Proceedings of the South Dakota Academy of Science, Vol. 97 (2018) 181 NEW MATERIAL OF AMYNODONTIDAE (PERISSODACTYLA) FROM BADLANDS NATIONAL PARK AND ITS IMPLICATIONS ON THE WHITNEYAN-ARIKAREEAN TRANSITION Ed Welsh1,*, Darrin Pagnac2, and Clint A. Boyd3 1Badlands National Park Interior, SD 57750 2South Dakota School of Mines and Technology Rapid City, SD 57701 3North Dakota Geological Survey Bismarck, ND 58505 *Corresponding author e-mail: [email protected] ABSTRACT An astragalus belonging to an indeterminate amynodontid, within the tribe Metamynodontini, is identified within collections from the Cedar Pass local fauna from the North Unit of Badlands National Park, South Dakota. This new specimen represents the youngest occurrence of Amynodontidae in North America. Prior to this study, Metamynodon makes its highest verified occur- rence in the underlying Scenic Member of the Brule Formation (Orellan North American Land Mammal Age). The Cedar Pass local fauna originates within the lower Poleslide Member of the Brule Formation and is considered to contain a medial Oligocene fauna (early Whitneyan North American Land Mammal Age). However, matrix affixed to the new specimen is primarily composed of sandstone and gravel consistent with the river channel deposits within the Sharps Formation, which overlies the Poleslide Member. Despite recent stratigraphic reclassification of underlying sediments from the basal Sharps Formation to the upper Poleslide Member, the associated Sharps fauna is distinctly characterized as Arikareean North American Land Mammal Age in character. Taxa previously recovered within the Arikaree channel deposits include the entelodont Daeodon, the canid Sunkahetanka, and the rodents Tamias, Proheteromys, and Hitonkala. Specimens of the castorids Capacikala and Palaeocastor were recovered from the underlying Poleslide siltstones. Prior referrals of the channel deposits at the top of Cedar Pass to the classic “Protoceras channels,” along with associated faunal components, confound our understanding of the Whitneyan-Arikareean transi- tion and highlight the need for further research into the fauna of the Sharps Formation in the North Unit of Badlands National Park. Keywords Aminodontidae, South Dakota, Whitneyan-Arikareean Transition 182 Proceedings of the South Dakota Academy of Science, Vol. 97 (2018) INTRODUCTION The Cedar Pass fauna, in Badlands National Park, is the most taxonomically diverse fauna within the White River Group, including representatives of nearly every taxonomic group reported from the White River Group. Specimens were collected in the late 1970’s to early 1980’s by field crews led by Philip Bjork, and those collections are currently reposited at the South Dakota School of Mines and Technology in Rapid City, South Dakota. Nearly all of the fauna was col- lected within a narrow horizon within the lower Poleslide Member of the Brule Formation, known unofficially as the Bjork Siltstone (Evanoff et al. 2010). A -pre liminary study by Korth (2014) describes the rodents from that collection, iden- tifying a transitional Orellan-Whitneyan North American Land Mammal Age (NALMA) fauna from the Bjork Siltstone. However, a single astragalus (BADL 42032: Figures 1C and 2) previously referred to Rhinocerotidae retains a sand- stone matrix matching the stratigraphically higher Arikaree channels, previously known as Sharps channels (Figure 3) (Evanoff et al. 2010; Benton et al. 2015). Here we reexamine BADL 42032 and correct the identification to an intedermi- nate, yet derived, taxon within the tribe Metamynodontini (Amynodontidae). Amynodontidae is a family within Rhinocerotoidea that has a narrow biochro- nologic range in North America from the middle Eocene to the early Oligocene. North American amynodontids had a relatively widespread geographic range in the late Eocene, particularly within the late Uintan and Duchesnean NALMAs, containing five known genera: Amynodon, Amynodontopsis, Megalamynodon, Metamynodon, and “Procadurcodon” (Wall 1998; Wilson and Schiebout 1981; Hanson 1996). There is a sharp decline in amynodontid diversity in the early Oligocene with only one known species, Metamyndon planifrons (Wall 1998). Oligocene occurrences of M. planifrons are primarily isolated to the Great Plains region, and nearly exclusive to the classically designated “Metamynodon sandstones” or “Metamynodon channels” within the lower Scenic Member (Figure 3) (Osborn and Wortman 1894; Scott 1941). Wall (1998) refers to a Whitneyan occurrence of this clade in South Dakota, but no field or specimen data have been published to verify this occurrence. Wall (1998) is likely referring to AMNH 1086, a large specimen from the “Protoceras channels” mentioned in Scott (1941). However, catalogue information from the AMNH is vague con- cerning the provenance of the specimen. Metamynodon sp. was reported from the Brule Formation in the Chalky Buttes area of Slope County, North Dakota (Hoganson and Lammers 1992; Murphy et al. 1993), but no distinct biochrono- logic association was reported, no specimen number (or holding institution) was provided. The specimen referred from North Dakota was relocated and identified as a rhinocerotid. Only one occurrence of M. planifrons is known from outside the Great Plains region. Manning et al. (1985) describe a skull of M. planifrons from the early Oligocene Byram Formation of Mississippi, with the biochrono- logic designation in calcareous nanoplankton zone NP22-23, early Oligocene (Manning 1997). Though Scott (1941), and subsequently Wall (1998), make reference to the presence of Metamynodon from the Poleslide Member of the Brule Formation, that occurrence is not noted in more recent publications on Proceedings of the South Dakota Academy of Science, Vol. 97 (2018) 183 biostratigraphy within the White River Group (Prothero and Whitlessey 1998; Prothero and Emry 2004). This was likely due either to the rarity of the taxon, which may indicate this taxon has little biochronologic significance, or that the reported occurrences from the Whitneyan are questionable or incorrect. Scott (1941) referred large limb elements to Metamynodon (AMNH 548), but Prothero (2005) refers the same specimen to Amphicaenopus (Rhinocerotidae) based on the morphology of the ulna. In light of all of this information, prior reports of Whitneyan occurrences of Metamynodon should be considered highly questionable. We consider the lithologic, stratigraphic, and biochronologic data associ- ated with BADL 42032 to slightly extend the range of Metamynodontini from the “Metamynodon channels” (Orellan NALMA) to the Arikaree channels (early Arikareean NALMA). We also address the difficulty in understanding the Whitneyan-Arikareean transition in the Big Badlands of South Dakota in regard to the challenges of correlating channel deposit collections and their comparative faunae. INSTITUTIONAL ABBREVIATIONS AMNH, American Museum of Natural History; BADL, Badlands National Park; SDSM, South Dakota School of Mines and Technology; UNSM, University of Nebraska State Museum; пин, Paleontological Institute, Russian Academy of Sciences. GEOLOGIC AND BIOCHRONOLOGIC SETTING Metamynodon was traditionally interpreted as having a semiaquatic mode of life, based on skeletal modifications and the fact that all known specimens seem to demonstrate confinement to aquatic facies (Scott 1941; Manning 1997; Wall 1998; Wall and Heinbaugh 1999). The matrix affixed to BADL 42032 is primar- ily fine greenish-gray sandstone with mixed coarse sand to gravel grains, which match sandstone channels attributed to valley-fill deposits of the Arikaree Group (Figure 3; Harksen 1974; Evanoff et al. 2010; Benton et al. 2015). The Whitneyan-Arikareean Transition is difficult to distinguish within the North Unit at Badlands National Park. Classically collected localities and referred specimens from South Dakota are more traditionally known from Sheep Mountain Table and areas to the south (Prothero and Whitlessey 1998; Prothero and Emry 2004). Faunae from the North Unit of Badlands National Park, particularly within the Cedar Pass area, are not well described or included in contemporary literature. Ward (1922) identified the channels at Cedar Pass as Protoceras beds, along with a handful of fauna, including the peccary Perchoerus robustus. P. robustus has more recently been synonymized with P. probus, with a range restricted to the Whitneyan of South Dakota and Nebraska, regardless of early Arikareean occurrences in the John Day Formation, Oregon (Prothero, 184 Proceedings of the South Dakota Academy of Science, Vol. 97 (2018) 2009). Macdonald (1951) also named the holotype of the large entelodont Archaeotherium (Pelonax) lemleyi from the Protoceras channels of Cedar Pass, again attributing the biochronology to the Whitneyan NALMA. A study of protoceratids by Patton and Taylor (1973) included an occurrence of Protoceras celer from the Protoceras channel, Poleslide Member of the Brule Formation near Cedar Pass. The revelation of specimens without associated fauna adds to confu- sion regarding the faunal component of the channels at Cedar Pass. It remains unclear if sediments contemporaneous to the “Protoceras channels” are present at Cedar Pass. It is possible that all of these channels are diachonous, represent- ing isolated pockets of Whitneyan or Arikareean faunae, but such a conclusion requires much more detailed studies of these deposits. Recently discovered field notes from Philip R. Bjork and Diane L. Thompson (formerly SDSM) have clarified stratigraphic