A book about fossil teeth would have been unthinkable before 1669. Three years earlier, a group of Tuscan fishermen caught a colossal white shark. As Brian Switek tells the story in his engaging book, Written in Stone: The Hidden Secrets of Fossils and the Story of Life on Earth, that shark excited the imagination of Medici Grand Duke Ferdinando II, a great patron of the sciences (1).
The shark's body was too large to transport and had begun to decompose anyway, so its head was cut off and sent to the Grand Duke, who chose his most talented resident anatomist for the privileged job of dissecting it (Figure i). Danish-born Nicolaus Steno (aka Niels Steensen) turned out to be the man for the job (1). As he poured over his dissection, Steno was struck by the uncanny similarity of the shark's teeth to what were then popular triangle-shaped stones called glossopetrae or “tongue stones” (2, 3) (Figure 1).
At the time, glossopetrae were used for all sorts of purposes: as antidotes to snake venom, treatments for epilepsy, amulets, and when ground into a fine powder, as toothpastes (2, 3). Roman philosopher Pliny the Elder thought glossopetrae dropped from the sky on moonless nights (4). Inspired by his shark dissection, Steno published his own explanation in 1669. He suggested that the corpuscular theory, which held that matter was made of tiny corpuscles, could explain how shark teeth turned to stone (5). When the teeth were buried in sediments, the corpuscles of minerals gradually replaced the corpuscles that made up teeth, transforming them into stone. This explanation is not so far from our modern understanding of how fossils form. In the case of fossil teeth though, most of the original mineral remains while mineral from surrounding sediments fills in tiny pore spaces within them in the process of permineralization.
That fossil teeth played such a starring role at the dawn of paleontology is no coincidence. To be sure, sharks lose a lot of teeth. But it is the fact that teeth are hard, compact, and composed of mineralized tissue that makes them prime candidates for preservation and fossilization. Indeed, most of the fossil record consists of teeth and that is also true of the human fossil record. This book is about what fossil teeth tell us about human evolution.