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 eight taxa, including; Odontaspis winkleri Leriche, 1905; Carcharias cf. whitei (Arambourg, 1952); Carcharias sp.; Anomotodon novus (Winkler, 1874); Cretalamna sp.; Otodus obliquus Agassiz, 1843; Hypolophodon sylvestris (White, 1931); Myliobatis dixoni Agassiz, 1843; and a chimaerid of indeterminate affiliation. Also present are lamnoid-type and carcharhinoid-type chondrichthyan vertebral centra. 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/Paleocene boundary in this region of the Gulf Coastal Plain. The evolutionary bottleneck in chondrichthyan diversity associated with the end-Cretaceous mass extinction event that has been documented from other parts of the world also appears to be strongly expressed in the Arkansas region of the Gulf Coastal Plain.

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Cainozoic research

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Martin A. Becker, Lauren C. Smith, & John A. Chamberlain Jr. (2011). Chondrichthyans from the Clayton Limestone Unit of the Midway Group (Paleogene: Paleocene) of Hot Spring County, Arkansas, USA. Cainozoic research, 8(1/2), 13–27.