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.

Conch structures, soft-tissue imprints and taphonomy of the Middle Ordovician cephalopod Tragoceras falcatum from Estonia

“…2a). Similar imprints have been documented by several authors in ammonoids, bactritids and Mesozoic and extant nautilids (Zaborski 1986, Hewitt et al 1991, Richter 2002, Richter and Fischer 2002, Klug et al 2008, Polizzotto and Landman 2010, Polizzotto et al 2015. In contrast, they have not yet been described to our knowledge in Palaeozoic nautiloids apart from a few Devonian orthocerids (Kröger et al 2005).…”

“…Nevertheless, the fact that they disappear slightly adorally suggests that these delicate structures might not always be preserved. We interpret these structures as pseudosutures, which are occasionally associated with drag bands but run parallel to the sutures instead of in the direction of growth (Polizzotto et al 2015). Note that the pseudosutures are distinct from the wrinkle layer ("Runzelschicht") well known in ammonoids and certain nautiloids (e.g.…”

Ammonoid Shell Microstructure

“…This may have made it possible to keep cameral and circumsiphonal fluids in a state of decoupling, which may have been beneficial from the physiological point of view (Mutvei 1967;Kulicki 1979;Kulicki and Mutvei 1988;Ward 1987;Weitschat and Bandel 1991). For further discussion of intracameral membranes, see Polizzotto et al (2015).…”

Buoyancy control in ammonoid cephalopods refined by complex internal shell architecture

“…Furthermore, this organ can facilitate pumping against the hydrostatic pressure of water, allowing functionality at a great range of depths 22 , 23 . Capillary membranes (i.e., the pellicle) line the chambers 24 – 27 , which allow liquid to be transported to the siphuncle even when they are decoupled 19 , 21 , 26 , 28 , 29 . The suture formed at the junction between each septum and the inner surface of the shell wall presents a repeating pattern easily observed in fossils (Fig.…”