THE MULTIPLE CAUSES AND ORIGINS OF BIOMINERALIZED SKELETONIZATION

Although the fossil record is clear on the Cambrian explosion being a complete ecological and evolutionary turnover, the view of it as mainly a biomineralization event has been difficult to stamp out. The almost simultaneous appearance of different shell minerals in different types of skeletons with different functions has made hypotheses of any single mechanism difficult to entertain. Biologically induced mineralization is probably as old as life itself (it is often a side effect of life processes) whereas biologically controlled mineralization requires sophisticated physiological mechanisms and appears later in evolution. The use of biominerals to strengthen animal skeletons is a further development of controlled mineralization that calls for an intricate integration of organic and inorganic materials. When skeletons appeared in animal evolution near the Precambrian-Cambrian boundary, a small selection of the many available biominerals were used for this purpose. The occurrence of particular biominerals in particular lineages has been interpreted in different ways: as phylogenetically constrained necessities, as temporary choices which can be inverted later in evolution, or as reflections of the initial conditions under which the particular skeletons evolved. The variety of skeletal structures and biominerals in the Cambrian, as well as an unexpected find of a modern analogue of Cambrian sclerite-bearing animals, suggest that the phylogenetic significance of sclerite shape is limited, that the choice of skeletal mineral reflects the environmental conditions under which the skeleton first evolved, and that, once established, the originally selected mineral is not replaced. Biomineralized skeletons most probably had multiple phylogenetic origins and evolved as a response to various environmental influences.