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.

Comparisons and discussion The sizes of these molars are larger than that of Nannocricetus described above. Furthermore, there is some size difference between the specimens from the upper lens and the lower one, i.e., those from L-YHC being somewhat smaller on average, but falling toward the small end of the U-YHC (Fig. 6). In spite of the difference, the homogeneous tooth characters show that the Yihachi cricetine should be considered one species. The smaller sized molars from the lower lens may be indicative of more primitive status. The cricetine represented by the described material significantly differs from contemporary genera such as Nannocricetus, Kowalskia and Colloides. It exhibits typical characters of Sinocricetus including relatively larger size, higher crown, more robust cusp, deeply and widely bifid M1 anterocone, an anterolophule spur present in some M1, variable development of mesoloph in upper molars, poorly developed mesolophid, and an obliquely directed metalophid and hypolophid in lower molars.

Sinocricetus is another cricetine endemic to northern China, and usually occurs in association with Nannocricetus and Kowalskia. Three species of the genus, S. zdanskyi, S. progressus and S. major, are known from the Late Miocene or Early Pliocene (Schaub, 1930; Wu, 1991; Qiu and Storch, 2000; Li, 2010). The Yihachi Sinocricetus is close to S. progressus in size, but slightly smaller than S. zdanskyi, and much smaller than S. major (Fig. 6). In morphology, it differs from S. zdanskyi, the type species known from the late Late Miocene of central Nei Mongol, in having lower crowned molars with less robust cusps, wider valleys, poorly developed mesolophs on upper molars, less separated anterocone(id) on M1 and m1, less frequent presence of anterolophule spur on M1, lower and weaker anterolophulid on m1, indistinct mesolophid on lower molars, and the M1 more commonly having 3 roots. The few S. major found in the Pliocene are easy to distinguish from the Yihachi form by their huge size, robust cusps, deeply divided anterocone on M1, the metaloph II on M2, distinctly separated anteroconid on m1, and by the developed mesolophid in lower molars. The Yihachi Sinocricetus is close to S. progressus not only in size, but also morphologically, such as the lower crowned molars with wider valleys, less inflated and separated anterocone on M1, weak anterolophul(id) on M1 or m1, and a more reduced mesoloph(id). Nevertheless, comparing the rich specimens from Yihachi with those of Bilike and Gaotege, a new species must be recognized. The new species differs from S. progressus in the absence of protoloph I from the majority of M1, having a poorly developed protoloth I on M2, less separated anteroconid and even occasionally two connections between the anteroconid and the two anterior main cuspids on m1, and most M1 and M2 with 3 roots. In addition, the published specimens (IVPP V15465.1-13) assigned to an indeterminate species of Sinocricetus from the early Late Miocene in Qinghai (Qiu and Li, 2008) are identical with the Yihachi materials both in size and morphology, and should be referred to this new species.