Dipodidae (rodentia, Mammalia) from the Miocene of Damiao, Nei Mongol, China

The rich micromammal fossils from Early, Middle, and Late Miocene horizons of the Damiao sequence in Nei Mongol provide an opportunity to study Miocene biostratigraphy and faunal turnover in Asia. In this study, we describe thirteen dipodid species from the Damiao sequence, including two new species, Sinozapus damiaoensis sp. nov. and Lophocricetus parvus sp. nov. Sinozapus damiaoensis sp. nov. is characterized by the metaloph connecting to the central hypocone on M1-M2, the absence of the anterior arm of the protoconid, and the posterior arm of the protoconid merging with the metaconid on m2. Lophocricetus parvus sp. nov. displays a set of transitional morphological traits between Heterosminthus and Lophocricetus. It resembles Heterosminthus in its small size, the presence of the mesocone and mesoloph on M1-M2, the vestigial pseudomesolophid, and the posterior crest of the protoconid on m1. Meanwhile, it has the characteristics of Lophocricetus, including the protostyle on M1 and the hypoconid-entoconid connection on the m1. These two new species represent the earliest records of their respective genera. The Damiao dipodid fossil records improve our understanding of Neogene dipodid diversity, and refine the biostratigraphic framework in the central Nei Mongol. The new fossil records reveal a shift from small, low-crowned, humid-adapted early taxa to ecologically diverse late Middle Miocene forms, including desert-adapted jerboas, indicating a trend towards regional aridification. Nonetheless, relict sicistines and zapodines suggest localized persistence of humid refugia within broader dry environments during the late Middle Miocene, which sheltered pliopithecids in the Mongolia Plateau.