Large burrow systems in marine Miocene deposits of the Betic Cordillera (Southeast Spain)

Large burrow systems occur in the Miocene marine deposits of the Almería-Nijar Basin (Betic Cordillera, Southeast Spain). They consist of galleries of dense burrow segments, locally forming irregular networks. Cylindrical, branched (Y-shaped) structures, with diameters ranging from 8 to 22 cm and minimum lengths of 30–60 cm are abundant. In addition, galleries longer than 70–80 cm are common, and burrow segments larger than 200 cm are locally present. Burrows have smooth walls, without pellet reinforcements, but locally possible scratch marks are present. Burrow fill is structureless and has a similar composition to the host rock. These structures are assigned to the ichnogenus Thalassinoides Ehrenberg, displaying similarities with T. suevicus. Identification of the possible tracemaker is not straightforward, but decapod crustaceans, probably shrimp or shrimp-like organisms, are the most likely candidates.