January 11, 1638, birthday of Danish physician Niels Steensen or latinized Nicolas Steno.
He popularized the first stratigraphical principles and recognized the true origin of fossils studying “tongue-stones” - petrified shark teeth.
Trained as physician and surgeon and a keen observer of the natural world, while visiting the Italian region of Tuscany, Steno recognized that sedimentary rock formations follow certain rules:
Layers of sedimentary rocks are arranged in a time sequence, with the oldest on the bottom and the youngest on the top, unless later processes disturb this arrangement (principle of superposition).
Layers of sedimentary rocks are deposited in a horizontal position if not being disturbed later (principle of original horizontality).
A stratum is deposited continuously unless some other solid body is in the way (principle of strata continuity).
Any geologic feature (like a fault, dike, or erosional surface) that cuts across another feature must be younger than the feature it disrupts (principle of cross-cutting relationships).
In 1666, during the dissection of a shark’s head, he noted the similarities of shark teeth with glossopetrae or tongue-stones found in the ground.
Steno was not the first to speculate about an organic origin of fossils, however, he was the first to propose a “theory” to explain how remains of sea animals could be petrified and become embedded in rocks.
The layers of a sedimentary rock were formed by sediment deposition in water. The now hard rock was once a soft mud.
An animal living in the sea (like a shark) would, after its death, sink slowly into the soft mud. Mud and water would seep into an animal’s hardest parts (like teeth and bones) and preserve them.
Movements of Earth’s crust push the mud with the fossil inside above water. Exposed to air, the mud dries and becomes hard rock.
The hardened layer is pushed back below water and new sediment covers it, forming a new layer.
In the end, all the layers are uplifted again, this time high enough to form a mountain, where a curious naturalist can find the fossils eroding from the rocks.
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