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Keywords:

  • Bioturbation;
  • environmental expansion;
  • ichnofacies;
  • Late Devonian;
  • mass extinctions;
  • trace fossils

A detailed ichnological analysis of the Upper Devonian–Lower Mississippian Bakken Formation of sub-surface Saskatchewan and the partially coeval Exshaw Formation of Alberta indicates the presence of an anomalous ichnofacies gradient. The distal Cruziana Ichnofacies, which in rocks of other ages is restricted to lower-offshore facies, here ranges from this setting to the lower shoreface. No archetypal Cruziana Ichnofacies is present in these deposits. This pattern is interpreted as resulting from the differential effects of the Late Devonian mass extinction in shallow-water ecosystems. The onshore expansion evidenced by ichnological data is consistent with the pattern displayed by the body-fossil record, which indicates a re-invasion of shallow-water environments by the Palaeozoic evolutionary fauna during the Late Devonian and into the Early Carboniferous. The ichnofauna studied is overwhelmingly dominated by deposit feeders, with suspension feeders being notably absent, further underscoring the importance of trophic type as a selectivity trait during mass extinctions.