Necklace with large tooth of an Otodus shark of 60 million years old.

Shark teeth, formerly also called tongue stones or (Latin) glossopetrae, are fossilized teeth of sharks. The teeth are the body parts of sharks that are best preserved as fossils. This is because the entire skeleton of sharks, with the exception of the teeth, consists of cartilage, which generally does not fossilize. The oldest known shark teeth belong to shark species that lived approximately 450 million years ago, while in the North Sea basin the most common shark teeth date from the Eocene and the Oligocene and are between 40 million and 65 million years old. Teeth of Megalodon, which lived between 1.5 and 16 million years ago, are the largest known with a maximum length of 17 cm and are very popular among collectors of fossil shark teeth. Smaller shark teeth are fairly common in sediments of fairly deep seas. These fossils wash up on various Dutch beaches, especially in Zeeland and Zeeuws Vlaanderen, such as at Cadzand. These teeth are mainly of Tertiary age (Eocene and Oligocene). Shark teeth were among the first fossils to be correctly identified. In the 16th century, the prevailing theory was that the Earth possessed an inner force to transform objects in its interior into the likeness of living animals and plants. However, the Swiss naturalist Conrad Gesner (1516 - 1565) published a drawing in 1558 in which fossil shark teeth are compared with those of present-day species. Later, the Danish researcher Nicolaus Steno (1638- 1686) would demonstrate that tongue stones, which had been found in the mountains, often showed signs of wear compared to teeth of a recently washed-up shark. He also discovered that the rocks in which they were found were originally soft. Steno's conclusion was that tongue stones were the teeth of sharks that had died long ago.