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.

The M1 (Fig. 2A, B) is reniform in outline, with a slightly narrower anterior part. The anterocone is wide and bifid anteriorly. The lingual and buccal anterocones are nearly equal in size, both equipped with a posterior spur that joins together into the anterolophule posteriorly. The anterolophule is low, joining the protocone posteriorly. The protoloph I is absent in 102 of 106 specimens, and blurred in the others. The protoloph II is always present in the available specimens, though short and low. Most specimens lack a mesoloph, but a very short or remnant mesoloph is present in 12 of the observed specimens. The entoloph is marked, but short, connecting the hypocone to the yoke of the protocone posterior arm and the protoloph II. The posteroloph extends to the posterior base of the metacone. Among the 15 specimens from the lower lens (L-YHC) that can be observed, 14 have 3 roots and only one has 4 roots, while of 46 specimens from the upper lens (U-YHC) only 4 have 3 roots but 42 are 4-rooted. The M2 (Fig. 2C, D) is subrectangular slightly wider anteriorly than posteriorly. The anteroloph is prominent, extending to the lateral sides of protocone and paracone, respectively. There is a short but prominent anterolophule, connecting the middle of anteroloph and the junction of the anterior arm of the protocone and the protoloph I. Both the protoloph I and II exist, though the protoloph II is less developed. A short mesoloph can be seen in 3 or 4 cases in the teeth either from U-YHC and L-YHC, and remnant mesolophs touching the metacone are frequently present in the samples. The entoloph is short, connecting the hypocone to the yoke of the posterior arm of the protocone and the protoloph II. The posteroloph is developed, joining the posterior base of the metacone. Most specimens have 4 roots (2 lingual and 2 buccal roots), and a few are 3-rooted (one anteroposteriorly-elongated lingual root sometimes with a vertical groove along the lingual side of the tooth, plus 2 buccal roots). There are more 3-rooted M2s in the lower lens than in the upper. The M3 (Fig. 2E, F) is subtriangular in shape. It has similar structures in the anterior portion as in M2. The hypocone and metacone are reduced to a ridge-like cuspule, enclosing a small pit posteriorly with the posteroloph. There are 3 roots.

The m1 (Fig. 2G, H) is elongated and tapers anteriorly. The anteroconid is narrow and equipped with a distinct anterolophid. It is slightly bifid in most specimens, and usually has a cleft at its apex in fresh teeth. The anterolophulid is low, joining the anteroconid to the junction of the metalophid I and the anterior arm of the protoconid. The metalophid I is developed in all cases, but the metalophid II is lacking. There is no sign of a mesolophid. The hypolophid is poorly developed, anteriorly directed, and joins the ectolophid. The posterolophid is relatively strong, running from the hypoconid to the postero-lingual corner of the tooth but failing to enclose the posterosinusid. The ectolophid is curved, connecting the hypoconid and the protoconid. There are two roots. The m2 (Fig. 2I, J) is rectangular in occlusal surface. It is very similar to the m1 in general morphology, except in lacking an anteroconid and having a wider anterolophid buccally than lingually. In addition, a low but distinct mesolophid can be observed in 3 specimens from L-YHC and 2 from U-YHC. The m3 (Fig. 2K, L) is subtriangular, tapering posteriorly. It is like the m2 in morphology, but the hypoconid and entoconid are rather reduced, and the posterosinusid is enclosed.