Why Would an Animal Swallow Stones? Grinding Food Without Chewing

A gastrolith is a stone swallowed deliberately and retained in the digestive tract, and the practice is well documented in living animals. Birds keep grit in a muscular gizzard where it grinds food that the beak cannot chew, and crocodiles, seals and some fish swallow stones as well. Applied to extinct animals, the term covers polished stones found inside or closely associated with skeletons, and the usual interpretation is that dinosaurs without grinding teeth used the same mechanism. The interpretation is attractive because many dinosaurs plainly could not chew, having teeth suited to stripping or cropping vegetation but not to processing it, which leaves an obvious question about how a large quantity of tough plant material was broken down.

Distinguishing a swallowed stone from an ordinary one is not straightforward:

• •Polish can be produced by water transport, by wind or by sediment movement as easily as by a stomach
• •The stone must be a type not found naturally in the surrounding rock, or it proves nothing
• •Position within the ribcage is the strongest evidence and is often disturbed before discovery
• •Modern bird gizzard stones are angular as often as rounded, which undermines polish as a criterion
• •Surface texture under a microscope differs between mechanisms, which is the most reliable test available
• •Many published examples fail these tests and the literature contains a great many doubtful cases

The case for the long-necked sauropods, where the idea is most often invoked, has been seriously challenged. Comparison with birds shows that a bird using stones to grind food carries a mass of them amounting to roughly one per cent of its body weight, and applying that ratio to a large sauropod predicts a quantity far larger than anything ever found associated with one. The stones actually reported with such skeletons are few and are frequently not the right rock type or position to be convincing. An alternative account holds that these animals processed plant material by fermenting it slowly in a very large gut, which their body size permits and which is how large herbivorous mammals manage the same problem, making grinding unnecessary. The stones found with some specimens may have been swallowed incidentally.

Several methods bear on the argument and each contributes something different. Tumbling stones in a machine with bone and plant material simulates a gizzard and produces wear surfaces that can be compared with the supposed fossil examples, and the results distinguish stomach polish from water polish reasonably well under a microscope. Surveying the surrounding rock establishes whether stones of that type occur naturally at the site, which is the decisive question in most cases. Counting the mass of stones found relative to the estimated body mass of the animal tests the comparison with birds directly. Examining living animals that swallow stones establishes what the practice actually looks like and has shown that the textbook picture of uniformly rounded polished pebbles is wrong. And re-examining published examples against these criteria has removed a substantial number from the accepted list.

The idea holds up better for some groups than for others. Certain bird-like predatory dinosaurs have been found with dense clusters of small stones in exactly the position a gizzard would occupy, in specimens preserved well enough that displacement can be ruled out, and those animals are close relatives of birds where the anatomy is expected. Marine reptiles including plesiosaurs are frequently found with stones inside, and the interpretation there is contested between grinding and ballast, with the ballast argument being that a stone in the gut of an animal living in water adjusts buoyancy and trim. Some living animals are proposed to use stones for exactly that reason. The honest position is that swallowed stones are confirmed in some extinct groups, disputed in others, and that a single explanation for all of them is unlikely.

Birds and several other living animals keep stones in the gut to grind food, and polished stones inside dinosaur skeletons have been read the same way. Polish alone proves nothing, since water and wind produce it too, so rock type and position in the ribcage carry the argument. The case for long-necked herbivores is weak, since the quantities found are far below what the comparison with birds predicts.