PaleoBios Contributions from the University of California Museum of Paleontology, Berkeley nIo. 6 October 15, 1968 RODENTS FROM THE HARTMAN RANCH LOCAL FAUNA, CALIFORNIA by Everett Lindsay no. 6 RODENTS FROM THE HARTMAN RANCH LOCAL FAUNA, CALIFORNIA BY EVERETT LINDSAY ABSTRACT Fossil rodents, represented by approximately 140 isolated teeth, were collected from the lower part of the Sespe Formation near Sespe Gorge, Ventura County, California. Age of the fauna is Uintan. The Hartman Ranch small mammal fauna includes Pareumys sp„, Namatomys fantasma n. sp., Griphomys sp., and Simimys sp. Namatomys fantasma shares morphological features with Simimys, suggesting these forms are phyletically related. N. fantasma has cheek tooth formula (4/4) identifying it as an eomyid while Simimys has cheek tooth formula (3/3) identifying it as a cricetid. Teeth of N. fantasma also resemble those of some sciuravids and zapodids, suggesting N. fantasma is intermediate in a lineage Sciuravidae------*»-Eomyidae------*¦ Cricetidae and Zapodidae. Pareumys and Griphomys are presently represented by only a few specimens. INTRODUCTION Fossils from Hartman Ranch, UCMP locality V-5814, Ventura County, California, include cylindrodontid, eomyid, ?geomyid, and cricetid rodents. Wei I-diversified small mammal faunas were previously reported from the Lower Sespe Formation by Chester Stock (1935), R. W. Wilson (1935, I940A, I940B, 1940C, 1949), and A. E. Wood (1956). Earlier described late Eocene faunas from the Sespe Formation were from sites in the Simi foothills near Fillmore, California. This is the first report of small mammals from the Sespe Formation near Sespe Gorge. I no. 6 2 20 • anterior 1211 17 12 ILL cylindrodontid eomyid cricetid Figure 1. Cusp terminology in upper cheek teeth 1-Paracone 2-Metacone 3-Protocone 4-Hypocone 5-Anterocone 6-Mesocone 7-protoconule 8-Metaconule 9-Protolophule 10-Metalophule 11-Mesoloph 12-Mesostyle 13-Mure 14-Posterior arm of protocone 15-Anterior cingulum 16-Posterior cingulum 17-Labial loph 18-Parastyle 19-Anterior basin 20-Central basin 21-Posterior basin • anterior cylindrodontid eomyid cricetid geomyoid Figure 2. Cusp terminplogy in lower cheek teeth 1-Anteroconid 2-Metaconid 3-Protoconid 4-Hypoconid 5-Entoconid 6-Mesoconid 7-Posteroconid 8-Metalophulid 9-Entolophulid 10-Lingual lophid 11-Mesolophid 12-Mesostylid 13-Mure 14-Posterior arm of protoconid 15-Anterior cingulum 16-Posterior cingulum 17-Transverse valley 18-Metalophid 19-Hypelophid 20-Transverse lophid 2 no. 6 Fossils were collected from calcareous, green, thin lenses of coarse-grained sandstone within the red shales near the base of the Sespe Formation. The sandstone lenses are lithologically similar to the Coldwater Sandstone that underlies the Sespe Formation and suggest local intertonguing of the Lower Sespe Formation with characteristic Coldwater Sandstone. Tortoise and rhinocerotoid fossils were collected from the Hartman Ranch locality also. Age of the Hartman Ranch local fauna is Uintan (late Eocene), correlative with the Tapo Ranch fauna, based on morphological similarity of Simimys sp. and Pareumys sp. from Hartman Ranch and the Tapo Ranch fauna. The rodent specimens were collected by underwater screenwashing using two sets of screen, 18-mesh and 25-mesh. Some of the matrix was digested with dilute acetic acid prior to screening. The following abbreviations are used in the text. UCMP = University of California Museum of Paleontology; LACM(CIT) = Los Angeles County Museum, collections formerly catalogued at the California Institute of Technology. All measurements are in millimeters. Figures 1 and 2, above, illustrate the position of morphologic terms used in the descriptions of rodent teeth. SYSTEMATICS Order RODENTIA Family CYLINDRODONTIDAEMiller and Gidley 1918 Pareumys Peterson 1919 Pareumys sp. Three isolated, relatively high-crowned cheek teeth, ?RP4 (UCMP 79271), ?LM3 (UCMP 79272), and LM3 (UCMP 79477) are identified as Pareumys sp. The premolar and M3 are relatively unworn, with the posterolabial corner broken away on both teeth. MJ is well worn. 3 no. 6 lescription of material: ?P^ - oval occlusal outline; paracone and letacone relatively close, metacone slightly labial relative to paracone; trotolophule long, directed transversely, strongly united to long and narrow protocone that is continuous with posterior cingulum; metalophule ihort, terminating medially, with weak union to posterior cingulum; :entral basin deep and wide; anterior basin deep and narrow; anterior cingulum low, continuous from protocone to termination anterior to paracone; parastyle absent. ?M3 - subquadrate occlusal outline, with anterolabial expansion; enamel island in posterior half of tooth; island transversely elongate with anterior and posterior sides convex anteriorly. M3 - oval occlusal outline; metaconid and protoconid prominent; hypoconid probably prominent (broken); entoconid small, mesoconid minute; metaconid and entoconid opposite anterior arm of labial cusps; anterior arm of protoconid high, directed transversely to its union with the metaconid; median transverse lophid (?mesolophid) high, uniting mesoconid and entoconid; posterolingual cingulum high, continuous from metaconid to posterior arm of hypoconid, passing through entoconid; isolated anterior and posterior lakes, separated by median transverse lophid; anterior lake deeper and wider than posterior lake; small lophule projecting into anterior lake from median protoconid, small lophule < projecting anteriorly into anterior lake from medial region of transverse lophid; anteroconid and anterior cingulum absent. Discussion: Pareumys sp. from Hartman Ranch is relatively high-crowned; occlusal measurements (Table 1) are comparable to other specimens of Pareumys from the Sespe Formation (Wilson, 1940C). The Hartman Ranch P is distinguished from Cylindrodon by its short metalophule. 4 no. 6 Figure 3. Pareumys sp. a-b. UCMP 79271, ?RP^ (a. occlusal view, _b. anterior view); £. UCMP 79272, ?LM3 occlusal view; j3. UCMP 79477, LM3 occlusal view. (scale line = 1 mm.) Family EOMYIDAE DepeYet and Douxami 1902 Namatomys Black 1965 Two species of Namatomys are presently known. N. Iloydi is known from two mandibles collected at the early Oligocene Pipestone Springs locality in Montana. One mandible has a complete dentition, the other is missing the M3. N. fantasma n. sp. from the Hartman Ranch local fauna is known only from isolated cheek teeth. Precise dental formula is essential for taxonomic identification since eomyids have cheek tooth formula 4/4, sciuravids have 5/4, fossil zapodids have 4/3, and cricetids have 3/3. All the families named above have low-crowned, lophate cheek teeth. The most significant characters uniting N. fantasma with N. Iloydi are presence of an anteroconid on P4, and cheek tooth formula. 5 no. 6 Namatomys fantasma* n. sp. Type: UCMP 77660 RP4, V-5814 Hartman Ranch, Ventura County, California. Hypodigm: Type and UCMP 66986-66988, 77648-77650, 77652-77654, 77656-77670. Stratigraphic and geographic range: Lower Sespe Formation in Sespe Gorge, Ventura County, California. Age: Uintan (late Eocene). Diagnosis: Anterocone (-id) on all cheek teeth except P 7 lower cheek teeth longer than wide; anterior arm of protoconid not elongated transversely to base of metaconid; metalophulid and entolophulid unite with posterior arms of protoconid and hypoconid; upper cheek teeth wider than long; paralophule and metalophule unite with anterior arm of lingual cusps; mesocone (-id) minute on all cheek teeth, except absent on M ' and M3; M3 reduced posteriorly. Differential diagnosis: Namatomys fantasma differs from Namatomys lloydi in stronger development of posterior metalophulid and weaker development of anterior arm of protoconid. N. fantasma differs from Simimys sp. from the Hartman Ranch fauna in possessing four upper and lower cheek teeth, smaller external dimensions, upper cheek teeth wider than long, and stronger development of mesocone (-id). Namatomys is the only known eomyid rodent with an anteroconid on P4. Sciuravus powayensis from the Poway conglomerate in San Diego County, California, is similar to NL fantasma but lacks an anteroconid on P4 and has five upper cheek teeth. P4 of Nl_. fantasma lacks an oppression facet on its anterior vertical face. Description: dP - paracone and metacone opposite the protocone and hypocone; posterior arm of protocone directed transversely, extending to median plane; protolophule and metalophule directed lingually, *Etymology: fantasma (L.) = ghost, phantom; referring to absence of associated material, hence an amorphous nature. 6 no. 6 Figure 4. a-j. Namatomys fantasma new species, a. UCMP 79504, RdP2* occlusal view. b. UCMP 77648, RP4 occlusal view. £. UCMP 77654, RM2 occlusal view. jd. UCMP 77655, RmI occlusal view. £. UCMP 77660, RP4 occlusal view. f_. UCMP 77666, LMX occlusal view. £. UCMP 77668, LM2 occlusal view. h. UCMP 77657, LM-* occlusal view. ±. UCMP 77660, RP4 labial view. j. UCMP 79482, RM2 occlusal view. k. Griphomys sp. UCMP 79479, LM3 occlusal view, (scale line = 1 mm.) 7 no. 6 uniting with anterior arm of lingual cusps; mesocone weakly represented by medial thickening on mure; mure between protocone and hypocone, uniting both cusps; mesoloph short; mesostyle minute, weakly uniting with labial loph that is directed posteriorly from paracone; anterior cingulum weakly joining anterior arm of protocone near median plane; parastyle small, elongated transversely; posterior cingulum low, terminating at median plane; cingular shelf labial to posterior cingulum on posterior margin; protoconule absent. P - subrectangular occlusal outline, wider than long; paracone and metacone opposite anterior arm of protocone and hypocone; posterior arm of protocone directed transversely extending labial to mure in 9 of 13 specimens, weakly uniting with paracone in 3 specimens; protolophule and metalophule directed transversely, uniting with anterior arm of lingual cusps; mesocone a slight thickening along mure in 10 of 13 specimens; mure uniting protocone and hypocone in 12 of 13 specimens; mesoloph short and stub-like in 12 specimens, prominent although never reaching mesostyle in 2 specimens; mesostyle small in 8 specimens, absent in 2 specimens; labial loph directed posteriorly from paracone in 5 specimens, absent in 5 specimens; anterior cingulum uniting with anterior arm of protocone near median plane, prominent labial to point of union, weakly developed litigual to median plane; posterior cingulum continuous from hypocone to posterolabial corner of tooth; protoconule absent; oppression facet absent from anterior vertical face. M' - subrectangular occlusal outline, slightly wider than long; largest tooth in upper dental series; paracone and metacone opposite anterior arm of protocone and hypocone; posterior arm of protocone with short extension labial to mure in 7 of 11 specimens; protolophule and metalophule directed transversely and slightly curved (convex anteriorly), uniting with anterior arm of lingual cusps; mesocone not discernible on mure; mure uniting protocone and hypocone in 9 of 11 specimens; mesoloph short and low; mesostyle small in 9 specimens, absent in 2 specimens; labial loph absent; anterocone small on anterior cingulum lingual to median plane; anterior cingulum uniting with anterior arm 8 no. 6 of protocone, prominent labial to anterocone, short and low lingual to anterocone, but continuous to base of protocone; parastyle small; posterior cingulum prominent on posterior margin, continuous from hypocone to posterolabial corner of tooth; protoconule absent. M - subrectangular occlusal outline, perceptibly wider than long; shorter in length than M '; paracone and metacone opposite anterior arm of lingual cusps; posterior arm of protocone with extension labial to mure strongly developed in 1 specimen, weakly developed in 7 specimens, and extension absent in 5 specimens; protolophule and metalophule directed transversely and curved (convex anteriorly), uniting with anterior arm of lingual cusps; mesocone a slight thickening on mure in 6 of 11 specimens, absent in 5 specimens; mure uniting protocone and hypocone in 10 specimens; mesoloph short in 10 of 13 specimens; mesostyle small; labial loph directed posteriorly from paracone in 2 of 11 specimens; anterocone small, lingual to median plane where anterior cingulum unites with anterior arm of protocone in 10 of 12 specimens; anterior cingulum prominent labial to anterocone, weakly developed lingual to anterocone; parastyle minute; posterior cingulum prominent on posterior margin, continuous from hypocone to posterolabial corner of tooth. MJ - subrectangular occlusal outline, slightly wider than long; smallest tooth in upper dental series; paracone and protocone prominent; hypocone minute; metacone absent; protolophule directed transversely, curved (convex anteriorly), uniting with anterior arm of protocone; central basin shallow, bounded anteriorly by paracone, protolophule, and protocone, bounded posteriorly by low posterior cingulum; posterior cingulum continuous along labial margin, uniting with base of paracone; mesoloph and mesostyle absent; anterocone minute on anterior cingulum where cingulum joins anterior arm of protocone; anterior cingulum long and narrow labial to anterocone, short and low lingual to anterocone; parastyle minute. 9 no. 6 P4 - obovate occlusal outline, narrow anteriorly; smallest tooth of lower dental series; anteroconid minute, isolated on medial plane in 9 of 10 specimens; metaconid transversely opposite and united posteriorly to protoconid by lateral extension of posterior arm of protoconid; entoconid transversely opposite and united to hypoconid by transversely- directed entolophulid that contacts posterior arm of hypoconid; mesoconid small, on mure; mure uniting protoconid and hypoconid on labial side of tooth; mesolophid short; mesostylid small, present on 9 of 10 specimens; lingual lophid low, directed posteriorly from metaconid, weakly joining mesostylid in 8 of 9 specimens; anterior cingulum absent; posterior cingulum continuous from posterior arm of hypoconid near median plane to postero lingua I corner of tooth; posteroconid isolated in 1 specimen. M] and M2 - rounded rectangular occlusal outline, longer than wide; largest tooth of lower dental series; anteroconid small, labial to median plane, weakly uniting with anterior arm of protoconid in 15 of 19 specimens, strongly uniting with metaconid in 3 specimens; metaconid and entoconid opposite protoconid and hypoconid and connected with posterior arm of labial cusps by transversely-directed metalophulid and entolophulid; mesoconid on mure, labial to median plane; mure weakly contacting protoconid and hypoconid in 16 of 19 specimens; mesolophid short; mesostylid minute, weakly contacting lingual lophid directed posteriorly from metaconid; anterior cingulum continuous from metaconid to protoconid, high lingual to anteroconid, low labial to anteroconid; posterior cingulum continuous from posterior arm of hypoconid to posterolingual corner of tooth . M3 - subrectangular occlusal outline, straight anterior surface, rounded posteriorly; anteroconid small in 15 of 18 specimens; anteroconid uniting with protoconid in 7 specimens, uniting with metaconid in 3 specimens, separate from protoconid and metaconid in 5 specimens; metaconid opposite protoconid, uniting posteriorly with transversely- directed posterior arm of protoconid; entoconid opposite and strongly uniting with hypoconid by curved, wide posterior cingulum (extension 10 no. 6 of posterior arm of hypoconid); entolophulid discernible as anterior branch of posterior cingulum in 7 of 18 specimens; mesoconid small, isolated in 13 of 18 specimens, weakly contacting hypoconid in 5 specimens; mesolophid long, not reaching mesostylid on lingual margin; mesostylid minute on 11 of 17 specimens, absent on 6 specimens; lingual lophid low, directed posteriorly from metaconid in 5 of 17 specimens; anterior cingulum continuous and.relatively straight between protoconid and metaconid, low labial to median plane. Discussion: Namatomys is the only known eomyid with an anteroconid on P4. Cricetid and zapodid rodents commonly have an anteroconid on the anterior lower cheek tooth identified as M] rather than P4. Simimys sp- from the Hartman Ranch local fauna has a distinctive median anteroconid on the anterior lower cheek tooth, designated Mj. The P4 of Namatomys and M] of Simimys are very similar, especially in the narrow anterior occlusal outline, small median anteroconid, absence of anterior cingulum, posterior union of protoconid and metaconid, and distinct mesoconid. It is suggested that P4 of eomyid rodents and M] of cricetid rodents may be homologous elements in the dental series. A general trend observed in middle Tertiary and later cricetid rodents is increased size of the anterior lower cheek tooth and reduction of the posterior lower cheek tooth. Solution to homology of lower cheek teeth in cricetid and other rodents awaits additional collecting, especially in late Eocene deposits. The sample of N_. fantasma includes 12 P4, 27 M-| and M2, and 20 M3. Number of teeth represented is probably biased by the collecting method (screen-washing), and the size of the screen relative to the size of the tooth. In the lower dental series, P4 is the smallest tooth, and M3 is the largest tooth. The M] and M2 of N_. fantasma are indistinguishable although in N. Iloydi the M-j is slightly narrower anteriorly relative to M2. Sciuravids, cricetids, zapodids and most eomyids have the paracone and metacone of upper cheek teeth uniting with the anterior 11 no. 6 arm of the protocone and hypocone while the metdlophulid and entolophulid of the lower cheek teeth unite with the posterior arm of the protoconid and hypoconid, as in Namatomys and Simimys. In N. fantasma the upper cheek teeth are wider than long, as in sciuravids and some zapodids; in Simimys and later cricetids the upper cheek teeth are longer than wide. Development of cusps and connections between the cusps on N. fantasma and Simimys sp. from Hartman Ranch suggest a close phyletic relationship. Namatomys seems a probable ancestor to the cricetids and zapodids, and Simimys is probably the earliest cricetid. Phyletic Relationships of Namatomys and Simimys I ¦I u o UJ \ Cncetodon \ / Simimys I Plesiosminthus Sciuravus Pauromys Late Eocene radiation of Nearctic rodents might include the transition of Sciuravus powayensis with cheek tooth formula 5/4 and absence of anteroconid on P4; to Namatomys fantasma with cheek tooth formula 4/4 and presence of an anteroconid on P4; to Simimys with cheek tooth formula 3/3 and presence of an anteroconid on the anterior lower cheek tooth. These three genera mark a transition from Sciuravidae to Eomyidae to Cricetidae. 12 no. 6 Family GEOMYIDAE (?) Gill 1872 Griphomys Wilson 1940 Griphomys sp. Figure 4 One complete M3 and another isolated fragmentary lower cheek tooth have two well-developed transverse lophs. These teeth resemble illustrations and descriptions of Griphomys, and are unlike other rodent teeth in the Hartman Ranch local fauna. M3 is undescribed in Griphomys; the Hartman Ranch specimen, UCMP 79479, is considered M3 because it is bilophodont and brachyodont, talonid is narrower than trigonid, and the posterior surface is rounded without an oppression facet. In addition, anterior cingulum is present; posterior cingulum and mesocanid are absent; vestige of a low, short iophid is present between the anterior cingulum and the protolophid about half-way between the protoconid and median plane. Another short vestige of a lophtd is directed anteriorly from the metalophid along the median plane. This tooth has the same size (Table 1) as other specimens of Griphomys from the Sespe Formation. In addition to Griphomys specimens from Hartman Ranch, the genus is known only from the Tapo Ranch fauna (Griphomys alecer at LACM(CIT) 207 and Griphomys near alecer at LACM(CIT) 202) and the Pearson Ranch fauna (Griphomys near alecer at LACM(CIT) 150) in the Sespe Formation. Family CRICETIDAE Rochebrune 1883 Simimys Wilson 1935 Wilson (1949) noted the following characters of Simimys: (a) presence of well-developed anterior cingula and anterior protocone crests in upper cheek teeth; (b) a small thickening on the anterior cingula of upper cheek teeth; and (c) cheek tooth formula 3/3. Simimys is known only from the Sespe Formation: S. murinus from LACM(CIT) locality 180, S. vetus from LACM(Clf) locality 207, S. cf. vetus from LACM(CIT) locality 202, S. simplex from LACM(CIT) 13 no. 6 locality 150, Simimys sp. from LACM(CIT) locality 127, and Hartman Ranch UCMP locality V-5814. Simimys sp. Simimys sp. from Hartman Ranch is represented by nine isolated cheek teeth. They are one M^, two M2/ three Mg, and three upper cheek teeth, probably M . Upper dentition of Simimys is based primarily on a skull (LACM 3529) with M in place and alveoli for only M . Upper cheek teeth from Hartman Ranch have oppression facets on the anterior vertical faces, excluding them from identification as M . Description of materia 1: M2 - subrectangular occlusal outline, longer than wide, slightly narrower posteriorly; anterocone minute, lingual to median plane on anterior cingulum; occlusal facet on anterior and posterior vertical faces; paracone opposite anterior arm of protocone, uniting anteriorly with transversely-directed anterior arm of protocone by low, weak protolophule; protoconule minute, a local thickening on anterior arm of protocone near median plane; posterior arm of protocone directed transversely, uniting with mure at base of mesoloph; metacone opposite hypocone and uniting with anterior arm of hypocone by low, transversely-directed metalophule; anterior arm of hypocone directed anterolabially as mure; posterior arm of hypocone directed transversely as low, wide posterior cingulum; posterior cingulum continues to base of metacone; mesocone a slight thickening at base of mesoloph on 1 specimen, absent on other specimen; anterior cingulum narrow, high labially and low lingually, continuous from paracone to protocone; parastyle minute, at termination of anterior cingulum, uniting with anterior arm of protocone in 1 of 3 specimens. M] - obovate occlusal outline, narrow anteriorly; anteroconid small, isolated on median plane, symmetrically located anterior to and between metaconid and protoconid; metaconid opposite and equal in size to protoconid, united posteriorly with transversely-directed 14 no . 6. Figure 5. Siraimys sp. a. UCMP 79270, LM2. b. UCMP 79646, LM2. c. UCMP 79269, LM2. _d. UCMP 79267,~W&i- ±- UCMP 79268, LM2- 1- UCMP 77647, LM3. (scale line = 1 mm.) All occlusal views. 15 no. 6 posterior arm of protoconid; entoconid opposite hypoconid and united to medial hypoconid by transversely-directed and slightly curved (convex anteriorly) entolophulid; mesoconid small, united to hypoconid -by posterior mure, not contacting protoconid; mesolophid long, directed transversely and flexed anteriorly, united to prominent mesostylid; mesostylid on lingual border, posterior and adjacent to metaconid; lingual lophid from metaconid absent; anterior cingulum absent; posterior cingulum high labially as posterior arm of hypoconid, low lingually where it terminates posterior to entoconid; ectolophid low on labial margin. M2 - subrectangular occlusal outline, narrow anteriorly; shorter in length and wider than Ml; anteroconid absent; metaconid opposite to and united with protoconid by transversely-directed posterior arm of protoconid; anterior arm of protoconid joining anterior cingulum; entoconid opposite hypoconid, weakly united to base of posterior arm of hypoconid by transversely-directed entolophulid; mesoconid small, strongly united to hypoconid, weakly united to protoconid; mesolophid short, directed transversely and flexed anteriorly, not uniting with mesostylid; mesostylid prominent, posterior and adjacent to metaconid; lingual lophid from metaconid uniting with mesostylid; anterior cingulum narrow, continuous from metaconid to protoconid on labial border, high medially at union with anterior arm of protoconid, low laterally; posterior cingulum high medially, low lingually at termination posterior to entoconid; ectolophid absent. M3 - obovate occlusal outline, narrow posteriorly; slightly smaller than M2; metaconid and protoconid prominent, hypoconid reduced; anteroconid and entoconid minute to absent; metaconid opposite protoconid and united posteriorly to.+ransversely-directed posterior arm of protoconid; anterior arm of protoconid joining anterior cingulum medially; mesoconid absent; anterior arm of hypoconid directed obliquely to mure, uniting with mesolophid; mesolophid long, not reaching lingual lophid; mesostylid absent; posterior arm of hypoconid 16 no. 6 directed transversely and continuing around posterior margin as posterolingual lophid; lingual iophid with slight notch in occlusal surface where mesostylid would be present; anterior cingulum continuous from metaconid to protoconid on labial border; anterior cingulum high near union with anterior arm of protoconid, low laterally; ectolophid absent. Discussion: Upper cheek teeth of Simimys sp. from Hartman Ranch are similar to M' of j>. murinus Wilson in (a) projection of the anterior arm of the protoconid to the anterolabial corner of the tooth where it unites with the parastyle, (b) a weak protoconule directed lingually from the paracone joins the anterior arm of the protocone, and (c) a minute protoconule is present on the anterior arm of the protocone. M^is not known injS. murinus. Simimys sp. is similar toj^. simplex Wilson in a paracone uniting with the anterior arm of the protocone, but a parastyle is not indicated on M^ of S. simplex. ^S. vetus Wilson differs from Simimys sp. in development of a prominent mure crest and mesocone. Lower cheek teeth of Simimys sp. are about equal in size (Table 1) to S. simplex and S. vetus. The latter two species are known from complete lower dentitions. Lower molars of S. simplex differ from Simimys sp. in absence of the isolated anteroconid on Mj, Mj is wider anteriorly, and M3 is wider posteriorly. Lower molars of ^5. vetus differ from Simimys sp. in lacking the posterior union of protoconid and metaconid, and M3 is not reduced posteriorly. Lower teeth of Js. murinus are not known. Simimys sp. from Hartman Ranch appears closest to^. murinus but specific identification is withheld, pending collection of more material/' Simimys is probably the ancestral cricetid, based on its reduced dental formula (3/3). Simimys was referred to ?Cricetidae by Wilson (1935) but frequently classified as a zapodid by other authors. There is no indication in Simimys for a fourth upper cheek tooth, a characteristic 17 no. 6 of Tertiary zapodids. Simimys is similar to later cricetid rodents in the development of an anteroconid on Mi and the reduced posterior half of M«. The protoconule of Simimys is possibly homologous with the anterocone of later cricetid rodents. In theory, the anterior cingulum of M' would not persist after P^ was lost, and the protoconule would expand anteriorly to occlude anterior to the anteroconid of Mi. The protoconule is prominent on M ' of S. murinus and minute on M2 of Simimys sp. from Hartman Ranch. Ail known middle and late Tertiary cricetid rodents lack a protoconule and have the anterocone uniting with the anterior arm of the protocone, similar to the union of the protoconule and the protocone in Simimys sp. from Hartman Ranch and S, murinus. The center of radiation for cricetid rodents is uncertain. Cricetids occur in the early Oligocene of Palearctica but are absent during the same interval in Nearcfica, Absence of cricetids during the early Oligocene of Nearctica is anomalous,ssince other small rodents (e.g. Adjidaumo, Ischyromys, Heliscomys, and Cylindrodon) are well represented at that time. Ecological limits of Oligocene small rodents are poorly known—Ischyromys was considered fossorial by Wood (1937), but recent study of uncrushed Ischyromys skulls by Black (1968) reveal the absence of many specializations interpreted by Wood. Evidence for ecologic replacement is lacking, although ischyromys was abundant in lower and middle Oligocene deposits, terminating in late Oligocene when the cricetid Eumys assumed dominance. Namatomys fantasma is a good morphological intermediate between Sciuravus powayensis and Simimys sp. Another possible ancestor of Simimys is Pauromys, a middle Eocene Nearctic sciuravid. Pauromys has a shortened and anteriorly narrowed P4 that is too reduced to be ancestral to Namatomys. Simimys may be derived from Pauromys through the loss of P4, but morphological similarity of anterior lower cheek teeth in Namatomys and Simimys plus the posterior reduction of the posterior lower cheek tooth in Simimys suggest that Simimys was 18 no. 6 derived from Namatomys through the loss of M3. Presence of Simimys and its presumed ancestor, Namatomys, in the same deposit at Hartman Ranch strongly suggest the early radiation of cricetids occurred in North America. ACKNOWLEDGEMENTS I am indebted to D. E. Savage, C. C. Black and J. Soiset for reviewing the manuscript and contributing helpful comments. All illustrations are by Owen Poe, staff artist of the University of California Museum of Paleontology. LITERATURE CITED Black, C. C. 1965. Fossil mammals from Montana, Pt. 2 Rodents from the early Oligocene Pipestone Springs Local Fauna. Carnegie Museum, Annals 38: 1-48. ________ 1968. The Oligocene rodent Ischyromys and discussion of the family Ischyromyidae. Carnegie Museum, Annals 39: 273-305. Dawson, Mary R. 1968. Middle Eocene rodents (Mammalia) from Northeastern Utah. Carnegie Museum, Annals 39: 327-370. Stock, Chester 1935. New genus of rodent from the Sespe Eocene. Geol. Soc. Am. BulL 4°: 61-68. Wilson, R. W. 1935. Cricetine-like rodents from the Sespe Eocene of California. Nat. Acad. Sci., Proc. 21 (1): 26-32. __________ 1940A. California paramyid rodents. Carnegie Inst. Washington, Publ. 514: 59-84. ______¦: . • ¦ 1940B. Two new Eocene rodents from California. Carnegie Inst. Washington, Publ. 514: 85-95. ______• • - 1940C. Pareumys remains from the later Eocene of California. Carnegie Inst. Washington, Publ .514: 97-108. _______' 1949. Additional Eocene rodent material from southern California. Carnegie Inst. Washington, Publ. 584: 1-25. 19 no. 6 Wood, A. E. 1937. The mammalian fauna of the White River Oligocene. Part II. Rodentia. Am. Phil. Soc, Trans. 28: 153-269. _____________ 1956. Mytonomys a new genus of paramyid rodent from the upper Eocene. J. Paleontol. 30: 753-755. PALEOBIOS, No. 6, 1968 20 no. 6 TABLE 1. MEASUREMENTS* length width Pr. width Hy. Pareumys sp. UCMP 79271 ?RP4 1.70 1.85 UCMP 79272 ?LM3 1.85 1.75 UCMP 79477 LM3 2.23 2.13 Griphomys sp. UCMP 79479 LM3 1.23 1.03 0.90 Simimys sp. UCMP 77646 LM2 1.40 1.24 1.21 UCMP 79269 LM2 1.39 1.25 1.19 UCMP 79270 LM2 1.26 1.20 1.07 UCMP 79267 LMi 1.45 0.75 1.06 UCMP 79268 LM2 1.30 1.00 1.15 UCMP 77647 LM3 1.31 1.10 0.90 UCMP 82686 RM3 1.26 1.13 0.83 UCMP 82687 RM3 1.15 0.88 0.72 * all measurements in millimeters Pr. = protoloph Hy. = hypoloph 21 no. 6 TABLE 2. MEASUREMENTS OF Namatomys fantasma n. sp.* N X S V OR P4 length 11 1.19*. 02 .0654 5.50 1.10-1.30 width 12 1.26±.02 .0744 5.90 1.15-1.38 M1 length 7 1.27±.02 .0624 4.91 1.18-1.35 width 7 1.34±.02 .0636 4.75 1.25-1.40 M2 length 11 1.18-.01 .0342 2.90 1.13-1.23 width 11 1.35±.02 .0666 4.93 1.25-1.45 M3 length 7 1.00±.04 .0956 9.56 0.90-1.20 width 7 1.10±.04 .0944 8.58 1.00-1.25 P4 length 9 1.16i.03 .0745 6.42 1.05-1.30 width Pr. 9 0.79±.01 .0292 3.70 0.75-0.85 width Hy. 9 1.06±.02 .0638 6.02 0.97-1.15 Mr length 17 1.29i.01 .0591 4.58 1.15-1.37 and width Pr. 15 1.19^.02 .0795 6.68 1.00-1.30 M2 width Hy. 18 l-.08±.02 .0861 7.97 0.90-1.23 M3 length 18 1.25±.01 .0595 4.76 1.15-1.35 width Pr. 17 l.lOi.Ol .0540 4.91 1.00-1.22 width Hy. 17 0.98±.01 .0505 5.15 0.90-1.08 * all measurements in millimeters N = number of specimens measured X = mean of sample S = standard deviation of sample V = coefficient of variability OR = observed range Pr. = protoloph Hy. = hypoloph 22