FIRST REPORT OF THE EDIACARAN CLOUDINID FOSSIL CONOTUBUS IN NORTH AMERICA AND ITS APPARENT PRESERVATION BY DIRECT PRECIPITATION OF IRON OXIDES AND IRON-RICH ALUMINOSILICATES

We report the first occurrence of the non-calcareous cloudinid fossil Conotubus outside of China. Along with the alga Elainabella and other as-yet-undescribed taxa, Conotubus cf. C. hemiannulatus is found in black shales of the Esmeralda Member of the Deep Spring Formation in Esmeralda County, Nevada. Conotubus was previously known only from the Gaojiashan Lagerstätte of South China. Its occurrence in the Deep Spring Formation of Nevada, along with Cloudina lower in the section, permits a rare Ediacaran biostratigraphic correlation between South China and Laurentia.

Microprobe analysis (Wavelength- and Energy-Dispersive X-Ray Spectroscopy) of several specimens suggest an unusual taphonomic pathway that appears to be different than that in the Gaojiashan Lagerstätte. There is a strong prevalence of iron-rich minerals other than pyrite in the tissues of preserved specimens. Iron-oxides and iron-rich aluminosilicates are the dominant authigenic minerals. While decomposing organic tissue, denitrifying bacteria have the enzymatic ability to concentrate Fe(iii) ions in their surrounding microchemical environment during the reduction of nitrate. During this process, Fe (ii) ions are oxidized to Fe(iii). The result can be the precipitation of a large variety of iron oxides, including ferrihydrite, goethite, and limonite (collectively, iron oxides), as well as iron-rich aluminosilicates. Wavelength- and energy dispersive X-ray spectroscopy show quantitative evidence of the complex mixing of these two end members, from more-or-less pure concentrations to 50/50 mixtures. These data suggest that the tube walls of Conotubus in the Deep Spring Formation were not preserved through pervasive pyritization (the dominant taphonomic pathway in the Gaojiashan Lagerstätte), but rather by various degrees of mixing of iron oxides and iron-rich aluminosilicates.