I have published a post “Meriones Gerbils in Mongolia: taxonomy and tails” covering some notes and photos which I put together in the process of trying to identify some gerbils from SW Mongolia that had white under-tails. That post refers to two references on Meriones gerbils that post-date local field guides and are worth a look noting that “Midday jird Meriones meridianus sensu lato is a widely distributed and highly variable species complex, whose taxonomy is still controversial despite several genetic and morphological studies” (Nanova et al, 2020):
Nanova OG, Lebedev VS, Matrosova VA, Adiya A, Undrakhbayar E, Surov AV, Shenbrot GI (2020). Phylogeography, phylogeny, and taxonomical revision of the Midday jird (Meriones meridianus) species complex from Dzungaria. Journal of Zoological Systematics and Evolutionary Research 2020:00:1–24. https ://doi.org/10.1111/jzs.12372.
Nanova O (2014). Geographical variation in the cranial measurements of the midday jird Meriones meridianus (Rodentia: Muridae) and its taxonomic implications. Journal of Zoological Systematics and Evolutionary Research, 52(1):75–85. doi:10.1111/jzs.12032