One of Steno's great realizations was that shells found on mountain heights were not sports of nature but organic remains. He argued that they had not been formed in place, and that they were not being formed like that today. The shells were often eroded, but never distorted like tree roots that grow in hard soil. They were perfectly formed, not irregular like crystals. And in places where they were broken, the separated pieces fit together. These arguments were ingenious, the first coherent theory of fossilization; but they were not accepted during his lifetime.

Steno's genius for anatomy provided him the tools to work on the mystery of fossils and the question of how seashells could be found in the rocks of mountains far from the sea. He hypothesized that layers upon layers of earth formed sediments in a sequence, recording a series of events and telling a story about the age of the earth. According to Steno, the stratum at the bottom is the oldest and that at the top is the youngest. Seashells, he said, found their way to mountaintops not by the great biblical flood, as many of his contemporaries believed, but by constant erosion and the sedimentation of soil. Steno published his discoveries in De Solido, after which he abandoned science, converted to Catholicism and spent the last 20 years of his life as an ascetic priest and eventually a bishop.