Fossil of a new mammal species from the age of dinosaurs discovered in Mongolia’s Gobi desert

A joint research team from Okayama University of Science (OUS) and the Institute of Paleontology and Geology, Mongolian Academy of Sciences (IPMAS) has discovered a fossil belonging to a previously unknown genus and species of mammal in Late Cretaceous (100–66 million-year-old) strata of Mongolia’s Gobi Desert. The mouse-sized animal was named Ravjaa ishiii, and the find was announced on April 4th at an OUS press conference. 
The species name honors Dulduityn Danzanravjaa, a revered 19th-century Buddhist monk, and the late Kenichi Ishii, former director of the Hayashibara Museum of Natural Sciences, who helped establish the Mongolia–OUS research partnership.
During the joint expedition in 2019 at the Baynshire Formation, the fossil-bearing deposit in the Gobi Desert, Mongolia, the team found a one-centimeter partial lower jaw. Analysis suggested the specimen is a member of Zhelestidae, a Cretaceous mammal family. Yet, its unusually tall molars and distinctive jaw shape differ from known relatives, and therefore the study erected a new genus and species. The discovery marks the first record of a zhelestid in Mongolia, showing the group also thrived far inland, not only along ancient coastlines as was previously inferred.
The suggested age of the Baynshire Formation corresponds to the early spread of angiosperms (flowering plants) in terrestrial ecosystems. The robust nature of the molars resembles those of seed  and fruit-eating mammals, providing an intriguing insight that early eutherians were already exploiting resources created by flowering plants. 
Tsukasa Okoshi (lead author, OUS doctoral candidate) said, “Due to the COVID-19 pandemic, the publication process took longer than expected, but we were finally able to establish the scientific importance of this specimen. We hope this research will serve as a starting point for further taxonomic studies of other small vertebrate fossils from the same site and era and will ultimately help uncover the rich biodiversity — including dinosaurs — that once inhabited the Gobi Desert during the age of dinosaurs.”
Professor Mototaka Saneyoshi (OUS) added, “Finding such a tiny fossil in the vast expanse of the Gobi Desert feels like a gift from the Gobi Desert. It’s nothing short of miraculous.”
Full results appear in Acta Palaeontologica Polonica 70 (1), where the paper was selected as the issue’s Editors’ Choice.

Tsukasa Okoshi, Ryuji Takasaki, Kentaro Chiba, Masahito Natori, Mototaka Saneyoshi, Akio Takahashi, Shota Kodaira, Shoji Hayashi, Shinobu Ishigaki, Buuvei Mainbayar, and Khishigjav Tsogtbaatar. 2025. New Late Cretaceous zhelestid mammal from the Bayanshiree Formation, Mongolia. Acta Palaeontologica Polonica 70 (1), 2025: 193-203 doi:10.4202/app.01213.2024