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Home , Amynodontidae, Archaeotherium, Daeodon, Metamynodon, Protoceratidae, Subhyracodon

Proceedings of the Academy of Science, Vol. 97 (2018) 181

NEW MATERIAL OF (PERISSODACTYLA) FROM NATIONAL PARK AND ITS IMPLICATIONS ON THE - 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 , South Dakota. This new specimen represents the youngest occurrence of Amynodontidae in . Prior to this study, makes its highest verified occur- rence in the underlying Scenic Member of the ( North American Land Age). The Cedar Pass local fauna originates within the lower Poleslide Member of the Brule Formation and is considered to contain a medial 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 , 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 “ 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 that has a narrow biochro- nologic range in North America from the middle 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 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 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 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 (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 and geographic information regard- ing specimens from the Cedar Pass Collection. While the information written in Bjork’s and Thompson’s field notes is rather incomplete, the field number associated with BADL 42032 links the specimen to a location in the proxim- ity of SDSM 675 (Dinohyus sp.) from Parris and Green (1969). Parris and Green (1969) identify the partial skull of Dinohyus sp. (now a junior synonym of Daeodon; Foss and Fremd, 1998) from the Arikaree channels at Cedar Pass. Other biochronologically significant specimens identified by Parris and Green (1969) include Tamias, Proheteromys, Hitonkala, Capacikala, and Palaeocastor. Parris and Green (1969) also identifiedPlesiosminthus from their fauna, but Green communicated to Martin (1974) that this identification was in error. Martin (1974) provided no correction to the identification of the misidenti- fied Plesiosminthus. Wang (1994) also references the large hesperocyonine canid Sunkahetanka from the same horizon. The majority of this fauna seems to corre- late well with the Wounded Knee Fauna, which is the most comprehensive early Arikareean fauna described from South Dakota (Macdonald 1963, 1970). The castorids Palaeocastor nebrascensis and Capacikala gradatus are known from the earliest Arikareean, with only P. nebrascensis extending into the late Whitneyan (Flynn and Jacobs 2008). The florentiamyid Hitonkala andersontau is restricted to the earliest Arikareean, and the basal heteromyid Proheteromys is known from the early Whitneyan to earliest Arikareean (Flynn et al. 2008). Daeodon is known from the Arikareean to Hemingfordian NALMAs, with an earliest Arikareean occurrence in South Dakota (Effinger 1998).Sunkahetanka geringensis is known from the early Arikareean (Wang 1994). A synthesis of the fauna suggests an earliest Arikareean occurrence for specimens known from the Arikaree channels, including BADL 42032 (Figure 4). An unpublished thesis provided a larger and more comprehensive fauna from the Sharps Formation at Cedar Pass (Santucci 1991). Most of the fauna identi- fied in that study from the Rockyford Ash, Lower Sharps, Middle Sharps, and Upper Sharps should be redesignated as Upper Poleslide under the current stratigraphic framework proposed by Evanoff et al. (2010) and Benton et al. (2015). Taxa identified from the Sharps Channels (=Arikaree channels) in that study included Peratherium spindleri (= youngi), Domnina greeni, Cynodesmus cooki (=Otarocyon cooki), Nothocyon roii (= Cynarctoides roii), Tamias Proceedings of the South Dakota Academy of Science, Vol. 97 (2018) 185

sp., Palaeocastor nebrascensis, Capacikala gradatus, Heliscomys sp., Hitonkola andersontau, Proheteromys bumpi, Eumys blacki (= Leidymys blacki), Scottimus sp., Elomeryx garbanii, Hadroleptauchenia shanafeltae (= Leptauchenia major), Pseudocyclopidius major (=Leptauchenia major), Nanotragulus intermedius, and sp. Most of the taxa identified by Santucci (1991) were previously and nearly exclusively known from the Wounded Knee Fauna of South Dakota (Macdonald 1963, 1970). Unfortunately, specimen numbers were not assigned to the aforementioned specimens until after the completion of the thesis, so confirming the taxonomic identifications is difficult. Therefore, these specimens will need to be revisited in the future because this fauna will be critical to under- standing the biochronology of the Sharps Channels, the Upper Poleslide, and the Whitneyan-Arikareean transition in the Cedar Pass area.

SYSTEMATIC Order PERISSODACTYLA Owen 1848 Superfamily RHINOCEROTOIDEA Gill 1872 Family AMYNODONTIDAE Scott and Osborn 1883 Tribe METAMYNODONTINI Kretzoi 1942 Metamynodontini indet.

Referred specimen. BADL 42032; Right astragalus. Figure 1C and 2 and Table 1.

Locality and Horizon. Cedar Pass, Arikaree channels, undifferentiated Arikaree Group. Badlands National Park, Jackson County, South Dakota, of America.

Description and Comparison. Overall dimensions of the astragalus are trans- versely widened. The lateral condyle is slightly larger than the medial condyle, without extending ventral to the articulation of the cuboid, as in Cadurcodon. The astragalar trochlea is significantly shallower and weaker articulation in compari- son to Cardurcodon and Paramynodon as well as the deep, well-developed trochlea in Rhinocerotidae. The navicular articulation is slightly inflected, forming a ridge with a broad, rectangular articulation for the cuboid. We hesitate to assign this specimen to a owing to the small sample size available of amynodontids, particularly with regards to postcranial elements. The astragalus in theTrigonias -Subhyracodon-Diceratherium lineage within Rhinocerotidae differs in possessing deep trochleae and tall, narrow proximal condyles, indicating a well-defined articulation with the tibia (Figure 1B). The navicular articulation is nearly flat, comprising almost the entire distal surface, while articulation for the cuboid is slender. SDSM 352 represents one of a few known specimens of M. planifrons with well- preserved postcrania (Figure 1A). BADL 42032 shares a distinct resemblance to SDSM 352, with some variation. BADL 42032 is smaller with a slightly deeper navicular notch. The articular surface for the sustentaculum is noticeably more 186 Proceedings of the South Dakota Academy of Science, Vol. 97 (2018) posteriorly extended on the plantar surface by a short pedestal and contains a distinct separation with the articulation for the cuboid. By contrast, the same feature in SDSM 352 is even with the plantar surface and conjoined with the articulation for the cuboid. The ChadronianMetamynodon chadronensis is known from a single specimen from South Dakota (Wood 1937), and more material was described from the Porvenir local fauna in the Vieja area of Texas (Wilson and Scheibout 1981). However, no postcranial material is known from M. chadronensis, making it impossible to make comparisons between BADL 42032 and that taxon. Wall and Manning (1986) designate two tribes within Amynodontidae that exhibit different morphologic variants: the lightly-built Cardurcodontini; and the graviportal Metamynodontini. “Procadurcodon” sp. is the only member of the Cadurcodontini known from both Asia and North America and is the only amynodontid known from the late Uintan Hancock Quarry, Clarno Formation, John Day area in Oregon (Hanson 1996; Lucas et al. 2004). Thus, it is reason- able to consider the possibility that BADL 42032 could represent a secondary immigration of Asiatic amynodontids. Gromova (1954) provides the most comprehensive guide to Asiatic amynodontids, particularly figures and descrip- tions of postcranial elements of Cadurcodon (Figure 1D). The trochlear groove in Cadurcodon is relatively deeper when compared to that of Metamynodon. The lateral condyle is large and proximodistally expanded so far that the distal por- tion of the condyle appears level with the distal ridge. Cadurcodon also exhibits a deeper navicular notch. That morphology differs substantially from that of BADL 42032 (Figure 1C), eliminating the possibility that the latter specimen is a member of the Cardurcodontini. Tissier et al. (2018) described a new genus of Amynodontidae, Sellamynodon zimborensis, from the Eocene and possibly Oligocene of Romania and Hungary. Tessier et al. (2018) also provided a phylogenetic analysis of Rhinocerotoidea. Their analysis placesSellamynodon as the sister taxon to Metamynodon, based entirely on cranial characters (Tissier et al. 2018). They included Megalamynodon within the Metamynodontini and placed Paramynodon as the basal taxon in the clade (Tissier et al. 2018). Of all the representatives of the Metamynodontini, outside of Metamynodon planifrons, only Paramynodon is known with associated postcrania (Colbert 1938). The astragalus of Paramynodon is morphologically more similar to basal Rhinocerotoidea (i.e. well-developed trochlea and proximal condyles). Considering the available, yet limited, comparative material, we refer BADL 42032 to Metamynodonini, but are wary of designating a specific identifica- tion. There is sharp morphological contrast with known Cadurcodontini, with more definitive similarities toMetamynodon than basal Metamynodontini (i.e. Paramynodon). BADL 42032 shows some variation from M. planifrons, but the limited sample size of postcrania for this species cannot eliminate the potential for ontogenetic variation. BADL 42032 could represent the only known postcra- nia of a juvenile individual of M. planifrons, demonstrating modifying articular surfaces through allometric growth. Individual variation within M. planifrons is also possible, considering the lack of available postcrania throughout museum Proceedings of the South Dakota Academy of Science, Vol. 97 (2018) 187 collections. Additionally, because postcranial material is lacking from M. chadro- nensis, it is unclear how much, if any, variation is present in the astragalus between M. chadronensis and M. planifrons. It is also possible that BADL 42032 represents a new species, unique to its respective biochronologic zonation; how- ever, there is no associated cranial material from the Arikaree channels to clearly distinguish a potential new species from the holotypes of the two known species of Metamynodon. The large graviportal rhinocerotidTeleoceras shares strong morphological affini- ties to Metamynodon (Prothero 2005). Teleoceras postcrania are more abundantly known, so we provide observations on morphologic variation to gain insight on the identification of BADL 42032. Harrison and Manning (1983) demonstrated high variability of carpal elements within Teleoceras fossiger from the Bixson Bone Bed (Hemphillian NALMA), Florida. The Harrison and Manning (1983) study focused on the magnum and unciform, with little mention of other carpal or tarsal elements. Cursory observations with more abundantly available rhinocero- tid material from Nebraska, housed in UNSM collections, provides insight into the aforementioned interpretations regarding the systematic ambiguity of BADL 42032. T. medicornutum, T. fossiger, and T. major were examined to determine potential variations corresponding in stratigraphic cline, ontogenetic variation, and simple individual variation. Noticeable individual variations were recognized within each of the species observed, but the observed variations were not as con- spicuously distinct as BADL 42032 to M. planifrons. T. fossiger demonstrates a similar plantar extension for the articulation of the sustentaculum in comparison to relatively older T. meicornutum, as is observed between BADL 42032 and M. planifrons. There is strong ontogenetic variation between juvenile and adult T. major. One significant ontogenetic alteration is a greater plantar extension of the articula- tion for the navicular in juvenile T. major, also noticed in BADL 42032. Other ontogenetic variations include: increased concavity with the navicular notch and proximal articulation surface for the calcaneum; broader articulation for the cuboid and a plantar narrowing to a more angular shape; deeper articulation for the fibula; and, the articulation for the sustentaculum is more separated from the articulation for the navicular. The ontogenetic variation observed withinT. major could be interpreted as analogous to the variation noted above between BADL 42032 and M. planifrons, lending preliminary support for the hypothesis that BADL 42032 is an immature individual of M. planifrons.

CONCLUSIONS

A paucity of postcranial material attributed to Metamynodon and other amyn- odontids made it difficult to identify BADL 42032 at the generic level. Specific identification will be exceedingly difficult without directly associated cranial or dental material, since those elements provide the primary characters that define those taxa. Observed variations within the rhinocerotid taxon Teleoceras did not provide a conclusive identity for BADL 42032, but were useful in delineating 188 Proceedings of the South Dakota Academy of Science, Vol. 97 (2018) the amount of ontogenetic variation observed within a single rhinocerotoid species. The range of ontogenetic variation observed with the examined spe- cies of Teleoceras could indicate that BADL 42032 is a juvenile individual of M. planifrons. Alternatively, BADL 42032 could represent a different species that demonstrates morphologic change corresponding with stratigraphic cline. There is also the possibility that BADL 42032 is an immigrant derived from Eurasian Sellamynodon or another closely related taxon, more derived in the clade Metamynodontini. There is a slight, but unlikely, possibility that BADL 42032 was reworked from older sediments. The Arikaree channels formed paleovalley fills within the upper surface of the Brule Formation. from the Poleslide could have been eroded and redeposited into the Arikaree valley-fill. There are fossils present within the Arikaree channels that exhibit polishing and rounding from fluvial transport, suggesting relocation, but not necessarily a geochronologic relocation. BADL 42032 does not demonstrate weathered features that would suggest transport from a far-removed distance, either geographically or geochronologically. Thus, we consider it unlikely that this specimen is reworked from significantly older deposits. We place BADL 42032 within the earliest Arikareean NALMA, based on the current biochronologic understanding of fauna reported from the Arikaree chan- nels at Cedar Pass (Figure 4), pending further study and evaluation. This is the youngest known occurrence of the family Amynodontidae in North America, since the last well-established appearance of the taxon Metamynodon is within the Orellan (Wood et al. 1941; Emry et al. 1987). The current fragmentary understanding of the biostratigraphic distribution of amynodontids during the Oligocene in North America may relate to their preferred habitat within or immediately adjacent to stream channels, which makes their presence or absence from faunae difficult to ascertain when those types of sediments are not preserved.

ACKNOWLEDGEMENTS

We would like to thank Rachel Benton and Megan Cherry (BADL) for access to the Cedar Pass Specimens. We would also like to thank Phil Bjork (formerly SDSM) and Carrie Herbel (UNSM), along with their associated field staff, for their diligent collection of the Cedar Pass Fauna. Gratitude is also extended to Ross Secord and George Corner from UNSM for access to rhinocerotid postcra- nia from various localities. We also thank Emmett Evanoff for his insight into the geology and stratigraphy of the Cedar Pass area. We would like to thank Rachel Short, who helped with the design of the anatomical figures. We would also like to thank our reviewers. The views expressed in this publication do not necessarily reflect the views of the National Park Service or the Department of the Interior. Proceedings of the South Dakota Academy of Science, Vol. 97 (2018) 189

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editor. Late and Cenozoic Mammals of North America: Geochronology and Biostratigraphy. University of California Press, Oakland, CA. Santucci, V.L. 1991. Stratigraphy of the Lower Sharps Channel Deposits, Cedar Pass Area, Badlands National Park. Master’s Thesis. University of Pittsburgh. Scott W.B. 1941. Part 5, Perissodactyla in W.B. Scott and G.L. Jepsen, editors. The mammalian fauna of the White River Oligocene. Transactions American Philosophical Society 28:747-980. Scott, W.B., and H.F. Osborn. 1887. Preliminary account of the fossil mammals from the contained in the Museum of Comparative Zoology. Bulletin of the Museum of Comparative Zoölogy at Harvard College 13:151-171. Short, R.A., S.C. Wallace, and L.G. Emmert. In Review. Morphology of a new species of Teleoceras, with comments on the genus. Bulletin of the Florida Museum of Natural History. Tissier J., D. Becker, V. Codrea, L. Costeur, C. Fărcaş, A. Solomon, et al. 2018. New data on Amynodontidae (Mammalia, Perissodactyla) from Eastern Europe: Phylogenetic and palaeobiogeographic implications around the Eocene-Oligocene transition. PLoS ONE 13(4): e0193774. https://doi. org/10.1371/journal.pone.0193774 Wall, W.P. 1998. Amynodontidae. Pages 583-588 in C.M. Janis, K.M. Scott, and L.L. Jacobs, editors. Evolution of Tertiary Mammals of North America Volume 1: Terrestrial Carnivores, Ungulates, and Ungulatelike Mammals. Cambridge University Press, Cambridge, England. Wall, W.P., and K.L. Heinbaugh. 1999. Locomotor adaptations in Metamynodon planifrons compared to other amynodontids (Perissodactyla, Rhinocerotoidea). National Park Service Paleontological Research 4:8-17. Wall W.P., and E.M. Manning. 1986. Rostriamynodon granger n. gen., n. sp. of amynodontid (Perissodactyla, Rhinocerotoidea) with comments on the phylogenetic history of Eocene Amynodontidae. Journal of Paleontology 60:911-919. Wang, X. 1994. Phylogenetic Systematics of the Hesperocyoninae (Carnivora: Canidae). Bulletin of the American Museum of Natural History 221:1-207 Ward, F. 1922. The Geology of a Portion of the Badlands. South Dakota and Natural History Survey 11:7-59. Wilson, J.A., and J.A. Scheibout. 1981. Early Tertiary vertebrate faunas Trans- Pecos Texas: Amynodontidae. The Pearce-Sellards Series 33:1-60 Wood, H.E. 1937. A New, Lower Oligocene, Amynodont Rhinoceros. Journal of Mammalogy 18(1):93-94. Wood, H.E, R.W. Chaney, J. Clark, E.H. Colbert, G.L. Jepsen, J.B. Reeside, C. Stock, and committee. 1941. Nomenclature and correlation of the North American continental Tertiary. Geological Society of America Bulletin 52(1):1-48. 192 Proceedings of the South Dakota Academy of Science, Vol. 97 (2018)

Figure 1. Astragali compared between: A, SDSM 352, Metamynodon cf. planifrons, left astragalus; B, BADL 42301, rhinocerotidae indet., right astragalus (image reversed); C, BADL 42032, Metamynodontini indet., left astragalus; D, пин No. 473-692, Cadurcodon ardynensis, left astragalus (image modified from Gromova, 1954; size approximate to scale). Scale bar = 5mm. Proceedings of the South Dakota Academy of Science, Vol. 97 (2018) 193

Figure 2. Left astragalus of BADL 42032. Views: A, anterior; B, posterior; C, proximal; D, distal; E, lateral; and, F, medial. Abbreviations: ACB, articular surface for cuboid; AF, articular surface for fibula; AN, articular surface for navicular; AS, articular surface for sustentaculum; AT, articular surface for tibia; BAS, base; DACL, distal articular surface for calcaneum; DR, distal ridge; PACL, proximal articular surface for calcaneum; TRG, trochlear groove; TRO-AST, trochlea. Scale bar = 5mm. Labeled anatomical features and terminology after Short et al. (in review). 194 Proceedings of the South Dakota Academy of Science, Vol. 97 (2018)

Figure 3. Stratigraphic column of the White River Group (modified from Evanoff et al. 2010) displaying the potential range of the Metamynodon channels and the strati- graphic position of origin for BADL 42032. Proceedings of the South Dakota Academy of Science, Vol. 97 (2018) 195

Figure 4. Biochronologic ranges of definitively identified taxa from the Cedar Pass Arikaree Channels, including the range extension of Metamynodontini into the early Arikareean. Black bars represent occurrences throughout North America, including South Dakota. Gray bar indicates North American ranges, excluding South Dakota. 196 Proceedings of the South Dakota Academy of Science, Vol. 97 (2018)

Figure 5. Comparative specimens of left astragali from Teleoceras major, Ashfall Fossil Beds State Historical Park, Nebraska. Adult (A, C, and E) and infant/juvenile (B, D, and E) in anterior (A-B), posterior (C-D), and proximal (E-F) views. Proceedings of the South Dakota Academy of Science, Vol. 97 (2018) 197

Table 1. Measurements of BADL 42032 and SDSM 352. See Figure 2 for abbreviations. Measurements (mm) BADL 42032 SDSM 352 Maximum Width 79.7 97.9 Proximodistal Length 69.2 63.0 Width at TRO-AT 55.5 56 Width at BAS 64.2 74.8 198 Proceedings of the South Dakota Academy of Science, Vol. 97 (2018)