Fossils and the Birth of Paleontology: Nicholas Steno (2 of 2)

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These exposed rock layers nicely illustrate Steno’s Law, with the youngest layers at the top and the oldest at the bottom.
Steno’s Law of Superposition
But how could fossils end up deep inside rocks? Steno studied the cliffs and hills of Italy to find the answer. He proposed that all rocks and minerals were originally fluid. Floating on the surface of the planet long ago, they gradually settled out of the ocean and created horizontal layers, with new layers forming on top of older ones. Molten rock sometimes intruded into the layers, reaching the top and spreading out into a new layer of its own. As the rocks formed, they could trap animal remains, converting them into fossils and preserving them deep within their layers. Those horizontal layers represent a time sequence with the oldest layers on the bottom and the youngest on top, unless later processes disturbed this arrangement. This ordering is now referred to as Steno’s Law of Superposition, his most famous contribution to geology.

Steno was not the only naturalist of his day to propose that fossils belonged to living creatures. Leonardo da Vinci and Robert Hooke, for example, also took up the same view. But Steno pushed the idea much further. He argued for the first time that fossils were snapshots of life at different moments in Earth’s history and that rock layers formed slowly over time. It was these two facts that served as the pillars of paleontology and geology in future centuries. And fossils ultimately became some of the key evidence for how life evolved on Earth over the past four billion years.

 

  • Rock layers image courtesy of David Smith, UCMP.  


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