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 Yihachi specimens seem to represent a new species of Rhinocerodon, which are larger-sized (Fig. 9), and show different morphologies from those of other known species. The Yihachi Rhinocerodon differs from R. pauli from the Late Miocene Pavlodar Formation in Kazakhstan in its larger size, in cone(id)s less merged into loph(id)s, smaller-sized anterocone(id) of the first molars, M1 with stronger protoloph I, earlier appearance of anterofossette with wear, more anteriorly directed metaloph I joining the paracone, and the presence of enamel pits on the occlusal surface, in m1 having a distinct buccal branch of anterolophid, a stronger metalophid II, the earlier occurrence of an anterofossettid, no buccal arm of the hypoconid, and in m2 having a closed posterosinusid. It also can be distinguished from R. seletyensis from the Late Miocene Kedey Formation in Kazakhstan by its larger size, the cone(id)s incompletely incorporated in the loph(id)s, the more developed M1 protoloph I, the M1 metaloph I connecting to the paracone, and the absent buccal arm of hypoconid but present buccal branch of the anterolophid on m1. R. irtyshensis from the Rytovo Formation of the terminal Miocene or basal Pliocene in Kazakhstan is characterized by its distinctly narrow and long m1 having a closely placed protoconid, anteroconid, and metacone, a broad and deep mesosinusid, a shallow posterosinusid, and a small mesostylid and ectostylid. These are easily differentiated from the Yihachi form. The Yihachi Rhinocerodon is close to R. abagensis from the late Late Miocene Baogeda Ula Formation in Nei Mongol in size, but differs in having less compressed cone(id)s, narrower anterocone(id)s in the first molars, in M1 having a more developed protoloph I that encloses an anterofossette with the anterocone, protocone, and paracone in the early stage of wear, a distinct connection between the metacone and paracone, in m1 having a distinct buccal anterolophid, and in m2 having an enclosed posterosinuid. The morphological differences from all the Kazakhstan and Nei Mongol species are sufficient to describe R. linxiamys nov. sp. from Yihachi.