top of page


Used Shark Teeth to Argue for Sedimentation in Rock Creation

Niels Stensen


Niels Stensen (or, Nicolas Steno, 1638-1686) was a Danish anatomist who served as a physician in the Medici court. Steno, as he was formally known, was an early anatomist of the brain and considered the father of modern geology due to his work on rocks and sediment strata. Later in life, Steno joined the priesthood and was eventually made a bishop.


Stensen studied mathematics and languages at his local grammar school. In 1656 he attended the University of Copenhagen where he studied medicine until his studies were interrupted when the Swedish king, Carl Gustav, laid siege to the city of Copenhagen. Stensen continued his medical studies in Amsterdam, then Leiden, in the Netherlands, and his experience and reputation quickly grew.






Stensen held remarkably advanced views on the study of anatomy and the brain for the time. Among his accomplishments, Stensen is responsible for proving that the heart is just a muscle like any other and not endowed with spiritual qualities as many believed. He also performed extensive investigations of the pineal gland of the brain, the lateral foramina, the vorticose veins in the eyes, the tarsal glands in the eye lids, the follicles of the ovary, the vitelline duct, and more. In 1665, in Paris, he delivered a renowned lecture on the brain that contained several criticisms of many contemporary theories. Not only did Stensen refute ancient theories on the brain’s function, but he also argued for improved dissection methods and he called for an update to the terms used for describing the parts of the brain. See here for more.


At this lecture, Stensen also criticized popular beliefs about the brain that had no basis in anatomical knowledge. At the time, many believed the mind was separate from the brain; it was commonly thought that the “mind” was the place where the soul and thoughts were housed, specifically in the pineal gland. Stensen controversially argued that this belief had no basis in fact. Stensen’s lecture ended with his call for a major reform of anatomical studies and practices. He emphasized that the brain’s complexity called for full-time researchers who could devote their lives to its study. (Go here for more on the debate on Rene Descartes, the pineal gland, and the seat of the soul.)


Despite his plea for full-time brain anatomists, Stensen began to apply his methodical approach to the seemingly unrelated study of earth and rocks. He moved to Florence, Italy in 1665, having accepted the post as personal physician to Ferdinand II de’ Medici. He joined the Academy of Experimental Sciences in Florence and developed an interest in geology and crystallography (Parent, 490), with a specific interest in what were known as glossopetrae,


Glossopetrae were the fossilized teeth of sharks found within rocks and were thought to resemble “snake tongues” due to their shape. While it was known that they were shark’s teeth, glossopetrae were found all over Europe, often in places far from water. Their source had been debated for centuries. Roman scholar Pliny the Elder noted glossopetrae in his Natural Histories: "We are told that Glossopetra, which is like the human tongue, does not form in the ground, but falls from the sky during the waning of the moon, and is indispensible for selenomancy,” the practice of divination by the moon (Book 37, Chapter 59). They were also thought to spontaneously appear in rocks.


However, in 1665, Stensen had the opportunity to dissect the head of a Great White shark caught near the Italian town of Livorno. Stensen methodically approached the problem of glossopetrae, and in the process laid the groundwork for modern geology practices. Stensen paired his observations of the shark teeth with his studies of the Tuscan landscape, and this led him to conclude that fossils were indeed the remains of living organisms trapped in rocks, and that rocks are produced through a process called sedimentation. Sedimentation is the process by which solid materials settle out of a fluid and eventually build up to form another solid layer. His studies were published in 1669 in his dissertation prodromus,“Of solids naturally contained within solids.” 


Niels Stensen’s contribution to geology would be one of his last scientific pursuits. Around the same time he made his fossil discovery he took an interest in theological studies and converted to Catholicism in 1667. By 1675 he became a priest and in 1677 Pope Innocent IX made him the bishop of Titiopolis. He traveled to several German cities while serving the Catholic Church, until he died in Schwerin under self-imposed poverty due to his religious renunciation.


Michael Gott

First Published 12/4/2014

The Dissertation Podromos




bottom of page