Ctenacanthids are an extinct family of prehistoric sharks.

Ctenacanths are known almost entirely from abundant fossils of their distinctive fin spines (body impressions or skeletal remains of these sharks are quite rare). The best-known genus is Goodrichthyes, known from a 7.5-foot (2.3-metre) specimen from Early Carboniferous deposits in what is now Scotland. Unfortunately, this specimen is contained in some 200 separate pieces of rock, and is thus rather difficult to interpret. The genus Ctenacanthus itself is represented by many species, almost all of them established on the basis of fin spines. The ctenacanths appeared in the Late Devonian (about 380 million years ago - slightly earlier than Cladoselache) and persisted until the Permian, with a few hanging on into the Triassic (about 250 million years ago). But there is no doubt that their heyday - in terms of diversity and abundance - was during the Carboniferous.

Ctenacanths 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).

The dorsal fin spines of Ctenacanthus sharks are found in marine deposits from Devonian through Permian time. As shown right, Ctenacanthids have a pair of rigid fin spines that emerge in front of each of two dorsal fins. The dorsal fin spines are ornamented with thin rows of dentine-like material that looks like strings of tiny beads in some areas. Ctenacanthid spines were sub-cylindrical and pointed and, in life, were supported by a wedge of cartilage that inserted into a groove on the posterior side of the fin spine.

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