Protistan Skeletons: A Geologic History of Evolution and Constraint

Abstract

The tests and scales formed by protists may be the epitome of lightweight bioconstructions in nature. Skeletal biomineralization is widespread among eukaryotes, but both predominant mineralogy and stratigraphic history differ between macroscopic and microscopic organisms. Among animals and macroscopic algae, calcium minerals, especially carbonates, predominate in skeleton formation, with most innovations in skeletal biomineralization concentrated in and around the Cambrian Period. In contrast, amorphous silica is widely used in protistan skeletons, and a majority of the geologically recorded origins of silica biomineralization took place in the Mesozoic and early Cenozoic eras. Amorphous silica may be favored in protist biomineralization because of the material properties of both silica itself and the organic molecules that template its precipitation. The predominace of carbonates and phosphates in macroscopic skeletons may, in turn, reflect the low quantities of dissolved silica in fresh and marine waters. The evolutionary success of diatoms has depleted silica levels in surficial waters since the Cretaceous Period, and fossils show that other biological participants in the silica cycle have responded both through altered habitat preferences and reduced use of silica in test construction. These natural instances of doing more with less might serve to inspire continuing innovations in biomimetic design.