Revision of the ichnogenus Sabellarifex Richter, 1921 and its relationship to Skolithos Haldeman, 1840 and Polykladichnus Fürsich, 1981

The trace fossil Sabellarifex Richter, 1921 is revised and a lectotype for the type ichnospecies, Sabellarifex eifliensis, is designated which unambiguously puts the ichnogenus Sabellarifex into synonymy with generally unbranched, vertical, tubular structures of the ichnogenus Skolithos Haldeman, 1840. Polykladichnus Fürsich, 1981, a vertically orientated trace fossil with upwarddirected Y- to U-shaped branching, remains a valid ichnotaxon, although some specimens in the type series of Sabellarifex eifliensis also show this feature. Further use of Sabellarifex is not recommended. The potential value of ichnotaxobases for simple, tubular, vertically orientated trace fossilsis discussed. Branching is considered an ichnotaxobase of high significance in simple, vertically orientated structures, thus relevant for ichnogeneric distinction. Wall-lining is considered less important but suitable for ichnospecific differentiation. Funnel-shaped apertures are not considered suitable ichnotaxobases in this case. This does not affect the classification of non-tubular, plug- or funnelshaped structures such as Conichnus or Bergaueria. In the course of evaluating ichnotaxobasesof simple, tubular, vertically orientated structures, Monocraterion Torell, 1870 is also revised. The morphology of the lectotype of Monocraterion tentaculatum clearly differs from Skolithos in showing radiating tubular structures. Their origin is unique and remains dubious hence Monocraterion should only be used for the type material. The palaeoecology of Skolithos and Polykladichnus is discussed. Marine Skolithos is best explained as a domichnion made by phoronids or annelids. Non-marine Skolithos may be produced by insects or spiders; sculptured terminations of Skolithos have hitherto only been observed in non-marine finds. If this may be diagnostic for all non-marine Skolithos remains open. Marine Polykladichnus are best interpreted as domichnia of polychaetes or cerianthid anemones. Non-marine Polykladichnus are most likely produced by insects or insect larvae.