The lower jaw of the ammonite Spathites

[Halichoeres]

The aptychus is a paired structure that has been known for a while to be associated with ammonites, although it's rare to have one preserved with the ammonite's shell, because aptychi are usually made of calcite, whereas shells are typically aragonite. They are both form of CaCO₃, but most fossilizing environments will preserve one or the other, but not both. This paper describes one found in association with a shell, and interprets it as a lower jaw, rather than as an operculum or hood, which is the way aptychi are often depicted in art. Here's a reconstruction:


This would have sat well inside the shell aperture and functioned to process food. In that sense it's more analogous to a squid's beak than a nautilus hood, although squid beaks are composed mainly of chitin.

Paper is open access (at least in the US): http://digitallibrary.amnh.org/handle/2246/6935