The new vertebrate microfossil assemblage from the Upper Cretaceous Bayn Shire Formation, the Gobi Desert, Mongolia

Tsukasa Okoshi;Hokuto Saito;Futo Kubo;Akio Takahashi;Mototaka Saneyoshi;Kentaro Chiba;Buuvei Mainbayar;Khishigjav Tsogtbaatar

Society of Vertebrate Paleontology 2022 annual meeting

Toronto, Canada

The fossil record of the Upper Cretaceous strata in Mongolia comprises an incredible array of small to large vertebrate fossils, representing a diverse ecosystem in this unique inland environment. Most of the exquisite vertebrate fossils come from the Djadokhta, Barun Goyot, and Nemegt formations, but those of the underlying Baynshire Formation are less abundant, especially with regards to small vertebrates. In this study, we report a new vertebrate microfossil assemblage that our group recently discovered from the Baynshire Formation to fill the significant gap in the fossil record of small vertebrates from this formation. The new assemblage was discovered at Bayn Shire, the type locality of the Baynshire Formation in the Gobi Desert, Mongolia. It was recovered from the lower fossiliferous beds of the outcrop, preserved in point bar deposits of a meandering river system. The majority of the assemblage is composed of dinosaur teeth (including tyrannosauroid, dromaeosaurine, sauropod, ankylosaurid and hadrosauroid) and indeterminate dinosaur bones and eggshells as well as crocodilian, turtle (including trionychid and adocid), and fish elements. Notably, this assemblage includes the remains of taxonomic groups not previously discovered from this formation: actinopterygian sinamiid dentaries, a squamate humerus, and a eutherian zhelestid dentary. The new taxonomic groups discovered from the assemblage contribute to a better understanding of the fauna in the Baynshire Formation and provide paleobiogeographic and evolutionary insights into these groups. Recent U/Pb dating on caliches in the Baynshire Formation suggests that this fossil assemblage represents the fauna during the Cretaceous Terrestrial Revolution (ca. 95 mya). It is hypothesized that the diversification of angiosperms drastically modified terrestrial ecosystems during this event, but little is known about how this event proceeded, partially due to the global scarcity of fossils during this time period. Further excavation and investigation of this assemblage will be an intriguing opportunity to document faunal changes during this important macroevolutionary event.