The Trace Fossil Record of the Burgess Shale  

The Burgess Shale is well known for its soft-bodied animals, but its ichnofossils have received scant attention.  Often considered to be rare, their relevance to environmental and palaeoecological interpretation has been questioned.  While macrofaunal vertical ichnofabrics seem to be restricted to localized bedding surfaces, such as in the Raymond Quarry Member, horizontal trace fossils display a wide variety of morphologies and are more widely represented than previously realized.  Several horizons preserve non-biomineralized organisms alongside biogenic structures.

Our census of ichnofossils from several Burgess Shale localities in Yoho and Kootenay National Parks, British Columbia, unravelled a wide range of morphologies recording the activities of both macrofaunal and meiofaunal benthic components.  The macrofaunal component is represented by several morphological groups: trackways, cruzianids, U-shaped and simple vertical structures, rosary-like structures, rosette structures, branching networks, pellet-infilled tubes, and a variety of simple, horizontal vermiform structures.  The low diversity of trace-fossil assemblages, which are often monospecific to paucispecific, suggests the absence of a mature endobenthic community.  For the most part, trace fossils are small, suggesting prevailing dysoxic bottom waters.  Most trace-fossil assemblages penetrate no more than 3 mm into the compacted sediment, and well-developed endobenthic ichnofabrics are strikingly absent.  This suggests a redox boundary almost coincident with the sediment water interface, which would produce anoxic pore-waters.  However, exceptional trace-fossil assemblages, such as the Diplocraterion assemblage in the Raymond Quarry, delineate distinctive stratigraphic surfaces in which the prevailing redox conditions were drastically altered.  Diplocraterion-like structures are incompatible with permanently anoxic pore waters, and indicate brief oxygenation events probably related to exhumation and erosion (i.e. Glossifungites ichnofacies). The meiofaunal benthic component is represented by minute biogenic structures which are particularly abundant in the Raymond Quarry and from Stanley Glacier, but absent in the Walcott Quarry.  Diminutive vermiform trails are common within or just below cuticular elements of Banffia, Hurdia, Odaraia, Sidneyia and Tuzoia, suggesting grazing behaviour on microbe-enriched detritus.  These trace fossils rarely extend into the surrounding sediment.  This micro-scale concentration of biogenic structures may be explained by the mediation of thin, flexible cuticles that provided optimum conditions for differential preservation buffering compaction effects.  In this sense, body fossils provided a unique taphonomic window for preserving meiobenthic trace fossils.

Trace fossils are instrumental to further expanding our understanding of the Burgess Shale palaeoecology and depositional environment.  Ichnological data provide robust evidence of the indigenous benthic community, revealing the presence of a hidden diversity and trophic level of trace-making organisms.  Trace fossil evidence also supports the idea of fluctuating oxygen conditions, revealing colonization windows in response to improved oxygenation.