CHONDRICHTHYANS FROM THE CLAYTON LIMESTONE UNIT OF THE MIDWAY GROUP (TERTIARY: PALEOCENE) OF HOT SPRING COUNTY, ARKANSAS

The Clayton Limestone Unit of the Midway Group (Paleocene) in southwestern Arkansas preserves one of the oldest chondrichthyan Cenozoic assemblages yet reported from the Gulf Coastal Plain of the United States. Present are at least ten taxa, including: Odontaspis winkleri Leriche, 1905; Carcharias cf. C. whitei (Agassiz, 1843); Carcharias cf. C. hopei (Agassiz, 1843); Anomotodon novus (Winkler, 1874); Cretalamna appendiculata (Agassiz, 1843); Otodus obliquus Agassiz, 1843; Hypolophodon sylvestris (White, 1931); Myliobatis dixoni Agassiz, 1843; Rhinoptera Cf. R. sherboni White, 1926; and a chimaerid of indeterminate affiliation. Also present are lamnoid-type and carcharhinoid-type chondrichthyan vertebrae. These taxa are relatively common in the Paleogene of the United States and also occur elsewhere globally. The Clayton chondrichthyan assemblage derives from an outcrop located only a few kilometers from a site exposing an assemblage of Maastrichtian chondrichthyans from the upper Arkadelphia Marl. Because these assemblages are closely spaced stratigraphically and geographically, they provide data on chondrichthyan taxonomic turnover across the Cretaceous/Paleogene boundary in this region of the Gulf Coastal Plain. Only a single species, Cretalamna appendiculata (Agassiz, 1843), thought by some to represent the ancestral lineage of Carcharodon megalodon (Agassiz, 1843), occurs in both assemblages. A small number of genera are found in both assemblages (e.g. Carcharias; Odontaspis) but the species of these genera in the Paleocene Clayton Limestone are different than the species of these same genera in the Maastrichtian Arkadelphia Marl. The evolutionary bottleneck in chondrichthyan diversity associated with the end-Cretaceous mass extinction event that has been documented from other parts of the world, thus appears to be also strongly expressed in the Arkansas region of the Gulf Coastal Plain. Whether the new Paleogene species of the K/P boundary-crossing genera represent post-extinction migration from other regions or events of local, post-extinction speciation will need further documentation.