Abstract
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.
Highlights
Trace fossils attributed to the burrowing activity of bivalves are common in the fossil record from the early Cambrian (e.g., Orłowski and Żylińska 2002) to the QuaternaryHandling editor: Mike Reich.30‐798 Kraków, Poland (Schäfer 1962) and include ichnogenera such as Protovirgularia M‘Coy, 1850; Lophoctenium Richter, 1850; Lockeia James, 1879; Ptychoplasma Fenton and Fenton, 1937; Siphonichnus Stanistreet et al, 1980; Oravaichnium Plička and Uhrová, 1990; Solemyatuba Seilacher, 1990; Scalichnus Hanken et al, 2001; Hillichnus Bromley et al, 2003; Saronichnus Pervesler and Zuschin, 2004; and Oblongichnus Bel Haouz et al, 2020
O. carinatum isp. nov. shows a great variation of morphology and transitional forms with O. hrabei are common
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
Summary
Trace fossils attributed to the burrowing activity of bivalves are common in the fossil record from the early Cambrian (e.g., Orłowski and Żylińska 2002) to the QuaternaryHandling editor: Mike Reich.30‐798 Kraków, Poland (Schäfer 1962) and include ichnogenera such as Protovirgularia M‘Coy, 1850; Lophoctenium Richter, 1850; Lockeia James, 1879; Ptychoplasma Fenton and Fenton, 1937; Siphonichnus Stanistreet et al, 1980; Oravaichnium Plička and Uhrová, 1990; Solemyatuba Seilacher, 1990; Scalichnus Hanken et al, 2001; Hillichnus Bromley et al, 2003; Saronichnus Pervesler and Zuschin, 2004; and Oblongichnus Bel Haouz et al, 2020. Aside from skeletal fossils, the Muschelkalk contains the richest trace-fossil assemblage of any Triassic succession worldwide (Knaust 1998, 2007; Chrząstek 2013; Stachacz and Matysik 2020), including abundant bivalve traces. Mayer (1954) interpreted stalk-like bodies from the Upper Muschelkalk of SW Germany as the casts of decayed worms and named them Triadonereis and their traces Triadonereites. These structures are identified as trace fossils, but despite their abundance, in the Lower Muschelkalk, their scientific determination remains difficult due to a partly unclear and unresolved ichnotaxonomic situation
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