AN UNUSUAL FRESHWATER BIVALVE FROM THE UPPERMOST CRETACEOUS OF MONTANA: A PREVIOUSLY UNEXPLORED ECOLOGICAL NICHE IN THE HELL CREEK FORMATION

The Lancian-age (NALMA) Hell Creek Formation contains a highly diverse assemblage of freshwater bivalves (Unionidae). Previous recognition of an unusual species by Morris and Williamson in 1988 was based on hinge plate fragments that were assigned to Pleiodon (Iridinidae). The discovery of more complete specimens now permits a new interpretation of a bivalve in a previously undocumented freshwater ecological adaptation. The undescribed taxon is rare. It is known only from eight specimens or fragments from six distinct localities in northern Garfield County, Montana. It occurs primarily in the upper half of the Hell Creek Formation, from about 57 to 12 m below its top (which approximates the Cretaceous-Paleogene boundary in this area). The taxon has been found in different species rich freshwater faunal associations. The taxon is easily distinguished from any other bivalve species in the Western Interior of North America by possessing a distinctively raised ligament platform that forms a vertical wall on the interior of an edentulous valve. The ligament surface is raised laterally to the dorsal commissure, which is characterized by a series of ridges and grooves that are continuous from one side of the beak to the other.

The presumed life orientation of the taxon would have the valves completely open, cojoined along the dorsal margin. Examples of this life mode are found in the living marine Ephippodonta (Galeommatidae) from the eastern Atlantic (Angola) and southwestern Pacific (Australia) Oceans. Galeommatids are known to crawl with valves dorsal and live in somewhat protected cavities between attached bivalves and the substrate or in burrows created by crustaceans. An obvious freshwater analog may not be possible, as other Hell Creek unionoid bivalves are active burrowers in mobile substrates. Although marine conditions are known to exist during the time of Hell Creek deposition in North Dakota, no other evidence suggests marine conditions extended into Montana (a distance of >300 km).