Numerous exquisitely preserved mammal fossils unearthed from Upper Cretaceous strata in the Gobi Desert of Mongolia have played a key role in understanding Mesozoic mammalian evolution. These splendid mammal fossils have been recovered mainly from the Baruungoyot and Djadokhta formations, but only two fragmentary remains of mammals have been collected from the underlying Bayanshiree Formation. Here we report a new species of Zhelestidae, Ravjaa ishiii gen. et sp. nov., based on a new mammal specimen recently discovered from the Bayanshiree Formation at the Bayan Shiree locality. The new specimen is represented by a well-preserved partial right dentary with the distal portion of an ultimate premolar and the first to the third molars. The gross observation and the phylogenetic analyses demonstrate zhelestid affinities: the subequally tall protoconid and metaconid, closely approximated hypoconulid and entoconid. Zhelestidae was widely distributed from Eurasia to North America in the Late Cretaceous but has not previously been recovered in the abundant Late Cretaceous mammalian fauna of Mongolia. This species represents the first zhelestid from the Mongolian Upper Cretaceous strata and contributes to further understanding of their paleobiogeographic and ecological insights.

Key words: Mammalia, Eutheria, Cretaceous, Bayanshiree Formation, Mongolia.

Tsukasa Okoshi [r23ndd7ap@ous.jp; ORCID: https://orcid.org/0009-0001-4884-315X], Graduate School of Science and Engineering, Okayama University of Science, Okayama, Japan. Ryuji Takasaki [ryuji.takasaki@utoronto.ca; ORCID: https://orcid.org/0000-0002-5093-7681], Department of Ecology and Evolution, University of Toronto, Toronto, Canada. Kentaro Chiba [chiba@ous.ac.jp; ORCID: https://orcid.org/0000-0003-2613-6187], Mototaka Saneyoshi [saneyoshi@ big.ous.ac.jp; ORCID: https://orcid.org/0000-0002-4296-6896], Akio Takahashi [takahashi@big.ous.ac.jp; ORCID: https://orcid.org/0000-0002-2766-7775], Shoji Hayashi [hayashi@big.ous.ac.jp; ORCID: https://orcid.org/0000- 0002-6459-6515], Department of Biosphere-Geosphere Science, Okayama University of Science, Okayama, Japan. Masahito Natori [natori@zool.ous.ac.jp; ORCID: https://orcid.org/0009-0005-6742-5358], Department of Zoology, Okayama University of Science, Okayama, Japan. Shota Kodaira [kodaira_shota@city.nagasaki.lg.jp; ORCID: https://orcid.org/0009-0001-5866-1683], Nagasaki City Dinosaur Museum, Nagasaki, Japan. Shinobu Ishigaki [ishigaki@ous.ac.jp; ORCID: https://orcid.org/0009-0003-1945-6286], Museum of Dinosaur Research, Okayama University of Science, Japan. Buuvei Mainbayar [Mainbayarb@mas.ac.mn; ORCID: https://orcid.org/0000-0003-0608-2871] and Khishigjav Tsogtbaatar [tsogtbaatar@mas.ac.mn; ORCID: https://orcid.org/0000-0001-5604-4318], Institute of Paleontology, Mongolian Academy of Science, Ulaanbaatar, Mongolia.