YUOMYS, A NEW ISCHYROMYOID RODENT GENUS FROM THE UPPER EOCENE OF NORTH CHINA

A new genus and species, Yuomys cavioides, tentatively referred to Ischyromyi- dae-Paramyinae, is described in the present paper. The material was collected from three Late Eocene localities: 1. Jentsen, Mienchi Distrct, Honan (field no. 5310- 5314); 2. Tung-chang, Chiyuan District, Honan (63022); and 3. Ula Usu, Inner Mon- golia. Suborder Protrogomorpha Zittel, 1893 Family Ischyromyidae Alston, 1876 ?Subfamily Paramyinae Simpson, 1945 Yuomys cavioides Li, gen. et sp. nov. Cricetodon schaubi, Woo and Chow, 1957, p. 268. Ischyromyoidea gen. et sp. nov., Chow, Li and Chang, 1973, p. 172, 174. Diagnosis An medium sized ischyromyid with hystricomorphous skull and sciu- rognathous lower jaw; masseter medialis arising from the snout and passing through the infraorbital foramen. Infraorbital foramen and incisive foramen large. Dental formula 1·0·2·3/ 1·0·1·3; cheek teeth brachodont, of paramyid type, but shared some primitive cylindrodontid characters (as Mysops-Parcumys). P~4 no hypocone, M~1, M~2 hypocone very developed; metaconule large; metaloph pointed to, but never in contact with protocone; M~3 the largest, with posterior half narrow and longer. P~4 large, trigonid with a closed basin and higher than talonid; hypolphid and ectolophid com- plete; hypoconid not hyposont, no mesoconid; three valleys of talonid deep and open. Lower molars similar to P~4, but with short metalophid II, trigonid unbasined, and valley occasionally closed. An initial mesostyle and mesostylid appeared on the molars. Type A shattered skull and associating lower jaw, with complete upper and lower teeth (left P~3 lost) (IVPP no. V 4796; field no. 5313). Paratype Anterior part of another skull, with right DP~4-M~2 and left P~3-M~1 (V 4802; 63022). Referred specimens: An upper and three lower jaw fragments (V 4797--4800; 5313, 5311); 6 isolated teeth (V 4801; 5317) and a lower jaw from Ula Usu (V 4803). Derivation of the name: Yuomys, Yu is an abbreviated name of Honan Province; cavioides indicated its hystricomorphous skull as in Cavia. Description The snout is relatively shorter and higher as in paramyids. The nasals long, terminating posteriorly about at M~1 and with W-shaped posterior suture. The premaxilla-maxilla suture runs from middle of incisive foramen upwards through the laterial surface of snout and extends posteriorly at a point as in that of nasal. The incisive foramen is large as in theridomyoids (such ns Sciuroides). On the ventral surface of maxilla, there is a distinct depression just anterior to P~3 as in Ischyromys troxalli and I. douglassi. Hartenberger (1973, P. 24) also described a more deep pit of Sciuroides cf. intermedius Which was considered probablement reservee au masseter anterieur superficiel. The anterior zygomatic root of maxilla is slender as in that of Pseudocylindrodon and Sciuroides and more or less anteriorly situated. On the ventral surface of zygomatic root, there is a shallow fossa paralell to the edge of root which may be the origin of anterior part of masseter lateralis. A relatively larger infraorbital foramen with a dorsal-ventral oval in shape (longer diameter: 6.4 mm. ca. 46% of snout height) appeared at the level of P~3. Just anterior to the foramen and extending somewhat dorsal to it, there is a distinct depression on the side of the snout as in Protoptychus. This depression (ca. 6×6 mm) should be the site of origin of the anterior part of medial masseter, so that Yuomys was a ty- pical hystricomorphous. The lacrimal is larger, sexangle in outline and situated postero- dorsally to infraorbital foramen. The lower jaw is deep and massive, of sciurognathous type. The masseteric tuber- cule is beneath the talonid of M~1 and with double ridges on the upper border of masseter fossa as in Saykonomys (Shevyreva, 1972). A large mental formen present below the diastema. The symphysis is longer, vertical and reversed comma (?) shaped. The cross-section of upper incisor is trianglar as in Protoptychus and Paramys atavus. The enamal cap of incisor extends well onto laterial surface and the pulp cavity is a narrow slit (fig. 1). The lower incisor is similar to the upper one. The upper cheek teeth are bunodont, but approach a four crested pattern. P~3 is a small, conical tooth, with a large outer cusp and a conspicous cingulum on its inner side. P~4 without hypocone and protocone situated slightly forward. The protoloph is complete and more transverse with no indication of a protoconule. The imcomplcte metaloph, ending by a large metaconule parallels with the protoloph. The anterior cingulum entire along the anterior side of tooth and bears a large parastyle. The posterior cingulum is short, extending only to the innero-posterior side of the meta- cone. The hypocone of M~1 well develop, larger than protocone and separate from the latter by a vertical groove extending from the occlusal surface down to the base of the crown. Metaloph somewhat converges to the protocone, but never connected with the latter. There is a little mesostyle which enlarged from M~1 to M~3 on the outer side of the central valley. M~2 similar to M~1. The last upper molar is the largest, with an incipient hypocone and narrower posteriorly; metaloph more oblique and pointed to the protocone, posterior valley broad, enclosed by tbe metaloph and posterior cin- gulum; internal groove situated posteriorly and only extending downwards half way of crown height. A DP~4 presehted in the paratype shows a similar molar pattern, except possessing a rather larger parastyle and a boarder anterior cingulum. The lower cheek teeth are more lophodont than upper ones. P_4 is large, not reduced as in seiuravids. The trigonid is bigher than talonid, basined and enclosed by the anterolophid and metalophid II. There is a complete hypolophid transvered the talonid basin and connected with ectolophid at a point before hypoconid. No mesoconid, hypoconulid less developed than in molars. The three valleys situated in talonid basin are open and deep. The lower molars are similar to the premolar, but with an incomplete metalophid II, and the valleys enclosed in some specimens. The metastylids more distinct in molars. Discusion 1. Yuomys, seems to be most closely akin to its Asiatic (?) Middle Eocene rela- tive, Petrokozlovia (Sheveryva, 1972), but it differs from the letter by (1) larger size, (2) P~4 without hypocone, (3) hypolophid complete and transverse to ectolophid, instead to hypoconulid (i. e. Tillomys-type), (4) less cylindrodont inclination and (5) double crest of masseter fossa on the lower jaw. Both these two Asiatic genera may be referred to Ischyromyidae-Paramyinae rather than Sciuravidae as classified by Sheveryva. 2. So far as is known, Yuomys is the only ischyromyoid genus possessing a bystricomorphous skull with sciuragnathous jaw. It may be compared with such forms as Rapamys, Knightomys, Protoptychus, paramyid of Michaux (1968), Saykonomys and Petrokozlovia etc. in having an enlarged infraorgital foramen, but in all these genera either their masseter muscle do not penetrat the foramen or without positive evidence to show that their skulls are hystricomorphous (as to Protoptychus, according to Wahlert (1972, p. 14), it was probably quite hystricognathous). Nevertheless, it is interesting to point out that the forms with enlarged infraorbital foramen, especially the Asiatic ones, whether represent a natural group of its own or merely parallel in certain morphologic features. 3. The cheek teeth pattern of Yuomys shares in some characters of both Ischy- romyidae and Cylindrodontidae. Judging from its larger size, structure of skull and others (such as brachyodont, terminal cheek teeth unreduced and the surface of the crest of lower cheek teeth unpeneplaned), the new genus seems to be closer to ischy- romyid-paramyines than to cylindrodontids or sciuravids. 4. A list summerizing known protrogomorphous rodents from Asia doing with some remarks is given in the Chinese text (see p. 61).