Typhlomys, Milne-Edwards, 1877

Qiu, Zhuding & Ni, Xijun, 2019, Platacanthomyids (Rodentia, Mammalia) From The Late Miocene Yuanmou Hominoid Locality Of Yunnan, China, Fossil Imprint 75 (3 - 4), pp. 383-396 : 386-389

Typhlomys aff. T. primitivus QIU, 1989

Text-figs 5 , 6

H o l o t y p e o f T. p r i m i t i v u s. Left M1 (IVPP

V 8818).

T y p e l o c a l i t y o f T. p r i m i t i v u s. Shihuiba,

Lufeng, Yunnan Province, China.

R e f e r r e d m a t e r i a l. Yuanmou: 1 M 1, 3 M 2, 1 M 3 (IVPP V 25884.1–5) from Loc. 9903; 1 M 1, 1 M 3, 4 M 1, 2 M 2 (IVPP V 25884.6–13) from Loc. 9904; 10 M 1, 11 M 2, 2 M 3, 6 M 1, 9 M 2, 1 M 3 (IVPP V 25884.14–52) from Loc. 9905; 14 M 1, 19 M 2, 7 M 3, 24 M 1, 14 M 2, 5 M 3 (IVPP V 25884.53–135) from Loc. 9906.

M e a s u r e m e n t s. See Tab. 2.

D e s c r i p t i o n. Molars are lophodont, with concave grinding surface, and unilateral hypsodonty of the upper molars. The first and second molars usually consist of six diagonal/transversal ridges and five synclines. The midridges/synclines have a mesial angle to the longitudinal axis of generally 45° to 55° on the upper molars and 25° to 45° on the lower molars. Synclines II on the upper molars are open both buccally and lingually in little-worn specimens, and synclines II and III on the lower molars are buccally open. The endoloph and the endolophid are well-developed, but the endoloph is interrupted at syncline II in the early stages of wear. The upper molars have 3 roots, and the lower molars are two-rooted.

The M 1 is long and narrow, with the lateral margins being slightly convergent anteriorly. The anteroloph is short and slightly anteriorly curved. The anterior extra ridge is present in all M 1, enclosing a small syncline Ia with the anterolophid. The protoloph and mesoloph are prominent and parallel in arrangement, and mostly free buccally in littleworn specimens. The metaloph is short, but longer than the anterior extra loph. The posteroloph is relatively thin and nearly transverse. Syncline Ia is present in all specimens, but variably developed. Synclines I and II are shallowly open buccally in most specimens. Syncline II is lingually closed in 8 out of 25 teeth. Syncline IV is small, open buccally in 2 M 1 s.

The M 2 is subrectangular in outline. It is similar to M 1 in structure, but the anteroloph is straight, and longer than the posteroloph. A continuous endoloph develops in early stages of wear, and an ectoloph forms with advanced wear of the tooth. Syncline Ia is variably developed, in 8 of 21 M 2 s it is distinct, in 4 specimens present as a discernible trace, in 9 completely absent.

The M 3 is subtriangular, with relatively transverse ridges and reduced posterior part. The protoloph, endoloph and ectoloph are complete and continuous. The mesoloph is present in most of the specimens. Among the 10 M 3 s, 1 has a small syncline Ia, and in 2 a trace of the syncline is discernible. In 3 from 8 M 3 s the metaloph is very short or incomplete, and in 4 it is absent. Syncline IV is absent in 5 out of 9 specimens .

The M 1 is distinctly longer than wide, with the lateral margins being slightly convergent anteriorly. The anterolophid is anteriorly curved. The anterior extra ridge is prominent and roughly anterobuccal-posterolingually oriented, frequently joining the anterolophid to enclose syncline Ia. The paraconid-metaconid connection is the longest diagonal/transversal ridge in the tooth, joining the anterolophid and endolophid to enclose long syncline I. The mesolophid is marked and free buccally. The hypolophid is nearly transverse and shorter than the mesolophid. The posterolophid is relatively thin, connecting to the hypoconid and entoconid to enclose a small syncline IV. Syncline I is laterally closed, while synclines II and III are buccally open. The endolophid is continuous, even in fresh teeth.

The M 2 is subrectangular, with a constricted posterobuccal corner. It has the same ridges and synclines as in M 1, but synclines Ia and IV are relatively small and shallow, and disappear in rather early stages of wear.

The M 3 is subtriangular, with reduced and curved posterior part. The paraconid-metaconid connection and the mesolophid are present in all 7 M 3 s. In the 6 specimens identified, a small syncline Ia is present or a trace of it is discernible. None of the M 3 shows a hypolophid or syncline IV .

D i s c u s s i o n. The platacanthomyid represented by the described specimens exhibits the following suite of dental characters: small molars with concave grinding surface; presence of anterior extra ridge and syncline Ia; less inclined mid-ridges/synclines; a tendency toward the development of endoloph and ectoloph on the upper molars, presence of a continuous endolophid on the lower molars, and the relative reduction of the third molars. These features agree with the diagnosis of Typhlomys given by Milne- Edwards (1877) and emended by Qiu (1989).

Typhlomys is a monospecific living genus distributed in the Oriental Region. Six fossil species of the genus, T. primitivus , T. hipparionum , T. intermedius , T. macrourus , T. cinereus and T. anhuiensis , have been documented from the late Cenozoic deposits in China ( Qiu 1989, Zheng 1993, Jin et al. 2009). The dental pattern of Typhlomys seems to be quite stable. The changes that took place between late Miocene T. primitivus and Recent T. cinereus are rather slight. Definition of these taxa is mainly based on size differences, and species distinction is subtle. For example, the Pleistocene species T. intermedius , T. cinereus and T. anhuiensis show close resemblance in size and morphology, and it is difficult to distinguish them when material is inadequate.

In morphology and dimensions, the platacanthomyid from Yuanmou matches Typhlomys primitivus from Lufeng. In general size, the molars of T. primitivus of the former fall within the range of the latter, but at the lower end ( Text-fig. 6 ). In morphology, minor differences of the Leilao taxon from the Shihuiba form are the presence of anterior extra ridge and syncline Ia in all M 1 s, in having syncline Ia in a few M 3 s, the absence of syncline IV in M 3. In spite of these differences, the Yuanmou platacanthomyid seems to have its closest affinities with T. primitivus from Lufeng. The slightly larger size, the presence of anterior extra ridge and syncline Ia in all M 1 s and in a few M 3 s, and the absence of syncline IV in M 3, may be interpreted as derived features for the Yuanmou taxon, which is treated as Typhlomys aff. T. primitivus here.

Typhlomys aff. T. primitivus is smaller than T. hipparionum from Lufeng. It is also smaller than Typhlomys cinereus and T. macrourus ( Zheng 1993) . In addition, it differs from the two Pleistocene species in lacking or having weakly developed syncline Ia in some M 2 s.

Figures

Text-fig. 5. Typhlomys aff. T. primitivus Q, 1989 from Leilao, Yuanmou. a) left M1 (IVPP V 25884.53), b) right M1 (IVPP IU V 25884.54),c) left M2 (IVPP V 25884.55),d) right M2 (IVPP V 25884.14),e) left M3 (IVPP V 25884.56), f) right M3 (IVPP V 25884.57), g) left M1 (IVPP V 25884.15), h) right M1 (IVPP V 25884.58), i) left M2 (IVPP V 25884.59), j) right M2 (IVPP V 25884.60), k) left M3 (IVPP V 25884.61), l) right M2 (IVPP V 25884.62).

Text-fig. 6. Scatter diagrams (length and width) of Typhlomys aff. T. primitivus measurements from Leilao, Yuanmou in comparison with those of T. hipparionum and T. storchi from Leilao, and of T. primitivus and T. hipparionum from Shihuiba, Lufeng.