Role of gastroliths in digestion questioned.

The discovery of large, polished stones inside the body cavities of some fossils of large plant-eating dinosaurs led many paleontologists to believe that the gastroliths--Greek for "stomach stones"-aided the creatures' digestion by grinding up tough vegetation. New analyses of the gastroliths in ostriches are casting doubt on that theory.

Many modern-day birds, which merely gulp their food because they have no teeth and can't chew, swallow stones that help grind grain, seeds, and vegetation in the creatures' gizzards. In ostriches, the muscle contractions in this gastric mill occur two to three times per minute, says Oliver Wings of Germany's University of Bonn.

In experiments in which Wings fed ostriches 2-centimeter cubes of various types of rock he found that sandstone blocks quickly crumbled in the birds' gizzards and that limestone lumps dissolved after just a couple of days. Tougher minerals such as rose quartz and ganite eroded more slowly. Interestingly, says Wings, none of the stones retrieved from the ostrich gizzards developed a highly polished surface such as that of the stones found within dinosaur fossils.

Typically, gizzard stones account for just over 1 percent of an ostrich's body weight. If that ratio held for a 50,000-kilogram Seismosaurus, the beast would have carried more than half a ton of rocks in its immense belly, says Wings. However, the Seismosaurus fossil found with the most gastroliths held only 15 kg of stones, the largest no bigger than a grapefruit.

Wings contends that it's unlikely such a small stash, dispersed through a vat-size stomach, could have significantly helped grind vegetation. Instead, he suggests, the stones may have been ingested accidentally. Or, he notes, dinosaurs may have habitually swallowed rocks for their mineral content, and the gastroliths that survived to be fossilized were those that were resistant to erosion and stomach acid.