Steno: solving the mystery
Tongue stones were very familiar to naturalists of the 1600s and played a pivotal role in the early history of palaeontology and the understanding of the origin of fossils. They were among the first fossils examined in the context of having an organic origin, not simply as elements belonging to the supernatural.
Steno (1638-1686), originally called by his Danish name Niels Stensen, was a noted physician interested in the natural sciences. He worked in Florence under the patronage of the Medici family.
In October 1666 a large shark was brought ashore by fisherman near Livorno in Tuscany. Grand Duke Ferdinand II ordered that it be taken to Florence for Steno to dissect. Steno's examination of the shark's head convinced him that tongue stones had to be the teeth of ancient sharks. However, Steno realised that there were some differences between tongue stones and the teeth of contemporary sharks, surmising that tongue stones came from sharks that must no longer exist. Furthermore, because tongue stones are collected on land, he suggested that the sea must formerly have covered the land in order for the sharks to live there.
In 1667 Steno published a book describing his studies of the shark's teeth and also proposing a theory for the formation of stratified rocks, which is close to what we now know to be correct. This work, entitled Elementorum Myologiae Specimen, seu, Musculidescripto Geometrica cui accedunt Canis Carchariae Dissectum Caput, et Dissectus Piscis ex Canum Genere, is a seminal volume in both palaeontology and geology (Cutler 2003).