A PHYLOGENETIC PERSPECTIVE ON THE EVOLUTION OF BIOMINERALIZATION IN FORAMINIFERA

Foraminifera is a monophyletic clade of predominantly marine single-celled eukaryotes that branches as a subclade within the more inclusive clade Rhizaria. Recent molecular analyses of Foraminifera suggest that the evolution of a calcareous test has independently evolved as many as five times. The mineralized component of the test wall in foraminiferans may be in the form of low-Mg calcite, high-Mg calcite, aragonite, or silica. Biomineralization is accomplished using two different pathways—Golgi-mediated secretion of membrane-bound bundles of crystals vs. extracellular precipitation on an organic template. Interpretations of the evolution of biomineralization that are based solely on molecular phylogenies are limited, however, because the initial character transformations that lead to the evolution of biomineralized test components arose in extinct stem lineages during the Paleozoic prior to the diversification of the major crown foraminiferal subclades. Cladistic analysis of both fossil and Recent foraminiferans, including representative fusulinacean, involutinacean, and paleotextulariid taxa, is used to examine the evolution of biomineralization in Foraminifera. An increase in morphological complexity is correlated to the evolution of a multi-chambered test. The evolution of large tests with complex internal structures—characteristic of modern photosymbiont-bearing taxa living in tropical to subtropical reef-associated habitats—is correlated with the independent evolution of a bilayered test wall in both extinct and crown foraminiferal groups.