Platacanthomys dianensis QIU , 1989

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 : 385-386

Platacanthomys dianensis QIU, 1989

Text-figs 3 , 4

H o l o t y p e. Left M 1 (IVPP V 8816).

T y p e l o c a l i t y. Shihuiba, Lufeng, Yunnan Province,

China.

R e f e r r e d m a t e r i a l. Yuanmou: 2 M 3, 1 M 2, 1 M 3 ( IVPP V 25883.1 – 4 ) from Loc. 9903; 1 M 3, 1 M 3 ( IVPP V 25883.5 – 6 ) from Loc. 9904; 2 M 1, 3 M 2, 2 M 3, 2 M 1 ( IVPP V 25883.7 – 15 ) from Loc. 9905; 10 M 1, 6 M 2, 9 M 3, 16 M 1, 5 M 2, 10 M 3 ( IVPP V 25883.16 – 71 ) from Loc. 9906 .

M e a s u r e m e n t s. See Tab. 1.

D e s c r i p t i o n. These teeth are unilaterally hypsodont and typically lophodont, with flat grinding surface, but with neither an anterior extra ridge nor a syncline Ia on the upper molars. The mid-ridges/synclines are distinctly inclined (with a mesial angle to the longitudinal axis being generally 55° to 70° on the upper molars and 45° to 60° on the lower molars). Synclines I, II and III on the upper molars, and synclines II, III and IV on the lower molars are buccally open. The endoloph is developed, but is interrupted by syncline II, while the endolophid is continuous. The upper molars have 3 roots each, and the lower molars are two-rooted.

The M 1 is longer than wide, with nearly parallel lateral margins. The anteroloph is slightly anteriorly curved, strong, but short. The protoloph and mesoloph are long, prominent and parallel in arrangement, being oblique lingually, and then turning transversely at the midline of the tooth. The metaloph is connected to the anterior part of hypocone. The straight posteroloph is relatively transverse, connecting with the metacone to enclose syncline IV in 4 of 9 specimens . Syncline I is the longest and deepest syncline of the tooth. Two out of 12 specimens show a connection between the protocone and hypocone, with completely lingually enclosed syncline II in one specimen .

The M 2 is similar to M 1 in outline and structure, but the anteroloph is long and straight. Syncline I is usually closed buccally in early stages of wear. Syncline II is lingually closed in one of 9 specimens. In 2 from 8 M 2 s, syncline IV is buccally closed in little-worn specimens .

The M 3 is subtriangular, with relatively transverse ridges and synclines. The posterior part of the tooth is reduced, showing narrow ridges and synclines, or incomplete ridges and synclines in some specimens. The endoloph is generally continuous, and syncline I is usually closed buccally. Among the 14 M 3 s, 6 have a weakly developed longitudinal connection between the ridges.

The M 1 is distinctly longer than wide, with the lateral margins being slightly convergent anteriorly. The anterolophid is short, weakly developed and anteriorly curved. The anterior extra ridge is the shortest diagonal ridge of the tooth, joining the anterolophid to enclose syncline Ia. The paraconidmetaconid connection, mesolophid and hypolophid are marked, and oriented roughly parallel to the anterior extra ridge, only slightly increasing the mesial angle from the anterior extra ridge to the hypolophid. The posterolophid is transverse, connecting with the entoconid to enclose syncline IV lingually. Syncline Ia is small, oval-shaped, usually with a stylid or a crest attached to the anterolophid, and consequently the syncline is separated by a short ridge when the tooth is in an advanced stage of wear. Syncline I is laterally closed in all specimens. The endolophid is developed, usually with a notch between the metaconid and mesoconid, and between the mesoconid and entoconid in a fresh tooth, but continuous in the early stages of wear. A low longitudinal connection buccal to the midline of the tooth is sometimes present in synclines II, III and IV, on 9 out of 17 specimens .

The M 2 is similar to M 1 in structure, but distinctly shorter, and with straight anterior margin. The anterior extra ridge is very short, joining the endolophid and the anterolophid to enclose a small and oval-shaped syncline Ia situated in the anterolingual corner of the tooth. The endolophid is complete and continuous. Of 6 M 2 s, 5 exhibit variably developed longitudinal connections, usually present in syncline II or III.

The M 3 is similar to M 2 in morphology, except for its somewhat reduced posterior part and more curved posterior margin. A small syncline Ia is present or discernible in 9 out of 10 identifiable specimens. The hypolophid and syncline 4 are relatively short, but still prominent and complete. Some M 3 s show a tendency to develop the longitudinal connections.

D i s c u s s i o n. The Leilao specimens are characterized by relatively large size, the flat grinding surface, rather oblique mid-ridges/synclines, upper molars lacking anterior extra ridge and syncline Ia, but having buccally opened synclines I, II and III, the lower molars possessing buccally opened synclines II, III, IV and developed endolophid, and the less reduced M 3 and M 3, which fit the diagnosis of the genus Platacanthomys . The form represented by these specimens closely matches, both in morphology and dimensions, P. dianensis , the only fossil species of the genus, from type locality Shihuiba of Lufeng ( Text-fig. 4 ). Minor differences are the less developed longitudinal connections between the ridges buccally to the midline of the lower molars. The weak development of longitudinal connections seems to be suggestive of more advanced status of Platacanthomys , because of the frequent presence of the connections on M 1 and M 2 in the late Miocene P. dianensis of Lufeng, Yunnan, but absence in the specimens of extant P. lasiurus housed in the Natural History Museum, London (also see Fejfar and Kalthoff 1999: 195). These differences would imply that the Leilao population is younger than the Shihuiba one.

Figures

Text-fig. 3. Platacanthomys dianensis QIU, 1989 from Leilao, Yuanmou. a) left M1 (IVPP V 25883.16), b) right M1 (IVPP V 25882.17), c) left M2 (IVPP V 25883.18), d) right M2 (IVPP V 25883.7), e) left M3 (IVPP V 25883.19), f) right M3 (IVPP V 25883.20), g) left M1 (IVPP V 25883.21), h) right M1 (IVPP V 25883.8), i) left M2 (IVPP V 25883.1), j) right M2 (IVPP V 25883.22), k) left M3 (IVPP V 25883.23), l) right M2 (IVPP V 25883.2).

Text-fig. 4. Scatter diagrams (length and width) of Platacanthomys dianensis measurements from Leilao, Yuanmou in comparison with those of the species from Shihuiba, Lufeng.