DISCOIDAL FOSSILS FROM THE EDIACARAN WOOD CANYON FORMATION, NYE COUNTY, NV, USA: NOT ALL DISCS ARE THE SAME.

The Ediacaran Period is well-known for the emergence of macroscopic complex life, including first evidence of embryos, macroscopic animals, an explosion of macroalgal morphological diversity, and the first purposeful trace fossils. With the relative scarcity of fossil deposits in the Ediacaran Period, however, even finding the occasional discoidal fossils may be considered sufficient for age constraints and correlation efforts. Unfortunately, such discoidal structures may form in a wide variety of ways, including true fossils and as abiotic mimics. The biological discoidal structures may form in at least two ways: as microbial colonies, or as the remains of more complex eukaryotic organisms. Here, we present data from the late Ediacaran lower Wood Canyon Formation in Nye County, Nevada, which has shown significant paleontological promise in recent years. Investigations in the Johnnie Range have revealed a diverse assemblage of pyritized tubicolous fossils, with rare examples showing the preservation of bilaterian-grade throughguts, limited specimens of erniettomorphs, and abundant trace fossils approaching the Cambrian boundary. Here, we report both in-situ and float examples of discoidal structures from sandstones near the Stirling Quartzite–Wood Canyon boundary. These structures have a range size from 3.57 to 47.5 mm in diameter and show the quintessential discoidal morphology of Aspidella, with visible concentric rings. Preserved in positive epirelief on the surfaces of desert-varnished coarse quartz sandstone beds, these specimens are generally found as isolated individuals—with the exception of multiple individuals found on smaller blocks of float. Similar discoidal structures have been reported from the region, including Nimbia from the Stirling Quartzite in Nye County, Nevada, and medusozoan strandings from the younger Zabriskie Quartzite in Death Valley, California. The studied fossils do not statistically differ in size from other Ediacaran discoidal structures, and in petrographic thin sections, there is no evidence of canals or other associated structures to suggest abiotic formation. Instead, the Wood Canyon discoidal structures are represented by quartz sand grains enveloped by microbial layers, giving us confidence to reconstruct them as possible holdfast structures, similar to Aspidella.