Ctenacanthid

Ctenacanthids are an extinct family of prehistoric sharks. They have a pair of dorsal spines ornamented with many fine rows of nodes. This gives the spines a distinctive comb-like appearance, hence the name (ctenacanthus = comb-spine). Instead of plate-like spines found in many other early sharks, the ctenacanthid spines were cylindrical and pointed. These spines, as well as details of their fin anatomy and the structure of their gill arches indicate that ctenacanthids share a common ancestor with the more advanced hybodonts of the Late Paleozoic and Mesozoic and the neoselachians (modern sharks).