Ctenacanthus is an extinct genus of prehistoric shark who lived during the Early Carboniferous period. Its remains have been found in Scotland and North America. Ctenacanthus "Agassiz 1835" is part of the order Euselachii "Hay (1902)". The genus Ctenacanthus is represented by many species, almost all of them established on the basis of fin spines. The dorsal fin spines of Ctenacanthus sharks are found in marine deposits from Devonian through Permian time.
The type specimen, C. major, is a fin spine from the Lower Carboniferous Avon Gorge near Bristol in England. Louis Agassiz referred a number of other materials to the genus Ctenacanthus. These include, inter alia, C. brevis (a spine he never saw and which he described based on a drawing by someone else), C. ornatus (probably an acanthodian), and C. tenuistriatus (at least part of which has also disappeared). In the following years, a number of other specimens were referred to Ctenacanthus, including many with concave posterior surfaces or broad, enamelled striations, as well as materials which had no conceivable relation to the type, such as Russian cephalopods.
In 1873, however, paleontologist Newberry reviewed Agassiz's diagnosis, but procured two specimens of C. major for his personal inspection. Unfortunately, by some preposterous coincidence, not only were both specimens mis-identified, but they both turned out (much later) not to be Ctenacanthus at all. Instead, it appears that they were both remains of Sphenacanthus. This genus had also been described by Agassiz, but the holotype material was so poor that Newberry apparently did not recognize the resemblance. Sphenacanthus spines are quite unlike those of Ctenacanthus and "have irregular, widely spaced ribs, sometimes with scattered tubercles but never closely pectinated like those in C. major, and the spine is concave or flat posteriorly." Maisey (1981: 13).
Given such contradictory indications, it is not surprising that Ctenacanthus became a nearly meaningless taxon. In the following decades any number of bits and pieces were referred to Ctenacanthus. It was even proposed that Ctenacanthus and Hybodus were synonymous. Things might have continued indefinitely in this way but for the discovery and gradual correlation of body fossils from the Cleveland Shale and elsewhere, as well as the careful sorting out of Late Paleozoic sharks by workers such as Maisey, Zangerl, and Lund.