THE FIRST APPEARANCE OF CONTROLLED EUKARYOTIC BIOMINERALIZATION IN THE NEOPROTEROZOIC FOSSIL RECORD

Biomineralization marks one of the most significant evolutionary milestones among the eukarya. Although metazoan mineralized skeletons are abundant in the post-Ediacaran fossil record, the advent of biologically-controlled mineralization, and the role of environment in prompting this innovation, are poorly understood. Here, we report crystallographic and geochemical evidence for controlled eukaryotic biomineralization in early Neoproterozoic (Tonian) apatitic scale microfossils (ASM) from the Fifteenmile Group of Yukon, Canada. High-resolution transmission electron microscopy reveals that the ASMs are constructed of a hierarchically-organized interwoven network of fibrous hydroxyapatite crystals each elongated along [001], indicating biological control over microstructural crystallization. New Re-Os geochronological analyses from organic-rich shale directly below the fossil-bearing limestone improve the age constraints on the fossiliferous unit by an order of magnitude. Mineralogical and geochemical data from these fine-grained strata indicate that locally dynamic redox conditions increased dissolved phosphate in pore and bottom waters, thus making hydroxyapatite biomineralization a permissible option for the ASM organism. This interplay between environment and evolution therefore may have opened an early window of opportunity for the first known biomineralizing eukaryotes.