Late Cretaceous Mammals from the Bayanshiree and Javkhlant Formations, Gobi Desert, Mongolia

Okoshi, T., Takahashi, A., Chiba, K., Takasaki, R., Nishimura, R., Saneyoshi, M., Buyantegsh, Ba., Mainbayar, Bu.,Tsogtbaatar, Kh.

The 6th International Symposium on Asian Dinosaurs in Japan 2025

Asian Dinoaur Association

Fukui, Japan

The Djadokhta and Baruungoyot formations of the Gobi Desert, Mongolia, have produced rich and exquisitely preserved Late Cretaceous fossils, yet the fossil record of the underlying Bayanshiree and Javkhlant formations remains scarce, particularly for small‑bodied vertebrates. This bias has obscured the understanding of faunal transitions in inland Asia during the Late Cretaceous. Since 2019, joint expeditions by the Institute of Paleontology, Mongolian Academy of Sciences, and Okayama University of Science have systematically targeted these formations. Here we present two recently discovered mammal specimens from the Bayanshiree and Javkhlant formations. The Bayanshiree mammal specimen was recovered from a layer that also yielded a diverse microvertebrate assemblage, including isolated remains of dinosaurs, crocodilians, turtles, amphibians, and fishes. The specimen represents a well-preserved partial right dentary with a partial ultimate premolar and the complete first to the third molars. The morphological features of the specimen, including the closely located hypoconulid and entoconid, suggest that it belongs to Zhelestidae, one of the major clades of non-placentarian Eutheria. This specimen has a unique set of features among known zhelestids, and thus, we recently erected a new taxon Ravjaa ishiii as the first zhelestid mammal from Mongolia. The Javkhlant mammal specimen is a partial left dentary experienced calcite infiltration and extensive fracturing, obscuring most of the anatomical features seen on the surface. However, micro X-ray computed tomography revealed that the specimen bears a large procumbent open-rooted incisor and several post-canine tooth roots, of which the first to third premolars preserved heavily damaged crowns. The characteristic incisor, together with its dentary morphology, suggests this specimen is referable to Zalambdalestidae, another major non-placentarian eutherian clade. Even with the new discoveries reported here, combined with published and unpublished reports of other teams, the mammal fossil record of the Bayanshiree and Javkhlant formations is highly limited compared to those of the overlying Djadokhta and Baruungoyot formations. However, it is worth noting that none of the mammals from the lower two fluvial-dominated formations are multituberculates, which are dominant mammals in the upper two aeolian-dominated formations. This putative mammalian faunal dissimilarity may be attributed to the depositional and inferred paleoenvironmental differences of the formations. Alternatively, the faunal dissimilarities might also reflect ecological competition between the eutherians and multituberculates since the herbivorous tendency in their diet has been suggested. The time range of these formations covers the early stage of the Cretaceous Terrestrial Revolution. The mammal fossils reported here underscore the importance of the microvertebrate assemblages in understanding the faunal transition in this particularly significant time bin for the reorganization of the terrestrial ecosystem in inland Asia.