Ammonoid internal shells?

Discussion in 'Cephalopod Fossils' started by Phil, Jun 29, 2007.

  1. Phil

    Phil Colossal Squid Supporter Registered

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    I had a chat to some chap last night who was of the opinion that hoplitid ammonites may have had internal shells based on external scarring he may have identified. Not all ammonites mind, but just that rather wonderful keeled, ridged and spined family.

    I'm not convinced but open minded. Any opinions anyone?
     
  2. Architeuthoceras

    Architeuthoceras Architeuthis Staff Member Moderator

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    Hoplitids are the ancestors of Placenticeras, the shells that the Ammolite in canada comes from, so the group obviously has a tendancy for nacreous shells. Some placenticeras shells are very large and I just cant see them being internal. I have seen some shell remains from the large placenticeras I find around here and I am quite sure the inside is alot smoother than the outside, seems an internal shell would be smooth inside and out. Still, I will keep an open mind, as many a so called truth has been shot down with the gain of new evidence.

    Any chance of a pic of this external scarring?
     
  3. monty

    monty Colossal Squid Staff Member Supporter

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    From my place of gross ignorance, I'm going to suggest that a smooth outside is more likely to be for streamlining as an external shell, and ornamentation might be more likely to serve the purpose of keeping an internal shell from sliding with respect to the animal flesh surrounding it.

    Of course, I just made that up, so even calling it a hypothesis is rather bogus, I'm mostly just trying to stir up the discussion!
     
  4. Phil

    Phil Colossal Squid Supporter Registered

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    Smooth shells may be useful streamlining, but then some of the hoplitids, and other families, had keels which I suppose may have channeled waterflow and possibly aided directional control. The ramshorn squid, Spirula, has a utterly smooth internal shell that still retains the phragmocone and siphuncle. I'm sure that if hoplitid ammonoids had tracing of muscle scarring on the outside of the shell, then someone would have noticed by now.

    I suppose there is also the problem of spines; it's hard to imagine spines growing through an enveloping fleshy mantle from the shell surface. Are there any other molluscs that do this does one think?

    That leads to another question, how exactly does Nautilus repair shell damage away from the secreting edge of the head?
     
  5. Architeuthoceras

    Architeuthoceras Architeuthis Staff Member Moderator

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    I dont have access to the following paper, but it may have some evidence for the mystery at hand.

    Doguzhaeva, L. A. and Mutvei, H., 1993, Structural features in Cretaceous ammonoids indicative of semi-internal or internal shells, in: The Ammonoidea: Environment, Ecology, and Evolutionary Change, Systematics Association Special Volume 47 (M. R. House, ed.), Clarendon Press, London, pp. 99-104.
     
  6. Phil

    Phil Colossal Squid Supporter Registered

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    Thanks Kevin!

    I've found the abstract but the article does not seem to be online unfortunately. The authors seem to conclude that there is some evidence that the shell was (at least) partly internal in at least three genera.

     
  7. Architeuthoceras

    Architeuthoceras Architeuthis Staff Member Moderator

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    Thanks Phil, looks like at least some of the ammonites may have had internal shell deposits on the outside of the shell. I wonder if the Hoplitids have any of these deposits?
     

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