Continuous replacement of teeth throughout their lifetimes is a well known phenomenon in modern sharks. The mechanism involves the continuous generation of embryonic teeth internally on the inner surface of the jaws, and their subsequent displacement on a moving mat of connective tissue in “assembly-line” fashion. The fully-formed teeth erupt from the gum at the outer jaw margin, are functional for a short time, and are shed as the next tooth in the series takes its place.
This study reports a specimen of a Devonian cladodont shark, Ctenacanthus, in which the dentition was not shed at the jaw margin, but instead migrated beneath the skin on the outside of the head. A survey of additional specimens demonstrates that tooth retention was present in cladoselachians as well, and may have been a feature of cladodont sharks in general.
Tooth retention in the cladodonts (the top-level predacious sharks of their day) seems to have been a problem that had to be overcome before the development of the modern cutting and gouging mechanism. It is suggested that the reorganization of the blood supply to the teeth, coupled with a sharp jaw margin, may have provided the mechanism for tooth loss and the eventual development of rapid tooth replacement.