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Abstract We describe > 200 ribbon-like macroscopic fossils from terminal Ediacaran strata at Mount Dunfee, Nevada, USA ∼ 115 m below the local placement of the Ediacaran–Cambrian boundary. They are preserved as casts and molds, composed of Fe-oxides and Fe-rich aluminosilicates in an aluminosilicate clay matrix. Measurements of 50 of the specimens provide a fossil size range of 0.22–0.74 mm-wide and 0.1–75.0 mm-long. Some specimens evidence original flexibility and appear to be fragmented, consistent with soft body preservation. They are therefore interpreted as body fossils, rather than trace fossils. Given this interpretation, we suggest that the fossils’ size range and ribbon-like morphologies are consistent with them being members of the problematicum Vendotaenia, which have not been previously reported from Ediacaran strata within the southern Great Basin. The phylogenetic affinity of vendotaenids is unresolved, but they are commonly interpreted as a form of eukaryotic macroalgae. This report firmly establishes vendotaenids in Ediacaran strata on Laurentia, broadening their known paleogeographic range during the end-Ediacaran Period. Additionally, the morphology of the fossils described here supports the notion that, although vendotaenids are reported from many Ediacaran paleocontinents globally, there was low macroalgal diversity at the end of the Ediacaran Period. 

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. 

Archaeocyaths, the first metazoan reef builders of the Phanerozoic, faced their ultimate demise during early Cambrian Age 4 but the exact timing of their local extinctions varied globally. In this study, we report archaeocyaths in strata that overlie the last robust archaeocyathan reefs of the western United States (Laurentian Cordillera). These are found in small microbial mounds in the upper unit of the Mule Spring Limestone near Split Mountain, Clayton Ridge, Nevada, as well as in storm beds in the Thimble Limestone Member of the Carrara Formation in Echo Canyon of the Funeral Mountains, Death Valley, California. Thin-section analysis revealed the presence of modular archaeocyaths, with Archaeocyathus being the only genus present. The small microbial mounds of the lowermost upper unit of the Mule Spring Limestone preserve frame-building Archaeocyathus in situ, a few meters above the well-known Bristolia beds of the Mule Spring Limestone. As some of the youngest known archaeocyaths of the western US, these occurrences represent the last gasp of archaeocyaths in the early Cambrian of Laurentia and one of the latest occurrences globally of archaeocyaths. We thus interpret Archaeocyathus in these units as an example of a dead clade walking—some of the last true archaeocyaths that locally persisted into the later Age 4 Cambrian following the disappearance of diverse archaeocyath reefs in western Laurentia. These last archaeocyath communities exhibited low diversity and disparity before the total extirpation of this reef-building hypercalcified sponge and their ecosystem. 

The geologically rapid appearance of fossils of modern animal phyla within Cambrian strata is a defining characteristic of the history of life on Earth. However, temporal calibration of the base of the Cambrian Period remains uncertain within millions of years, which has resulted in mounting challenges to the concept of a discrete Cambrian explosion. We present precise zircon U–Pb dates for the lower Wood Canyon Formation, Nevada. These data demonstrate the base of the Cambrian Period, as defined by both ichnofossil biostratigraphy and carbon isotope chemostratigraphy, was younger than 533 Mya, at least 6 My later than currently recognized. This new geochronology condenses previous age models for the Nemakit–Daldynian (early Cambrian) and, integrated with global records, demonstrates an explosive tempo to the early radiation of modern animal phyla.