Many dinosaurs had narrow, pointed teeth that were more suited to tearing off plants rather than chewing them. So many researchers assumed the giant animals were helped by stones they swallowed, which acted as a kind of gastric mill.

But Oliver Wings of the Institute of Earth Sciences at the University of Tubingen, and Martin Sander from the University of Bonn have cast doubt on the theory.

For the study, the scientists offered stones such as limestone, rose quartz and granite as food to ostriches -- relatives of dinosaurs.

After the ostriches had been slaughtered, the scientists investigated the gastric stones. It became clear the stones wore out quickly in the muscular stomach and were not polished. That suggests they were not remains of a gastric mill, Sander said.

The researchers believe, instead of stones, the intestines of dinosaurs were formed in such a way that food was retained for a very long time to improve the digestive process.

The study is detailed in the current issue of the Proceedings of the Royal Society.

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