A trilobite fauna in a storm bed in the Poleta Formation (Dyeran, Lower Cambrian), western Nevada, U.S.A.

摘要

An unusual occurrence of trilobites is present at the TH Pit where a unique combination of fossil concentration and weathering are important factors. This site is in the upper part of the middle member of the Poleta Formation, 64 m above the base of the Dyeran Stage in the Montezuma Range, Esmeralda County, Nevada. The fossil bearing bed is a 30-cm-thick, poorly bedded siltstone associated with meter-scale, fine grained sandstone storm beds. The unweathered fossiliferous rock is very hard calcareous siltstone which fractures irregularly across the contained trilobite sclerites. Pre-Miocene weathering has modified this rock to a punky, non-calcareous siltstone that splits cleanly around internal and external molds of trilobite sclerites. The boundary between the weathered and unweathered parts of this bed is sharp and crosses stratification with irregular convex surfaces. The fauna of this unusual bed is limited to trilobites which occur as a bioclastic mixture of isolated sclerites, often broken, plus articulated trilobites. Some of the trilobites are variably twisted, suggesting that this layer is the result of a storm event. The fauna includes predominately dominately olenelloids,Teresellus goldfieldensis n. gen. and sp. andElliptocephalus praenuntius (Cowie, 1968), with a common ptychopariid,Keeleaspis? terhaari n. sp. and a rarer corynexochid,Polliaxis hanseni n. sp. These trilobite species are known elsewhere in the Esmeralda Basin at the same stratigraphic position but not all together at the same site.