud.

It's unlikely that Gallimimus pursued prey that required cutting, ripping, or tearing--such actions probably would have damaged the beak's delicate structures. The gastroliths, or stomach stones, that have been found in some ornithomimids provide another clue that the creatures didn't consume large animals, says Makovicky. Gastroliths are similar to modern birds' gizzard stones, which help grind vegetation and hard-shelled invertebrates into a more easily digested pulp.

Furthermore, ornithomimids couldn't have gotten their nourishment from fleshy fruits because plants that bore them hadn't yet evolved, says Dale A. Russell, a vertebrate paleontologist at North Carolina State University in Raleigh.

Russell suspects that all of the toothless ornithomimids may have been filter feeders, foreshadowing a feeding strategy employed by a wide variety of modern aquatic birds.

"I'd never considered that if one were to design a Mesozoic duck, it would look like an ornithomimid," says Russell.