BENGTSON, Stefan: Biomineralized skeletons – when, where, and why did they evolve?

Biomineralization can be usefully thought of as biologically induced vs. biologically controlled. Biologically induced mineralization is probably as old as life itself (it is often a side effect of life processes) whereas biologically controlled mineralization generally 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. A number of animal groups appearing in the Early Cambrian evolved dermal sclerites of various structure and composition. The variety of sclerite shapes 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. The question why some animals evolved mineralized skeletons appears to be easier to answer than the question why others did not.