The nonmarine mollusks of the late Oligocene–early Miocene Cabbage Patch fauna of western Montana III. Aquatic mollusks and conclusions

Abstract

The aquatic molluscan fauna of the late Oligocene–early Miocene Cabbage Patch beds of western Montana includes eight taxa, six of which are new at specific or subspecific level,Sphaerium bakeri, Valvata paula, Planorbula powelli, Biomphalaria haydeni, Lymnaea tumere, andLymnaea vetusta ambigua.Two are referred to previously known taxa,Viviparus nanusandLymnaea shumardi.The generaBiomphalaria, Planorbula, andViviparusno longer occur in western Montana, nor in the Columbia River drainage.Planorbula powelliis the earliest unequivocalPlanorbulafrom North America. Of the 18 supraspecific taxa of this molluscan fauna, eight (44%) are now displaced, all but one to the east and/or south. Climatic interpretations derived from modern habitats of the generaBiomphalariaandPolygyroideasuggest a late Oligocene–early Miocene Mean Annual Temperature of 10°C, with January means not less than 5°C, and July means about 15°C. Environmental analysis suggests a subhumid to subarid shrub and grassland vegetation similar to modern environments with > 50 cm Mean Annual Precipitation, surrounding marshy shallow lakes subject to seasonal desiccation. Effective moisture varied from slightly greater than the present for the late Oligocene to a latest Oligocene pluvial interval followed by subarid conditions for the early Miocene. Geographically, the drainage of this area was into a closed and isolated basin during the late Oligocene–early Miocene. Molluscan assemblages are found to support the tripartite division of the Cabbage Patch beds that has been based on mammals and should be useful in correlation between basins.