Male "ghost sharks"—eerie deep-sea fish known as chimaeras that are related to sharks and rays—have a strange rod jutting from their foreheads, studded with sharp, retractable teeth. New research appearing in Proceedings of the National Academy of Sciences reveals these are not merely lookalikes, but real rows of teeth that grow outside the mouth.
Male sharks use teeth to hold female sharks during copulation, but ghost shark teeth are small and not suited for that purpose, so they evolved another set of teeth on their forehead to do the same job.