Field of Science

The Horns of Ammon











  1. The fractal nature of the chamber sutures suggests to me that they were optimized for some sort of transfer across the suture. We see all sorts of other fractal structures such as capillary retes for maximizing surface area to maximize diffusion of heat or gases.

    One possible thing to diffuse (or pump) would be oxygen: this could allow more aerobic activity during dives into anoxic waters.

    A quick perusal via google doesn't seem to show this being suggested anywhere. Do you know of anything like this?

  2. Aren't they solid shell, though? It would seem a bit difficult to diffuse oxygen through aragonite.

  3. Shell is composed of an organic matrix with aragonite deposits. Because the matrix determines where the crystals develop, it would be simple to have pores in the sutures. They would not have to be large to permit gas diffusion.

    Alternatively, it might be that heat diffusion is needed: perhaps to warm or cool gas. Or it might be that attachment points need to be maximized.

    If you maximize surface area with a fractal pattern, there is probably a reason. Those are the first three I can think of.

  4. it would be simple to have pores in the sutures

    But that would destroy their suitability for one function we know they did have, containing the gas for buoyancy within the chambers (remember, there's no mantle behind the living chamber, so the animals couldn't have sealed over the septa after making a new living chamber). The siphuncular tube, on the other hand, did have extensive pores, presumably to control the removal of gas/fluid from the chambers.

  5. Why would pores in the septa destroy suitability for containing gas any more than pores in the siphuncular tube? Pores do not have to permit gas to flow: they can be plugged with permeable materials that can let gas diffuse.


    The siphuncular tube is filled with siphuncle; the chambers are presumably devoid of living tissue.

  7. The pores of the outermost septum could be plugged by living tissue, since it is right up against the body.


    Not necessarily, and certainly not for at least part of the animal's life. The body was not directly attached to the septum of the body chamber itself: the attachment muscle scars are located on the dorsal wall of the chamber in front of the septum (not incompatible with body-septum contact, but not requiring it). Living Nautilus have a fairly large fluid cavity between the body and the septum (but, as discussed in the post, what applies to Nautilus didn't necessarily apply to ammonoids). Still, The body would have had to move forward when the time came to begin growing a new septum, so there would have had to have been a fluid cavity present at that period at least. At least some authors have suggested that, if the ammonoid body was coleoid-like, it may have been relatively small compared to the size of the body chamber, and able to shift a fair way back and forth within the chamber to extend or retract the body from the shell aperture.

    I've been trying to find out whether the gas permeability of ammonoid septa has been directly tested, but so far no luck. The septa were nacreous in composition, but I don't know how permeable nacre actually is (not very, I suspect, but I could well be wrong). Whatever the rate of gas diffusion through the septa, I'd be surprised if it exceeded rates through the siphuncle, and I can't see how rapid diffusion through septa could be anything other than counter-productive. Most suggestions of a relation between septal complexity and diffusion rates (for instance, in this paper) seem to be saying the opposite: that complex septa would have reduced diffusion rates of gas in and out of the chambers via the siphuncle.


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