Hybrid nature of a new Jurassic-Cretaceous worm burrow indicated by microbial mediation of its wall formation

A new trace fossil Macroterebella hoffmanni nov. igen., nov. isp. occurs in Oxfordian and Aptian limestones of Romania, in the Central Dobrogea and the Rarău Mountains, respectively. It is a tubular, branched, and winding burrow (5–14 mm in diameter) displaying a thick wall (0.8–2 mm) with a micropeloidal texture. The Dobrogea burrows contain abundant calcite pseudomorphs after dolomite in the wall. A ferruginous halo occurs around burrows from Rarău. The burrow lumen resulted from burrowing by the tracemaker, most likely a polychaete worm of the family Terebellidae, while the wall is nonconstructional, and its formation was microbially mediated. Terebellids produce mucous-lined burrows, which are attractive for microbial activity which is geochemically important for metal adsorption and mineral nucleation. The micropeloidal texture of the Macroterebella wall is the result of bacterially mediated precipitation and possibly influenced by extracellular polymeric substances (EPS). Some microbes, especially sulphate-reducing bacteria producing extracellular polymeric substances (EPS) are able to mediate the formation of dolomite, and some microbial cells may be nucleation sites for dolomite. The lack of dolomite and the presence of a ferruginous halo around the burrows in the Rarău specimens may reflect different environmental geochemical conditions within these burrows compared with those from Dobrogea. This study confirms that microbes and organic matter in the mucous lining of burrows in a carbonate environment may play an important physicochemical role in the final appearance of trace fossils. Macroterebella nov. igen. can be considered as a trace fossil with a hybrid bioturbation/biosedimentary nature.