Large chambered sponge borings on a Late Cretaceous abrasion platform at Cracow, Poland

A new ichnospecies of the bioeroding sponge ichnogenus Entobia, i.e., E. cracoviensis isp. n., is distinguished by having a single, large, isolated chamber and radiating canals. It occurs in a rockground surface on a Turonian or Santonian abrasion platform that is cut into Oxfordian limestones as exposed at Bonarka, Cracow. The morphology of the new ichnospecies is compared with fossil and modern sponge boring morphologies. In every case, the borings lack their upper parts and are roofless. Three models are constructed for the depositional history of the rockground and its colonization by the E. cracoviensis tracemaker: (1) that the roof has been removed by physical erosion, causing, or subsequent to, the death of the sponge; (2) that the lack of the roof is primary (biological), the roof having been removed by the sponge itself; and (3) that the boring sponge was psammobiontic, initiating its boring beneath a thin sand deposit, where there was no need to maintain a roof to the boring. The third model, based on living species of Aka, fits the details of preservation best, and is considered to represent the most likely scenario.