90-Million-Year-Old Mammal Fossils Discovered in the Gobi Desert

Scientists have discovered a new genus and species of ZheLestid mammals from fossil sites in the Bien Series Formation located in the Gobi Desert, Mongolia.

This new species, named ravjaa ishiii, existed during the Cretaceous period approximately 90 million years ago.

The ancient mammals are mouse-sized; the ZheLestidae family of Eutherian mammals was widely found across Eurasia and North America in the late Cretaceous period.

“The remarkably preserved mammal fossils found in late Cretaceous deposits of the Gobi Desert have been crucial for understanding the evolution of Mesozoic mammals,” stated Tsukasa Okosi, a PhD candidate in Science at Okayama University, and a collaborator on the research.

“These extraordinary mammal fossils were primarily excavated from the Balungoyacht and Djadokhta formations, with only two fragmentary fossils obtained from the underlying Baianshiree formation.”

The new Zhelestid fossil was uncovered in 2019 in the Bayan Shiree area within the Bayanshiree Formation.

This specimen consists of the distal section of a premolar and part of a mandible, measuring 1 cm from the first to third molar.

“Discovering such a tiny fossil in the vastness of the Gobi Desert feels like a gift from nature. It’s truly miraculous,” remarked a professor from Okayama University of Science.

“Their exceptionally tall molars and unique jaw structures set them apart from known relatives, prompting us to establish a new genus and species,” explained the paleontologist.

“The robust nature of their molars is reminiscent of seed and fruit-eating mammals, which provides fascinating insights into how early mammalian species may have exploited plant resources.”

The research team asserts that ravjaa ishiii is the first Zhelestid identified from the Bien Series formation, indeed marking the first discovery of its kind in Mongolia.

“ravjaa ishiii may represent the oldest member of the Zhelestids, as the currently oldest known Zhelestids were found in Uzbekistan, indicating that this group’s emergence dates back to the early/late Cretaceous boundary,” the researchers noted.

Their study is published online in the journal Acta Palaeontologica Polonica.

____

Tsukasa Okoshi et al. 2025. A new Late Cretaceous Zhelestid mammal from the Mongolian Baiansilli formation. Acta Palaeontologica Polonica 70(1): 193-203; doi: 10.4202/app.01213.2024