Nicolas Steno ( Danish: Niels Stensen) (January 10, 1638 - November 25, 1686) was a pioneer both in anatomy and geology.

After having completed his university education in Copenhagen, the city of his birth, he set out travelling in Europe; in fact, he would be on the move for the rest of his life. In the Netherlands, France, and Italy he came into contact with prominent physicians and scientists, and thanks to his eminent power of observation he very soon made important discoveries. At a time when scientific studies consisted in the study of ancient authorities, Steno was bold enough to trust his own eyes, even when his observations differed from traditional doctrines.

Steno first studied anatomy, beginning with a focus on the muscular system and the nature of muscle contraction. He used geometry to show that a contracting muscle changes its shape but not its volume.

However, in October 1666, two fishermen caught a huge shark near the town of Livorno, and Duke Ferdinand ordered its head to be sent to Steno. Steno dissected it and published his findings in 1667. Examination of the teeth of the shark showed a striking resemblance to certain stony objects, called glossopetrae or "tongue stones," that were found in certain rocks. Ancient authorities, such as the Roman author Pliny the Elder, had suggested that these stones fell from the sky or from the moon. Others were of the opinion, also going back to ancient times, that fossils naturally grew in the rocks. Steno's contemporary Athanasius Kircher, for example, attributed fossils to a "lapidifying virtue diffused through the whole body of the geocosm."

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