As a medical student, Steno (1638–1686) entirely belonged to the seventeenth-century cultural context in which the problem of glossopetrae, or fossilized sharks’ teeth, was given special attention by a number of scholars. In his Canis (1667), Steno entered the realm of geological studies and advanced, on the basis of his medical knowledge and chemical expertise, a hypothesis concerning the animal origin of glossopetrae. Here, I show that the Canis offers an excellent text in which to understand the changes in geological studies between the Aristotelian and Cartesian frameworks. Thus, based upon the legacy of the medical and chemical traditions, Steno’s synthesis of previous views on the animal origin of glossopetrae with Descartes’ particle theory becomes a very good example for showing the nature of the transition between Renaissance and early modern geology.