DIVERSITY AND ECOLOGICAL COMPLEXITY IN ORGANIC-WALLED MICROFOSSIL ASSEMBLAGES FROM THE MID-NEOPROTEROZOIC CHUAR GROUP, GRAND CANYON, ARIZONA

Compilations of Proterozoic fossil diversity suggest that eukaryotes diversified through the early and mid-Neoproterozoic, but suffered a sharp decline prior to Sturtian glaciation. Organic-walled microfossil assemblages from the lower half of the ~770 to >742±6 Ma Chuar Group, Grand Canyon, Arizona, provide an unusually detailed glimpse of mid-Neoproterozoic eukaryotic communities just before their apparent collapse. Previous light microscopy studies of these assemblages reported the presence of at least two dozen species; SEM studies newly reported here have revealed the presence of several additional species and provided surprising new details about the morphology of previously described species. New species include one characterized by an outer wall composed of square ‘pillows’ closely packed together, and another covered in ~2 µm long elliptical structures tentatively interpreted as organic scales. The species formerly assigned to Trachysphaeridium laminaritum and previously interpreted as having a wall covered in hemispherical shallow depressions instead appears to possess a wall consisting of a reticulated meshwork of ~0.3 µm wide fibers arising from an inner layer and surrounded by a thin outer envelope. Specimens previously assigned to Lophosphaeridium laufeldi possess a wall composed of similar fibers that likewise extend from an inner layer and are covered by an outer envelope; in many specimens three or four fibers merge to form a single point, and a few specimens exhibit both single fibers and a reticulated meshwork similar to that of cf. T. laminaritum. Thus, specimens of cf. L. laufeldi and cf. T. laminaritum are tentatively interpreted as collectively recording developmental stages of a single species. SEM studies have also revealed the presence of ~0.2 µm diameter round holes irregularly distributed in the walls of specimens from a variety of species. These are similar to holes made today in fungi and algae by predatory vampyrellid amoebae and are thus tentatively interpreted to reflect predation by amoeboid protists. Together with the vase-shaped microfossils found in uppermost Chuar Group strata, these organic-walled microfossil assemblages indicate the presence, at least locally, of a diverse and ecologically complex biota several million years before global glaciation.