Patterns in skeletal biomineralization

Mineralized skeletons evolved many times in the eukaryotes, including dozens of times in animals. These numerous independent acquisitions provide an opportunity to test hypotheses about the origin and evolution of skeletons, including factors controlling the choice of mineralogy and the reasons why mineralized skeletons evolved. Earlier work showed that the choice of carbonate mineralogy (aragonite vs. calcite) in animals reflects the chemistry of seawater at the time the skeleton evolved. We have since expanded this dataset to include other mineralogies and other eukaryotes and have identified >80 acquisitions of mineralized skeletons. Several patterns have emerged. First, acquisitions among animals are clustered, with more than half appearing in the early Cambrian, and another 25% in the Ordovician through Devonian. A smaller cluster of acquisitions occurred in the mid-Triassic to Jurassic, with nearly all in the cnidarians and annelids. In contrast, skeletal acquisitions in non-animal eukaryotes are distributed throughout the Phanerozoic, with no obvious clustering in time. This supports the view that ecological factors affecting only animals (e.g., the appearance of carnivores), rather than physical factors affecting all marine organisms (e.g., increased seawater [Ca²⁺]), drove biomineralization in animals. Second, acquisitions of phosphatic skeletons are concentrated in the Neoproterozoic and Cambrian, perhaps reflecting higher seawater phosphate at this time. Finally, the distribution of mineralogies tends to be non-uniform: 73% of the acquisitions in the animals and all of the acquisitions in seaweeds were carbonate, whereas all of the acquisitions in the stramenopiles were silica. Whether this reflects functional constraints, e.g. related to unicellular vs. multicellular organization, or phylogenetic constraints, e.g. reflecting underlying homology of genes involved in the nucleation and/or inhibition of these particular minerals, is not clear.