First Hybodontiform Record Reveals Chondrichthyan Role in Carnian Food Webs of the Upper Triassic Reifling Basin, Austria

Abstract TheUpper TriassicReifling BasinoftheNorthern Calcareous AlpsinAustria has yielded rich marine fossil assemblages, offering rare insights into complex basinal food webs during the CarnianPluvial Event (CPE), a critical interval of major global environmental upheaval. Despite extensive sampling, chondrichthyan fishes had not been recovered until now. Here we report the first unambiguous chondrichthyan record from the Reifling Basin, represented by a single indeterminate hybodontiformtooth from the lower Carnian of Scheiblingbach. Still largely embedded in matrix, the specimen was examined through micro-CT scanning and digital segmentation. The tooth, indicative of a small- to medium-sized durophagous generalist likely occupying a mid-trophic position, adds to the scarce chondrichthyan record from the Alpine Upper Triassicduring the CPE. While the Scheiblingbachtooth bears some resemblance to Polyacrodus and Acrodus , the deeply unresolved taxonomy and likely polyphyletic nature of these and other historically problematic hybodontiform genera preclude confident identification, exemplifying the persistent difficulty of assigning isolated hybodontiformteeth to specific taxa. Nonetheless, the tooth from Scheiblingbach refines our understanding of Triassic hybodontiform diversity and distribution and highlights the urgent need for future research to overcome long-lasting taxonomic challenges that continue to constrain our perception of early Mesozoic chondrichthyan dynamics.