POSSIBLE MICROMETAZOAN COPROLITES FROM THE JOHNNIE FORMATION (LATE NEOPROTEROZOIC) OF THE MOJAVE DESERT, CALIFORNIA

The late Neoproterozoic Johnnie Formation crops out in Death Valley and nearby ranges of the southern Mojave Desert. It overlies the Noonday Dolomite, which is considered to be the cap carbonate laid down at the end of the last Proterozoic glaciation, and is overlain by the Stirling Quartzite, which has yielded Ediacaran fossils in its uppermost beds. The Johnnie Formation has not been radiometrically dated, but is at least roughly correlative with the Doushantuo Formation of China: postglacial, but predating the widespread appearance of the Ediacara biota. In a section exposed in the Nopah Range of southern Inyo County, California, a prominent horizon in the lower half of the formation is composed of stromatolitic limestone, with domical and columnar stromatolites present. Acid dissolution of stromatolite samples reveals extremely thin layers of acid-resistant material, interspersed with the stromatolitic banding and apparently authigenic. Scanning electron microscopy of these layers shows Fe-containing microbial pseudomorphs, as well as ovoid bulges, between 250 and 350 microns in their widest dimension, forming short chains or small clusters and apparently deforming each other. X-ray spectroscopy reveals that both the bulges and the surrounding matrix are aluminosilicate clays, but that the bulges are significantly richer in iron than the surrounding matrix. The shape of the bulges is very close to that of certain late Proterozoic trace fossils described as fecal pellets, including both unnamed fossils and specimens identified as Palaeopascichnus and Neonereites. I provisionally interpret the specimens from the Johnnie Formation as fecal pellets made by unknown grazing metazoans with a complete gut.