Balanoglossites Mägdefrau, 1932 from the Middle Triassic of Germany: part of a complex trace fossil probably produced by burrowing and boring polychaetes

The ichnogenus Balanoglossites Mägdefrau, 1932 is re-described from its type area and is interpreted as part of a complex trace fossil comprising both burrow and boring components. The type ichnospecies, B. triadicus, consists of predominantly deep U- or Y-shaped tunnel elements, whereas B. ramosus n. isp. encompasses irregularly ramified galleries. Both ichnospecies occur frequently in firmground and hardground substrates with sporadic epikarst features. In the underlying softground, they may integrate into Labyrintichnus terrerensis Uchman & Álvaro, 2000, a network of irregular galleries and communicating shafts. On the omission surface, the tunnels are linked to elongated or winding grooves in hard or firm substrates. Further elements of the complex trace fossil include lobate burrows similar to Zoophycos and Rhizocorallium, mainly occurring in firmground and softground, shallow to deep U-shaped pouches and winding tunnels (Maeandropolydora sulcans Voigt, 1965) in hard and firm substrates. Finally, some tunnel portions and well preserved top surfaces of the ichnofabrics are scattered with the faecal pellets Coprulus oblongus Mayer, 1952. C. oblongus has a rounded shape and a homogeneous structure and belongs to the Coprulidae n. ichnofam. Based on the numerous fingerprints preserved in the complex trace fossil, polychaete worms are identified as possible tracemakers. This is in accordance with the close combination of borings and burrows, a tunnel system highly variable in shape and size, blind ending side-branches and tunnel terminations, association with scolecodonts and exceptionally preserved polychaete remains. In addition, different commensals may have contributed to the modification of the complex trace fossil.