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Many birds and crocodiles swallowed small stones, which collected in a muscular pouch in the digestive tract, called the gizzard. Squeezed by the muscles of the gizzard, the stones helped crush tough plant food before it reached the stomach, and thus aided digestion. Some scientists thought dinosaurs also had gizzard stones. What is the main idea of the paragraph?
Many birds and crocodiles swallowed small stones, which collected in a muscular pouch in the digestive tract, called the gizzard.