THE EARLIEST PROBOSCIDEAN IN SOUTH DAKOTA FROM THE MIDDLE MIOCENE (BARSTOVIAN) FORT RANDALL FORMATION

The “Proboscidean Datum” (the earliest appearance of the group) for the Great Plains has been calibrated between 14.2-14.0 Ma. Proboscideans (gomphotheres, mastodons, and elephants) are common constituents of Great Plains paleofaunas by the late Barstovian NALMA (15.0-12.5 Ma). Late Barstovian proboscidean remains are most commonly found in the Valentine Formation of Nebraska, but have also been reported from the Madison Valley beds of Montana, Barstovian paleofaunas from central Wyoming, and most notably from correlative assemblages from the Pawnee Creek Formation in northeastern Colorado. Previously, the earliest occurrences of Proboscidea from South Dakota were from the Wolf Creek and Mission assemblages of early Clarendonian age. The earliest record of proboscideans in South Dakota is now recorded from the Fort Randall Formation.

The Fort Randall Formation consists of several hundred feet of fluvial channel sands and overbank silts and clays intermittently exposed in southern South Dakota and into Nebraska. The Fort Randall Formation unconformably overlies the Cretaceous marine shales of the Pierre Group, and is overlain by the Valentine Formation. The highly resistant Bijou Quartzite commonly caps exposures of Fort Randall Formation serving as a useful marker bed. The Fort Randall Formation contains a diverse and well-documented late Barstovian fossil mammal assemblage, including extensive records of insectivores and rodents.

SDSM 78602, a large fragment of tooth enamel from a gomphotheriid proboscidean molar, was recovered from McConnell/Trenholm Site (V2006-1), approximately 1.5 miles from the Fort Randall Fm. type section. The fragment represents a portion of the characteristic “trefoil” molar cusp (diagnostic of the family) and lateral portion of the tooth crown. While the specimen is not of exceptional quality, its recovery from the Fort Randall Formation extends the record of Proboscidea in South Dakota to the latest Barstovian.