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Scheltema - The Evolution of Exoskeleton Nucleation in Shelled Fossils and Recent Molluscs

The Evolution of Exoskeleton Nucleation in Shelled Fossils and Recent Molluscs
Amálie H. Scheltema
Biology Department, Woods Hole Oceanographic Institution MS #34, Woods Hole, MA 02543, USA, ascheltema@whoi.edu
Nucleation sites of CaCO3 in exoskeletons of Recent molluscs can be related to exoskeletons in Paleozoic molluscs and mollusc-like taxa. Nucleation sites have been identified from the literature. They can be divided into taxa with and without a periostracum that is secreted in a groove of the mantle edge. In those taxa without a periostracum: (1) Nucleation is within epidermal cells. The resulting sclerites are retained in cuticle secreted by epidermal cells over the general body surface (Aplacophora, geologic age unknown; Polyplacophora, Halkieria, Cambrian). (2) Nucleation and growth is within epidermal cells as sclerites that accreted in rows held by body cuticle, later mineralized to form a solid shell (Maikhanella multa, Cambrian). In taxa with cuticle restricted to periostracum: (3) Nucleation is within individual cells of the epidermis. The prisms grow out of the cells, fuse together, and then fuse to periostracum (Nautilus, monoplacophorans? Paleozoic). (4) Periostracum is in three layers, outer and middle formed within the periostracal groove, the inner by mantle epithelium. The vacuolated middle layer is the site of nucleation and prism growth (Amblema, Mytilus, Recent). (5) Nucleation is on the periostracum from ions in the extrapallial fluid between the epithelial cells of the outer mantle fold and periostracum/shell (most Recent Conchifera). The great diversity of extant molluscs is attributed in part to the evolutionary loss of sclerites and development of periostracum and a fluid-filled extrapallial space, allowing greater freedom of movement within the shell and in the outside environment.


next up previous
Next: Tang - Detecting stability Up: Special Symposium - From Previous: Roopnarine et al. -
Peter Roopnarine 2005-04-12