Date of Award
Doctor of Philosophy (PhD)
Study of forty fossiliferous sites in late Wisconsinan sediments from the Missouri Coteau district in central North Dakota has defined the molluscan fauna of the period of geologic time between 12,000 and 9,000 C14 years before the present. The fauna includes twenty-eight taxa of nonmarine gastropods and bivalves and is dominated by branchiates. The modern molluscan fauna of the Missouri Coteau district in North Dakota lacks four species which were resident in the late Wisconsinan and contains no unionids, one additional aquatic species, and is strongly dominated by pulmonate gastropods.
The geology of the Missouri Coteau has been interpreted as being the sedimentary result of the stagnation of the marginal fifty to one hundred miles of the Wisconsinan ice sheet.
The interpretation of the paleoecology of the late Wisconsinan molluscan fauna derives from geologic, paleoautecologic, and paleosynecologic data. It is hypothesized that at the time the late Wisconsinan continental ice sheet stagnated in central North Dakota, the climate was less extreme and more humid than at present. Ponds and streams existed on the ice-cored topography of the Missouri Coteau. They were connected by a drainage system tributary to the Missouri River and populated by an abundant, branchiate-dominated molluscan fauna.
Fishes, probably of the families Percidae and Centrarchidae, lived in the ponds and rivers and unionid bivalves successfully populated many of the water bodies. Calcareous algae (Chara) and undoubtedly other species of aquatic vegetation colonized the clear, warm water of the drainage system. The ice-basined, drift-insulated lake was the most common lithotope in which fossil mollusks were preserved. The entire drainage system was supplied by greater rainfall and snowmelt than now occurs on the Missouri Coteau (probably in the order of 20 to 30 inches per year) and the precipitation - evapotranspiration ratio was positive. The humid climate changed toward the more arid climate of today before the ice core of the Coteau melted. Comparison with modern molluscan assemblages characteristic of defined habitats in Minnesota indicates that the mollusks which lived on the ice-cored Missouri Coteau, could have occupied small streams, medium-hardwater lakes, and/or hardwater prairie lakes. In consideration of the population dominance by branchiates, I believe that hard water prairie lakes are not likely to have occurred on the late Wisconsinan Missouri Coteau.
In order to evaluate this hypothesis the terminus of the Martin River Glacier in south-central Alaska was studied. A molluscan fauna of ten fresh-water and thirteen terrestrial taxa was discovered on and in front of the stagnant terminus of the glacier. Twenty-three potential habitats for mollusks were studied and defined. Twelve of these were found to contain mollusks. The aquatic mollusks were found to occupy cool water (as low as 5°C) and warm water (above 10°C) and to be able to withstand very low concentrations of dissolved solids and high turbidity. Branchiate gastropods and bivalves were found to favor turbid environments next to the glacier and to populate successfully ice-basined, clear, warm lakes on the glacier. Terrestrial snails and slugs were found as much as a kilometer out on the drift-covered stagnant glacial terminus separated from the glacier ice by only 11 inches of alder-leaf-litter. Terrestrial snails were found in spruce forests only once. They were not found under willow bushes, young alder shrubs (less than 7 years in age), or on outwash gravel surfaces. Acid peat growths, and ponds developed in them, were also barren of mollusks. Edaphic conditions are assumed to account for their absence in these potential habitats.
The Alaskan studies lend credence to the hypothesis erected to explain the paleoecology of the late Wisconsinan molluscan fauna of the Missouri Coteau in that they prove that mollusks are vigorous pioneers near and on stagnating glaciers, that less than a foot of super glacial drift is sufficient to insulate terrestrial molluscan habitats, and that six feet of superglacial drift is sufficient to insulate ice-basined, clear, warm lakes from their containing ice walls. The studies further emphasize that terminal glacial features such as end moraines, outwash fans, dead-ice moraines, kettles, and superglacial lakes are the result of a non-glacial climate and the inference of glacial climate from such features is a fundamental error. The employment of fossil nonmarine mollusks as indicators of past water quality was brought under serious question by the studies reported here. The fact that aquatic mollusks live successfully in Alaskan waters which contain less than 1 ppm dissolved solids brings into question the practice of employing mollusks as paleolimnologic indicators of chemical aspects of their environments. I believe the use of mollusks as indicators of past water quality of their habitats must be discontinued.
Tuthill, Samuel J., "A Comparison of the Late Wisconsinan Molluscan Fauna of the Missouri Coteau District (North Dakota) with a Modern Alaskan Analogue" (1969). Theses and Dissertations. 302.