The science of optics is shedding light on rocks found in dinosaurs' digestive systems. The rocks, called gastroliths, are thought to have helped to grind up food in the animals' gizzards.

Roger Johnston, Kim Manley and Cheryl Lemanski of the Los Alamos National Laboratory, New Mexico, have developed a technique which can identify gastroliths by the way in which they scatter light. As a result of their technique, palaeontologists may learn more about the behaviour of dinosaurs (Optic Communications, vol 74, p 279).

The study of rocks which are suspected to be gastroliths is controversial. Geologists usually require several pieces of evidence before they will accept that a rock was used by a dinosaur to aid its digestion. First, the stone must be unlike the rock found in its geological vicinity. Secondly, it should be rounded and polished, because inside a dinosaur's gizzard any genuine gastrolith would have been acted upon by other stones and fibrous materials. Lastly, the stone must be found with the bones of the dinosaur which ingested it.

The last criterion has been a sticking point. Stones found without bones have been dismissed by palaeontologists as objects polished by water or wind.

Now Johnston and his colleagues have found a way of measuring the surface roughness of suspected gastroliths. This has allowed them to determine whether the smooth surfaces are the same as those of genuine gastroliths. The technique involves shining laser light onto a rock sample and detecting the manner in which the light is scattered. The direction and intensity of the reflected light depends on several factors, including the average depths of the pits on the surface, and the angle of the walls of the pits. In the technical jargon, different sufaces have different 'angle/intensity scattering curves'.

Johnston and his colleagues scattered light from known gastroliths, suspected gastroliths and fluvial rocks, and compared the scattering curves. They found that the gastrolith curves were distinct from the others. The technique, therefore, has the potential to identify positively many suspected gastroliths.

According to the researchers, palaeontologists will now be able to study many rocks which they had previously ignored. Bones are much more fragile than the stones, so many skeletons may have completely disintegrated, leaving the gastrolith as the only obvious clue to the dinosaur's existence. Johnston says that if the gastrolith's validity can be verified, it may be possible to trace the gastrolithic rock back to its original source. This may provide important information on how dinosaurs migrated, he says. Because the number of suspected gastroliths is large, the objects could provide significant new insight into the lives of dinosaurs.

New Scientist, 06 Apr 91, Volume 130, Issue 1763