1.8" Fossil Plesiosaur (Elasmosaurus) Gastrolith - Montana

This is an interesting, 1.8" wide gastrolith from the stomach of a Plesiosaur (Elasmosaurus sp.). It was collected from the Pierre Formation in Montana. This specimen is Late Cretaceous in age, or approximately 75 million years old.

Gastroliths are rocks/stones that are swallowed and left in the gastrointestinal tract to assist with the grinding of food. Among living animals, gastroliths are predominantly found in birds (most commonly chickens, ostriches and other ground dwelling birds). Gastroliths can also be found in toothed whales, crocodiles, penguins and other animals. Surprisingly enough, they are also found associated with the stomach regions of dinosaur (sauropods, ceratopsians and plesiosaurs) fossils.

Very often round stones polished by water are mislabeled as gastroliths. Unless these stones have been found directly associated with a skeleton as this one was, we don't believe they can be interpreted as gastroliths. True gastroliths also don't tend to have a smooth surface, they tend to have lots of small "divots" where they've knocked together inside of the animals gastrointestinal tract.