PATTERNS IN THE EVOLUTIONARY ACQUISITIONS OF MINERALIZED SKELETONS IN EUKARYOTES

Mineralized skeletons evolved many times within eukaryotes, providing multiple independent data points for testing hypotheses about skeletal evolution. Earlier work showed that acquisitions of aragonite vs. calcite skeletons in animals are clustered in time and correspond to the mineralogy favored by seawater at that time. We have since expanded this dataset to include other mineralogies and other eukaryotes and have identified >80 acquisitions of mineralized skeletons in eukaryotes. Several patterns have emerged. First, the phylogenetic distribution of skeletal mineralogies tends to be non-uniform: most skeletal acquisitions in animals and archaeplastidans are of carbonate, for example, whereas most acquisitions in rhizarians and stramenopiles are of silica. Second, acquisitions of phosphatic skeletons are concentrated in the Neoproterozoic and Cambrian, and, with one possible exception, occurred only in non-photosynthetic taxa. Finally, more than 60% of animal acquisitions occurred in the early Cambrian, whereas those of other 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 widespread biomineralization in animals. Given the frequent acquisition of mineralized skeletons across the tree of eukaryotes it is puzzling that mineralized skeletons are not common until several hundreds of millions of years after the origin of the eukaryotic crown group. We offer several ideas for why this might be.