What Is a Gastrolith? Swallowed Stones and What They Were Doing

Birds have no teeth and process food in a muscular gizzard, and many species swallow grit and stones that sit in that chamber and grind plant material as the muscles contract, which substitutes mechanically for chewing. The stones wear against each other and against the food, becoming rounded and polished, and are periodically expelled and replaced. Crocodilians also carry stones, and the interpretation there is contested, with ballast for buoyancy control proposed alongside or instead of digestion. Seals and sea lions swallow stones in quantity for reasons that remain genuinely unclear. Applied to extinct animals, the reasoning is that a herbivore with teeth unsuited to grinding must have processed food somewhere, and a gizzard full of stones would do it, which is why the idea was attached enthusiastically to sauropods, whose peg-like teeth clearly stripped vegetation without chewing it.

Distinguishing a genuine gizzard stone from an ordinary pebble that happens to lie nearby requires several kinds of evidence:

• •Position within the body cavity of an articulated skeleton, which is the strongest single indicator and is rare
• •Rock type foreign to the surrounding sediment, indicating the stones were transported from elsewhere rather than deposited with the body
• •A polished surface texture distinct from the rounding produced by water transport, which is examined microscopically and is harder to interpret than it sounds
• •Size and quantity appropriate to the animal, since a handful of small pebbles in a very large herbivore does not amount to a grinding apparatus
• •Absence in most specimens of the same species, which argues against the behaviour being general
• •Experimental comparison, including tumbling experiments and studies of ostrich gizzard stones, which established what genuine gizzard polish looks like

The idea that sauropods used stone-filled gizzards was widely accepted and has largely been abandoned for those animals. Detailed work compared the supposed sauropod gastroliths with stones recovered from ostrich gizzards and found that the polish and wear patterns differed, that the quantities associated with sauropod skeletons were far too small relative to body mass to grind a useful amount of vegetation, and that stones were absent from the great majority of well-preserved specimens including articulated ones where they would have been found. The alternative explanation for how these animals processed food is that they did not process it mechanically at all, relying instead on an enormous fermenting gut in which microbes broke down plant material over a long retention time, which is how many large herbivores work and which requires no grinding. Some theropod groups closer to birds do preserve convincing gizzard stones, so the behaviour existed among dinosaurs, and its distribution is narrower than was assumed.

The history is a clean example of how a plausible mechanism can propagate through a field on the strength of its reasonableness. The argument was sound in structure, since the animal needed to process food somehow and stones would have done it, and the supporting observations were weak, consisting largely of polished stones found in the vicinity of skeletons in sediments where rounded pebbles occur anyway. Once the interpretation was published it was repeated, and specimens were labelled accordingly in collections, which created an appearance of accumulated evidence that was really accumulated citation. Testing it required somebody to specify what a genuine gizzard stone should look like and then check, which is unglamorous work of exactly the kind that corrects a field. The same pattern recurs across palaeontology, where behaviour must be inferred from static remains and where a compelling story is easier to generate than to falsify, and the specimens most vulnerable to it are those where the evidence is an association rather than a direct trace.

Birds swallow stones that grind food in a muscular gizzard, and the same explanation was applied enthusiastically to dinosaurs. Testing it requires stones inside an articulated body cavity, of rock foreign to the surrounding sediment, with the right polish and in sufficient quantity. Sauropods fail those tests and are now thought to have fermented plant material in a large gut instead, while some bird-like theropods genuinely did carry gizzard stones.