Abstract
Birch mice (family Sicistidae) are small dipodoid rodents distributed in regions surrounding the Qinghai-Xizang Plateau and extending across the Palearctic. In China, members of the genus Sicista are rarely recorded, and their systematics remain poorly resolved. As part of the Second Xizang Plateau Expedition by the Kunming Institute of Zoology, Chinese Academy of Sciences, systematic surveys conducted in southern Xizang and the western Tianshan Mountains yielded two previously unrecognized species. Two specimens from southern Xizang were found to occupy a deeply divergent phylogenetic position within Sicistidae. Morphological assessments and molecular phylogenetic analyses of both extant and fossil Sicistidae, along with total-evidence dating and ancestral distribution reconstruction, identified these specimens as representatives of an ancient extant lineage that diverged from Sicista approximately 20.38 million years ago. This lineage is designated as a new genus, defined by the new species Breviforamen shannanensis gen. et sp. nov. Furthermore, 11 specimens from the Tianshan Mountains are described as a second new species, Sicista brevicauda sp. nov., based on diagnostic morphological and genetic features. Ancestral distribution reconstructions, combined with fossil records, indicate an early Miocene origin for Sicistidae across a broad region spanning the "Gobi" Desert to parts of North America. Climatic deterioration and increasing desertification during the mid-Miocene likely drove southward dispersal of Breviforamen gen. nov. into southern Xizang prior to the complete formation of the Yarlung Zangbo River. Overall, these findings broaden current understanding of Sicistidae diversity, elucidate the origin and dispersal patterns of the family, and highlight the presence of an ancient relict lineage in China.