EXPANDED DIVERSITY OF SCALE MICROFOSSILS FROM THE MID NEOPROTEROZOIC FIFTEENMILE GROUP, YUKON

Diverse scale-like microfossils occur in carbonates and cherts of the pre-Sturtian (811.5 ± 0.2 Ma to 717.4 ± 0.1 Ma) Fifteenmile Group of the Yukon Territory. Because they are made of apatite, the fossils can be freed from carbonate by dissolution, making it possible to document their full morphological complexity and diversity by use of SEM. The enigmatic Fifteenmile fossils, 10-30 microns in maximum dimension, display a diversity of intricate morphologies: ovoid, hexagonal and square scales all occur, most composed of a hexagonal mesh-like network of interwoven struts but some with a solid surface. Central bosses occur in some taxa, tooth-like microstructures in others. In all, 23 species have been identified in the Fifteenmile limestones, 17 of them newly described. A wide variety of extant protists form scales, including basal green algae and species within the greater haptophyte+centrohelid clade. Phosphate biomineralization, however, is rare, with elevated Ca and P documented in only a single green algal species. Additionally, neither square nor hexagonal shaped scales have been observed in any modern or fossil scale-forming protists. Thus, the Fifteenmile fossils cannot be unambiguously allied to an extant taxon, and it is possible that the scales were formed by now extinct stem group protists. To date, these microfossils have not been reported from any other locality, but the potential to recover additional assemblages via carbonate dissolution increases the prospect that more occurrences will be discovered. Regardless of their precise taxonomic affinity, the appearance of heavily armored and morphologically complex cell-covering structures points to a changing ecological landscape in pre-Sturtian seas.