Discovery of an ancient Himalayan birch mice lineage illuminates the evolution of the family Sicistidae (Rodentia: Dipodoidea) with descriptions of a new genus and two new species

Birch mice (family Sicistidae) inhabit regions surrounding the Qinghai–Xizang Plateau and extend throughout the Palearctic. In China, <i>Sicista</i> are rarely recorded, and their systematics remain poorly understood. During an examination of specimens collected from southern Xizang and the western Tianshan Mountains as part of the Second Xizang Plateau Expedition by the Kunming Institute of Zoology, Chinese Academy of Sciences, we identified two specimens from southern Xizang with a unique phylogenetic position within Sicistidae. To clarify their taxonomic and evolutionary status, we conducted morphological comparisons and phylogenetic analyses of both extant and fossil Sicistidae, along with total-evidence dating and ancestral distribution reconstructions. Our results indicate that these two specimens form an ancient extant lineage within Sicistidae that diverged from its sister group (<i>Sicista</i>) approximately 20.38 million years ago. We recognize this lineage as a new genus, defined by a new species, <i>Breviforamen shannanensis</i> <b>gen. <i></b>et<b></i> sp. nov.</b> Furthermore, eleven specimens from the Tianshan Mountains are described as a second new species, <i>Sicista brevicauda</i> <b>sp. nov.</b> Ancestral distribution reconstructions combined with fossil records suggest that Sicistidae originated in a region spanning the Gobi Desert region to North America during the early Miocene. Subsequent climatic deterioration in the mid-Miocene and increasing desertification in Asia likely drove Breviforamen gen. nov. to migrate southward into southern Xizang, China, prior to the complete formation of the Yarlung Zangbo River. Overall, this study broadens our understanding of Sicistidae diversity, elucidates the family's origins and dispersal patterns, and highlights the presence of an ancient relict lineage in China.