Preservation of Burgess Shale Fossils  

Investigations of the preservation of fossils from Walcott’s Burgess Shale and other Burgess Shale-type localities address a variety of questions including: Why and how are soft tissues preserved? What impact have decay and preservation on the morphology that survives, and on the interpretation of these Cambrian animals? Does an understanding of depositional setting and preservation help us to predict where other soft-bodied fossils might be found? Why are occurrences of soft-bodied fossils relatively common in Cambrian rocks compared to younger strata? In researching such questions, the focus of enquiry has ranged from regional to microscopic, from environmental context to chemical composition.  Common factors have emerged as work on Cambrian soft-bodied fossils has progressed – the survival of organic material appears to be fundamental, for example.  But the concept of ‘Burgess Shale-type preservation’ has become less useful as additional discoveries have broadened the range of preservational styles known to be involved in the preservation of Cambrian soft-bodied fossils.  The preservation of organic materials associated with phosphatic gut traces and clay minerals, formed both early and late, is now known from a range of stratigraphic and environmental settings (some of them post Palaeozoic) in addition to the Burgess Shale.  Moreover, Cambrian soft bodied fossils with tissues preserved in calcium phosphate, pyrite and silica have been documented.  I will review the range of preservational styles among Cambrian soft-bodied organisms and consider the extent to which the Burgess Shale-type preservational window extends beyond the Middle Cambrian.