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Five marsupial species are recognized from the Brule Formation at two localities in southwestern North Dakota: Fitterer Ranch and Obritsch Ranch (middle Oligocene; Whitneyan North American Land Mammal Age [NALMA]). The herpetotheriidsHerpetotherium fugaxCope, 1873a,Copedelphys superstesnew species, and the peradectidNanodelphys hunti(Cope, 1873b) are represented at both localities. A fourth species isH. sp., cf.H.merriami(Stock and Furlong, 1922), represented by a single specimen from Fitterer Ranch, being limited elsewhere to the later Arikareean NALMA. A fifth species is represented by two isolated lower cheek teeth, interpreted as m1s, from Fitterer Ranch that are unique in lacking a trigonid (only two cusps present) while having a well-developed talonid. These specimens are referred to an indeterminate herpetotheriine species. The new species ofCopedelphysis distinct from other species of the genus in that the anterior two lower molars are enlarged relative to the posterior molars. Overall, this new species is more similar in proportions to the latest Eocene (Chadronian)C.titanelix(Matthew, 1903) than the Oligocene (Orellan and Whitneyan)C.stevensoni(Cope, 1873b). This study adds a third and fourth Whitneyan marsupial fauna from the Great Plains region of North America, increases the known diversity of Whitneyan marsupials, and provides further evidence that marsupial diversity during the late Paleogene in North America was relatively stable until the late early Arikareean NALMA.UUID:http://zoobank.org/b8534802-b9ed-4120-baca-fc72917f7d6a

A large sample (>100 specimens) of the fossil rodentThisbemysWood, 1959, from the early Eocene (late Bridgerian; Br3) Washakie Formation of Wyoming is described. Two species are recognized: a new species,T.intermediusn. sp., andThisbemyscf.T.uintensis(Osborn, 1895). The large sample size of the former has allowed for a detailed study of variation in size and morphology within the species based on tooth-wear. As a result, some systematic changes in the species included in this genus are suggested: (1)T.brevicristaOstrander, 1986, is limited to the holotype, and all specimens previously referred toT.brevicristaare likely referable toT.plicatusWood, 1962; (2) statistical analysis supports the separation ofT.plicatusfromT.corrugatusWood, 1959, based on its smaller size and less-crenulated cheek teeth, which corroborates their existence at different horizons (T.plicatusearlier Bridgerian [Br2],T.corrugatuslate Bridgerian [Br3]); (3)Paramys delicatiorLeidy, 1871, is suggested as the senior synonym ofT.plicatusWood, 1962; (4)T.perditusWood, 1962, is limited to the early Eocene (Washatchian: Wa1-6); and (5) all specimens previously referred toT.niniWood, 1962, except the holotype, are referred toT.perditus(fide Korth, 1984; contra Anderson, 2015).UUID:http://zoobank.org/59c101e2-cd55-4dce-aeea-3da48c4fbd53

Eleven species of rodents are described from the Diamond O Ranch local fauna from the Beaverhead Basin, southwestern Montana. Of the species recognized, seven have been previously reported from the Chadronian of Montana or adjacent Wyoming and Saskatchewan or Texas: Ardynomys occidentalis Burke, 1936; Pseudocylindrodon sp., cf. Pseudocylindrodon neglectus Burke, 1935; Eutypomys parvus Lamb, 1908; Adjidaumo minimus (Matthew, 1903); Paradjidaumo disjunctus Korth, 2013; Paradjidaumo nanus Emry and Korth, 2013; and Aulolithomys vexilliames Korth and Emry, 1997. Isolated molars of two different indeterminate species of Ischyromys Leidy, 1856, are also recognized. Two new species are described: the ischyromyine Spurimus hoffmani and the eomyid Metanoiamys norejkoi. Both these species represent the latest occurrence of these genera, which are elsewhere limited to the earlier Eocene. The rodents from Diamond O Ranch suggest an early Chadronian age for the fauna; however, other components of the fauna mak...

Nine rodents are described from the Whitneyan (middle Oligocene) White Hills fauna of Montana. Of these, three new genera are described: the aplodontid Altasciurus, the cylindrodontid Lophicylindrodon, and the castorid Montanacastor. Prosciurus relictus (Cope, 1873) is named as the type species of Altasciurus. The other genera are unique to this fauna and are based on new species; Lophicylindrodon expiratus and Montanacastor simplicidens. In addition, two more new species are described: the eomyid Orelladjidaumo exiguus and the cricetid Willeumys argosorus. The rodent fauna from White Hills is believed to be Whitneyan in age because of the occurrence of Eumys brachyodus Wood, 1937, and Leptodontomys douglassi (Burke, 1934); both elsewhere known from the Whitneyan. The rodent fauna is unique for several reasons: 1) a much lower diversity of rodents than in comparably aged faunas; 2) the number of unique species (four of nine); 3) the relict occurrence of a cylindrodontid (otherwise latest occurren...

The cylindrodontid rodent Pseudocylindrodon Burke, 1935, formerly included seven named species, but is here restricted to the type species P. neglectus Burke, 1935, two additional North. American species (P. citofluminis Storer, 1984, and P. lateriviae Storer, 1988), and the Asian species P. mongolicus Kowalski, 1974. The other three species previously assigned to Pseudocylindrodon are here referred to the new genus Dolocylindrodon: D. medius (Burke, 1938), D. tobeyi (Black, 1970), and D. texanus (Wood, 1974) based on a combination of cranial and dental morphology. Two new species of Dolocylindrodon are named from the Chadronian part of the Climbing. Arrow Formation of southwestern Montana: Dolocylindrodon vukae (type species of the genus) and Dolocylindrodon rahnensis. Additional specimens of D. medius from the early Chadronian McCarty's Mountain fauna of Montana are briefly discussed. The recognition of a new genus and two new species of cylindrodonts increases the known diversity of this famil...

Recently discovered specimens of the marsupialHerpetotherium merriami(Stock and Furlong, 1922) from the John Day Formation, Oregon, are described. The species was previously known only from a single (type) specimen. These additional specimens have allowed for a revised diagnosis of the species based on the presence of an additional stylar cusp on the upper molars, as well as the relative size of the stylar cusps. This new material also allows for an examination of the variability within the species and establishes a time range for the species, which extends through most of the Arikareean (late Oligocene: Ar1–Ar3: late Rupelian-Aquitanian).Herpetotherium merriamiis distinct within the genus because it is among the larger of the species, but one of the latest occurring, whereas the general pattern of the genus is a reduction in size through time.

The ischyromyid rodent Thisbemys Wood, 1959, is known from the lower Eocene (Wasatchian North American Land Mammal Age [ = NALMA]) through the middle Eocene (Uintan NALMA) of western North America....

Twenty-one species of small mammals are recognized from the Observation Quarry fauna including four new species: the lipotyphlans Lanthanotherium observatum (Erinaceidae), Antesorex wilsoni (Soricidae), and Scalopoides hutchisoni (Talpidae), and the rodent Mioheteromys subterior (Heteromyidae). Due to more complete material, emended diagnoses are provided for the plesiosoricid Plesiosorex greeni Martin and Lim, 2004, and cricetid rodent Copemys lindsayi Sutton and Korth, 1995. Of the 21 species recognized, seven are unique to this faunal assemblage, six are restricted to the Hemingfordian, seven are known elsewhere only from the Barstovian or later, and one is known from both the Hemingfordian and Barstovian. The small mammals suggest that the Observation Quarry contains a transitional Hemingfordian-Barstovian fauna with a greater number of species from the latter, supporting an early Barstovian age.

The large Miocene beavers (Castoridae) Anchitheriomys Roger, 1898, and Amblycastor Matthew, 1918, have been often considered as synonyms (Flynn and Jacobs 2008). Most recently, they have been distinguished as separate genera based on the morphology of the dentary: Amblycastor restricted to North America and Anchitheriormys limited to Eurasia (Mors and Hulbert 2010). However, the dentary that was cited as evidence for the separation lacks cheek teeth and its specific identification cannot be verified. In addition, a previously undescribed specimen from the type area of “Amblycastor” fluminis Matthew, 1918, demonstrates that the morphologies used to separate these genera are not consistent with the North American specimens; thus, the two genera are synonymous and the North American species should be referred to Anchitheriomys.

A new genus and species of sciuravid rodent, Perasciuravus mcintoshi, is named from the late Bridgerian (early Eocene) of the Washakie Formation, southcentral Wyoming. The most distinguishing characters of the species (and genus) are that it retains the primitive condition of the zygomasseteric structure for rodents, characteristic of sciuravids, but has a more specialized dental morphology of the cheek teeth (complete ectolophids and hypolophids on the lower molars; complete metalophs and protolophs on the upper molars) typical of the later-occurring family of geomyoid rodents, the Eomyidae. Perasciuravus is viewed as morphologically transitional between the Sciuravidae and the Eomyidae. Its occurrence also overlaps the temporal range of the Sciuravidae (Wasatchian [early Eocene] to Duchesnean [early-late Eocene]) and predates the first occurrence of the Eomyidae (Uintan [middle Eocene]).

Eight species of rodents are described based on previously unreported fossil material from the early Eocene Adobe Town Member of the Washakie Formation of Wyoming. Among these are two new species: the sciuravid Pauromys turnbulli and the cylindrodont Pareumys flynni. The overall rodent fauna consists of twelve species. The greatest change in the rodent fauna is between the lower and middle units of the Adobe Town Member; six species having their last occurrence in the lower unit and three species having their first occurrence in the middle unit. This coincides with the boundary of the Bridgerian and Uintan North American Land Mammal Ages.

The John Day Formation of Oregon is one of the richest and best studied assemblages in North America. Including seven members and spanning about 20 million years, there are over 150 vertebrate species known from the John Day Formation. The rodent faunas of John Day have not been as well studied as larger mammals, with many families having received little attention in the last hundred years. Here, twenty one species of rodents are described, based on new discoveries and previously unpublished specimens from the John Day Formation. The new material includes four new genera and species; the eutypomyid Allotypomys pictus, the anchitheriomyine castorid Microtheriomys brevirhinus, the eomyid Proapeomys condoni, the heteromyid Bursagnathus aterosseus; and six new species: the sciurine sciurid Miosciurus covensis, the eomyids Apeomys whistleri and Neoadjidaumo arctozophus, the heteromyids Proheteromys latidens and Trogomys oregonensis, and the sicistine dipodid Plesiosminthus fremdi. The problematical eo...

Three species of Yoderimyinae (Eomyidae: Rodentia) are recognized from the lower part of the White River Formation (early to medial Chadronian) in the Flagstaff Rim area, Wyoming. The new material allows an improved diagnosis for the subfamily. The enamel microstructure of Yoderimyinae supports its inclusion in the Eomyidae.A new genus, Zemiodontomys, is established for Yoderimys burkei Black, and new material, including upper dentition, is referred to this species. This genus differs from Yoderimys in having higher crowned and more lophodont teeth and in lacking P3. A second new genus, Litoyoderimys, is established for Yoderimys lustrorum Wood, and a new species, L. auogoleus, is referred to the genus. This genus has lower crowned, more cuspate teeth than Yoderimys.Through early and medial Chadronian time, evolution in yoderimyines includes the following morphologic transformations: increase in size; increase in crown height and lophodonty of cheek teeth; reduction of P3 (from double-rooted, to single-rooted, to absent); increase in relative size of P4 and p4; and increased longitudinal torsion of the mandible.

Until recently, few well-described rodent faunae from the Whitneyan North American Land Mammal Age [NALMA] were known, hindering studies of rodent diversity, biogeography, and evolutionary patterns during the Oligocene. This study describes a new Whitneyan rodent assemblage from the Obritsch Ranch paleontological locality in the Little Badlands region of North Dakota. Specimens were collected from three stratigraphically restricted sampling intervals within the middle to upper Brule Formation, resulting in the recognition of fourteen rodent species, five of which are elsewhere known to first appear in Whitneyan faunae. Described is one new species, the eomyid Paradjidaumo obritschorum, and the first cranial material of the heteromyid rodent ProharrymysKorth and Branciforte, 2007. The rodent fauna from the upper two sampling intervals at Obritsch Ranch and the uppermost fauna recently described from the nearby Fitterer Ranch paleontological locality share four taxa in common with the late Whitneyan Blue Ash local fauna from southwestern South Dakota, indicating these two North Dakota rodent faunae are also from the late Whitneyan. Increasing knowledge of Whitneyan rodent faunae in North America reveals unusually high survivorship of rodent species from the older Orellan NALMA into the Whitneyan NALMA and much geographic variation in the diversity, distribution, and relative abundance of different rodent families between individual Whitneyan rodent faunae. Those factors help explain prior difficulties in differentiating Orellan and Whitneyan rodent faunae and in identifying biostratigraphically useful rodent taxa for the Whitneyan. Overall, Whitneyan rodent faunae from North America display an increase in the diversity of aplodontiids, cricetids, and sciurids and a decrease in eomyid and ischyromyid diversity relative to the Orellan.

The Cook Ranch local fauna is from the Sage Creek Basin in southwestern Montana. Fossil mammals have been described from the Cook Ranch local fauna as early as the 1930s. The age has been identified as early Oligocene (late Orellan) based on the mammalian fauna. Twelve species of rodents are recognized herein from Cook Ranch. Two new species are described: an eomyid Metanoiamys woodi and a cricetid Wilsoneumys focarius. The aplodontiid Campestrallomys annectensKorth, 1989a, is referred as a new combination, Ninamys annectens (Vianey-Laud et al., 2013), and distinguished from North American species of CampestrallomysKorth, 1989a, based on dental characters (e.g., partial ectoloph on upper cheek teeth, morphology of hypolophid on lower cheek teeth). This is the first recognition of this genus in North America of this otherwise Eurasian genus. A neotype for the heliscomyid Heliscomys gregoryi is assigned, and it is demonstrated that the species is distinct from other species of the genus based on size and style and stylid development of the molars.The bulk of the species present are typical of other North American Orellan faunas but one species, Adjidaumo minimusMatthew, 1903, is elsewhere limited to the Chadronian (latest Eocene) and another species, Agnotocastor praetereadensStirton, 1935, is elsewhere limited to the Whitneyan (early middle Oligocene). These identifications extend the known ranges of both species.

Korth, William W., Robert J. Emry, Clint A. Boyd, and Jeff J. Person. Rodents (Mammalia) from Fitterer Ranch, Brule Formation (Oligocene), North Dakota. Smithsonian Contributions to Paleobiology, number 103, vi + 45 pages, 17 figures, 15 tables, 1 appendix, 2019.—Eighteen rodent species are recognized from the Fitterer Ranch fauna of North Dakota on the basis of more than a thousand collected specimens. Of the species recognized, four are new: the prosciurine aplodontiids Prosciurus hogansoni and Altasciurus leonardi, the heliscomyid Heliscomys borealis, and the cricetid Eumys lammersi. A previously described castorid from this fauna, Oligotheriomys primus Korth, 1998, is considered a synonym of “Eutypomys” magnus Wood, 1937, on the basis of the recovery of lower dentitions but is retained in the genus Oligotheriomys. A single specimen is questionably referred to Microparamys, a genus elsewhere limited to the Eocene (Clarkforkian-Chadronian land mammal ages). The rodent fauna appears to be a combination of predominantly Orellan and Whitneyan species (early Oligocene), suggesting that the section might transcend the Orellan-Whitneyan boundary. However, the rodent fauna does not alter significantly from the lowest to the highest horizons.

Four species of mylagaulid rodents are described from the Tesuque Formation, Espanola Basin of New Mexico: Alphagaulus vetus (Matthew, 1924), Ceratogaulus robustus, new species, Notogaulus minor, new genus and species, and Notogauhis major, new species. Ceratogaulus robustus is distinguished from other species of the genus by its much heavier dentary and relatively wider premolar. Notogaulus is separable from other mylagaulines in its unique features of its cranium (ovate rugose areas on the nasals; shape, thickness, and porosity of bone on the postorbital processes), and relatively primitive dental morphology (no preferred separation of the branches of the parafossette on P4). The recognition of a distinct genus of mylagaulid from New Mexico demonstrates that by the middle Miocene (Barstovian) derived mylagaulines had evolved regionally into distinct genera.

Two species of the eomyid rodent Paradjidaumo are recognized from the Chadronian (latest Eocene) White River Formation in the Flagstaff Rim area of Wyoming: P. hansonorum from early Chadronian levels and P. nanus, new species, from higher levels representing the middle Chadronian.

Additional specimens of the problematical rodent Pipestoneomys Donahoe, 1956, have allowed for recognition of a new family, Pipestoneomyidae. A new genus and species of pipestoneomyid is recognized from the late middle Eocene (Duchesnean North American Land Mammal Age; Bartonian), Argorheomys septendrionalis, which is morphologically more primitive than Pipestoneomys and demonstrates that this new family has been distinct since the Duchesnean. The Pipestoneomyidae share a number of derived characters with the Geomorpha, especially the two-part inner layer of incisor enamel of the Eoymidae. The Pipestoneomyidae differ from the Eomyidae in lacking the basic “omega” pattern of the cheek teeth of the former, so are in the Eomyoidea as a distinct family.

The White River Formation at Flagstaff Rim in central Wyoming is a key sequence for understanding the biochronology and chronostratigraphy of the Chadronian North American Land Mammal Age. The lower part of this section represents early Chadronian time, an interval whose fauna has not been well known. From these early Chadronian levels 15 species of rodents are discussed and/or described. Of these, three are new species, all belonging to the Eomyidae: Namatomys erythrus, Metanoiamys paradoxus, Cristadjidaumo skinneri. These rodent assemblages add substantially to our knowledge of early Chadronian faunas, and show successional changes in the rodent taxa from the earliest Chadronian through middle Chadronian.

A new genus and three new species of early aplodontoid rodent are described from the Orellan (early Oligocene) of Montana, Brachygaulus nicholsi (type species), Brachygaulus leistneri, and Brachygaulus xerobothrus. A fourth indeterminate species is also recognized but not named. The occlusal dental morphology of Brachygaulus has a number of similarities to that of the earliest mylagaulids, the Promylagaulinae (early Arikareean: late Oligocene). However, the brachydont cheek teeth of Brachygaulus are more similar to primitive contemporaneous basal aplodontoids (=Prosciurinae). Brachygaulus represents an ancestral morphotype for the Mylagaulidae, demonstrating that the family can be derived from a prosciurine-like ancestor independent of the Meniscomyinae.

Specimens previously identified as the Hemphillian cricetid Basirepomys pliocenicus (Wilson, 1937) (=Peromyscus pliocenicus Wilson, 1937) by Korth and De Blieux (2010) have been re-examined, and several distinct species are here recognized. Specimens from the early Hemphillian Rome fauna of Oregon are referred to a new, more primitive species of Basirepomys Korth and De Blieux, 2010, here named Basirepomys romensis, new species, which differs from other species of the genus in having lower-crowned cheek teeth and incomplete alternation of cusps on some molars. Specimens from the early Hemphillian Juniper Creek and Little Valley faunas of Oregon are referable to Paronychomys Jacobs, 1977, rather than Basirepomys, and represent a new species, Paronychomys shotwelli, which is distinguished from other species of the genus by its larger size and relatively longer lower third molar. A specimen previously referred to Peromyscus pliocenicus from the early Hemphillian of Kansas is referable to Paronychomy...

The species of Eumys Leidy, 1856, are reviewed. Six species are recognized and emended diagnoses are presented for the genus and all species. The enigmatic species, Coloradoeumys galbreathi Martin, 1980, is considered a new junior synonym of Eumys elegans Leidy, 1856; Coloradoeumys Martin, 1980, therefore becomes a new subjective synonym of Eumys Leidy, 1856. Additional specimens of Eumys cricetodontoides White, 1954, and Eumys parvidens Wood, 1937, are recognized and described. A new species, Eumys euryodus, from the Whitneyan of Nebraska is described. Size and proportional measurements of the molars, along with features of the occlusal morphology, are the major factors used to distinguish the species of Eumys.

Four species of rodents (two heteromyids and two cricetids) and one lagomorph are identified from the late Tertiary Sevier River Formation of Utah. The heteromyids include a new genus and species of heteromyine, Metaliomys sevierensis, which is intermediate in morphology between the Clarendonian and early Hemphillian Diprionomys Kellogg and the extant genera Liomys and Heteromys. A single specimen is referred to Diprionomys sp., cf. D. minimus (Kellogg). The cricetid Paronychomys lemredfieldi Jacobs is known from the Hemphillian of Arizona. The second cricetid is referred to a new genus Basirepomys. Peromyscus pliocenicus Wilson from the Hemphillian of California is designated as the type species of the new genus, to which the new species B. robertsi from Utah is referred. Basirepomys is viewed as intermediate between Peromyscus and the basal neotomyine Repomys May from the late Hemphillian and Blancan. The only lagomorph in the fauna is Hypolagus vetus (Kellogg). Four of the taxa recognized from...

Leptarctus mummorum, new species, from the late Clarendonian of northwestern Kansas is described for the unusual mustelid genus Leptarctus Leidy, 1856. It is distinguished from other species of the genus by its larger size, temporal crests meeting along the central axis of the skull, relatively longer rostrum and bullar processes, depression anterior to the orbit, oblique ridge along the orbital wall, upper third premolar (P3) with an accessory internal accessory cusp, and upper fourth premolar (P4) with a greatly enlarged parastyle. It appears that previous suggestions that Leptarctus was strictly herbivorous and arboreal are not supportable based on the morphologies of the skull and dentition.

Previously, it was believed that there was a dramatic turn-over in the fauna of beavers (Rodentia, Castoridae) from the Barstovian to the Clarendonian from Nebraska. Stratigraphically controlled collections of fossil castorids from the Valentine Formation, which cross this boundary, now indicate that a complete change in the castorid fauna did not occur, but instead a more gradual change and replacement of earlier taxa with more advanced taxa occurred. The range of Eucastor tortus and Monosaulax skinneri is extended from late Barstovian into the latest Barstovian Devil's Gulch Member and the range of the otherwise Clarendonian species of Prodipoides is extended downward into the late Barstovian Crookston Bridge Member. The skulls of Monosaulax skinneri and Eucastor valentinensis are described in detail for the first time. The cranial morphology of M. skinneri is primitive for Castoroidini and that of E. valentinensis is specialized for tooth-digging behavior as in the Nothodipoidini. A new genus, Temperocastor, is proposed for E. valentinensis based on its primitive morphology of the cheek teeth and derived cranial morphology. Temperocastor represents the most primitive nothodipoidine.

A new genus and species of paramyine ischyromyid, Metaparamys dawsonae, is described from the Chadronian of Wyoming. The new genus is distinguished from other paramyines in having the ectolophid on lower molars complete between protoconid and hypoconid and without a mesoconid, hypocone and metaconule on upper molars relatively larger than in Paramys, lower incisor wide with convex anterior surface, and masseteric scar on mandible more anterior than in other paramyines. The Uintan species Paramys compressidens Peterson is also assigned to Metaparamys. The Bridgerian P. delicatior Leidy appears to be a likely ancestor for Metaparamys based on the cross-sectional shape of the lower incisor. The occurrence of M. dawsonae is one of the latest records of a paramyine rodent in North America.

The cranium of the Hemingfordian beaver Anchitheriomys nanus Korth differs from the previously described skull of the Barstovian A. fluminis (Matthew) in being smaller and having less procumbent incisors, grooves on the palate, and the ethmoid and optic foramina more posterior in position on the skull. These features are viewed as being primitive in A. nanus relative to A. fluminis. The basicranium of A. nanus is preserved (unknown for A. fluminis) and demonstrates the derived castorid character of lacking the stapedial foramen in the auditory bulla, but has the apomorphic features of an elongated anterior process and a concave external meatus of the bulla (evidently a functioning part of the glenoid fossa). The paroccipital process is greatly broadened, another unique feature among castorids. The cranial morphology of the earlier A. nanus suggests that the anchitheriomyines are more closely related to the advanced castorids (Castorinae and Castoroidinae) than the primitive agnotocastorines, with...

The skull and mandible ofNothodipoidesKorth are described. There are a number of adaptations present that are shared with other rodents with tooth-digging behavior (procumbent, elongated, and flattened incisors; anteriorly tilted occipital; strongly arched upper diastema). A new tribe of Castoroidinae, Nothodipoidini, is proposed forNothodipoidesandMicrodipoidesKorth and Stout, based on the fossorial adaptations of the skull, and their generally smaller size. It appears that fossorial adaptations have occurred in castorids at least three times: the Palaeocastorinae (Whitneyan through Arikareean), the primitive beaverMigmacastor(Arikareean), and the Nothodipoidini (late Barstovian through the Clarendonian).

Ten species of geomyoid rodents are recognized from the early-early Arikareean Ridgeview local fauna of Nebraska: three heliscomyids, four florentiamyids, one heteromyid, and two species of the probable geomyid Tenudomys Rensberger, 1973. Two new genera are described, the heliscomyid Tylionomys and a harrymyine heteromyid Proharrymys. Two previously described Arikareean species are referred to Proharrymys Heliscomys schlaikjeri Black, 1961 and Proheteromys fedti Macdonald, 1963 (new combinations). Four new species are named: the heliscomyids Heliscomys macdonaldi and Tylionomys voorheisi, the heteromyid Proharrymys wahlerti, and the probable geomyid Tenudomys ridgeviewensis.Heliscomyids are more diverse in the Ridgeview local fauna than in any other Arikareean fauna in North America. This increases the known diversity of this family during a time interval from which only a single species was previously recognized. Larger florentiamyids are poorly represented in the fauna, but the sample is large enough to demonstrate that Florentiamys kennethi Wahlert, 1983 is a junior synonym of F. tiptoni (Macdonald, 1970). A large sample of the small florentiamyid Kirkomys Wahlert, 1984 is sufficient to demonstrate that Proheteromys nebraskensis Wood, 1937 is synonymous with Kirkomys milleri Wahlert, 1984, resulting in the new combination Kirkomys nebraskensis.The only heteromyid in the fauna, P. wahlerti, is believed to be the basal harrymyine from which the Hemingfordian Harrymys can be derived.

A new genus of sciurid, Hesperopetes, contains three species: H. thoringtoni (type species), H. jamesi, and H. blacki. H. thoringtoni is from the early Chadronian (late Eocene) White River Formation of central Wyoming and represents the earliest recognized species of sciurid. The other species are from the latest Whitneyan or earliest Arikareean (late Oligocene) Blue Ash local fauna of South Dakota. Hesperopetes is closest in dental morphology to Oligopetes Heissig, which is known only from the early Oligocene of Europe. Hesperopetes may also represent the earliest occurrence of a lineage leading to pteromyine sciurids (“flying” squirrels). It is impossible at this time to construct a phylogeny of North American “flying” squirrels due to their scarcity in the fossil record and their undetermined relationships with Old World taxa.

Hopkins (2004) recently reviewed the species of Ansomys Qiu (1987). She recognized only two species from North America, A. nexodens (Korth, 1992) from the Orellan (early Oligocene) of Montana, and A. hepburnensis Hopkins (2004) from the Barstovian (middle Miocene), also of Montana. This left a considerable time gap between the known record of North American species, which led her to suggest a series of migration events between Eurasia and North America throughout the Tertiary. Recently, Kelly and Korth (2005) named an additional species of Ansomys, A. nevadensis from the Hemingfordian (early Miocene) of Nevada, thus partially filling the time gap between the previously described North American species. Kelly and Korth (2005), however, noted that A. nevadensis was morphologically closer to the Asian middle Miocene species A. orientalis Qiu (1987) rather than the other North American species. The fossil specimens described below are from the latest Whitneyan or earliest Arikareean of South Dakota (late Oligocene), once again, filling another time gap in the North American record of Ansomys. They also represent the easternmost occurrence of the genus in North America. Dental terminology used follows that of Wood and Wilson (1936) with modifications of Rensberger (1975). All specimens cited are part of the research collections at the Carnegie Museum of Natural History (CM).

Recently discovered fossil material from the late Arikareean of western Nebraska is referable to a new genus and species of castorid, Priusaulax browni. This species is the earliest record of a castoroidine beaver in North America. The skull possesses a combination of derived castoroidine features (divergent tooth rows, loss of stapedial foramen, premolars larger than molars, grooved palate) and primitive agnotocastorine features (elongated rostrum, procumbent incisors, contact between lachrymal and jugal bones). Morphologically and temporally, Priusaulax is transitional between the earlier agnotocastorines with more primitive characters and the later castoroidines with more derived characters.

A new species of squirrel, Sciurus olsoni, from the late Miocene (Clarendonian) of Nevada, is distinguished from other species of Sciurus by its smaller size and minor differences in the cheek teeth. It is the first Miocene record of the extant genus Sciurus, and the first record of a tree squirrel (tribe Sciurini) in North America between the early Miocene (Arikareean) and the Pleistocene. This late Miocene occurrence in North America suggests that the history of tree squirrels in North America was probably continuous through the Neogene, and the intermittent fossil record is due both to preservation bias and to their never having been abundant as living elements of fossil faunas; they are rare even where they do occur in deposits that are otherwise very richly fossiliferous. The later Tertiary and Pleistocene radiation of tree squirrels may have originated in North America.

Peratherium sp. cf. P. innominatum, a species with an early Wasatchian to Duchesnean range, is present in the Turtle Basin Local Fauna. Two new genera and species of rodents are described: Cristadjidaumo mckennai (Eomyidae) and Passaliscomys priscus (Heliscomyidae). The remainder of the rodent fauna consists of species that are similar to Duchesnean faunas from both the northern Great Plains (Saskatchewan) and from southern California. The probable occurrence of Simimys and Griphomys suggests a slightly greater similarity to the fauna of southern California.

Migmacastor procumbodens, gen. et sp. nov. is described from the late Arikareean of Nebraska based on a nearly complete skull. The new taxon exhibits various incisal specializations (e.g., incisors...

Citation for this article: Korth, W. W. 2017. First record of Hesperopetes Emry and Korth, 2007 (Rodentia, Sciuridae), from the Orellan (early Oligocene). Journal of Vertebrate Paleontology. DOI: 10.1080/02724634.2017.1318890.