Middle Triassic bivalve traces from central Europe (Muschelkalk, Anisian): overlooked burrows of a common ichnofabric

The ichnotaxonomy, producers and ethology of the bivalve trace fossil Oravaichnium Plička and Uhrová, 1990 are revised, and the mode of formation is discussed. The ichnogenus is compared with other mollusc trace fossils such as Protovirgularia, Lockeia and Ptychoplasma, as well as the common, cosmopolitan trace fossils Planolites and Palaeophycus, which are morphologically similar to Oravaichnium. A lectotype of Oravaichnium hrabei Plička and Uhrová, 1990 is defined and illustrated. Oravaichnium carinatum isp. nov. from the Middle Triassic of Poland and Germany is described and interpreted as a bivalve burrow. It differs from the relatively rare O. hrabei by a carinate rather than subquadrate cross section. However, O. carinatum isp. nov. shows a great variation of morphology and transitional forms with O. hrabei are common. Similarly, transitional forms of Oravaichnium with other bivalve ichnogenera, especially Protovirgularia, also occur. The studied Triassic ichnoassemblage clearly indicates that bivalve burrows are much more common than previously believed and are represented by repichnia, fodinichnia and cubichnia. The occurrence of similar ichnofabrics containing Oravaichnium in other Triassic succession of the Germanic and Tethys basins and elsewhere suggests a much wider distribution than hitherto known. It is evident that bivalves, most likely nuculids, participated greatly in bioturbation, and the Middle Triassic infaunalisation is one of the most important steps in Phanerozoic evolution of ichnocoenoses.