A Cold-Water Brachiopod Fauna from the Upper Cisuralian (Lower Permian) Kharnuden Formation at the Hovsugol Section, Southeastern Mongolia: Systematic Description and Biogeographical Implications

This study presents a systematic and biostratigraphic analysis of a cold-water brachiopod fauna from the upper Cisuralian (lower Permian) Kharnuden Formation exposed at the Hovsugol section in southeastern Mongolia. The fauna comprises 40 species assigned to 28 genera, including three newly described taxa: Paramarginifera pentagona n. sp., Linoproductus planiconvexus n. sp., and Jilinmartinia hovsugolensisn. sp. Biostratigraphic evidence tentatively indicates a Kungurian age, with possible extension into the Artinskian. The assemblage likely originated along the southern margin of the Siberian Craton within the Paleo-Asian Ocean (PAO). The fauna shows strong biogeographic affinities with contemporaneous assemblages from the Beishan orogenic belt (northwestern China), Inner Mongolia, South Primorye (Russia), and selected Japanese terranes, but differs markedly from faunas of Transbaikal and central–northeastern Mongolia. These contrasts support the existence of the Sino-Mongolian Seaway, interpreted as a transitional marine corridor within the closing PAO. The assemblage is characterized by elements of the northern Boreal realm, dominated by cold-adapted and endemic taxa and lacking paleotropical elements. Paleoecological indicators suggest cool-temperate conditions at relatively high paleolatitudes combined with geographic isolation. These results provide new insights into Cisuralian faunal provincialism, marine connectivity, and the reorganization of ocean circulation during the final stages of Paleo-Asian Ocean closure.