Humans aren’t supposed to dine on pebbles unless stuck in a variation of the folk story Stone Soup. However, several animals – both prehistoric and modern – ingest rocks. Is there any method to this rock-eating madness or are animals across the planet blindly chomping down on anything in their path?
When rocks are consumed by an animal, the rock takes on another name – it becomes a gastrolith. If you haunt the paleontology section of your local library, you might be familiar with the connection between gastroliths and several types of dinosaurs. Sauropods often consumed rocks in order to aided in the digestion of food.
Ingested gastroliths served as substitutes for molars, particularly in animals without a fully evolved set of teeth that could chew food thoroughly enough for digestion. The rocks tumbled around in the digestive tract as they ate, separating and rending the flesh of birds, plants, or tiny fish nabbed by the predator.
Over time, the gastroliths became smooth as the rocks continually collided into each other within an animal, with craggy portions of the rocks chipped off and digested or pulverized into dust.
A couple of signs stuck out when paleontologists initially discovered these gastroliths amongst fossil remains. The smooth quality of the stones initially caused alarm. Under further observation, the rounded stones did not match stones in the immediate surrounding area or local mineral deposits, suggesting the fossilized animal carried the rocks along before death.
When the supply of gastroliths used for mastication becomes too smooth and thus unable to tear apart, animals regurgitate or pass the now-smooth rocks and consume craggier ones found along the ground, beginning the process anew.
Gastroliths don’t just help animals without an evolved set of teeth digest their food, gastroliths can also act as a ballast system for some aquatic creatures. Ingesting the rocks created a center of balance, allowing massive aquatic creatures like plesiosaurs to move through the water with ease.
Modern gastrolith users include crocodiles and many types of birds, including ostriches and chickens. Due to their smaller size, birds populate their gizzards with grit, very tiny rocks, and pieces of shells. The gizzard, a strong muscular region separate from the stomach of a bird, becomes embedded with tiny, sharp objects over time. Imagine a pulsating muscle with hundreds of tiny teeth — that's the modern gizzard.
The contraction and relaxation of the muscle fulfills a similar function as teeth would in chickens. Gizzards somehow became a tasty fried treat for those living in the Southeastern United States, and, in turn, makes accidental gastroliths out of humans if the gizzards are not thoroughly cleaned prior to cooking.
Top image via wwarby/Flickr. Early 20th Century painting of a diplodocus by Charles R. Knight, with the painting in PD. Images of gastroliths within fossilized remains by Ryan Somma/Flickr. Image of fried chicken gizzards by Takaokun/Flickr.