The rosetted trace fossil Dactyloidites ottoi (Geinitz, 1849) from the Cenomanian (Upper Cretaceous) of Saxony and Bavaria (Germany): ichnotaxonomic remarks and palaeoenvironmental implications

The rosetted trace fossil Dactyloidites ottoi (Geinitz, 1849) is discussed based on new occurrences from the Cenomanian Regensburg Formation (glauconitic sandstones, Bavaria) and on a careful re-description of the type material and other specimens from quartz-rich sandstones of the Cenomanian Oberhaslich Formation of Saxony. The feeding trace consists of a fan-shaped spreiten structure originating from a central, vertical to oblique shaft leading downwards into the sediment. The branching radial elements (up to 20, 4-6 mm wide) are subhorizontal protrusive vertical spreiten, mostly forming incompletely circular rosettes with radii of 200A degrees-270A degrees and diameters between 30 and 75 mm. The ichnotaxonomic classification of the trace has been discussed controversially. Gyrophyllites kwassizensis Glocker, 1841 from contemporaneous nearshore glauconitic sandstones in the Bohemian Cretaceous Basin is similar to D. ottoi, but its radial elements are club-shaped and unbranched and do not show any spreiten. The validity of the ichnogenus Haentzschelinia Vyalov, 1964 (type ichnospecies Spongia ottoi) is questioned because the essential behaviour expressed by this form is basically the same as in Dactyloidites Hall, 1886. Different size, age, depositional environment, shape of the rosettes (circular vs. fan-shaped) and number of radial elements are not valid criteria for an ichnogeneric separation. Thus, the classification of S. ottoi in Dactyloidites Hall, 1886 is reasonable. The potential tracemaker of D. ottoi was a worm-like organism systematically reworking the sediment for organic food particles, supported by studies on the feeding behaviour of the modern lugworm Arenicola marina: when nutrient-rich sediments occur at living depth, the lugworm directly ingests the surrounding sediment by generating radial tunnels originating from the central shaft. This feeding strategy results in rosetted structures very similar to D. ottoi. The ichnospecies has a proven range from the Jurassic to the Neogene and predominantly occurs in shallow-water, nearshore to deltaic, nutrient-rich siliciclastic settings (lower Skolithos and upper Cruziana ichnofacies). The narrow palaeoenvironmental window in which the trace may form (shallow-marine settings, local presence of organic-rich sediments at living depth) is furthermore closed by taphonomic constraints: high sedimentation rates towards the top of shallowing-upward cycles support the preservation of D. ottoi, while low accumulation rates during transgressive conditions enhance the chances of their destruction by subsequent bioturbation. Thus, both the formation and the preservation of D. ottoi are related to narrow palaeoenvironmental and specific taphonomic conditions, readily explaining the rarity of the ichnotaxon.