Helicoprionidae
The Helicoprionidae are an extinct, poorly known family of bizarre holocephalids within the poorly understood order Eugeneodontida. Members of the Helicoprionidae possessed a unique "tooth-whorl" on the symphysis of the lower jaw and pectoral fins supported by long radials. The closest living relatives of the Helicoprionidae and all other eugeneodontids are the ratfishes. The anatomy of the tooth-whorl differed amongst genus and species, some possessing complete spirals, others possessing halved spirals, and some with wedged half-spirals. Each tooth-whorl is thought to be adapted to a different type of prey, and a different predation strategy.
