Nicolaus Steno could not have guessed that he would one day be known as a father of modern geology on the fateful day in October 1666 when he opened a package containing a great white shark’s head.
Steno (born Niels Stensen) was a Danish anatomist working in Italy when Duke Ferdinand had the animal’s head—part of an unusually large specimen brought in by two Italian fisherman—delivered to him. Steno was still early in his career, but he was skilled at dissection and had already made notable observations and discoveries related to muscles, glands and the brain.
Steno dissected the shark’s head, as the duke requested, and published drawings and a description of the animal’s jaw. Examining the plentiful teeth, he noted that they closely resembled glossopetrae, or “tongue stones,” triangular pieces of rock that had been found in many locations but were not well understood. Theories about the origins of tongue stones ranged from bizarre (they fell from the sky on moonless nights) to spiritual (they were serpent teeth turned to stone by St. Paul). Many people believed they simply “grew” in the rock.
Steno’s sketches offered strong evidence that these mysterious tongue stones were actually fossilized shark teeth. Although this may seem like a small contribution now, at the time, fossils were not understood as they are today. In fact, the term fossil was used generally to describe anything dug up from the earth–whether it had ever been alive or not. The connection Steno drew would go a long way toward narrowing the definition of fossil and supporting the idea that teeth, shells, bones and other durable parts of living creatures could be turned to stone over time by natural processes.
It also opened the door for Steno to begin studying and writing about geologic processes, which would lead to the biggest contributions of his scientific career. In 1669, he finished De Solido, a 78-page publication that was intended to be an introduction to a longer work but which ended up making a huge splash on its own.
De Solido contained not one, not two, but three of the basic principles of modern geology:
- The Principle of Original Horizontality – layers of sediment are deposited horizontally due to the action of gravity
- The Law of Superposition – newer layers of sediment and rock are found on top of older layers
- The Principle of Lateral Continuity – sediment spreads out laterally in all directions, forming continuous layers
Understanding the general principles of how layers, or strata, of the Earth form, allows geologists to piece together the story of geologic events over time. Layers that are not horizontal, for example, must have been tilted or folded by a force, such as an earthquake, after they formed. Steno’s principles are the basic underpinnings of the branch of geology known as stratigraphy.
Steno’s unlikely path into the heart of geology is a reminder that scientific inspiration can come from many places if we are keen observers. As a young medical student, Steno had worried because he found himself unable to focus his attention on a single discipline. Thank goodness he didn’t.
Learn about the Law of Interfacial Angles, another one of Nicolaus Steno’s key contributions to geology, in our module Defining Minerals
Read more details about Nicolaus Steno’s life from NASA Earth Observatory
Written by Christine Hoekenga
Christine is a freelance writer, editor, and content strategist, specializing in science and nature. She holds an Bachelor's degree in Environmental Science and Media Studies and a Master's of Science Writing. She has been working in science communication and education for nearly a decade as a journalist, an organizer for conservation groups, and a museum educator. Before joining the Visionlearning team, she served as the New Media and Online Community Manager for the Webby award-winning Smithsonian Ocean Portal. Christine is assisting Visionlearning with developing new modules and glossary terms, managing the blog, and outreach through social media.