The internal structure of cephalopods is of particular importance in their classification, particularly among fossil forms. Because the shell is aragonite (as, of course, are those of most molluscs) fossils usually occur as internal molds, with the shell dissolved away. The photos below show closer views of the specimens shown above. The one at left shows the suture or line of juncture between the internal shell wall and the last septa. This suture exists because the septa are added to the shell sequentially as they are needed. Fossilization typically involves filling the camerae with sediment and then dissolving away the shell. The suture would then be visible at the surface of the resulting internal mold. The photos at right and below show two other related features of the septa and chambers. The expanded opening that connects each pair of adjacent chambers through them is called the septal neck. In life, an organic tube (the siphuncle) connects all the septal necks. The function of this system is to allow removal of residual body fluids from the chambers as they form, giving the phragmocone a positive buoyancy with respect to the body and keeping the animal oriented properly in the water. It was assumed for many years that the cephalopods could rapidly pump water into and out of the chambers to control their overall buoyancy and thus depth, but this has now been proven wrong.
In some cases cephalopod fossils are preserved as molds of isolated chambers (photo below left Carter Coll. Gg3 -- Devonian of western Tennessee), but the many chambers or even the entire skeletal mold also holds together in many cases (below right Carter Coll. Gg2 -- Ordovician of southwestern Ohio). Notice that the lower of the specimens on the left shows the mold of the face of a septum, with the position of the septal neck quite obvious. The bottom-left photo (Carter Coll. Gg1 -- Ordovician of northern Kentucky) also shows the septal neck, and the one to its right (Carter Coll. Gg8 -- Ordovician of northern Alabama) shows a longitudinal cut of an endoceroid cephalopod with a very large-diameter siphuncle.
All the cephalopods on this page have simple septa that make straight or smoothly curved sutures with the shell wall. Such sutures are informally called nautiloid type after the genus Nautilus. Several orders of cephalopods in the Paleozoic had nautiloid sutures, for example, the Endoceroida, with their large-caliber siphuncles, but only the order Nautiloida in the post-Paleozoic had/has them. More correctly these are called orthoceratitic sutures.
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