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Anaptychus is a type of marinefossil. It is a hard anatomical structure, a sort of curved shelly plate, now understood to be part of the body of anammonite. Pairedaptychi have, on rare occasions, been found at or within theaperture of ammonite shells. The aptychus was usually composed ofcalcite, whereas the ammonite shell wasaragonite.

Aptychi can be found well-preserved asfossils but usually quite separate from ammonite shells. This circumstance led to them being initially classified asvalves ofbivalves (clams), which they do somewhat resemble. Aptychi are found in rocks from the Devonian period through to those of the Cretaceous period.^[1]

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

When only a single plate is present, as is sometimes the case, the term "anaptychus" is used.

Aptychi seem to have most often existed as bilaterally-symmetrical pairs and were first described (incorrectly) as being the valves of bivalvemollusks. Aptychi are now considered to be a two-valved closing hatch on the shells of extinctammonites or a double-platejaw-piece similar to that of some moderncephalopods.^[2][3][4][5]

Set near to or against the shell's terminal opening (the living chamber), the aptychi usually consisted of two identical butmirror image valves. Some authors consider the aptychus to be a jaw apparatus (mandibles), while others believe them to be pairedopercula. If the latter is the case, then aptychi may have had a function similar to the head shield of modernnautiluses.

• Morphological Terminology: the Aptychus from "North American Late Devonian Cephalopod Aptychi".Kirtlandia46 (August 1991):49-71. By Calvin J. Frye and Rodney M. Feldmann.

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