CHONDRICHTHYANS FROM THE ARKADELPHIA FORMATION (UPPER CRETACEOUS: UPPER MAASTRICHTIAN) OF HOT SPRING COUNTY, ARKANSAS

Erosion of the upper Arkadelphia Formation by the Ouachita River in Hot Spring County, Arkansas, has revealed a diverse and abundant, late Maastrichtian chondrichthyan fauna representing at least 17 species: Squatina hassei, Ginglymostoma lehneri, Plicatoscyllium derameei, Ondontaspis aculeatus, Carcharias cf. C. holmdelensis, Serratolamna serrata, Squalicorax kaupi, Galeorhinus girardoti, Rhinobatos casieri, Ischyrhiza avonicola, Ischyrhiza mira, Sclerorhynchus sp., Ptychotrygon cf. P. vermiculata, Raja farishi, Rhombodus binkhorsti, and Dasyatis sp. All of these species are widely known from the Upper Cretaceous of North America. An extremely rare species, Schizorhiza cf. S. stromeri, currently known only from Mexico and Africa, is also present. The chondrichthyan remains occur almost entirely as teeth scattered by river activity across the surface of the marl and marly clay of the Arkadelphia Formation and can be collected from within localized accumulations of river sediments. Also present with the chondrichthyan remains are teeth from actinopterygians and reptiles as well as molluscan steinkerns, echinoid spines, and fragments of branching corals. This fauna expands the known geographic distribution of late Maastrichtian chondrichthyans in North America, and provides an important biostratigraphic connection between fauna of the Gulf Coastal Plain and Western Interior Seaway. It also supports the idea of a significant chondrichthyan turnover across the Cretaceous–Tertiary boundary.