Typhlomys storchi, Qiu & Ni, 2019

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 : 390-394

Typhlomys storchi n. sp.

Text-figs 6 , 8

H o l o t y p e. Right M 1 (IVPP V 25886 ); dimensions:

2.75 × 1.30 mm (length × width).

E t y m o l o g y. This new species is dedicated to Gerhard

Storch.

T y p e l o c a l i t y. Loc. 9905, Leilao, Yuanmou,

Yunnan Province, China.

T y p e s t r a t u m. 30–150 cm grayish bands of pebbly sandstone, Xiaohe Formation.

A g e. Late Miocene.

P a r a t y p e. 1 left M 1 (IVPP V 25887.1) from Loc.

9905; dimensions: 2.05 × 1.30 mm (length × width).

R e f e r r e d m a t e r i a l. 2 isolated teeth (1 M 3 from

Loc. 9906, 1 M 2 from Loc. 9903; IVPP V 25887.2–3).

M e a s u r e m e n t s f o r r e f e r r e d m a t e r i a l.

M 3: 1.60 × 1.25 mm; M 2: 2.00 × 1.35 mm (length × width).

D i a g n o s i s. Large-sized species of Typhlomys . Molars with concave grinding surface, six inclined diagonal ridges separated by five synclines, continuous endoloph(id)s and ectoloph(id)s incorporated with the anteroloph(id)s and posteroloph(id)s to circle the occlusal surface without buccal opening of synclines II and III. Anterior portion of M 1 distinctly extending forward.

S t r a t i g r a p h i c r a n g e. Typhlomys storchi n. sp. is known so far only from Leilao, the type locality.

D e s c r i p t i o n. The available molars (M 1, M 3, M 1, M 2) are lophodont, with concave grinding surface, slight unilateral hypsodonty of the upper molars, and have six diagonal/transversal ridges separated by five synclines. The endoloph and the endolophid are rather developed, and the synclines are laterally closed in all stages of wear.

The M 1 is subrectangular in occlusal view, with nearly parallel lateral margins. The anteroloph is pronounced and anteriorly curved. The anterior extra ridge is prominent, enclosing a crescent syncline Ia with the anterolophid. The protoloph and mesoloph are long and strong, roughly parallel to the anterior extra ridge in direction. The mesoloph has a mesial angle to the longitudinal axis of 53°. The metaloph is short, but longer than the anterior extra ridge. The posteroloph is nearly transverse. Synclines II and III are marked, and in parallel arrangement; there is no trace of a buccal opening of the synclines. Syncline IV is close to syncline Ia in size.

The M 3 is an elongated-triangle in occlusal view, with the lateral margins being slightly convergent posteriorly. It has a reduced posterior part and relatively transverse ridges. The anterior extra ridge is fused with the thick anteroloph lingually. The protoloph and mesoloph are conspicuous and nearly transverse in direction. The metaloph is short and slightly curved backward. A trace of syncline Ia is discernible at the anterobuccal corner of the tooth. Syncline II is the longest furrow on the tooth. Syncline IV is more distinct than syncline Ia. The M3 has 3 roots.

The M 1 is much longer than wide, with the lateral margins converging and distinctly extended anteriorly. The tooth is in very early wear, but shows continuous endolophid and ectolophid. No deep buccal opening of synclines II and III is visible as seen in other Typhlomys . The anterior extra ridge is long and pronounced, but does not touch the anterolophid in this stage of wear. The paraconid-metaconid connection is the longest diagonal/transverse ridge on the tooth, with a mesial angle to the longitudinal axis of about 40°. The mesolophid is nearly transverse in the lingual area, and then turns anterobuccally at the middle of the tooth width. The hypolophid is short and transverse, joining with the posterolophid to form a continuous loop posteriorly. Syncline Ia is long but narrow, due to the thickening of the endolophid anteriorly. Syncline I is the longest syncline on the tooth. Synclines II and III are similar in shape and parallel in arrangement, but the former is longer than the latter. Syncline IV is short and transverse. M 1 has two roots.

The M 2 is subrectangular in occlusal outline, with a straight anterior margin and slightly curved posterior margin. The tooth is in early wear, and shows continuous endolophid and ectolophid, which are incorporated with the anterolophid and posterolophid to circle the occlusal surface. As in the M 1, there is no trace of a buccal opening of synclines II and III. The anterior extra ridge is short, joining the endolophid and the anterolophid to enclose an elongated oval syncline Ia. The paraconid-metaconid connection, the mesolophid and the hypolophid are roughly parallel in arrangement, but successively reduced in length. The mesolophid has a ~60° mesial angle to the longitudinal axis. Synclines I, II and III are similar in shape and orientation. Syncline IV is short, but longer than syncline Ia, slightly curved backward. M 2 has two roots.

D i s c u s s i o n. The described specimens are referred to the genus Typhlomys , due to the lophodont molars with concave grinding surface, six diagonal/transverse ridges and five synclines, mid-ridges/synclines less inclined to the longitudinal axis, developed endoloph in the upper molars and continuous endolophid in the lower molars. In these characters, they resemble Typhlomys , but differ from Neocometes and Platacanthomys .

The huge dimensions ( Text-fig. 6 ) of these specimens are striking, even larger than those of Typhlomys macrourus , the currently largest known form of the genus from Longgupo of Wushan, Chongqing ( Zheng 1993). The relative anterior extension of M 1 is also unique in Typhlomys . In addition, the specimens possess morphological features different from all known species of Typhlomys , for example, the presence of continuous endoloph(id)s and ectoloph(id)s, and the lack of buccal opening of synclines II and III. The four molars from Leilao obviously represent a new platacanthomyid. Therefore, it is assigned to a new species of Typhlomys , although the material is rather scarce.

A possibility of the species described here belonging to a new genus of the Platacanthomyidae cannot be precluded, due to the huge size, the presence of continuous endoloph(id)s and ectoloph(id)s, the distinct extension of anterior portion on M 1, and the lack of buccal opening of synclines II and I in molars. Nevertheless, it is considered inadvisable to create a new genus in view of the inadequate material.

Figures

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.

Text-fig. 8. Typhlomys storchi n. sp. from Leilao, Yuanmou. a) left M1 (IVPP V 25887.1, paratype), b) right M3 (IVPP V 25887.2, reversed), c) right M1 (IVPP V 25886, holotype, reversed), d) right M2 (IVPP V 25887.3, reversed).