Location: Florida, USA, Bone Valley Formation
Age: 4.9-5.3 million years ago, Neogene Period (Pliocene Epoch)
The fossil in the photograph was made by cells that usually construct teeth, but those cells grew on the back of a flat, wing-finned ray or skate. For millions of years rays and their relatives have repurposed tooth-forming cells to make hooks and spikes, called dermal thorns, in their skin.
In a growing animal, only certain kinds of cells can make certain kinds of body tissue. For example, there are special cells that make bones, special cells that make teeth, and special cells that make the coloring in skin. The bodies of skates and rays repurpose some of their tooth-forming cells to form dermal thorns like these ones in the photograph.
First, tooth-forming cells form the spiky part of the dermal thorn. Then, a subset of those cells lay down a hard, shiny material called enameloid, which is similar to the enamel on the outside of human teeth. Other cells lay down other tooth-forming minerals to make the base of the dermal thorn. Finally, bone-forming cells make attachment bone on the bottom of the structure.
Dermal thorns help protect rays and skates. Multiple species can form identical dermal thorns. Without any other parts of the skeleton, it is impossible to identify exactly which species made the pair of dermal thorns in the photograph.
Specimen Number: UF/TRO 17915
Sire, Jean-Yves, and Ann Huysseune. “Formation of dermal skeletal and dental tissues in fish: a comparative and evolutionary approach.” Biological Reviews 78(2003):219-249.
Reif, Wolf-Ernst. “Morphogenesis and histology of large scales of batoids (Elasmobranchii)” Paläeontologische Zeitschrift 53(1979)26-37.
Kemp, Normal E. “Chapter 2: Integumentary System and Teeth.” In Sharks, Skates, and Rays: the Biology of Elasmobranch Fishes, edited by William C. Hamlett, 43-68. Baltimore: Johns Hopkins Press, 1999.
Note: If you are looking for more information on dermal thorns, it might help to know that this fossilized dermal thorn and structures like it are also formally called odontodes, or dermal denticles.
Bonus: Here is a picture of dermal thorns in a living ray.