Hybodont Shark Spine

Such spines, protruding from in front of the dorsal and pectoral fins, are derived from enlarged skin denticles and are made of tooth material, not bone or cartilage.

Here's what Robert L. Carroll says in his book, VERTEBRATE PALEONTOLOGY AND EVOLUTION:

Fin spines are a prominent feature of both ancient and modern sharks. Spines, like teeth, were apparently derived from single denticles that became greatly enlarged. Spines typically consist of an inner trunk of dentine, the base of which becomes deeply inserted in the body in advanced genera, and an outer mantle of ornamented enameloid material....

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