A new helical trace fossil from the Lower Devonian of Spitsbergen (Svalbard) and its palaeoenvironmental significance

In Devonian molasse deposits (Old Red Sandstone) of Spitsbergen, vertical and regularly helical, tubular trace fossils are found predominantely in pedogenically overprinted calcareous clay to siltstones. Host rocks are interpreted as alluvial plain deposits with lacustrine intercalations and marine ingressions. Skolithos helicoidalis isp. nov. generally occurs as tubes with a round to oval cross section, continuously coiling downward from distinct bedding planes to a depth of 50 to 70 cm. Morphological variations with regard to cross-sectional shapes, amplitude of coiling and whorl distance are referred to primary trace heterogeneity, sediment composition, and secondary influences such as post-sedimentary deformation, diagenesis and lateral tectonic compression. Commonly occurring at the upper extent of fluvial, lacustrine and marginal marine beds, Skolithos helicoidalis isp. nov. probably marks periods of slow or no sedimentation, independent of facies. Its frequent association with pedogenic structures such as carbonate glaebules and slickensides suggests a plant origin (rhizolith). Alternatively, the extraordinary morphological regularity and wide distribution across facies could also point toward an animal, possibly of euryecological arthropod origin.