Paludicola 6(2):31-40 June, 2007 © 2007 by the Rochester Institute of Vertebrate Paleontology MAMMALS FROM THE BLUE ASH LOCAL FAUNA (LATE OLIGOCENE), SOUTH DAKOTA. RODENTIA, PART 1: FAMILIES EUTYPOMYIDAE, EOMYIDAE, HELISCOMYIDAE, AND ZETAMYS William W. Korth Rochester Institute of Vertebrate Paleontology, 265 Carling Road, Rochester, NY 14610 ABSTRACT More than twenty species of rodents have been listed from the Blue Ash local fauna of South Dakota in preliminary surveys, but only four species have been formally identified and described. Species of rodents from the families Eutypomyidae, Eomyidae, and Heliscomyidae are described here. There are one species of eutypomyid, Eutypomys wilsoni (new species); three species of eomyids, Paradjidaumo sp., Leptodontomys sp. cf. L. douglassi, and Zophoapeomys indicus (new genus and species); and two heliscomyids, Heliscomys medius (new species) and Heliscomys sp. An additional specimen of the problematical rodent Zetamys sp. is also cited. This increases the number of described taxa from the Blue Ash fauna to seven. There appears to be some ambiguity as to the age of the fauna, latest Whitneyan or earliest Arikareean. Unfortunately, the species described here (and previously) are unique to this fauna and cannot be used to correlate with other North American faunas to determine the exact biostratigraphic age. INTRODUCTION Barstovian of North America (Hopkins, 2004), and the genus of a new squirrels is known elsewhere only from The Blue Ash fauna from Fall River County, the Chadronian (Emry and Korth, in press). The South Dakota was first mentioned by Martin (1974) specimens described below, and those of the species who identified it as Whitneyan in age and presented a previously described from the Blue Ash fauna, were preliminary faunal list that included over 20 species of collected from several anthills. Because of this, the rodents (Table 1). He suggested that the fauna was vast majority of the specimens are isolated cheek teeth. transitional between typical Orellan and Arikareean However, the sample consists of nearly 1000 individual faunas of the Great Plains. The geology of the area specimens. This paper is intended to be the first in a was described by Simpson (1985), who similarly series of systematic descriptions of the rodents from recognized approximately the same number of rodent the Blue Ash fauna. taxa from what he called the Blue Ash Channel. Abbreviations.—Dental measurements: L, Simpson (1985:fig. 2, 3) determined that the Blue Ash anteroposterior length; W, transverse width; H, crown Channel rested on top, and cut into rocks of the height. Institutions: CM, Carnegie Museum of Natural Whitney Member of the Brule Formation, source of the History. Dental terminology follows that of Wood and type Whitneyan Land Mammal Age (Wood et al., Wilson (1936). Upper teeth are indicated by capital 1941: Prothero and Emry, 2004). He was unable to letters, lower teeth by lower-case letters. determine whether fossils from the Blue Ash Channel were latest Whitneyan or earliest Arikareean. Bailey SYSTEMATIC PALEONTOLOGY (2004) came to a similar conclusion when he compared Order Rodentia Bowdich, 1827 a number of early Arikareean faunas and estimated that Family Eutypomyidae Miller and Gidley, 1919 the Blue Ash fauna was near the Whitneyan- Eutypomys Matthew, 1905 Arikareean boundary in age. Eutypomys wilsoni n. sp. Only four species of rodents have been formally (Figure 1A, B) described from the Blue Ash local fauna thus far: the aplodontid Ansomys (Korth, in press), two new sciurids Type Specimen—CM 76293, left M1 or M2 (Emry and Korth, in press), and the problematical Referred Specimen—CM 76292, left P4. Zetamys sp. (Martin, 1974). All of these species are Diagnosis—Small species, similar to E. parvus in unknown from elsewhere, so they cannot be used in size; accessory lophules on cheek teeth as high and any faunal correlation. Generically, Zetamys is known pronounced as major lophs (major lophs nearly only from the early Arikareean Gering fauna (Tedford indistinguishable); upper cheek teeth proportionally et al., 2004), Ansomys is known from Orellan to elongated (longer than wide) as in E. montanensis; M1 31 32 PALUDICOLA, VOL. 6, NO. 2, 2007 markedly longer than P4 (P4 longer or slightly shorter TABLE 1. Rodents previously listed from the Blue Ash fauna. in length than M1 in other species); cheek teeth higher Martin, 1974 Simpson, 1985 crowned than in other species. Aplodontidae Etymology—Patronym for R. W. Wilson. Prosciurus sp. Prosciurus sp. Description—The upper premolar is roughly Pelycomys sp. ?Haplomys indet. square in outline, being nearly equal in length and ?Allomys sp. ?Prosciurine indet. Haplomys sp. width (L = 2.52 mm; W = 2.55 mm). The buccal cusps ?Meniscomys sp. (paracone and metacone) are rounded and there is a Sciuridae deep valley along the buccal margin of the tooth Squirrel indet Squirrel indet between them. A distinct parastyle (anterocone) is Protosciurus Protosphermophilus present at the anterobuccal corner of the tooth. This Miospermohilus cusp is anteroposteriorly compressed, smaller than the Castoridae major buccal cusps, and extends the buccal corner of Agnotocastor castorid gen. et. sp. new the tooth farther forward than the rest of the anterior castorid gen. et. sp. indet Eutypomyidae margin. There is a narrow valley on the occlusal Eutypomys surface between the parastyle and the paracone. The Eomyidae mesostyle is doubled on the margin of the tooth Small eomyid Adjidaujmo cf. douglassi between the paracone and metacone, the posterior of Paradjidaumo Paradjidaumo trilophus Cricetidae the two being the larger. The lingual cusps are Eumys brachyodus Eumys brachyodus crescentic and slightly smaller than the buccal cusps. Leidymys cf. blacki Eumys cf. parvidens There is only a shallow emargination between the Geringia mcgregori Eumys indet. lingual cusps. The central basin of the tooth and the Scottimus cf. kellamorum ?Leidymys cf. blacki Leidymys sp. indet. narrow valley between the paracone and parastyle are Geringia cf. mcgregori filled with numerous lophules. A metaloph and very Scottimus exiguus short protoloph are recognizable, but are very irregular. Scottimus longiquus There is no obvious mesoloph or other complete loph Scottimus sp. indet. Heliscomyidae on the occlusal surface of the tooth, which is a complex Helisomys ?Helisomys sp. indet. of numerous lophules. A posterior cingulum forms the Heteromyidae posterior margin of the tooth connecting the hypocone Proheteromys and metacone posteriorly. The lophules are only Geomyidae Tenudomys slightly lower than the protoloph and metaloph. There Florentiamyidae is no indication of a distinct conule on either of the Florentiamys ?Florentiamys sp. indet major lophs of the tooth. The tooth shows the Family uncertain beginnings of wear only on the occlusal surface, and Zetamys sp. _______________________________________________________ there is no recognizable facet on the anterior side of the tooth to accommodate a P3. of the transverse lophs extends only about half the M1 is larger than P4. It is markedly longer and width of the tooth, joining the posterior cingulum only slightly wider (L = 2.95 mm; W = 2.73 mm). M1 lingual to its center. There are also several shorter is longer than wide, a proportion known only in one anteroposterior lophs that connect the transverse lophs. other species of Eutypomys, E. montanensis. The four These lophs are as high and thick as the transverse major cusps of the tooth are relatively more reduced lophs. There is no indication of conules. The tooth is than in P4. The buccal cusps (paracone and metacone) clearly higher-crowned lingually than buccally are only small swellings along the buccal margin of the (unilateral hypsodonty). The height of the crown at the tooth, and the lingual cusps (protocone and hypocone) protocone is apporoximately 90% of the width of the are strongly obliquely compressed, entirely tooth (H = 2.42 mm). incorporated in lophs, and are not distinguishable on Discussion—Eutypomys wilsoni is distinguished the occlusal surface as cusps. The anterior and from all other species of the genus by its small size, posterior cingula run the entire width of the tooth from being smaller than all others except the Chadronian E. the buccal to the lingual cusps, and form the anterior parvus and the Duchesnean species, E. acares and E. and posterior margins of the tooth. Across the center obliquidens, described by Storer (1988). The occlusal of the tooth are six transverse lophs that run the width pattern of the cheek teeth of E. wilsoni is similar to that of the tooth. Both the paracone and metacone have of Orellan and later species of the genus, with lophs running lingually from them across the occlusal accessory lophules as high and distinct as the major surface, which may be interpreted as the protoloph and lophs. Proportions of the upper molars are similar to metaloph; but they are not any higher or wider than the those of Arikareean E. montanensis (Wood and remainder of the transverse lophs. The most posterior KORTH—BLUE ASH FAUNA, SOUTH DAKOTA 33 montanensis has not been reported. According to the published figures of E. montanensis (Wood and Konizeski, 1965:text-fig. 2; Macdonald, 1970:fig. 25), the crown height is about the same as E. thompsoni. Storer (1988) argued that Eutypomys acares had greater dental similarity to E. inexpectatus and the Chadronian E. parvus than other species of the genus (also see Russell, 1972; Storer, 1978), and that they were slightly more primitive than the Orellan and later species of Eutypomys. He viewed E. obliquidens as an early, more specialized species, not directly related to any other. E. wilsoni is most similar to E. montanensis than other species of the genus because of the proportions of the upper molars. Only in E. montanensis are the upper molars wider than long, as in E. wilsoni (Macdonald, 1970:table 31). This distinctive feature appears to unite the latter two species together, distinct from all of the earlier species.