MORPHOLOGY AND PRESERVATION OF <i>GAOJIASHANIA</i>, AN ENIGMATIC TUBULAR FOSSIL FROM THE UPPER EDIACARAN DUNFEE MEMBER, DEEP SPRING FORMATION, NEVADA, USA

Abstract The upper Ediacaran stratigraphic record hosts fossil assemblages of Earth’s earliest communities of complex, macroscopic, multicellular life. Tubular fossils are a common and diverse, though frequently undercharacterized, component of many of these assemblages. Gaojiashania cyclus is an enigmatic tubular fossil and candidate index fossil found in upper Ediacaran strata globally and is best known from the Gaojiashan Lagerstätte of South China. Here we describe a recently discovered assemblage of Gaojiashania fossils from the Ediacaran Dunfee Member of the Deep Spring Formation of Nevada, USA. Both body and trace fossil affinities have been proposed for Gaojiashania; we present morphological and biostratinomic evidence for a body fossil affinity for the Dunfee specimens. Additionally, previous studies have highlighted that Ediacaran tubular fossils are characterized by a wide range of preservational modes, including association with pyrite, apatite, or clay minerals and preservation as carbonaceous compressions. Petrographic, SEM, and EDS data indicate that the Dunfee Gaojiashania specimens are preserved as ‘Ediacara-style’ external, internal and composite molds, in siltstone and sandstone with a clay mineral-rich matrix of both aluminosilicates and non-aluminous Mg- and Fe-rich silicate minerals that we interpret as authigenic clays. Authigenic clay-mediated fossilization of unmineralized tissues, including moldic preservation in heterolithic siliciclastic strata, as indicated by the Dunfee Gaojiashania, may be linked to the prevalence of both silica-rich and ferruginous seawater conditions prior to both the radiation of silica-biomineralizing organisms and the rise of ocean and atmospheric oxygen to modern levels. In this light, clay authigenesis may have played a critical role in facilitating multiple modes of Ediacaran and Cambrian exceptional fossilization, thus shaping the stratigraphic distribution of a range of Ediacara macrofossil taxa.