REEVALUATION OF SPECIES CONCEPTS OF HELL CREEK FORMATION (CRETACEOUS) UNIONIDAE, MONTANA

The Hell Creek Formation represents a landscape of numerous rivers and lakes, which were home to a variety of freshwater bivalves. The dominant constituent of this assemblage were members of the freshwater mussel family Unionidae, yet the traditional focus of Hell Creek Formation studies has concentrated on either dinosaurs or mammals. In light of the dominance in relative numbers of freshwater mollusks in the Hell Creek Formation, they have been comparatively less studied. These unionids were first described by Whitfield using the Brown collections in the early 1900s, but received minimal reevaluation until Russell in 1975. Both the Whitfield and Russell publications concentrated on naming species based on general species descriptions without numerical analysis, and not on evolutionary relationships. This project, applies rigorous character trait analysis combined with cladistical methods to further explore the species diagnoses selected unionids. Eight species, or four sets of sister species were selected based on their similarity to their sister species and the difficulty in identifying accurately species within species sets. The species chosen were derived from three genera, and consist of Plethobasus aesopiformis, Pleth. biesopoides, Plesielliptio brachyopisthus, Ples. postbiplicatus, Ples. gibbosoides, Ples. whitfieldi, Proparreysia letsoni, and Prop. verrucosiformis. Each species was carefully measured for a diverse array of external and internal shell character traits. The traits that were measured were chosen to both represent a general description of each specimen and their potential to be used as a tool in distinguishing species for practical identification. Statistical analysis clearly shows the possible separation of species of generally similar morphology. Ongoing cladistical analysis has the potential to show relationships between genera as well. Analyzing the relationships between the Hell Creek Formation genera of unionids is crucial to critically examining their placement into both extinct and extant genera. Once completed, this study will create a well-defined rubric for discriminating unionid species in the Hell Creek Formation that can be utilized for further study on these species as well as other similar unionids.