Systematics of organic-walled microfossils from the ca. 780–740 Ma Chuar Group, Grand Canyon, Arizona

he ca. 780–740 Ma Chuar Group, Grand Canyon, Arizona, provides an exceptional record of life during the diversification of crown-group eukaryotes, just prior to the first Cryogenian glaciation. We document in detail the assemblage of organic-walled microfossils preserved in fine-grained siliciclastics throughout the unit. In contrast with earlier studies, we primarily used SEM to document fossil morphologies, augmented by transmitted light microscopy, FIB-SEM, and TEM. This resulted in the discovery of new species and the recognition of broad-ranging, intraspecific biological and taphonomic variation in other species. Twenty-two species and five unnamed morphotypes are described, including three new species: Kaibabia gemmulella, Microlepidopalla mira, and Volleyballia dehlerae; two new combinations: Galerosphaera walcottii and Lanulatisphaera laufeldii; and 17 previously described forms. The possible colonial green alga Palaeastrum dyptocranum Butterfield in Butterfield, Knoll, and Swet...