Stomach Stones

Article | Updated 10 years ago

Fossilised dinosaur skeletons have been found with stones (gastroliths) in their stomach region.

Birds swallow sharp, rough stones that help grind up food in their muscular second stomach, called a gizzard. This ensures they extract as much nutrition from their food as possible. Crocodiles also swallow stones, as did extinct long-necked plesiosaurs, but the function of these stones is unclear. The stones may help with controlling buoyancy but the total weight of stones in these animals is very small relative to the whole weight of the animal.

Theropod stones

Oviraptor skeletons have been found with gastroliths that have a rough surface, similar to the gastroliths of modern birds. As well, these fossils have virtually identical amount of stones relative to body size as modern birds. This is evidence that Oviraptor had a gizzard and that this feature is not unique to birds but rather is a more basic theropod character (like feathers) that is retained in birds — modern theropods.

Sauropod stones

It is quite rare to find sauropod skeletons with gastroliths. Those that have been found have gastroliths with smooth, polished surfaces as well as a small amount of stones relative to their body size. So scientists think these giant dinosaurs lacked a gizzard-type stomach.

Stones fossilised inside the stomach of this Psittacosaurus

Psittacosaurus skeleton
The gastroliths of this plant-eating dinosaur, from the Early Cretaceous of Mongolia, have been preserved. This dinosaur’s teeth were not suitable for grinding and gastroliths would have helped with digesting its food.