Preservation of erniettomorph fossils in clay-rich siliciclastic deposits from the Ediacaran Wood Canyon Formation, Nevada
Preservation of erniettomorph fossils in clay-rich siliciclastic deposits from the Ediacaran Wood Canyon Formation, Nevada
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DOI：
10.1098/rsfs.2020.0012
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发表时间：
2020-06
期刊：
影响因子：
4.4
通讯作者：
J. G. Hall;Emily F. Smith;Nobumichi Tamura;S. Fakra;Tanja Bosak
中科院分区：
文献类型：
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作者：
J. G. Hall;Emily F. Smith;Nobumichi Tamura;S. Fakra;Tanja Bosak
三维保存的埃迪卡拉纪化石出现在砂岩层中。内华达州蒙哥马利山脉埃迪卡拉木峡谷组（WCF）的砂质硅屑沉积层包含两种形状和大小相似的化石形态：一种具有毫米级的山脊和明显的下边界，而另一种则缺乏这些诊断特征。我们将这些解释为近形虫的近形虫，具有不确定分类亲和力的软体生物。我们通过岩石学、拉曼光谱、x射线荧光显微探针和x射线衍射研究了这两种深晶的铸造和模具保存。所有化石及其周围的沉积基质均含有石英颗粒、富铁绿泥石和白云母。脊状化石的石英颗粒比无脊状化石大70%左右，表明脊状化石中粘土矿物丰度较低。绿泥石和白云母可能来源于蒙脱石和高岭石前体，它们经历了下绿片岩相变质作用。据推测，在脊状化石周围的沉积物中，高岭石和蒙脱石含量丰富，这使得一个连续的、独特的、富含粘土和干酪根的底界得以保存。石英在WCF的脊状化石和其他地区的埃尼特形化石中普遍存在，也表明这种矿物在砂岩和粉砂岩矿床中埃尼特形的三维保存中起着重要作用。
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.