Cameral membranes, pseudosutures, and other soft tissue imprints in ammonoid shells

Abstract

Ammonoids occasionally show subtle structures linked with the attachment or contact of the soft body with the shell. Only some of these structures were mineralized and thus are rarely preserved. We describe mainly three different kinds of structures, namely (i) cameral membranes, (ii) muscle imprints (excluding muscle attachment structures) and (iii) blood vessels. Cameral membranes (i) have been discovered only in a small fraction of ammonoid species and are close to the siphuncle or cut off parts of the chambers or separate the chamber volumes vertically. Pseudosutures commonly run subparallel to the normal sutures, although they are fainter. They likely formed during the forward movement of the septal mantle prior to the insertion of a new septum. Drag lines (ii) also developed in the course of this process or may represent imprints of muscle fibers. Blood vessels (iii) also left imprints in ammonoid shells.