Before I leave the Palaeozoic cephalopods for a while, I have to sneak in this one last post. As I believe I've well and truly established by now (see earlier posts here, here and here), externally-shelled cephalopods in the Palaeozoic showed a far greater diversity of basic morphologies than their Mesozoic and Caenozoic successors - coiled gyrocones, long straight orthocones, short fat brevicones. By the beginning of the Mesozoic, almost all cephalopod shells were planar coils. A few orthoconic orthocerids lingered into the Triassic (and some ammonoid families did later experiment with different arrangements), but, overall, the coil was king.
I have also referred in association with the posts linked to above to why this was probably so - buoyancy management. The cephalopod shell, with its inbuilt flotation chambers, is a marvellous thing indeed, and was doubtless a crucial factor in allowing some cephalopods to become the biggest animals in the Palaeozoic. An exogastric (i.e. away from the venter) coil brings the centre of buoyancy more or less directly above the animal. Straight-shelled forms, of which there were many during the Palaeozoic, faced more of a challenge in this regard. Simply extending and enlarging the shell would have increased the potential buoyancy, but with the animal's buoyancy shifted towards the back end and its mass centred towards the living chamber at the front, orthoconic cephalopods with simple shells would have ended up floating permanently head-downwards with their arses sticking towards the sky - a rather inconvenient position for doing anything much. Some alternative approach was required to allow the shell to remain horizontal.
One approach that was used by a number of Palaeozoic cephalopods, such as orthocerids, was the formation of cameral deposits. Cameral deposits were mineralised layers coating the insides of the chambers (Teichert, 1964a). They became progressively thicker as they approached the apex of the shell, thus counter-balancing the weight of the living chamber at the front. They were also generally thicker ventrally than dorsally, to keep the animal upright.
If we were to assume that Palaeozoic cephalopod anatomy was just like that of a modern Nautilus (a completely unwarranted assumption, but one that has been made all too often), explaining cameral deposits poses a major dilemma. In Nautilus, the only part of the soft anatomy extending behind the living chamber is the siphuncle, a backward extension of the mantle. The siphuncle is a narrow cord running (more or less) through the centre of the chambers. Otherwise, the chambers are devoid of tissue, and the internal walls are bare (Stenzel, 1964). If orthocerids and such had the same arrangement, then the cameral deposits would have had to have been laid down in each successive living chamber before that chamber was closed off by the development of a new septa and forward contraction of the mantle. Though such an arrangement has indeed been suggested in the past, Teichert (1964a) pointed out that it was probably impossible. In many orthocones, the cameral deposits are so well-developed that the most apical chambers are entirely or nearly entirely filled by them. If they had been laid down before the formation of the next chamber, there would have been no room left in the shell for the animal itself! Also, when cameral deposits growing from opposing walls of the chamber meet in the middle, they are generally divided by a thin line, a pseudoseptum. It seems more likely that orthocerids and many other Palaeozoic cephalopods differed from modern Nautilus in possessing a "cameral mantle", a further extension of the mantle that lined the inner walls of the chambers*. While a cameral mantle may have been an ancestral feature for cephalopods (in light of the presence of cameral deposits in a number of phylogenetically disparate lineages, though some authors, e.g. Kolebaba, 2002, have recognised an order Pallioceratida defined by the presence of cameral mantle), it has not been preserved in any living cephalopod.
*A third alternative, suggested by some authors such as Mutvei (2002), is that the cameral deposits were not laid down during the lifetime of the animal at all, but instead represent post-mortem deposits formed by minerals precipitating from water penetrating the chambers. If so, they would be completely irrelevant to the animal's lifestyle. I agree with Teichert (1964a) that this seems unlikely considering the even, regular arrangement of the deposits.
An alternative solution to cameral deposits was employed by groups such as endocerids. Endocerids (which include the largest of all orthocones) had very large siphuncles, sometimes occupying nearly half the diameter of the shell (in another example of how Palaeozoic cephalopods may have differed in anatomy from modern cephalopods, Teichert [1964a] suggested that the siphuncular space in such forms may have been large enough that not only the mantle but also some of the visceral mass probably extended back past the living chamber). Instead of forming cameral deposits, endocerids weighted the apex of the shell by mineralising the siphuncle itself. The siphuncular space became filled with endocones, conical mineral layers stacked one into the next like a series of waffle cones (Teichert, 1964b). A hollow tube running through the centre of the endocones probably contained the living tissue. Because the siphuncle was such a sturdy structure, it is not uncommon for endocerids to be preserved as isolated pieces of siphuncle, with no trace of the more delicate external shell. Some structurally very distinctive groups, such as the Allotrioceratidae with two stacks of endocones pressed into the siphuncle alongside each other, are only known from such fragments of siphuncle (Teichert, 2004b), and what the remainder of the animal looked like is a complete mystery.
I do have to end this post on something of a complaint. The Endocerida, as recognised by Teichert (1964b), contains an assortment of families united primarily (as far as I can tell) by the presence of endocones. However, elsewhere in the same volume, Teichert (1964a) refers to the presence of endocones in some members of at least two other cephalopod orders, the Discosorida and Orthocerida. At least one of the families included by Teichert (1964b) in the Endocerida, the Narthecoceratidae (then known only from isolated siphuncles), has been transferred to the Orthocerida after the discovery of more complete specimens (Frey, 1981). So it would appear that all cephalopods with endocones are endocerids - except for when they are not. The stench of potential polyphyly hangs heavy in the air...
Frey, R. C. 1981. Narthecoceras (Cephalopoda) from the Upper Ordovician (Richmondian) of southwest Ohio. Journal of Paleontology 55 (6): 1217-1224.
Kolebaba, I. 2002. A contribution to the theory of the cameral mantle in some Silurian Nautiloidea (Mollusca, Cephalopoda). Bulletin of the Czech Geological Survey 77 (3): 183-186.
Mutvei, H. 2002. Connecting ring structure and its significance for classification of the orthoceratid cephalopods. Acta Palaeontologica Polonica 47 (1): 157–168.
Stenzel, H. B. 1964. Living nautilus. In Treatise on Invertebrate Paleontology pt K. Mollusca 3 (R. C. Moore, ed.) pp. K59-K93. The Geological Society of America and The University of Kansas Press.
Teichert, C. 1964a. Morphology of hard parts. In Treatise on Invertebrate Paleontology pt K. Mollusca 3 (R. C. Moore, ed.) pp. K13-K53. The Geological Society of America and The University of Kansas Press.
Teichert, C. 1964b. Endoceratoidea. In Treatise on Invertebrate Paleontology pt K. Mollusca 3 (R. C. Moore, ed.) pp. K160-K189. The Geological Society of America and The University of Kansas Press.