First Bashkirian (Carboniferous, Early Pennsylvanian) marine chondrichthyan assemblage from eastern North America and its implications for Carboniferous faunal turnover

Global Early Pennsylvanian (Bashkirian) chondrichthyan records are scarce, and none have previously been reported from the eastern United States. This interval is critical in chondrichthyan evolutionary history, marking a pronounced faunal turnover from the Late Mississippian. Here, a novel assemblage is described from a transgressive condensed section of nodular limestone within the Upper Pottsville Formation at the Union Chapel Mine (Stephen C. Minkin Paleozoic Footprint Site), Walker County, Alabama, U.S.A. These specimens represent the first wholly marine Bashkirian chondrichthyan fauna from eastern North America. The assemblage includes Glikmanius occidentalis, Stethacanthus sp., a eugeneodontiform tentatively attributable to Caseodontoidea, and indeterminate durophagous holocephalans, and constitutes one of the most diverse Bashkirian chondrichthyan faunas reported from North America. The late Bashkirian occurrence of G. occidentalis extends its stratigraphic range below its previously reported oldest occurrence in the Moscovian. The presence of a non-symphyseal pavement tooth from the Caseodontoidea supports prior hypotheses that the Eugeneodontiformes diversified near the Serpukhovian-Bashkirian boundary. Broader comparisons of Bashkirian faunas with older and younger assemblages indicate a general decline in chondrichthyan alpha diversity across the Mississippian-Pennsylvanian boundary. This pattern may reflect environmental and tectonic drivers, including global cooling that led to the onset of Late Paleozoic Gondwanan glaciation, in addition to the closure of the Rheic Ocean, which together may have contributed to faunal turnover and subsequent ecological restructuring of chondrichthyan assemblages.