Glossopetrae, also known as Tongue stones, are shark teeth. Pliny the Elder (AD 23-79), the great Roman naturalist and compiler of the encyclopedic but credulous 37-volume Natural History, believed that they fell from the sky during lunar eclipses. They were later thought to be the tongues of serpents that Saint Paul had turned to stone while visiting the islands of Malta.
Glossopetrae were widely believed to have magical properties, most notably the ability to counter-act toxins of many kinds - from venom injected via snakebite to poison slipped by a would-be assassin into a king's chalice of wine. To work their magic, glossopetrae needed only to be held against a snake-bitten body part or plunked into a suspect glass of wine and any poisons therein would be quickly and irreversibly detoxified. Due to these marvelous supposed capabilities, many nobles and statesmen of the Middle Ages retained these 'tongue stones' as amulets, either worn about the neck as a pendant or kept secreted away in special pockets reserved for this purpose. The supposed magical properties of glossopetrae are, of course, without foundation and the result of pure wishful thinking; one wonders how many medieval politicians' suspicious deaths or illnesses are attributable to undue faith in the power of glossopetrae.
Danish geologist and anatomist Neils Stensen (1638-1686) - commonly known as Nicolaus Steno - was among the first naturalists to discern the true nature of glossopetrae. Our story begins in October 1666. This was during Isaac Newton's annus miribilis (great year, in which he single-handedly revolutionized optics, characterized the behavior of 'universal' gravitation, and invented the calculus) and a mere month after much of London burned to the ground, thereby bringing the last major outbreak of Black Plague to an abrupt end. Stensen was 29 years old and employed as medical anatomist and physician to Ferdinand II, the Grand Duke of Tuscany, when the head of a large white shark caught off Leghorn (now known as Livorno) was brought to the court at Florence. It fell to Stensen to examine it. Although the shark's head was badly desiccated and many of the teeth were missing, Stensen dissected the jaws, eye, and ear, describing these structures in detail in a report published in 1667. (Stensen also noted the pores that peppered the snout of a shark's head, but it fell to his student, Stephano Lorenzini, to describe in detail the mucus-filled ampullae [tubules] that now bear his name; Lorenzini published his description of these pores in 1678, but it was not until 1967 that their electrosensitive function was revealed through experiments by Sven Dijkgraaf and his student Adrianus Kalmijn.) Stensen noted that the teeth of his specimen closely resembled in form and structure the glossopetrae he had been studying. From this, he reasoned that the thousands of glossopetrae found within rocks and inundating European museum collections MUST be shark teeth. But how could shark teeth come to be imbedded in solid rock and in such staggering numbers?
Stensen puzzled over these mysteries for many months. Up to that time, the earth was believed to be only a few thousand years old and rocks were created as they had been found. Fossil remains were thought to be accidental aggregations of minerals that just happened to assume shapes that resemble familiar objects. Stensen observed that his shark specimen had hundreds of teeth and that new ones formed continually as old teeth wore down and fell out. Stensen went on to postulate (incorrectly) that all solids are derived from liquids and that the form of a solid indicates the motions of the liquids that produced it. From this, Stensen argued that, when one solid lies embedded in another, one can determine which hardened first by noting the impress of one object on another. Thus, fossil shells and sharks' teeth were solid before the strata that entomb them because they impress their form upon the surrounding sediment, much as we make footprints in wet sand. Further, since marine fossils virtually identical to modern forms are often found high in mountains and far from the sea, fossils in general must be derived from solid parts of once-living animals and the earth itself must have an extensive history. Stensen presented these findings in an influential book published in 1669.
Stensen was converted to Catholicism in 1667 and ordained as a minister in 1675. In 1677, he went on to become the Titular Bishop of Titiopolis (a pagan-held area in which he could not actually reside - hence the 'titular' part of his title - in what is now part of Turkey), travelling about ministering to the scattered Catholic remnants of northern Germany, Denmark, and Norway. As an ordained Catholic bishop with a bent for historical geology, Stensen was among the first to make science acceptable to the Church. Although he was mired in the beliefs of his time, Stensen has been hailed as "the father of modern geology and paleontology", for his keen powers of observation and inquiring mind changed our view of the world in ways that are deceptively simple yet profound.