Cricetids (Rodentia, Mammalia) from the Late Miocene Yihachi locality of Gansu, China

QIU Zhu-Ding, Lawrence J. FLYNN, WANG Ban-Yue, LI Lu

Vertebrata Palasiatica 2026, 64 (1): 1-25. DOI: 10.19615/j.cnki.2096-9899.251117

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Our purpose in this paper is to describe the hamster-like rodents (Cricetidae) from a Late Miocene age site in Linxia Basin, Gansu Province, and discuss their significance for the changing ecology of central Asia. The micromammal site known as Yihachi was introduced previously (Qiu et al., 2023; Qiu and Li, 2023), when its squirrels were discussed in some detail. We take this opportunity to describe the more abundant cricetids. There are four genera, common Nannocricetus and Sinocricetus plus the less abundant living Mesocricetus. A few specimens represent the high-crowned and lophodont Rhinocerodon. The cricetids and other faunal elements indicate an early Late Miocene age, and the pattern of occurrence of the hamster species is consistent with a picture of a changing paleoenvironment due to increasing effects of the East Asia monsoon system. After the disappearance of older and archaic genera, Yihachi represents growing endemism in the Late Miocene of northern China due to increasing seasonal rain and the declining average temperature.

Fig. 7 Molars of Mesocricetus fengi sp. nov. from Yihachi, Gansu
A. l M1 (IVPP V34189, holotype); B. r M1 (V34190.2); C. l M2 (V34190.3); D. l M2 (V34191.1, reversed);
E. l M3 (V34190.4); F. r M3 (V34190.5); G. l m1 (V34190.6); H. r m1 (V34190.7);
I. l m2 (V34190.8); J. r m2 (V34190.9); K. l m3 (V34190.10); L. r m3 (V34190.11), M. right mandibular fragment with m1-2 (V34190.1)
A-M. occlusal views; A1, G1, M1. lingual views; A2, G2, M2. buccal views. Scale bars: + for A-L, * for M

The M1 (Fig. 7A, B) is kidney-shaped in outline, with a slightly convex lingual edge and a concave buccal edge. The anterocone is large and equally bifid anteriorly. Both the lingual and the buccal anterocones have a posterior spur, and the buccal spur bends to join the lingual spur at its mid-way. The anterolophule is low and short but distinct, longitudinally extending from the protocone to the junction of the two spurs. An anterolophule spur is absent. The protoloph I is lacking in all the 4 observable specimens. The protoloph II and the metaloph I are always present, though short and low. A metaloph II is blurred. The posteroloph extends to the posterobuccal base of the metacone. The entoloph is short and zigzag-shaped, arranged in an X-form with the protoloph II and the metaloph I. The M2 (Fig. 7C, D) is rectangular with projecting posterior margin. The buccal anteroloph is prominent, connecting with the anterior arm of the protocone and ending in a bulge at the anterobuccal corner of the tooth, while the lingual anteroloph is of a residual trace in 3 specimens and absent in 2 cases. A protoloph I is absent, but a protoloph II and a metaloph I are always present. The entoloph is short, arranged in an X-form with the protoloph II and the metaloph I. The posteroloph is marked, extending from the hypocone to the posterior base of the metacone. Neither a protosinus nor a posterosinus is present. The M3 (Fig. 7E, F) is subtriangular. It is similar to the M2 in the anterior portion, having a pronounced buccal anteroloph, no or residual lingual anteroloph. The hypocone and metacone are reduced, but still conspicuous. The protoloph II connects either with the metaloph I or the hypocone.

The m1 (Fig. 7G, H) is elongated and tapers anteriorly. The anteroconid is variably split anteriorly and connected posteriorly. The anterolophid is poorly developed. The anterolophulid is short but conspicuous, joining the connection of anteroconid with the junction of the metalophid I and the anterior arm of the protoconid. The metalophid I is developed but the metalophid II is absent in all cases. The hypolophid (or anterior arm of entoconid) is anteriorly directed to meet the ectolophid. The ectolophid is short, joining the protoconid with the hypoconid. The protoconid, hypoconid, and entoconid converge on the center of the tooth via the posterior arm of the protoconid and the anterior arms of hypoconid and entoconid. The posterolophid is prominent, extending to the posterolingual corner of the tooth. The buccal portions of the mesosinusid and posterosinusid are distinctly shallower than the sinusid. The m2 (Fig. 7I, J) is very similar to the m1 without anteroconid. The buccal anterolophid extending from the metaconid is prominent and protrudes anterobuccally, while the lingual anterolophid is completely lacking. The anterior arm of the protoconid solely joins the anterolophid. The metalophid I and the anterior arm of the protoconid are separately connected to the anterolophid. A metalophid II is absent. The posterior arm of protoconid, the anterior arms of hypoconid, and the hypolophid I are converged on the central of the tooth. The posterolophid is prominent, extending and tapering to the base of entoconid posterolingually, and enclosing the posterosinusid. The m3 (Fig. 7K, L) is similar to the m2 in outline, except for its relatively larger length and narrower posterior part with a more curved posterior margin. It is similar to the m2 in morphology, with strong buccal anterolophid, absent lingual anterolophid, the matelophid I merged with anterolophid, and the anterior arm of the protoconid joining the anterolophid solely. The hypoconid and the entoconid are slightly reduced, but still marked. The development and arrangement of the crests in the middle part are variable, there is a pseudomesolophid present in 3 specimens, and a short crest enclosed the mesosinusid lingually in 5 cases.