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

a r t i c l e i n f o 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. The exceptional preservation and diversity of the soft-bodied animals from the Burgess Shale have made it one of the most well-known and celebrated fossil sites in the world. Since renewed interest in the Burgess Shale biota in the 1980s (Briggs et al., 1994 and references therein) a number of new, geographically widespread, Early and Middle Cambrian soft-bodied biotas have been described. These soft-bodied faunas provide important information on the timing and nature of the apparently rapid development of modern phyla, known as the 'Cambrian explosion' (Fortey, 2001). One of the most spectacular sites, the Early Cambrian Chengjiang biota of Yunnan, China (Hou et al., 2004), yields animals that are common to the Burgess Shale but also various unique taxa (e.g. Fuxianhuia Hou, 1987). The Pioche Shale of Nevada, USA, yields a less abundant soft-bodied biota than the Burgess or Chengjiang deposits, but importantly spans the Early–Middle Cambrian boundary and so has the potential to offer important insights into changes in the diversity and preservation of soft-bodied faunas at this time (Lieberman, 2003). This study contributes to our understanding of the taphonomic pathways that led to the preservation of Burgess Shale-type biotas. The mechanism by which …