POSSIBLE SCALE-BEARING PROTISTS IN THE MID-NEOPROTEROZOIC CHUAR GROUP, GRAND CANYON, AND UINTA MOUNTAIN GROUP, UTAH

Early to mid-Neoproterozoic rocks host a variety of organic-walled microfossils, macrofossils, mineralized scales, and vase-shaped tests thought to record the taxonomic and ecological diversification of eukaryotes. Here we report a new fossil from the <770–742±6 Ma Chuar Group, Grand Canyon, and the <770 Ma Uinta Mountain Group, Utah, that consists of circular clusters, ~9–33 µm in diameter, composed of numerous overlapping scales, ~2–7 µm long and ~1–3 µm wide. Scales are commonly folded, suggesting they were organic in life. With rare exceptions, the scales do not show radial cracks, nor is there evidence for variation in opacity or other breakage that would suggest they were hollow bodies. The circular clusters of scales maintain a relatively constant thickness in cross-section, suggesting the clusters were not composed of a three-dimensional mass of scales in life but rather represent a spherical body that was surrounded by a layer of scales. Relative thickness estimates suggest there were two to three scales arranged with their flattened sides parallel to each other and tangential to the spherical body. It is possible the scales are the remains of prokaryotes, but their flattened shape, resistant composition, ordered arrangement, apparently restricted distribution in time, and the absence of evidence for fission make this interpretation unlikely. More likely, the scales were formed by and surrounded a protistan cell. Organic scales are found in a variety of modern eukaryotic clades, including the prymnesiophytes, chrysophytes, and prasinophytes. Their widespread distribution and variety of form suggest they evolved convergently numerous times in eukaryotes, making it difficult to assess homology between the fossils and any particular modern group. Nonetheless, it may be possible to infer a function for the fossil scales based on comparisons with modern forms. One commonly held view is that scales deter predators. This is consistent with other evidence for the appearance of microbial predators by ~750 Ma, including vase-shaped microfossils interpreted as testate amoebae, a diversity of mineralized fossil scales that likely also functioned as protistan cell armor, and evidence in numerous Chuar Group acritarchs for ‘drill holes’ likely made by a predatory amoeba.