INTERPRETING THE INFLUENCE OF THE LAST INTERIOR SEAWAY IN NORTH DAKOTA (PALEOCENE, CANNONBALL FORMATION) IN NONMARINE STRATA (FORT UNION GROUP): REEVALUATION OF A CORBULID USING AN AMAZONIAN ANALOGUE

Corbula mactriformis was described in 1856 by Meek and Hayden from the “Fort Union Lignite Group” in North Dakota. Paleoecological inferences about this species have been limited, but its presence has generally been taken to mean local marine conditions, even though it was most typically found in unabashedly freshwater snail and clam assemblages in the Tongue River Formation. This marine connection was enhanced by its identification in the partially laterally equivalent Cannonball Formation by Stanton in 1920 and Cvancara in 1966. Undocumented suggestions have been made by the above authors and Taylor in 1975 that differences in C. mactriformis morphology may exist between the marine and nonmarine strata or that unnamed forms may also be present. In nonmarine associations, C. mactriformis has a wide distribution in the eastern Williston Basin and is known from adjoining counties in southwestern and central North Dakota and in the Estevan area of southeastern Saskatchewan. It occurs from the upper part of the Ludlow Formation (~61 Ma, Torrejonian NALMA) to the lower part of the Sentinel Butte Formation (~Ravenscrag Formation) (~57 Ma, Tiffanian-4 NALMA), but is most common in the Tongue River Formation. Of the 40 documented localities, five contain only C. mactriformis, and at eight others, it is associated with only one other freshwater taxon. These species-limited associations are known from the entire range of the taxon. The marine record of C. mactriformis consists of three positive and three questioned identifications from central North Dakota and from two localities in Perkins County, South Dakota. The species is not known from the tongues of the Cannonball Formation. C. mactriformis has subsequently been assigned C. (Potamomya), C. (Pachyodon), and Bicorbula. Present studies indicate that Meek’s 1876 recognition of Pachyodon may be most appropriate. Pachyodon is thought to be endemic to northwestern South America in the foreland basin behind the Andes in the Neogene in a huge fresh to brackish water system, which is consistent with an alluvial system intertonguing with North American interior marine deposits during the time of the Laramide Orogeny. Strontium isotopic studies of South American Pachyodon indicate its complete tolerance of freshwater. Recognizing Pachyodon mactriformis would significantly extend the geological range of this genus.