The title of today's post, offhand, is a hideously contrived allusion to something that I suspect many (most?) of you will not recognise. Those of you that do recognise it, possibly wish that you didn't. Nevertheless, I'll leave it to each of you to decide for yourself whether or not this post would have been improved by the inclusion of kabuki-inspired haute couture, or chariots pulled by topless busty Amazons in lieu of horses.
Lamellorthoceras, to introduce the star of today's post, is a genus of straight-shelled cephalopods from the Lower and Middle Devonian of northern Africa. It was not a large cephalopod. Like most straight-shelled Palaeozoic cephalopods, fossils of Lamellorthoceras represent pieces of the original shell rather than the entire thing (making judging its size when alive a bit tricky), but even with a generous estimate I don't think we're talking about anything more than a few centimetres long. Lamellorthoceras forms the core of a small, mostly Devonian family, the Lamellorthoceratidae, distinguished by a very interesting feature. Like other cephalopods, the shell of lamellorthoceratids was divided into a series of chambers, with a fleshy siphuncle presumably running the length of the shell. In most other cephalopods, the chambers around the siphuncle were more or less hollow, filled with gas to give the shell buoyancy. In fossils of lamellorthoceratids, however, the chambers are filled with thin lamellae arranged in a radial pattern between the shell and the siphuncle. This is so unusual compared to other cephalopods that the lamellorthoceratid Arthrophyllum was initially described as a type of coral! Genera of lamellorthoceratids have been distinguished based on the overall shape of the shell, and by the structure of the lamellae. Arthrophyllum, for instance, has simple straight lamellae in transverse section, while the lamellae of Lamellorthoceras are wavy and/or bifurcating.
In a previous post on this site, I discussed some of the implications of such structures, called cameral deposits, for the soft anatomy of fossil cephalopods. If we were to assume that all fossil cephalopods had much the same anatomy as our only real living model, the pearly nautiluses of the Nautilidae, then cameral deposits present us with a real problem. In Nautilus, the siphuncle is sealed away from each chamber by a structure called the connecting ring, and the walls of the chambers are devoid of living tissue. The siphuncle serves to control the buoyancy of the shell by controlling the ratio of fluid to gas in the chambers, but this fluid is only secreted or absorbed via pores in the connecting rings. The only part of the nautilus shell where mineral deposits are being actively laid down is in the anterior body chamber where the living animal is housed. For fossil cephalopods to have been laying down mineral deposits within the chambers behind the body chamber, there would have had to have been outgrowths of the mantle still present in the chambers. The siphuncle could not have been an isolated unit the way it is in Nautilus. Unfortunately, the connecting rings of nautilids are delicate structures that do not preserve easily as fossils, so seeing whether they were present in lamellorthoceratids is not as simple as just looking for them. Nevertheless, Kolebaba (1999) claimed after close examination of the Upper Silurian Nucleoceras that the connecting rings of lamellorthoceratids were at least open dorsally.
However, some researchers (e.g. Mutvei 2002) hold a quite different interpretation of what the cameral deposits meant for the living animal: absolutely nothing. Perhaps they were not a feature of the living cephalopod at all, but represent sediment build-up in empty shells after the animal's death. This would have interesting implications for the lamellorthoceratids, if their primary claim to fame was a taphonomic illusion! Evidence for the inorganic origin of the cameral deposits cited by Mutvei (2002) include their different chemical make-up from the main shell, often more similar to the surrounding matrix, and specimens preserved flattened in shales with no sign of cameral deposits. However, cameral deposits are not laid down haphazardly within a shell as one might expect if they were post-mortem artefacts, but more or less consistently between specimens. Deposits growing out from opposing chamber walls and septa do not merge seemlessly, but remain separated by breaks in the deposits ('pseudosepta') that may represent tissue membranes. Flattened shale specimens may indicate an original absence of cameral deposits, or they may represent preferential dissolution of the cameral deposits under those preservation conditions. It is also possible that cameral deposits present during life may have provided nuclei for further sediment deposition after death.
Needless to say, our views on the presence in life of cameral deposits could also have strong implications for our understanding of these animal's lifestyles. If the intra-cameral lamellae of Lamellorthoceras were present in life, the shell would have held little, if any, space for buoyant gas. As such, it probably would not have had the swimming abilities of modern cephalopods; instead, it may have had a more benthic lifestyle.
Kolebaba, I. 1999. Sipho-cameral structures in some silurian cephalopods from the Barrandian area (Bohemia). Acta Musei Nationalis Pragae, Series B, Historia Naturalis 55 (1-2): 1-16.
Mutvei, H. 2002. Connecting ring structure and its significance for classification of the orthoceratid cephalopods. Acta Palaeontologica Polonica 47 (1): 157–168.
Stanley, G. D., Jr & C. Teichert. 1976. Lamellorthoceratids (Cephalopoda, Orthoceratoidea) from the Lower Devonian of New York. The University of Kansas Paleontological Contributions 86: 1-14, 2 pls.
Sweet, W. C. 1964. Nautiloidea – Orthocerida. In Treatise on Invertebrate Paleontology pt. K. Mollusca 3. Cephalopoda – General Features – Endoceratoidea – Actinoceratoidea – Nautiloidea – Bactritoidea (R. C. Moore, ed.) pp. K216-K261. The Geological Society of America and the University of Kansas Press.