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. 8A, B) is olive-shaped occlusally. The distinctly larger lingual main cusps have a curved margin lingually, while the buccal ones taper buccally. The anterocone is simple, narrow and crest-like. There are two anterolophules extending from the anterior arm of the protocone and the paracone (protoloph I), respectively. Both anterolophules enclose a long and longitudinal anterofossette with the anterocone, protocone, paracone and the weak protoloph II. The metaloph I is strong and broadly connects with the paracone. The entoloph is thick and strong, joining the paracone. The entoloph and the metaloph I enclose a broad and long posterofossette with the hypocone, metacone, and the weak metaloph II. The posteroloph is short but distinct. The sinus, anterosinus, and mesosinus are broad and deep, and the protosinus is distinct. There are two small enamel lakes in the central and the posterior part of the tooth. The M3 (Fig. 8C) is subrectangular. The anteroloph is pronounced, transverse and confluent with the protocone. There is no lingual anteroloph. The protoloph I is short and thin, joining the anteroloph and dividing the anterosinus into an enclosed anterofossette and an open buccal anterosinus. The entoloph is strong, connecting the hypocone with the paracone, and arranged in an X-form with the protoloph II and the metaloph I. A posterofossette is present, enclosed by the anterior arms of hypocone and metacone and the strong posteroloph. The sinus and mesosinus are broad and deep, the anterosinus is distinct but the posterosinus is indistinct.

The m1 (Fig. 8D) is subrectangular in occlusal surface, with smaller lingual main cusps mesial to the buccal ones. The anteroconid is small and simple. It forms a “sickle-shaped structure” with the protoconid and the metaconid by the connections of the strong anterolophulid and the metalophid I. There is a short buccal branch of anterolophid. The metalophid II is thin and low, joining to the protoconid and closing a round anterofossettid with slight wear. The hypolophid (or anterior arm of entoconid) is thick and meets the ectolophid anteriorly. The ectolophid is prominent and connects the protoconid with the hypoconid. The hypoconid, entoconid, hypolophid and the prominent posterolophid form a “horse-shoe structure”. The sinusid, mesosinusid, and posterosinusid are broad and deep, and the protosinusid is distinct. There is no buccal anterior arm of hypoconid, but an anterobuccal tubercle at the side of the hypoconid. The m2 (Fig. 8E) is rectangular. The anterolophid is marked, extending from the metaconid to the anterobuccal corner of the tooth and joining the base of protoconid. There is no lingual anterolophid. The anterior arm of the protoconid (anterolophulid) is short and joins the anterolophid. A metalophid II is absent, but a spur is present behind the metaconid. The entoconid connects the protoconid to the short hypolophid. The ectolophid joins the junction of the posterior arms of protoconid and hypolophid. The posterolophid is distinct, extending from the hypoconid and tapering to the base of entoconid posterolingually, and closing the posterosinusid. The m2 has two roots.