- Solnhofen Limestone
- Eichsatt, West Germany
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. Set near 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.