CORRELATING THE SOUTH DAKOTA WHITE RIVER GROUP AND ITS CONTINENTAL MOLLUSKS TO NORTH DAKOTA: A BEGINNING UP THE SLIPPERY SLOPE OF UTILIZING HISTORIC FAUNAS

White River Group pulmonate gastropods begin their less than well-documented record in 1853 with a meeting of the players at an outlier of Chadron Formation north of the famous Mauvaises Terres of South Dakota. The majority of the eight types from Poeno Springs described by John Evans and B.F. Shumard (1854 and in Meek, 1876; planorbid, physid, and lymnaeids; four of five types lost, one figured); F.B. Meek and F.V. Hayden (1860; planorbids; one of two lost, neither figured); and Meek (1876, one lymnaeid figured) remain problematic.

Our work has shown the upland surface exposures around the Poeno Creek headwaters are of the Chadron Formation. The relocated fossil pulmonates are in sec. 9, T. 2 N., R. 17 E. (Horse Tooth 7.5' Quad., Pennington Co.). The unknown historic location (Poeno Springs) is referred to Locality L3033. We have collected from discrete carbonate areas similarly described by Hayden in 1855 as on "an elevated ridge composed of white aluminous and calcareous marl, containing slabs and concretions fully charged with fossil shells."

Apparently similar pulmonate fossils were identified by T.-C. Yen (Hansen, 1953) from Chadron-like lithologies atop buttes in southwestern North Dakota. Yen's identifications from White (Colgrove of Hansen, not Cosgrove of Murphy et al., 1993; White Butte W and E 7.5' Quads., Hettinger Co.), LeFor (Long of Hansen; LeFor 7.5' Quad., Stark Co.), and Straight Buttes (White Butte NW 7.5' Quad., Stark-Hettinger Cos.) extended the range of three Poeno Springs taxa to North Dakota deposits that can be undoubtedly identified as the South Heart Member of the Chadron Formation. In their review of the White River Group-capping buttes of southwestern North Dakota, Murphy et al. (1993) placed Yen’s (Hansen, 1953) identifications in a table, but they did not find fossils at these buttes.

In 1977, Hartman and R.C. Holtzman collected pulmonates from White Butte (Locality L0079; sec. 17, T. 136 N., R. 93 W.). The present collection is exclusively of variably preserved siliceous lymnaeid internal and external molds. Specimens range in size from slightly more than 1 mm to about 8 mm. Potentially, four species are present, but assigning any to the Poeno Springs fauna is premature even though preservation and first indications suggest comparability to certain described lymnaeids.