A new genus and species of zhelestid mammal has been identified from the fossilized remains found in the Bayanshiree Formation in Mongolia’s Gobi Desert.
Life reconstruction of Ravjaa ishiii (foreground), depicted on the foot of the hadrosaurid dinosaur Gobihadros. Image credit: Kohei Futaka.
Named Ravjaa ishiii, the new species roamed Earth during the Cretaceous period, around 90 million years ago.
The ancient mammal was the size of a mouse and belonged to Zhelestidae, an eutherian mammal family widely distributed from Eurasia to North America in the Late Cretaceous.
“Numerous exquisitely preserved mammal fossils unearthed from Late Cretaceous layers in the Gobi Desert of Mongolia have played a key role in understanding Mesozoic mammalian evolution,” said Okayama University of Science doctoral candidate Tsukasa Okoshi and colleagues.
“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.”
The new zhelestid fossil was found in 2019 at the Bayan Shiree locality of the Bayanshiree Formation.
The specimen is a 1-cm-long partial lower jaw with the distal portion of an ultimate premolar and the first to the third molars.
“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,” said Okayama University of Science’s Professor Mototaka Saneyoshi.
“Its unusually tall molars and distinctive jaw shape differ from known relatives, and therefore we erected a new genus and species,” the paleontologists said.
“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.”
According to the team, Ravjaa ishiii is the first zhelestid from the Bayanshiree Formation and in fact the first discovered in Mongolia.
“Ravjaa ishiii potentially represents the oldest member among zhelestids or as old as the currently known oldest zhelestids collected in Uzbekistan, suggesting the emergence of this group occurred around the Early/Late Cretaceous boundary to early Late Cretaceous,” the researchers said.
Their paper was published online in the journal Acta Palaeontologica Polonica.
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Tsukasa Okoshi et al. 2025. New Late Cretaceous zhelestid mammal from the Bayanshiree Formation, Mongolia. Acta Palaeontologica Polonica 70 (1): 193-203; doi: 10.4202/app.01213.2024
