Tilting marks: a wave-produced tool mark resembling a trace fossil

Small waves can move by repeated tilting objects over considerable distances oblique to nearly parallel to wave crests. This was observed for dead, gaping, articulated bivalves of the genus Mytilus. Only larger-than-average waves, about every 20th wave, affected the shells, but even these waves did not actually reach the sediment surface. The wave energy led to a right-left tilting and a small lateral shift of the bivalve producing a sedimentary structure (tilting mark) by pushing aside the sediment when tilted. Tilting marks typically consist of symmetrical walls and a central furrow displaying a transverse ornamentation. Tilting marks resemble the lower part of the trace fossil Scolicia, and could be misinterpreted as such. Tilting marks were observed in very shallow water.