390 FOSSIL INVERTEBRATES/Cephalopods (Other Than Ammonites) Figure Form and structure of the living Nautilus (Reproduced from Doyle P (1996) Understanding Fossils Chichester: John Wiley.) is more typical of the Ammonoidea (see Fossil Invertebrates: Ammonites); simpler sutures are found in the Nautiloidea and, where present, in the Coleoidea Experiments have shown that the watch-glass shape imparts significant strength to the shell, and it is suggested that the water depth at which implosion will occur may be in excess of 500 m, based on experiments using living Nautilus The camerae are joined by a continuous porous pipe known as the siphuncle, which passes through each septum by means of a ‘bottle-neck’ arrangement, the septal neck (Figure 2A) This neck has many forms, most of which point forwards (prochoanitic) or backwards (retrochoanitic), with grades in between, depending on the taxon Ultimately, the siphuncle joins all the chambers through the septal necks to the body chamber These septal connectors are often modified with complex structures – difficult to interpret in a functional sense – which are common features of fossil nautiloids In fact, apart from living Nautilus, which has a simple siphuncle, many of the higher nautiloid taxonomic groups are distinguished by such structures Characteristically, the siphuncle is placed centrally in the living Nautilus and several nautiloid groups, but is marginal in other nautiloids and in ammonoids and shell-bearing coleoids The prime function of the camerae is to provide buoyancy, and it is the role of the siphuncle to regulate the mixture of gas and liquid within each camera in order to create a buoyant state Regulation of buoyancy is such that during a diurnal cycle, based on recent studies, ectocochleate cephalopods are able to move through the water column In some cases, the camerae can be modified to improve the buoyancy, with additional shell aragonite being laid down within each chamber – cameral deposits – to aid the balance of the shell Similar deposits are found within the siphuncles of some fossil nautiloids – endosiphuncular deposits – although, again, they are absent from the relatively uncomplex Nautilus Cephalopod Classification The cephalopods are generally treated as a class within the Phylum Mollusca (see Fossil Invertebrates: Molluscs Overview), but it is here that consensus ends The class Cephalopoda is further subdivided into subclasses, but there is considerable debate about the relative ranks of the constituent cephalopod groups For example, following the Treatise on Invertebrate Paleontology some authors propose a division of the Nautiloidea into four separate subclasses in their own right: Endoceratoidea, Actinoceratoidea, Orthoceratoidea, and Nautiloidea (in a restricted, yet still very broad, sense) A simple classification, involving subdivision into three more-or-less equal groups, is employed here Class Cephalopoda Cuvier, 1797 (Cambrian– Holocene) Fully marine free-swimming selfbuoyant molluscs with a differentiated head and well-developed sensory apparatus Subclass Nautiloidea Agassiz, 1847 (Cambrian– Holocene) Cephalopods with coiled or straight (orthocone) ectocochleate shells with simple chamber walls; the siphuncle is often complex, marginal to central; living examples have multiple arms; tetrabranchiate