UPPER CRETACEOUS DAKOTA FORMATION CONTINENTAL MOLLUSKS – RESURRECTION OF WESTERN INTERIOR SEAWAY EASTERN SEABOARD OCCURRENCES

Mid- to late-19h century geological studies of the (upper) Dakota Formation in its type area of Nebraska (NE) and Iowa and elsewhere in Nebraska, Minnesota (MN), and subsequently in Kansas resulted in the collection of a few fossil freshwater mussels and snails. Fossils can be interpreted as from the Woodbury (Omadi) Member and have been consistently interpreted as (mid) Cenomanian. Fossil-bearing sediments are apparently rare. Hayden found the first mussels from bluffs several km west of Dakota City, Dakota County (NE). Relocation of these and other topotypic specimens has been unsuccessful to date. Although the thick, 10-cm-long specimen that Meek (1871, 1876) named “Unio (Baphia?) nebrascensis” and later assigned to Margaritana (now Margaritifera) is distinct in the pantheon of Cretaceous mussels, its placement in this extant pandemic genus and the Margaritiferidae awaits additional specimens that show internal valve features. A similar hefty, articulated specimen was discovered by Upham (1888) from minor ("unmapped") exposures along the Mississippi River just down from the confluence with Two River in Morrison County (fossils at the University of Minnesota).

White (1894) named the elongate Unio barbouri (similar to Ligumia) (Unionidae) and continental snails from external and internal molds produced from hard, ferruginous clastic matrix in Jefferson County (NE). Vintage plastertypes are preserved at the University of Nebraska-Lincoln (UNL) and the National Museum of Natural History (NMNH), but not all of the type-producing matrix discovered by Hicks (1885) has been seen (not found at UNL or NMNH).

Dakota fossil assemblages are variously composed of freshwater, freshwater-tolerant, brackish, and marine taxa indicating various habitats and depositional mixing on a low floodplain with limited clastic influx produced from minor uplands resulting in deltaic progradation within entrained distributary river systems (with some avulsions) over a few million years of sea-level fluctuations.