Hermit crabs loved ammonite shells, too. Plenty of other [organism] in [organism] goodness here.
Laelaps again: Ammonoid pearls! Sadly there will be no fossilized ammonite-pearl necklaces, since these were the sort of pearl accretions that just result in entombed parasites in the wall of the shell, but that very fact means we can infer coevolution with those parasites over geological time. Lovely.
3D ammonite radula reconstruction (with video!) using an X-ray technique rather than traditional sectioning revealed that the jaws and teeth of the type examined are... pretty wimpy, actually. The original article suggests that the unusual ammonoid jaw assembly is an adaptation to feeding mainly on plankton in the water column.