Shell Layering - Why?

[Fujisawas Sake]

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?

[Architeuthoceras]

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.

[Fujisawas Sake]

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.

[Architeuthoceras]

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

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

[Graeme]

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