Shell Layering - Why?

Discussion in 'Physiology and Biology' started by Fujisawas Sake, Jan 11, 2007.

  1. Fujisawas Sake

    Fujisawas Sake Larger Pacific Striped Octopus Supporter Registered

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    Most molluscs have two calcareous shell layers, the outer prismatic layer and inner nacreous layer. What I was wondering is if there are any theories as to why this might be? I have a Cretaceous ammonite fossil that still has some of the nacreous layer intact, and I was wondering if there might be any evolutionary advantage to having calcium crystals in the shell in two distinct layers (one that gives us that oh-so-pretty Mother of Pearl of which we are so fond).

    Thoughts?
     
  2. Architeuthoceras

    Architeuthoceras Architeuthis Staff Member Moderator

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    A smooth inner "nacreous" layer for the mantle to be next to, and the outer "prismatic" layer to face the elements and/or corrosive sea water.

    I read somewhere that fossil spirulids have 3 layers, another smooth layer outside because the shell is internal, will have to look this up.8-)
     
  3. Fujisawas Sake

    Fujisawas Sake Larger Pacific Striped Octopus Supporter Registered

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    Layering and shell structure has taken all sorts of evolutionary turns, especially with the advent of the pulmonate gastropods. The texture of the nacreous layer is a good call.
     
  4. Architeuthoceras

    Architeuthoceras Architeuthis Staff Member Moderator

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    Oops!:oops:

    Looks like what I remember isnt what I remember at all. An orthocone formerly referred to Bactritida (?Ammonoidea) is referred to Spirulida (Coleoidea) because of the lack of a nacreous layer. Seems the Spirulida and this Orthocone have 2 prismatic layers.

    Doguzhaeva, L., Mapes, R., and Mutvei, H., 1999, A Late Carboniferous Spirulid Coleoid from the Southern Mid-Continent (USA) Shell Wall Ultrastructure and Evolutionary Implications, in: Advancing Research on Living and Fossil Cephalopods, edited by Oloriz and Rodriguez-Tovar. Kluwer Academic/Plenum Publishers
     
  5. Graeme

    Graeme Vampyroteuthis Registered

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    I would have thought that the inner layer would cater for movement within the shell, while reducing friction.
     

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