CRETACEOUS FRESHWATER RAYS (CHONDRICHTHYES, RHINOBATOIDEI) AND THE DEVELOPMENT OF AN INTEGRATIVE APPROACH TO NONMARINE BIOSTRATIGRAPHY

North American land-mammal age-concepts have been applied to nonmarine Cretaceous rocks (e.g., Aquilan, Judithian, Edmontonian, Lancian). Recent analysis of Coniacian and Santonian nonmarine microvertebrate localities indicate that in many localities mammals make up less than one percent of the recovered material. Although isolated mammal teeth can be taxonomically useful, it is inappropriate to base biostratigraphic zonations on the rarest elements in the fauna. This is particularly true of riverine localities which contain abundant and diverse vertebrates and invertebrates, but where mammals may be completely absent from the fauna. Integrative biostratigraphic zonations that incorporate as much of the fauna as possible would be more useful. An example of potentially biostratigraphically useful specimens are the isolated teeth of freshwater rays. Ray teeth are abundant from the Albian through Maastrichtian and show marked morphologic trends. Specimens from the Cedar Mountain Formation (late Albian) are small and have shallow crowns. Ray teeth from the Dakota Formation (Cenomanian) are more diverse and demonstrate a tendency, one which continues throughout the Late Cretaceous, to increase the packing of the teeth resulting in more flattened sides to the crowns. Ornamentation, the nature of the transverse crest, crown shape, and other characters continue to evolve during the Cretaceous suggesting that rays can be of considerable biostratigraphic utility. Coupled with other common elements in the nonmarine Cretaceous such as ostracodes, fish vertebrae, and herptiles jaws, robust biostratigraphic zones can be developed based on the most common recovered elements. Mammals should be incorporated into zones, but only as one component within a broad integrative concept of nonmarine biostratigraphy.