Two new dwarf Entobia ichnospecies in a diverse aphotic ichnocoenosis (Pleistocene/Rhodes, Greece)

This study explores and extends the lower size range of the sponge boring ichnotaxon Entobia, thereby introducing two new ichnospecies – E. mikra and E. nana– both reaching only a fraction of a millimetre in size. They are components of a diverse aphotic ichnocoenosis, comprising a total of 18 ichnotaxa, recorded in skeletons of the cold-water scleractinian Lophelia pertusafrom Lower Pleistocene deposits of Rhodes. Recent analogs are studied and morphometrically analysed from various aphotic, cold-temperate settings in the NE Atlantic. The first new ichnospecies, E. mikra, is characterised by solitary or clustered, irregular cavities ranging from 60 to 335 μm in size with a mean of only 171 μm, and comprises elongate, branched and anastomosing forms. The traces posses a characteristic botryoidal microsculpture with etching scars measuring 3.6 to 8.1 μm at a mean of only 5.6 μm. The scars are locally encountered in various growth stages resembling initial rims, semi- and nearly-closed etching cells. The other new ichnotaxon, E. nana, is slightly larger with solitary or clustered cavities measuring 102 to 756 μm in diameter at a mean of 298 μm, and show a bulged to weakly botryoidal surface texture. The chambers posses varying numbers of apophyses and exploratory threads which terminate bluntly or lead to neighbouring chambers. Both new ichnospecies are assumed to represent the work of unknown boring micro-sponges, given the lack of alternatives among potential heterotroph bioerosion agents in aphotic environments, and considering the compelling resemblance in the botryoidal microsculpture of E. mikra (recording the activity of archaeocyte etching cells) and the presence of interconnected chambers in the case of E. nana. The possibility that they are merely juvenile stages of larger entobians is recejcted because of missing intermediate growth stages and the Gaussian size class frequency distribution, mirroring normal interspecies variability and population structure. The ultimate proof can only be drawn by histological analyses or by finding siliceous sponge spicules – if present at all in the case of the responsible sponges – allowing for a taxonomical evaluation of the trace maker and shedding new light on the actual size range and diversity of boring sponges