Field of Science

Open Query – What are Cephalopod Shells For?

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

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 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?


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.


  1. What's wrong with using it for protection? Is it that the septa would have no use, then?

  2. Hmmm, original function of the septate shell? Good question. Without knowing anything about basal cephalopods, cephalopod development, or phragmocone microstructure (I suppose I'm what many cable news channels would call an "expert"), here's a few ideas to falsify/entertain:

    - It strikes me that segmentation (ontogenetic/serial homologous segmetation anyway) in metazoans is closely tied to developmental pathways. Admittedly this gets a bit circular...but perhaps one solution for shell growth in basal cephalopods was to repetitive addition of sections rather than simple marginal addition?

    - Structural support? The early cephalopod shells seem pretty thin relative to say average gastropod/bivalves, although maybe this is a Mesozoic Marine Revolution thing? Still, since they lacked torsion and were (apparently) relative active (?) perhaps they required internal reinforcement in their shells?

    Just some thoughts, surely you and others have a better perspective on this.

    I might as well mention that "exaptation" has caught on on certain uncharted backwaters of the intertoobz.

  3. I really can't html my way out of a paper bagcan I?

  4. My first thought was Neil's suggestion of structural support, but if the shell chambers are not actually occupied by the animal then what benefit is this? Also, wouldn't this still require marginal addition for the basal chamber that is actually occupied by the growing animal?

    A segmentary origin, hmm... sort of like a "vertical polyplacophoran".

    It seems the animal actually "wanted" chambers for some reason, but any use other than something related to gas regulation is hard for me to comprehend. So I guess I haven't really contributed anything here.

    regards -- ted

  5. O.k. i have to ask a question now that you are on cephalopoda.

    BTW, I have been enjoying your blog for quite a while now. (I am a bio-hobbyist, raised by a biology/zoology teacher, so I get into weird life forms and unusual modes of living.)

    While i was perusing your link to Palaeos, I stumbled across this living cephalopod: "Spirula".

    It looks like it has an internal shell with dorsal siphuncle. As I recall my paleontology classes, that is a sign of an Ammonite type of cephalopod.

    It's position seems to be uncertain, and I am in a heated discussion with several of my friends as to whether this might indeed be a a "Living Fossil" of the Ammonites.

    What would be the kind of questions that might answer this question?

  6. Even if the entire shell isn't occupied, it is still serving some (presumably) protective function for the basal cephalopods so it would make sense to keep it sturdy and intact (to me anyway).

    I thought about drawing the polyplac analogy but of course their segmentation is fixed rather than additive right? What about the Halkierids?

    While the phylogenetic position of Spirulids is contentious (or used to be anyway) all the molecular phylogenies that I've seen have Spirula comfortably nested within Coleoidea. Also the shell of Spirula coils in a reverse mode compared to ammonites (endogastric vs. ectogastric) right? A living ammonite would be pretty awesome but I think Spirula along with it's similarly imposterous cousin Argonauta is just trying to punk us.

  7. Thanks for all your comments.

    I don't think segmentation would have anything to do with it, because (a) molluscs are not, nor to the best of anyone's knowledge have they ever been, segmented (the seriality found in a few molluscs such as polyplacophorans, neopilinids and, yes, if muscle scars are to be believed, some of the potential ancestors of cephalopods relates to a small number of organ systems and doesn't reflect any real underlying segmentation), and (b) the septa of cephalopods are arranged dorsoventrally rather than anteroposteriorly. This latter is not so obvious from modern cephalopods, because cephalopods have undergone a 90 degree rotation in their body axes, but is fairly clear in stem-cephalopods.

    I'm intrigued by Neil's idea of structural support. Cephalopods did have stronger-shelled ancestors, but maybe the septa allowed the ancestral cephalopods to reduce the weight of the shell without sacrificing structural strength? Greater energy efficiency while retaining protection from predators. This may also have allowed cephalopods to attain greater size than their limpet-like ancestors. It also may explain why early mini-cephalopods had such closely-packed septa, as if the septa were too widely spaced predators could still easily attack the more delicate sections between septa. The great thing, too, is that I can imagine how one would test this idea by comparing the structural strength of the different shell designs.

    Steve - I'm sorry, but there is no chance that Spirula is living ammonoid. As Neil noted, Spirula is in a fairly derived position in the coleoid family tree (among the squid), as well as having the shell internal rather than external, different details of shell structure from ammonoids, etc. As I noted in the ascocerid post, the coiled shell has evolved multiple times among cephalopods (nautilids and ammonoids both became coiled independently from straighter ancestors) because it's one of the most efficient arrangements for maintaining stable buoyancy.

    Argonauta is even more of a fraud than Spirula, because the 'shell' of Argonauta isn't even a shell in the usual molluscan sense - it's an egg-case. The female secretes it to contain her eggs, then discards it. Argonauta males and non-breeding females are fairly ordinary octopods.

  8. Shucks! I've been punked by a sneaky squid.

    Thanks for your patience in responding to a lowly hobbyist!

    Keep up the good blogging!

  9. Well, if you want to take the FEA approach I can have one of the computer nerds er, genii in my lab work his/her magic.

  10. I agree with Chris's comments about segmentation. We usually think of the shell's open end (aperture) as the front and the pointy end (apex) as the back, and this may have been true of later orthocones. But Kröger 2008 uses muscle attachment scars from the earliest fossil cephalopods to argue that their serially repeated muscles ran parallel to the last septum. That implies that the animal's head and foot both emerged from the aperture, which was ventral, while the apex was dorsal.

    KRÖGER, BJÖRN. 2007. Some lesser known features of the ancient cephalopod order Ellesmerocerida (Nautiloidea, Cephalopoda). Palaeontology 50 (3): 565-572.

  11. I like the idea of testing the structural support hypothesis by measuring the weight saved by thinning the outer shell of the septate portion of the conch. (Of course, it should be noted that every portion of the outer shell was at one time in the animal's life part of the body chamber and thus unsupported by septa for some period; the outer wall could not start out thick during the body chamber period and then be secondarily thinned later in life -- it had to be secreted at whatever thickness was demanded functionally at any and all times in the animal's life. Thus it's not clear that we can expect much of a thinning or weight savings.) However, the preservation of Paleozoic cephalopods -- especially these Cambrian bugs -- is as molds and casts, or at best as calcite (or chert?) replacements. Would such fossils even allow us to measure shell thickness accurately and precisely?


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