Calcium carbonate biomineralisation in disparate systems : common mechanisms?
This study considers the extent to which four calcium carbonate biomineral systems share common characteristics. Within the sample set, there is a range of ultrastructures and two types of calcium carbonate polymorph (calcite and aragonite). The mini survey includes three invertebrate systems: two members of the Phylum Brachiopoda; the articulated brachiopod Terebratulina retusa (Subphylum Rhynchoenlliformea) and the inarticulated brachiopod Novocrania anomala (Subphylum Craniiformea), and a member of the Mollusca, the bivalve Mytilus edulis. The fourth, outlying vertebrate system, is the eggshell of the domestic fowl, Gallus gallus. The minor element composition of each of the four systems is considered in the context of mineral ultrastructure. The shell of T. retusa comprises two layers; a primary layer of acicular calcite and an underlying secondary layer composed of calcite fibres. In thin section, a variation between the upper and lower portions of the secondary layer is evident. The concentrations of magnesium, sulphur and strontium are significantly greater in the primary layer of the shell. Magnesium concentration also differs between the upper and lower regions of the secondary layer with higher concentration in the upper portion of the secondary layer. The shell of N. anomala consists of two layers; a primary layer of acicular calcite and a secondary layer of calcite semi-nacre. N. anomala has a high magnesium calcite shell. The concentration of minor elements does not differ significantly between the primary and secondary layers. Two calcium carbonate polymorphs occur in the M. edulis shell with an outer calcite layer and an inner aragonite layer. Magnesium concentration is higher in the calcite layer while strontium concentrations are greater in the aragonite layer. Sodium concentration gradually decreases across the calcite layer from the outer surface to the calcite/aragonite boundary and increases in the aragonite layer.