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The AAPG/Datapages Combined Publications Database

Rocky Mountain Association of Geologists

Abstract


The Mountain Geologist
Vol. 49 (2012), No. 1. (January), Pages 19-34

Additions to the Vertebrate Faunal Assemblage of the Middle Miocene Fort Randall Formation in the Vicinity of South Bijou Hill, South Dakota, U.S.A.

Darrin Pagnac

Abstract

Recent slumping within the vicinity of South Bijou Hill (Charles Mix County, South Dakota) has exposed new outcrops of the middle Miocene Fort Randall Formation allowing for additional insight into stratigraphy and paleontology. Collecting efforts have greatly expanded the known faunal list which now includes several new taxonomic occurrences within the Fort Randall Formation. These new discoveries have permitted reexamination of the precise age of the formation and its contained faunal assemblage, and have extended the temporal range of several taxa.

Additions to the faunal assemblage of the Fort Randall Formation are described from three new localities. Additional specimens of previously described taxa, including the shrew Plesiosorex donroosi and the jumping mouse Megasminthus tiheni, offer additional insight into morphological variation. Taxa previously unknown from the Fort Randall Formation are described, including an eagle, the garter snake Thamnophis, the horses Archaeohippus and Cormohipparion quinni, the beavers Monosaulax valentinensis and Eucastor, and the oldest occurrence of a gomphotheriid proboscidean (elephant relative) from South Dakota.

Based on these newly described faunal constituents, the latest Barstovian (Ba2) age for the Fort Randall Formation and its faunal assemblage is confirmed, based in part on the co-occurrence of C. quinni, M. valentinensis and Eucastor. Additionally, the depositional hiatus between the Fort Randall Formation and the overlying Valentine Formation appears to encompass a relatively short temporal span. The Fort Randall Formation contains a faunal assemblage quite similar to that from the Egelhoff and Norden Bridge quarries in the lowermost Cornell Dam Member of the Valentine Formation, suggesting a relatively close temporal relationship.


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