Some examples of aptychi (top right:
Oppelia from Late Jurassic of Solnhofen, Germany; bottom left: aptychi (recto and versus) from Late Jurassic of Lombardy, Italy), and conceptual scheme of their function.
An aptychus is a type of marine fossil, a hard anatomical structure like a curved shelly plate, which was part of the body of an ammonite. Paired aptychi have, on rare occasions, been found at or within the aperture of ammonite shells.
Aptychi are often found well-preserved as fossils, but are only very rarely found connected to ammonite shells. This circumstance led to them being initially classified as the valves of bivalves, which they somewhat resemble. Aptychi are found in rocks from the Devonian period through to the those of the Cretaceous period. The aptychus was usually composed of calcite, whereas the ammonite shell was aragonite.
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 as to which species of ammonite many aptychi belong.
When only a single plate is present, as is sometimes the case, the term "anaptychus" is used.
[edit] Function
Aptychi seem to have most often existed as bilaterally-symmetrical pairs, and were first described (incorrectly) as being the valves of bivalve mollusks. Aptychi are now considered to be either: (1) a two-valved closing hatch on the shells of extinct ammonites; or (2) a double-plate jaw-piece similar to that of some modern cephalopods.[1][2][3][4]
Set close to or against the shell's terminal opening (the living chamber), the aptychi usually consisted of two identical but mirror image valves. Some authors consider the aptychus to be a jaw apparatus (mandibles), while others believe them to be paired opercula. If the latter is the case, then aptychi may have had a function similar to the head shield of modern nautiluses.
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Pair of fossil aptychi (image is 1 cm across)
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[edit] References
