Prodistylomys lii, Bi & Meng & Wu & Ye & Ni, 2009

Bi, Shundong, Meng, Jin, Wu, Wenyu, Ye, Jie & Ni, Xijun, 2009, New Distylomyid Rodents (Mammalia: Rodentia) from the Early Miocene Suosuoquan Formation of Northern Xinjiang, China, American Museum Novitates 3663, pp. 1-20 : 11-12

Prodistylomys lii , n. sp.

Figures 6–7 , table 1

HOLOTYPE: IVPP V16015.1 , right fragmentary mandible with broken i2, and m1–m3.

REFERRED MATERIAL: IVPP V 16015.2, right M3; V 16015.3, right p4.

STRATIGRAPHIC AND GEOGRAPHIC PROVENANCE: Suosuoquan Formation of Chibaerwoyi, Xinjiang. Early Miocene.

ETYMOLOGY: Named for Professor Li Chuankui for his many contributions to the study of Asiatic rodents.

DIAGNOSIS: Similar to P. xinjiangensis in size, but differs in having well-developed roots, the absence of the metaflexid, and less hypsodonty.

COMPARATIVE DESCRIPTION: The only preserved upper cheek tooth V16015.2 is a heavily worn M3 (fig. 7–A). The M3 trigon is kidney shaped, whereas it is trapezoidal in Distylomys burqinensis . The talon is much narrower than the trigon. At this stage of wear, the flexi are already fused and form a fossette. No cement is present in the fossette. The two strong roots of M3 are fused.

The mandible (fig. 6) is similar to that of the type species, P. xinjiangensis . The diastema is short and shallow. The mental foramen opens dorsally and lies on the dorsal side of the diastema. In contrast, in P. xinjiangensis the mental foramen opens labially and occurs near the level of the dorsal shelf. The lower incisor is short, extending posteriorly to below m2 as in P. xinjiangensis and D. qianlishanensis , whereas it extends as far as m 3 in D. burqinensis . The tip of the lower incisor is broken and its cross section is subrectangular with flat anterior and lingual walls. The enamel thins and extends a short distance onto the labial and lingual walls.

The p4 is not preserved in the holotype, but the alveolus indicates presence of three roots (fig. 6). The only known p4 is IVPP V16015.3 (fig. 7B). The trigonid is shaped like an isosceles triangle with gently concave labial and lingual margins. The anterolophid is short and blunt. The hypoflexid extends only halfway down the tooth, whereas the mesoflexid is much longer, extending to the root. The flexids are U-shaped and have no cement. In P. xinjiangensis , the flexids are V-shaped and cement filled. The talonid is wider than the trigonid; the talonid is slightly broken posteriorly, but what remains suggests absence of a metaflexid. This p4 has 3 roots. Although the p4 crown morphology is similar to that of dp4 of P. xinjiangensis except for the absence of the posterolophid, we prefer to consider this tooth as p4 because the roots are not splayed apart. If so interpreted, it is probable that p4 and dp4 have similar general crown morphology.

The lower molars have the same bilophodont occlusal pattern as P. xinjiangensis , but lack the posterolophid. The flexids extend only to the alveolus, clearly showing welldeveloped roots in all the lower molars. In P. xinjiangensis , the flexids extend all the way into the tooth base, indicating reduced roots.

COMMENTS: In the Chibaerwoyi locality, fossils were mostly collected from beds A and B (7–12 m, and 14–21 m from the base of the formation, respectively). P. lii was collected from Bed A. The exact stratigraphic level of P. xinjiangensis was uncertain, but most likely was collected from Bed A. However, P. lii is morphologically more primitive than P. xinjiangensis in having stronger roots, or less hypsodonty, and lack of posterolophid, suggesting an earlier occurrence.

Figures

Fig. 6. Occlusal (A), labial (B), and the lingual (C) views of the mandible of Prodistylomys lii (IVPP V16015.1).

Fig. 7. Prodistylomys lii. A. occlusal (A1) and labial (A2) view of right M3 (IVPP V16015.2). B, occlusal view of right p4 (IVPP V16015.3).