Ptychoplasma conica isp. nov. - a new bivalve locomotion trace fossil from the Lower Jurassic (Hettangian) alluvial sediments of Sołtyków, Holy Cross Mountains, Poland

A new locomotion (repichnion) trace fossil, Ptychoplasma conica isp. nov., which is composed of chains of hypichnial mounds, is described from Hettangian alluvial sediments in Central Poland. Its occurrence is limited to amalgamated crevasse sand stones. The trace fossil is associated with freshwater bivalves belonging probably to Unionidae. This trace fossil reflects rhythmic (?diurnal) movement of the tracemaker in accordance with the direction of flow in the crevasse channel, where the forward movement took place in the shallow part of a sandstone layer and was interrupted by resting episodes in deeper sediment layer along the mud-sand inter ace. Episodic flood events forced bivalves to produce escape structures, moving from deeper (previous) to upper (later) levels of lateral movements. Some vertical burrows with bivalve body fossils preserved at the bottom suggest a taphonomic burial. P. conica ranges from Late Triassic to Hettangian.