Evolutionary patterns in skeletal biomineralization*

Mineralized skeletons evolved many times within eukaryotes, providing multiple independent data points for testing hypotheses about skeletal evolution. We have identified >80 acquisitions of mineralized skeletons in eukaryotes along with their time of first appearance and their mineralogy, and have found several interesting patterns. First, the distribution of skeletal mineralogies tends to be non-uniform: most animals and archaeplastidans, for example, chose carbonate, whereas most rhizarians and stramenopiles chose silica. Whether this reflects functional constraints, e.g. related to uni- vs. multicellularity, or phylogenetic constraints, e.g. related to homologous genes involved in nucleation/inhibition of these minerals, is unclear. Second, acquisitions of phosphatic skeletons are clustered in the Cambrian, perhaps due to high PO₄^3- in the oceans at this time. Finally, more than 60% of animal acquisitions occurred in the early Cambrian, whereas those of other eukaryotes are distributed more uniformly in time, supporting the view that factors affecting only animals, e.g., the appearance of carnivores, rather than factors affecting all marine organisms, e.g., increased Ca²⁺, drove widespread biomineralization in the early Cambrian.