Environmental implications of a beetle assemblage from the Gervais Formation (early Wisconsinan?), Minnesota
The Gervais Formation of northwestern Minnesota is the oldest known Quaternary stratigraphic unit in the Red River Valley. Wood from the fossiliferous silt and peat member is > 46,900 yr old. The age of the unit is uncertain but may be early Wisconsinan. A well-preserved beetle assemblage consisting of extant species is described from the fossiliferous horizon. Presently, most of the species are widely distributed and occur in Minnesota. The remainder have restricted northern distributions. Included in this group is the weevil Vitavitus thulius, known only from two localities in Northwest Territories. The sedimentary environment indicated by the beetles was a small lake, rich in aquatic and semi-aquatic vegetation. The margins were characterized by open areas and spruce woodland. Species with arctic-alpine distributions indicate a cold environment, but whether it was regional or local in extent could not be determined. If regional, a climate similar to that of the tundra-forest transition zone is postulated with mean July temperatures in the range 11° to 14°C. If the cold environment was restricted, however, the climate may have been similar to that of the Lake Superior region with a mean July temperature of 17°C.