mi LIBRARY OF THE UNIVERSITY OF ILLINOIS AT URBANA-CHAMPAIGN 2 5603 FI V.I6 y Ul^lVEHSlTY OF ItltNOlS LIBRARY AT URSANA-CHAMPAIGN GEaOGY trfitl^swH^Vn on or before the Latest Date stamped below. Theft mutilation, and underlining of books are ;a"ons for disciplinary action and may ^esu t in dismissal from the Un.vers.ty. L161 — O-1096 / FIELDIANA . GEOLOGY Published by FIELD MUSEUM OF NATURAL HISTORY Volume 16 May 26, 1970 No. 17 A New Pai'euviys (Rodentia: Cylindrodontidae) from the Duchesne River Formation, Utah Craig C. Black Carnegie Museum, Pittsburgh ' INTRODUCTION During the summer of 1968, Dr. John Clark and Mr. Orville Gil- pin of Field Museum of Natural History fFMNH) and their assist- ant. Mr. Tom Guensburg, collected in the Duchesne River Formation west of Vernal. Utah. Fossil mammals are quite rare in this unit (see faunal list. Black and Dawson. 1966. pp. 334 337) particularly from the middle and upper, or Halfway and La Point Members (Kay. 1934). They were quite fortunate, therefore, in finding a number of mammalian specimens in the La Point Member, including the two jaws described here as a new^ species of the genus Pareumys, a genus previously known only from the Uintan. It has generally been recognized that there is a considerable dif- ference between the fauna of the Randlett Member at the base of the Duchesne River Formation and that from the La Point Member at the top. The fauna from the Randlett Member bears resemblance to the Myton fauna which occurs stratigraphically below it (Black and Dawson. 1966). The fauna from the La Point Member is not as well known as that from the Randlett but it has few genera in com- mon with the fauna from the upper part of the Uinta Formation. It should be emphasized, however, that the La Point Member fauna also shows little resemblance to faunas from the Chadron Formation in Nebraska, South Dakota, and Montana. At the present time, there is some controversy over the age of faunas from the Duchesne River Formation and the correlation of these faunas with others in Texas and California (Black and Dawson. 1966; Clark et al., 1967; Wilson et al., 1968). This confusion stems, in part, from an inade- quate knowledge of the mammals from the Duchesne River Forma- tion and the precise stratigi-aphic position of specimens already de- scribed. •Present address: Museum of Natural History, University of Kansas. Library of Congress Catalog Card Number: 76-12U571 Publication 1097 453 The Libr MAY 15 GEOLOGY LIBRARr'"™' '= 454 FIELDIANA: GEOLOGY, VOLUME 16 Family Cylindrodontidae Genus Pareumys Peterson, 1919 Pareumys guensburgi^ n. sp. Figures 1, 2 Type.— FMNR PM 14978, left mandible with P4-M3. Hypodigm.— Type and FMNH PM 14979, left mandible with M2. Horizon and Locality. — Duchesne River Formation, La Point Member, Late Eocene, SE I4 sec. 23. T. 4 S., R. 19 E., SLM, Uintah Co., Utah. Diagnosis. — Largest species of genus; jaw massive as in Cylindro- don: P3-M3 higher crowned than in Pareumys grangeri and P. milleri; P4 reduced, smallest tooth of series; Mj M3 present three-lophed con- dition after slight wear; hypolophid joins ectolophid; posterolophid as strong as hypolophid. Description. — There is considerable difference in size between the two mandibles here referred to Pareumys guenshurgi. Nevertheless, they are considered to represent the same species because the M2 in the smaller jaw is so nearly identical in size and structure to that of the type. In addition, the incisor structure is the same in each speci- men although the incisor in the type is larger than that in PM 14979. Morphologically, the two mandibles are identical. The masseteric fossa ends sharply below the posterior end of M2 (fig. 2C) with the ventral masseteric ridge carried forward in a thick knob below the middle of M2. There is only a single mental foramen just anterior to and below P4 in PM 14979. In PM 14978, this area has been destroyed. The lower cheek teeth of P. guenshurgi are higher crowned than those of any other species of Pareumys. In this respect they parallel the condition seen in Pseudocylindrodon. This increase in crown height is mostly effected through increase in the height of the ringing enamel of the teeth without an increase in the depth of the occlusal pattern. The crown pattern is thus lost after moderate wear even though the teeth still retain a considerable height of crown. This approach to hypsodonty is carried further in Cylindrodon and is quite different from the hypsodonty seen in mylagaulids oi' beavers where the entire crown pattern shares in the increase in crown height. ' Named for Tom Guensburg, discoverer of the two specimens. BLACK: NEW PAREUMYS 455 Fig. 1. Pareumys guensbergi, FMNH PM 14978, type, occlusal view of left P4-M3, X 10. The fourth lower premolar is the smallest tooth of the series. It is almost circular in occlusal outline with the protoconid and meta- conid appearing as a single cusp when worn. There is only the slight- est indentation in the enamel on the anterior face of P4 to indicate a separation of these cusps (fig. 1). The hypoconid buttress swings forward as it does also on Mi and M2 and to a lesser extent on M3. The entoconid crest is prominent on P4 and fuses with the posterior end of the ectolophid. The entoconid is set off from the metaconid and the posterolophid until the tooth has become extremely worn. The crown patterns of Mi and M2 were probably quite similar (figs. 1, 2A), although Mi is too heavily worn to be certain of this. M2 displays three prominent cross crests in the type (PM 14978). The less worn M2 in PM 14979 shows that the anterior crest is com- posed of the anterior cingulum (or metalophid I) plus a very short posterior protoconid arm (metalophid II) which does not reach the internal slope of the metaconid. The trigonid basin is quite shallow and opens into the central basin of the tooth on M2 as it does also on Mg. There is no metastylid on M2 or M3 and it was probably absent on Mi as well. The entoconid on M2-M3 is completely set off from both metaconid and posterolophid during early wear stages. The anterior entoconid slope fuses with the posterior metaconid slope with moderate wear and then with further wear the entoconid and posterolophid become joined. The posterolophid is swollen on all molars and at early wear stages a hypoconulid is clearly discernible. The posterior third of M3 is somewhat reduced through shortening of the posterolophid. The lower incisor is heavy and broadly triangular in cross-section. The anterior face of the incisor is slightly rounded with the enamel overlapping about one-third of its lateral face and one-fifth of its medial face. 456 FIELDIANA: GEOLOGY, VOLUME 16 Measurements in millimeters FMNH FMNH PM 14978 PM 14979 P4 - Ms occlusal 9.50 P4 anteroposterior 1.90 transverse metalophid 1.50 transverse hypolophid 1.90 Ml anteroposterior 2.30 transverse metalophid 2.10 transverse hypolophid 2.05 M-. anteroposterior 2.60 2.70 transverse metalophid 2.40 2.40 transverse hypolophid 2.50 2.40 Ms anteroposterior 2.40 transverse metalophid 2.50 transverse hypolophid 2.40 I anteroposterior 2.70 1.90 transverse 2.50 1.70 Relatiojiships. — The genus Pareumys is quite distinct from later members of the Cyhndrodontidae and certainly could not have been ancestral to them. This view has been expressed by Wilson (1940, p. 106) but was based upon somewhat different criteria. Wilson argued that the incomplete metaloph on P4-M2 of the California Pareumys material ruled out this genus as being ancestral to Pseudo- cylindrodon or Cylindrodon. This was true for the latter two genera as understood at that time, but additional material of Pseudocylin- drodon neglectus from Pipestone Springs, Montana (Black, 1965), shows a few specimens with a metaconule-posterior cingulum con- nection such as is seen in the California specimens of Pareumys. In Oligocene species of Pseudocylindrodon the metaloph is never inter- rupted completely as in Wilson's material of Pareumys. This incom- plete condition of the metaloph is seen, however, in some Uintan specimens of Pseudocylindrodon (Black, in press) although it appears to be an infrequent variant in that population. It would appear that variation of metaloph development is typical of the middle Eocene Fig. 2. Pareumys giiensburgi, A, FMNH PM 14978, type, lateral view of mandible, X 4; B, FMNH PM 14979, lateral view of mandible, X 5; C, same, occlusal view of M.., X 10. B ^H^ \ V 1 V 457 458 FIELDIANA: GEOLOGY, VOLUME 16 Mysops (Wilson, 1938) and of the late Eocene species of both Pareu- mys and Pseudocylindrodon whereas a complete metaloph had been established in the early Oligocene Cylindrodon and nearly established in the early Oligocene Pseudocylindrodou . In this character Pareu- mys appears to have diverged from the Pseudocylindrodon -Cylindro- don line and emphasized the metaconule-posterior cingulum attach- ment over the metaloph-protocone attachment. There is an even more fundamental difference between these two lineages, however, and this involves a reduction in the size of P4 rela- tive to Ml and M2 in the Mysops to Pareumys line. Another lineage leading to Pseudocylindrodoyi and Cylindrodon developed from My- sops which maintained the size of P4 nearly equal to that of the molars. The relative size of M3 also varies in these two lineages, with M3 in Pareumys much nearer the size of M1-M2 than in Pseudo- cylindrodon and Cyliyidrodon . Pareumys differs from the Oligocene Ardynomys in a number of respects and was probably not ancestral to that group either. There is no closure of the central valley or elongation of the posterior meta- conid slope in Pareumys as in Ardynomys, the buccal valley is wider and M1-M3 are more elongate than in Ardynomys, and the characters of P4 already mentioned also distinguish Pareumys from Ardynomys. Within Pareumys itself, P. guenshurgi has P4 more reduced than in P. milleri, P. grangeri, or P. troxelli. Also, P. guenshurgi is much the largest species of the genus. In other details of the dentition, P. guenshurgi is quite similar to P. milleri and could have evolved from the Uintan C species. As far as is known at present, P. guens- hurgi is the last representative of the genus. Acknowledgements. — I would like to thank Dr. John Clark who made the specimens of Pareumys guenshurgi available to me for study. This research was carried out with the aid of NSF Grant GB-7801. Publication was supported by a grant from the Gulf Oil Corporation. REFERENCES Black, Craig C. 1965. Fossil mammals from Montana. Pt. 2. Rodents from the early Oligo- cene Pipestone Springs local fauna. Ann. Carnegie Mus., 38, pp. 1-48, figs. 1-6. In press The paleontology and geology of the Badwater Creek area, central Wyoming. Pt. 5. The cylindrodont rodents. Ann. Carnegie Mus. BLACK: NEW PAREUMYS 459 Black, Craig C. and Mary R. Dawson 1966. A review of late Eocene mammalian faunas from North America. Amer. Jour. Sci., 264, pp. 321-349, fig.s. 1 4. Clark, J., J. R. Beerbower, and K. K. Kietzke 1967. Oligocene sedimentation, stratigraphy, paleoecology and paleoclimatol- ogy in the Big Badlands of South Dakota. Fieldiana: Geol. Mem., .5, pp. 1-158, figs. 1-53. Kay, J. LeRoy 1934. The Tertiary formations of the Uinta Basin, Utah. Ann. Carnegie Mus., 23, pp. 357-371, 5 pis. Wilson, J. A., P. C. Twiss, R. K. DeFord, and S. E. Clabaugh 1968. Stratigraphic succession, potassium-argon dates, and vertebrate faunas, Vieja Group, Rim Rock Country, Trans-Pecos, Texas. Amer. Jour. Sci., 266, pp. 590-604, figs. 1-2. Wilson, R. W. 1938. Review of some rodent genera from the Bridger Eocene. Part II. Amer. Jour. Sci., 35, pp. 207-222, figs. 5-12. 1940. Pareumys remains from the later Eocene of California. Carnegie Inst. Wash., Publ. No. 514, pp. 97-108, 2 pis. n