Raman Imaging of Early and Middle Cambrian Soft-Bodied Arthropods from the Pioche Shale, Nevada, USA  

The post-burial processes which preserved Burgess Shale organisms are controversial.  Typically, fossils consist of collapsed carbonaceous sheets, coated by diagenetic aluminosilicate films.  These highly compressed, layered fossils are typically just micrometres thick.  Models of the preservational process focus on either the carbonaceous compressions or aluminosilicate films as the main preservational agent.  The composition of aluminosilicates associtated with Burgess Shale-type fossils differs from aluminosilicates in the surrounding matrix, which implies that the aluminosilicate films are diagenetic, not sedimentary, in origin.  The aluminosilicates cannot have been derived from the surrounding sediment, either by forcible injection into the body cavity during burial, or by leaking into decay-generated voids in organisms.  A number of scenarios have been postulated to account for these aluminosilicates.

Over the past decade, elemental mapping has emerged as a useful chemical tool to determine the composition of fossil material.  However, energy dispersive X-ray spectroscopy (EDS) cannot identify mineral phases as a standalone technique.  Raman micro-spectroscopy, another non-destructive technique, is capable of complementing elemental data with more informative molecular structural information.  Raman micro-spectroscopy is based on the fundamental vibrational motions of molecules, molecular ions, and crystals; this allows it to elucidate the mineralogy and carbonaceous composition of Burgess Shale-type fossils.  Raman imaging simultaneously collects spectra from individual areas of the sample, so the distribution of each material present can be visualized.  We used Raman imaging to identify the distribution and mineralogy/carbonaceous composition in various Cambrian species, including Tuzoia and Anomalocaris, as preserved within the Burgess Shale-like Pioche Shale.  The imaging analysis produced various 2D and 3D images showing the spatial arrangement between carbonaceous materials and hematite throughout the fossil.  Significantly, in agreement with recent studies, the fossil mineralogy is different from the host rock.  This gives us a better understanding of preservational modes in Cambrian lagerstätten.