Sulcolithos variabilis igen. et isp. nov.: grooves on firm and hard bedding surfaces

The trace fossil Sulcolithos variabilis igen. et isp. nov. is described from the Middle Triassic (Ladinian) Muschelkalk Group of Germany. The groove-like structures occur on the surface of limestone beds that provided frm to hard substrate to the burrowing and/or boring organisms. The grooves have an elongate, straight or curved course and can be compound with subsurface burrow systems of Balanoglossites. Similar trace fossils have been reported from Late Ediacaran to Holocene deposits, and seem to be prevalent in Ordovician, Triassic and Jurassic time. Based on indirect evidence (e.g. morphology, interrelationship with Balanoglossites, substrate property), S. variabilis is attributed to the combined burrowing and boring activity of polychaetes. This interpretation is supported by direct evidence, such as the occurrence of exceptionally preserved polychaete remains in the traces, both in the Triassic material from Germany and the Ordovician specimens from Anticosti Island, Canada. However, morphologically similar traces in soft and frm substrate can also be produced by other organisms (e.g. bivalves) and represent behavioural convergence.