Two views of the same "Petrodus", a dermal denticle from the skin of ancient sharks. The petrodi consists of a round base that is larger at the top than at the bottom and a cone shaped upper section.  The base was located within the skin and the upper section protruded through the skin. petrodi consist of trabecular dentine beneath a very thin layer of orthodentine.  These denticles formed a sort of skin armor that helped protect the shark.  When they were originally named they were assumed to be from a single species of shark, but we now know that they occur on a large number of species.  therefore, the name "Petrodus patelliformis" is no longer considered correct. (Nomina Dubia) 
Approximate Age: 310-312 million years old.
Location of Discovery: Washington County., Illinois