On Diplocraterion Torell 1870 and the significance of morphological features in vertical, spreiten-bearing, U-shaped trace fossils

A detailed study of the morphological features of vertical, U-shaped spreitenburrows, including the ichnogenera Diplocraterion, Corophioides and Polyupsilon showed that the latter two must be regarded as synonyms of Diplocraterion Torell 1870. Out of the numerous ichnospecies belonging to the three former ichnogenera only five merit ichnospecific separation: D. polyupsilon, D. parallelum, D. biclavatum, D. helmerseni and D. habichi. This decision is based upon the classification of their morphological features as either "significant" or "accessory," reflecting the significance of the behaviour patterns. Significant features (vertical U-tube, spreite as such), reflecting highly significant behaviour, are used for classification at the ichnogeneric level. Distinctive accessory features (burrow outline, bi- or unidirectional growth of the spreite), reflecting behaviour at a lower level of significance and often controlled by the environment, are used for classification at the ichnospecific level.