What is "aptychus"

Aptychus \Ap"ty*chus\, n. [NL., fr. Gr. 'a priv. + ?, ?, fold.] (Zo["o]l.) A shelly plate found in the terminal chambers of ammonite shells. Some authors consider them to be jaws; others, opercula. [1913 Webster] ||

n. A marine fossil consisting of a hard anatomical structure like a curved shelly plate, once part of the body of an ammonite.

An aptychus is a type of marine fossil. It is a hard anatomical structure, a sort of curved shelly plate, now understood to be part of the body of an ammonite. Paired aptychi have, on rare occasions, been found at or within the aperture of ammonite shells. The aptychus was usually composed of calcite, whereas the ammonite shell was aragonite.

Aptychi can be found well-preserved as fossils, but usually quite separate from ammonite shells. This circumstance led to them being initially classified as valves of bivalves (clams), which they do somewhat resemble. Aptychi 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 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.