A new classification concept for U-shaped spreite trace fossils

In the course of a review of the ichnogenera Diplocraterion Torell, 1870, Rhizocorallium Zenker, 1836 and Ilmenichnus Hecker, 1980, a new classification concept for these U-shaped spreite structures is proposed. The terms single-spreite lamina and double-spreite lamina structures are introduced for their classification. Single-spreite lamina structures are characterized by having one spreite lamina between the U-limbs (U-spreite lamina) resulting from the displacement (growth) of the U-limb from either proximal to distal (protrusive) or vice versa (retrusive). Double-spreite laminae are the result of the combination of a spreite lamina between the U-limbs (U-spreite lamina) plus an additional, predominantly retrusive shift of the U-limb (limb-spreite lamina). Strictly based on behaviour of the trace fossil producers, the ichnotaxonomically most relevant criteria (ichnotaxobases) of U-shaped spreite burrows are their orientation towards the bedding plane and the presence of a single-spreite lamina or double-spreite laminae. As a consequence, the newly proposed classification scheme unites simple, U-shaped structures with single-spreite lamina and with a vertical or inclined orientation under Diplocraterion (made by suspension-feeders). Wedge-shaped structures with double-spreite laminae remain under Rhizocorallium (made by mixed deposit- and suspension feeders), whereas horizontal structures are assigned to Ilmenichnus (made by deposit-feeders). The new combination Diplocraterion ultimum (Saporta & Marion, 1883) represents oblique, singlespreite lamina burrows. Many forms previously assigned to Rhizocorallium jenense are oblique single-spreite lamina burrows, now assigned to Diplocraterion ultimum (Saporta & Marion, 1883). Rhizocorallium devonicum Hecker, 1930 has priority over Rhizocorallium irregulare Mayer, 1954. The validity of Rhizocorallium commune Schmid, 1876 is questioned and its further use is not recommended as it is based on a very vague and non-diagnostic description only. Horizontal to slightly oblique spreite structures should be assigned to Ilmenichnus Hecker, 1980, which leads to the combination of Ilmenichnus devonicus (Hecker, 1930). Two new ichnospecies, Ilmenichnus atherus and Ilmenichnus multilobatus, are introduced for horizontal, uni-lobed, spreite structures with a knobby exterior, and multi-lobed, spreite structures, respectively.