Aptychus

The Aptychus is the scoop-shaped two-part lower jaw of the Ammonites, an extinct group of cephalopods. In addition to its primary function as the lower jaw a secondary function as an operculum ( lid ) is probable.

In contrast to the aragonitic housing, the calcitic Aptychen were often obtained in the fossilization. However, since a common housing and maintaining Aptychus is rare, the function of the Aptychus is not precisely known. Besides being used as a grave tool, with which the animal was searching the soft substrate of the seabed for food, the jaw apparatus could also have served as a protective flap that most of the living chamber locked in danger.

The other ammonites have jaw apparatus as the one-piece horny Anaptychus other cephalopods ( Cephalopoda ).