Microfauna of the Yagaan Khovil Fossil Locality: New Data on Upper Cretaceous Ostracods

The “Yagaan Khovil” locality, situated in the central region of Southern Mongolia, is a prolific Upper Cretaceous site yielding diverse paleontological remains, including vertebrate fossils, dinosaur eggshells, and footprints. In our previous research, the sedimentary characteristics and lithofacies were identified, indicating that the depositional setting closely resembles that of the Nemegt Formation (Maastrichtian) (Buyantegsh et al., 2023). The objective of the present study is to refine the paleoenvironmental reconstruction and stratigraphic positioning of this region. To achieve this, 13 microfaunal samples were collected from mudstone and sandstone layers to determine the ostracod assemblages. Taxonomic identification was performed based on carapace morphology, internal microstructures, and valve shapes. Preliminary results revealed a non-marine ostracod assemblage in four samples, including of four species belonging to three genera: Talicypridea reticulata, Talicypridea biformata, Candona altanulaensis, and Cypridea cavernosa. These taxa are widely distributed across the Upper Cretaceous continental deposits of the Nemegt Basin and Central and Northern China, serving as significant biostratigraphic indicators. Species of the genus Talicypridea, specifically T. reticulata and T. biformata, are considered to have originated during the Bayanshiree age (Cenomanian to Santonian), are absent during the Djadokhta age (Campanian), and subsequently diversified within the Barun Goyot and Nemegt formations (Khand, 2000). These species are widely distributed across Central Asia, including China (Ye et al., 2022). Notably, T. biformata has also been documented in the Maastrichtian deposits of India (Bhatia et al., 1996). Candona altanulaensis was first described from the Late Cretaceous Nemegt Basin at the Altan Uul IV locality (Szczechura and Blaszyk, 1970) and has been further identified in Nemegt-aged deposits at Bugin Tsav and Tsagaan Khushuu (Szczechura, 1978). This species was also reported from the Maastrichtian Lameta Formation in the Jabalpur region of India (Khosla and Sahni, 2000). Furthermore, Cypridea cavernosa has been defined at various Nemegt Basin localities, including Nogoon Tsav, Bugin Tsav, and Altan Uul IV (Szczechura, 1978), as well as in borehole samples from the Jiaozhou Formation in Shandong Province, East China (Du et al., 2021), and similar horizons in India (Bhatia et al., 1996). The identification of these four ostracod species across three genera provides crucial biostratigraphic evidence for the Maastrichtian age of the Late Cretaceous. Consequently, we propose that the geological age of the Yagaan Khovil locality be assigned to the Campanian-Maastrichtian interval.