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. 5A, B) is kidney-like in outline, with a slightly convex lingual edge and a concave buccal edge. The anterocone is wide and nearly equally bifid. The lingual and buccal anterocones are normally equipped with a posterior crest that joins into the anterolophule posteriorly. The anterolophule is low, extending from the protocone. A weak buccal anterolophule spur is present in 8 out of the 38 specimens. When the anterolophule spur is present, the buccal anterocone is connected to the spur by its posterior crest. The protoloph I is absent in 30 of the 36 specimens, and very low or blurred in the others. The protoloph II is always present, though short and low. All specimens have a low mesoloph, reaching the buccal border in 8 teeth, halfway in 11, and short or joining the metacone in the others. There is no metaloph I, nor a distinct metaloph II. The entoloph is short, connecting the hypocone to the yoke of the posterior arm of the protocone and the protoloph II. The posteroloph extends and merges into the posterobuccal base of the metacone. Out of the 20 observable teeth, 16 are 3-rooted, and 4 are 4-rooted. The M2 (Fig. 5C, D) is rectangular. The anteroloph is prominent, gentler buccally than lingually. There is a short but prominent anterolophule, connecting the middle of anteroloph with the junction of the anterior arm of the protocone and the protoloph I. Specimens have a protoloph I and protoloph II, though the latter is poorly developed in a few teeth. A mesoloph exists in all the teeth, but it is generally low and poorly developed, long in 3 cases, half-length in 4, and short or touching the metacone in 18. There is no metaloph I, but a very weak and low metaloph II is present in 9 out of the 26 specimens. The entoloph is short but marked, connecting the hypocone to the junction of the posterior arm of the protocone and the protoloph II. The posteroloph is distinct, extending from the hypocone and fusing into the posterior base of the metacone. Among 19 observable teeth, 13 have 3 roots (a vertical groove along the anteroposteriorly elongated lingual root in a few teeth), and 6 are 4-rooted. The M3 (Fig. 5E, F) is subtriangular. It is similar to the M2 in the anterior portion, but the posterior part is rather reduced, with smaller metacone than hypocone. There is no distinct mesoloph. The prominent metaloph I encloses an enamel pit with the posteroloph and the anterior arm of the hypocone. There are 3 roots.

The m1 (Fig. 5G, H) gently tapers anteriorly. The anteroconid is wide, slightly bifid in 2 out of the 19 specimens, incipiently twinned in 3, and single-cusped or crest-like in the remainder. A cleft at its apex can be seen only in unworn or very fresh teeth. The anterolophid is conspicuous, nearly extending to the mesiolateral bases of protoconid and metaconid, respectively. The anterolophulid is very weak, joining the anteroconid with 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 distinct mesolophid. The hypolophid is short and weak, anteriorly directed and joining the ectolophid. The posterolophid is relatively developed, extending from the hypoconid to the posterolingual base of the entoconid and enclosing the posterosinusid. The ectolophid is curved, connecting the hypoconid to the protoconid. The m2 (Fig. 5I, J) is rectangular in occlusal surface. The buccal anterolophid extending from the metaconid is strong and protrudes anterobuccally, while the lingual anterolophid is completely lacking. The metalophid I is fused with the anterolophid. The anterior arm of the protoconid joins the anterolophid. A metalophid II is absent. The posterolophid is prominent, extending and tapering to the base of entoconid posterolingually. The posterosinusid is enclosed lingually. The m3 (Fig. 5K, L) gently tapers posteriorly and has a curved posterior edge. It is similar to the m2 in morphology, except for the reduction of the posterior portion, especially the entoconid. A pseudomesolophid from the posterior arm of the protoconid is present in some specimens. The strong posterolophid encloses the posterosinusid with the hypoconid and entoconid.