SYSTEMATICS

Genus Mimomys Forsyth Major, 1902¹

Arvicoline rodent with rooted molars and anteroconid complex on m1 having a cap that is skewed lingually and with prominent wings, a "Mimomys Kante²" and an islet derived from a buccal reentrant in early stages of wear; M3 with one triangle and a basined posterior loop (islet with wear) or two triangles and hooked posterior loop. Cementum and dentine tract development are trends within the genus.

Mimomys (Mimomys) orientalis Young, 1935

Holotype and Type Locality: complete right m1 of a young individual, little worn, now lost according to Zheng and Li (1986); from locality 34, lower level (sands) at Pinglu, southern Shanxi, more than 300 km south southwest of Yushe.

Distribution and Age: The type locality is considered "Youhean" in age, now late Yushean (Late Pliocene, pre-Nihewanian), and a referred specimen is from Weinanliuhe, Youhe, Shanxi Province. Zheng and Li (1986) reported Yushe specimens purported to come from around Haiyan and Jizi Gou, which is middle to late Pliocene.

Diagnosis: Length of m1 ranges from 2.8 to 3.1 mm, cement absent from reentrant angles (traces of cement in one specimen according to Young 1935), islet present in the anteroconid complex until about half worn, dentine tracts moderately developed (as in M. hajnackensis).

Referred specimen from Nan Zhuang Gou: F:AM 116247, a left dentary with incisor, m1 and m2, and with m3 alveolus but lacking the posterior part of the jaw (Figure 1). Cheek teeth are half worn, so that the enamel islet of m1 is nearly removed by wear. A dense area in the dentine with a minute remnant of enamel still shows the position of the islet. The m1 is 2.9 mm long. There is no cement in the reentrants of F:AM 116247. The full extent of dentine tracts is not known due to wear, but on the buccal posterior salient angle, the tract is comparable to the modest tract of unworn specimens (see the buccal views of specimens described by Zheng and Li (1986), fig. 3 ).

The matrix found in crevices of F:AM 116247 appears to indicate a rather dark gray to brownish gray poorly sorted mudstone with fine sand and abundant black grains. The bone is dark brown with light brown highlights where thinner. The occlusal surface of the tooth enamel is medium gray, while the dentine is orange-brown. These contrast in the second specimen.

Subgenus Cromeromys Zazhigin, 1980

Enamel islets in m1 and M3 not present, cement present in all species and well developed in later forms, "Mimomys Kante" present on the buccal primary wing in older forms and progressively migrates anteriorly as a distinct triangle of the cap of the anteroconid complex, dentine tracts progressively more developed with time, M3 short, with completely confluent central triangles and wide posterior loop not extended posteriorly.

Mimomys (Cromeromys) gansunicus Zheng, 1976

Holotype and Type Locality: IVPP V-4765, a right m1 from an adult with incomplete root development from Jingou, Heshui, Gansu Province.

Distribution and Age: The few Chinese specimens known to Repenning in the 1980s are from early Nihewanian age deposits, early Matuyama Chron equivalent, late Pliocene. Repenning considered this species to be very close to Mimomys tornensis of Europe and to Mimomys populations from Siberia incorrectly called M. intermedius by Zazhigin (1980), both of which range from late Pliocene into early Pleistocene. Field work in Yushe Basin in 1988 produced abundant M. gansunicus from the Haiyan Formation, early Matuyama Chron (Flynn et al. 1997).

Diagnosis: Length of m1 ranges from 2.9 to 3.1 mm, "Mimomys Kante" absent or weakly developed on the posterior part of the cap of the anteroconid complex, cement well developed, dentine tract on the posterior buccal salient angle very long and breaking the enamel pattern of the occlusal surface at a slightly earlier stage of wear than does the tract on the buccal face of the anteroconid complex.

Referred specimen from Nan Zhuang Gou: F:AM 116248, a right dentary with all cheek teeth and part of the posterior portion of the jaw. The specimen is an adult, with roots fully formed on all molars, but wear is only about one quarter on the molars with the dentine tract of the anteroconid complex not quite reaching the surface. The dentine tracts are slightly less extensive than those of M. tornensis, suggesting somewhat greater geological age, consistent with early Matuyama Chron equivalence in China.

The matrix found in crevices of F:AM 116248 appears to indicate a light reddish tan well sorted sandstone composed of very fine to fine sand with almost no black grains. The bone is white with tan stains, and on the occlusal surface of the teeth the enamel is light gray but is mottled dark gray low on the sides of the teeth. The dentine and cement are white. The bone and possibly the enamel have been bleached by exposure, and the bone is gray (not dark brown) beneath its surface.

¹ Repenning loved footnotes. He would drive editors crazy, inserting comments copiously. I follow his example in this and in his usage of Mimomys subgenera. Later Mimomys (Cromeromys) has many derived features that make its diagnosis easy. Mimomys (Mimomys), on the other hand, is a cluster of primitive Mimomys species that show the generic characters but with little else uniting them. They tend to be larger species, so his rule of thumb was that average length of m1 is 3 mm or more – but this is an average, and individuals, like the one described here, can be less than 3 mm.

² Diagnoses and terminology follow Repenning (2003, for example). Mimomys Kante is an extra structure at the anterior end of the first lower molar (m1), a vertical ridge on the buccal primary wing of the anteroconid complex.