Open Query – What are Cephalopod Shells For?

Cross-sectioned reconstruction of the Late Cambrian cephalopod Plectronoceras, from Palaeos.com.

I'm continuing with the "nautiloid" theme here (previous posts on Palaeozoic cephalopods can be found here and here). The primary reason why Palaeozoic cephalopods have been occupying my mind lately is that I am currently working my way through the Treatise of Invertebrate Paleontology volume on "nautiloids" (as explained in an earlier post, the lumping of all non-ammonoid, non-coleoid cephalopods under the name of "nautiloid" is a really unfortunate example of paraphyletic lumping, which is why I'm insisting on the quotation marks). For those unfamiliar with them, the Treatise of Invertebrate Paleontology (or simply the Treatise to its friends) is a series of volumes cataloguing the known genera of fossil invertebrates, and each volume also includes a series of introductory essays describing the anatomy, palaeoecology, etc. of the group it covers. I've commented before that there is something incredibly purgatorial about a Treatise volume. It's a long, painful, torturous slog that hammers you both mentally and physically*, but when you finally come out the other end there can be no doubt that you've done something really worth achieving.

*Anyone who doubts that a book is able to challenge you physically has not had to carry a bag holding the entire 1000 pages plus in three volumes of the Treatise section on crinoids, nor been assualted by a falling copy of the 1100 pages of Roewer's Die Weberknechte der Erde.

The question that is currently residing in my mind also, in a roundabout sort of way, relates to the recent publication of the fossil avialian Epidexipteryx ('Les'). Typically for a Nature article, the revolutionary nature of that fossil has been more than a little exaggerated, but it has publicised the growing consensus among palaeontologists that feathers were not originally used for flight when they first evolved. It is more likely that they were originally used for insulation, and only later became used for other functions such as display and flight*. Gould & Vrba (1982) actually coined a term for this phenomenon, 'exaptation', which was meant to refer to cases where a feature that originally evolved for one function was co-opted for use in another function, as opposed to 'adaptation', when a feature originally evolved directly for its current function. The term 'exaptation' in itself never really caught on, because almost all 'adaptations' are in some way 'exaptations'. However, the verb form 'exapted' remains useful when describing examples of such a process.

*I'm going to speculate a little and suggest that the use of feathers for display may have even been a necessary prerequisite for their use for flight, because the planar surface required for flight may have been less likely to be selected for insulation than for maintaining a stereotyped form for display. This may be why flightless maniraptorans such as Caudipteryx possessed planar tail and arm fans.


Cross-section of the Late Cambrian cephalopod Yanheceras, showing the closely-spaced septa in this 12 mm shell. Image from Palaeos.com again.

What is the connection between cephalopods and the origin of feathers? As note before, fossil cephalopods can be distinguished from all other molluscs by their unique shell structure, with the shell divided into a series of internal chambers. In most shelled cephalopods, the chambers would have been/are mostly hollow and filled with gas whose volume can be adjusted to control shell buoyancy. But how and why did this unique structure evolve in the first place? Contrary to first impressions, the chambers were probably not used as floats when they first appeared. The really early cephalopods, such as plectronoceratids and ellesmeroceratids, were small subconical shells, generally only a few centimetres in length. Despite their small size, their shells were divided into relatively large numbers of chambers, with the dividing septa packed close to each other. Even if the chambers were filled with gas (which they may not have been - it is currently unknown just when in cephalopod evolution the chambers became gas-filled), it is unlikely that the volume of the chambers would have been enough to lift the weight of the shell. Plectronoceratids, etc., were almost certainly benthic (Furnish & Glenister, 1964). Because these early forms are mostly outside the cephalopod crown group, there is currently no way of knowing whether they shared any of the soft-body features associated with modern cephalopods, such as tentacles and the siphon, or whether they still had a more primitive, superficially snail-like (though untorted) morphology.

Holland (1987), if I understand correctly, suggests that the septate shell, either by increasing relative buoyancy (even if it did not make the animal actually buoyant) or by changing the distribution of the shell's weight, may have made cephalopod locomotion more energy-efficient, allowing greater mobility. Does this seem like a likely explanation? Can any of you think of an alternative? And how would you suggest we test any likely explanations?

REFERENCES

Furnish, W. M., & B. F. Glenister. 1964. Paleoecology. In Treatise on Invertebrate Paleontology pt K. Mollusca 3 (R. C. Moore, ed.) pp. K114-K124. The Geological Society of America and The University of Kansas Press.

Gould, S. J., & E. S. Vrba. 1982. Exaptation; a missing term in the science of form. Paleobiology 8 (1): 4-15.

Holland, C. H. 1987. The nautiloid cephalopods: a strange success. Journal of the Geological Society of London 144 (1): 1-15.

Moore, R. C. (ed.) 1964. Treatise on Invertebrate Paleontology pt K. Mollusca 3. The Geological Society of America and The University of Kansas Press.