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.

Fig. 3 Size ranges and averages of length and width in the first molars of various species of Nannocricetus from Yihachi, Gansu and other localities in China
Measurements of Lantian N. primitivus, N. wuae, Ertemte N. mongolicus and N. qiui are cited from Zhang et al., 2008, 2011; Wu, 1991, and Li et al., 2018, respectively
Numbers inside the parentheses are specimen numbers

Fig. 3 shows size ranges with number of specimens and the average of length and width of the first molars of Yihachi materials, with the type specimens of other known species. Obviously, the Yihachi Nannocricetus is closer to N. primitivus and N. wuae in size but smaller than the other species on average. Morphologically, it differs from N. mongolicus, the type species of the genus, in the M1 having frequent residual mesoloph(id), retention of the 3-rooted M1-2 in some cases, in M3 having more reduced posterior part, and m1 having narrower and less bifid anteroconid and less developed anterolophulid. Nannocricetus wuae from Damiao 02, early Late Miocene of Nei Mongol, is a relatively primitive species of the genus (Zhang et al., 2011; Kaakinen et al., 2015). It is characterized by the absence of a posterior spur on the less bifid anterocone of M1, the presence of a single-cusped anteroconid one m1 in all cases, the frequent of remnant mesoloph(id) on molars, and the 3-rooted upper molars. Nannocricetus qiui from Zanda, Xizang of Early Pliocene is a more derived species (Li et al., 2018). It differs from the Yihachi taxon in its larger size and higher crowned molars without mesoloph(id), in having a wider and more bifid anterocone on M1, a distinct posterosinus on M1-2, an elongated anteroconid portion on m1, more pronounced anterolophule(id) on M1 and m1, and a cuspidate posterolophid on m1-2. The Yihachi specimens, however, show a set of characters that correspond to N. primitivus including being close in size, a moderately bifid M1 anterocone and slightly bifid m1 anteroconid, a moderately developed anterolophule(id) on M1 or m1, presence of short or residual mesoloph(id) in some molars, and a 3 or 4-rooted M1-2, and thus are referred to the species.