Shell microstructures of Cambrian molluscs replicated by phosphate

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Abstract
The original microstructure of the aragonistic and calcite shells of Cambrian molluscs is frequently visible at high magnifications on the surfaces of phosphatic internal moulds. More rarely, the nature of the original microstructure may be determined by examining the internal surfaces of phosphate layers which coated the shells, or by examining the phosphate fillings of the tunnels made by endolithic algae within the shells. Because the original aragonitic microstructures of molluscan shells are almost invariably destroyed by recrystallisation in rocks older than the Carboniferous, the discovery of replicated shell microstructures has provided a new understanding of the evolution of the molluscan shell. Most of the common molluscan microstructures — spherulitic prismatic aragonite, tangentially arranged fibrous aragonite, crossed-lamellar aragonite, nacre and foliated calcite — had probably appeared by at least the beginning of the Middle Cambrian, and most are found in representatives of the Class Monoplacophora. A simple biochemical model is used to explain the convergent evolution of nacreous and foliated structures from prismatic ones.