THE FIRST COMPREHENSIVE OLIGOCENE HERPETOFAUNA FROM THE WHITE RIVER GROUP OF SOUTH DAKOTA, REPRESENTED IN THE CEDAR PASS LOCAL FAUNA

The Cedar Pass Local Fauna from the White River Group of South Dakota comprises a diverse and complex faunal assemblage. Although predominantly mammalian, recent studies have discovered that a highly diverse and substantial herpetofauna was present. Some of the more representative specimens from the Cedar Pass Local Fauna include cf. Scaphiopus (Anura), Peltosaurus (Anguidae), Aciprion (Iguanidae), Rhineura hatcheri (Amphisbaenidae), and an assortment of boid taxa.

Two additional iguanids have been identified: Cypressaurus and Leiocephalus. Previously, Cypressaurus had been observed from North Dakota and Saskatchewan, Canada (Chadronian). Fragmentary material of Leiocephalus has been documented from Wyoming (Orellan), but the Cedar Pass specimens consist of more complete dentigerous cranial material. Another recent discovery is the posterior portion of an anuran skull that closely resembles the extant taxon Rana. Compared closely to Rana pipiens (leopard frog), the primary morphologic difference is a variation in the parasphenoid. If the referral of this specimen to Rana is accurate, this specimen represents the first Oligocene occurrence of Ranidae in North America, making it the oldest record of a ‘true frog’ on this continent (prior oldest record is from the Hemingfordian). Re-examination of these lower vertebrates has increased insight into the environment and resulting faunal diversity in North America during the Oligocene.

The Cedar Pass Local Fauna offers an important contribution to the North American Oligocene herpetological record. This is significant for three reasons: 1) With the exception of a single, indeterminate specimen (Arikareean), no Oligocene iguanids from South Dakota had been reported until 2013; 2) There are only two sites in North America where the Whitneyan herpetofauna is observed, the Cedar Pass Local Fauna and the I-75 Local Fauna in Florida; and, 3) An expansive herpetofaunal diversity is contrary to the accepted convention of declining diversity at the Orellan-Whitneyan boundary. The interpretation of a declining herpetological diversity during the Orellan-Whitneyan is erroneously based on a lack of collected material from this interval.