Preservation of early and Middle Cambrian soft-bodied arthropods from the Pioche Shale, Nevada, USA

Abstract

The Early–Middle Cambrian Pioche Shale of Lincoln County, Nevada preserves a diverse array of soft-bodied ecdysozoans that are well-known from the Burgess Shale soft-bodied fauna of British Columbia, Canada. At Pioche certain genera occur both above and below the Middle Cambrian boundary, allowing a comparison of the nature of their preservation across this interval. In order to investigate the mineralization of this fauna, SEM analysis of mineral textures and element mapping of Anomalocaris, Tuzoia and Canadaspis specimens using energy dispersive X-ray spectroscopy (EDS) was undertaken. Results of these analyses show that Early Cambrian specimens are generally preserved in botryoidal hematite which is dark red in hand specimen while specimens from the Middle Cambrian are preserved as kerogenized carbon films with associated oxidized pyrite crystals and framboids. Comparison with other Cambrian soft-bodied biotas reveals that the nature of preservation of the Middle Cambrian biota at Pioche is remarkably similar to that described for the Chengjiang soft-bodied fauna, China. There does not seem to be an adequate analogue for the Early Cambrian preservation of soft-bodied organisms at Pioche. The preservation of the fauna is discussed in relation to changing environmental conditions at the Early–Middle Cambrian boundary.