INTERPLAY OF ECOLOGIC, TAPHONOMIC AND EVOLUTIONARY CONTROLS ON ICHNOFAUNAS FROM BURGESS SHALE-TYPE DEPOSITS

The presence of trace fossils in many classic Cambrian Burgess Shale (BS)-type deposits provides uncontroversial evidence of an in situ benthic community. Based primarily on distinct preservational styles and secondarily by morphology and size range, a variety of trace fossil assemblages can be recognized. Of these, the most diagnostic are (1) biogenic structures associated to body fossils typically reflecting an ecologic link (i.e. bioturbators exploiting food resources) and/or taphonomic overprint (i.e. shielding effect of a carapace) and (2) burrows and tubes containing their own producers. These two types of assemblages seem to be a distinctive ichhnologic signature of BS type-deposits recording ecologic and taphonomic conditions uncommon during the rest of the Phanerozoic. Burgess Shale-type ichnofaunas reveal a complex interplay of taphonomic, ecologic and evolutionary controls. In addition to the dominance of oxygen-deficient conditions and the position of the redox discontinuity surface very close to or at the sediment-water interface, early diagenetic processes and biomat development played a substantial role in the morphology and preservability of biogenic structures in BS-type deposits. The association of trace fossils and large, non-biomineralized carapaces may respond to ecologic (e.g. the “Tuzoia garden model”) or taphonomic (e.g. the “sheltering carapace model”) controls. Additionally, the absence of deep-tier colonization in early to middle Cambrian dysoxic settings opened a taphonomic window for the exceptional preservation of the small, delicate surficial and shallow-tier trace fossils so characteristic of these deposits. These conditions coexisted with intensely bioturbated deposits in more oxygenated, shallower-water deposits, pointing to the unusual environmental and taphonomic conditions of BS-type settings.