An Early Miocene regurgitalite containing basking shark remains from Slovenia: insight into cetorhinid paleoecology of the Central Paratethys

A bromalitic concentration from the Lower Miocene (upper Burdigalian, calcareous nannoplankton Zone NN4) deposits of Meljski Hrib near Maribor, northeastern Slovenia, preserves at least 25 fragmentary basking shark gill rakers together with polygonal calcified-cartilage tesserae and disarticulated echinoid remains. Comparative morphology, geometric morphometric analysis of gill-raker bases, and micro-computed tomography of associated skeletal elements support assignment of the material to Keasius cf. K. septemtrionalis. This represents the first confirmed occurrence of Keasius from Slovenia and extends the known paleogeographic distribution of the genus into the southeastern margin of the Central Paratethys. The close association of cetorhinid remains with abundant echinoid material and the absence of acid etching on gill-raker surfaces indicate that the assemblage is rather a regurgitalite than a coprolite. The associated invertebrate fauna indicates deposition in an outer-shelf to upper-slope environment (approximately 150–250 m water depth). This taphonomic and paleoenvironmental context is most consistent with opportunistic scavenging by a benthopelagic vertebrate predator, plausibly a lamniform or hexanchid shark. The assemblage provides rare fossil evidence of trophic interactions involving planktivorous basking sharks and macrophagous vertebrates in Miocene deep-water ecosystems and provides new insights into the paleoecology and biogeographic history of cetorhinid sharks within the Paratethyan realm.