Aptychus

An aptychus is either a two-valved closing hatch on the shells of extinct ammonites or a jaw-piece of some modern cephalopods. When only a single plate is present, the term anaptycus is used.

Set against the shell's terminal opening (the living chamber), the aptychus was made of aragonite, just as the shell was, and was composed of two identical valves. It is unclear what the exact function of this apparatus was. Some authors consider the aptycus to be a jaw apparatus, while others believe them to be opercula (mandibles). It may have had a function similar to the head shield of modern nautiluses.

Aptychi are often found well-preserved as fossils, but are rarely found connected to ammonite shells. This let to them being initially classified as a type of bivalve. They are found in rocks from the Devonian period through to the those of the Cretaceous period.

There are many forms of aptychus, varying in shape and the sculpture of the inner and outer surfaces, but because they are so rarely found in position within the ammonite shell it is often unclear as to which species of ammonite many aptychi belong.

References

* [http://www.calfrye.com/aptychi/morphological_terminology.htm Morphological Terminology: the Aptychus] from "North American Late Devonian Cephalopod Aptychi". "Kirtlandia" 46 (August 1991):49-71. By Calvin J. Frye and Rodney M. Feldmann.