Preservation of erniettomorph fossils in clay-rich siliciclastic deposits from the Ediacaran Wood Canyon Formation, Nevada.

J G Hall,T Bosak,E F Smith,N Tamura,S C Fakra

doi:10.1098/rsfs.2020.0012

Abstract

Three-dimensionally preserved Ediacaran fossils occur globally within sandstone beds. Sandy siliciclastic deposits of the Ediacaran Wood Canyon Formation (WCF) in the Montgomery Mountains, Nevada, contain two fossil morphologies with similar shapes and sizes: one exhibits mm-scale ridges and a distinct lower boundary and the other is devoid of these diagnostic features. We interpret these as taphomorphs of erniettomorphs, soft-bodied organisms with uncertain taxonomic affinities. We explore the cast-and-mould preservation of both taphomorphs by petrography, Raman spectroscopy, X-ray fluorescence microprobe and X-ray diffraction. All fossils and the surrounding sedimentary matrix contain quartz grains, iron-rich chlorite and muscovite. The ridged fossils contain about 70% larger quartz grains compared to the ridgeless taphomorph, indicating a lower abundance of clay minerals in the ridged fossil. Chlorite and muscovite likely originated from smectite and kaolinite precursors that underwent lower greenschist facies metamorphism. Kaolinite and smectite are inferred to have been abundant in sediments around the ridged fossil, which enabled the preservation of a continuous, distinct, clay- and kerogen-rich bottom boundary. The prevalence of quartz in the ridged fossils of the WCF and in erniettomorphs from other localities also suggests a role for this mineral in three-dimensional preservation of erniettomorphs in sandstone and siltstone deposits.

Highlights

Three-dimensional cast-and-mould preservation of soft-bodied organisms occurs rarely in the fossil record, but is a common taphonomic mode during the Ediacaran and lower Palaeozoic ([1] and references therein)
Two different interpretations of the lifestyle of precursor organisms are proposed for erniettomorphs from Namibia: some have suggested that these organisms were semi-infaunal, and used sediment as a ballast (e.g. [20,52,53,54]), while others have disputed this and suggested that erniettomorphs were infilled with sand post-mortem and pre-burial (e.g. [55])
Clay minerals adsorbed onto the surfaces of organisms during the early stages of decay, delaying the decay and preserving carbonaceous matter at thin, but visible organomineral interfaces that separated the decaying organisms and the surrounding sediment

Summary

Introduction

Three-dimensional cast-and-mould preservation of soft-bodied organisms occurs rarely in the fossil record, but is a common taphonomic mode during the Ediacaran and lower Palaeozoic ([1] and references therein). Because exceptionally preserved fossils from this time may include deeply diverging animals, understanding the fossilization mechanisms during the Ediacaran and early Phanerozoic has garnered much attention These fossils, commonly preserved in shale and siltstone deposits, have inspired numerous hypotheses about fossilization mechanisms such as pyritization [2,3,4], silicification Three-dimensional cast-and-mould preservation—a taphonomic mode that may have preserved diverging metazoan clades during the Ediacaran [9,10]—is comparatively less well understood than other taphonomic windows. This mode of preservation has been hypothesized to require pyrite This mode of preservation has been hypothesized to require pyrite (e.g. [11,12]), silica [1] and/or aluminosilicates (e.g. [13,14,15]) to cement the mould and/or act as the moulding surfaces

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