The Katian (Late Ordovician) represents a critical phase of the Great Ordovician Biodiversification Event (GOBE), and was marked by the diversification and pronounced provincialism of brachiopods across the tropical and temporal Proto-Tethys. Taxonomic revisions of brachiopod assemblages from Mongolia and Russia remain challenging because of restricted accessibility to historical fossil sites. However, modern analytical techniques offer promising avenues to extract new data from limited material. Here, we re-examine the rhynchonellide brachiopod Salairella latecostellata-a morphologically distinctive taxon with atrypide-like outline-from the Bayankhongor region of central Mongolia. Serial grinding of two topotypes reveals their internal morphology, facilitating a systematic revision of the genus. Salairella had a restricted distribution to what is now the Altai Mountains, Siberia, and Mongolia. This contrasts with the coeval shallow water Altaethyrella Fauna, which dominated the regions of modern China and Kazakhstan during the Katian. This palaeobiogeographical disparity infers a latitudinal gradient-driven brachiopod endemism within the Katian Proto-Tethys. The Altai Mountains region then emerges as an evolutionary hotspot and transitional zone for rhynchonellide brachiopods preceding the Late Ordovician Mass Extinction (LOME).Yuchen Zhang* [yuchenzhang@nigpas.ac.cn], State Key Laboratory of Palaeobiology and Petroleum Stratigraphy, Nanjing Institute of Geology and Palaeontology, Chinese Academy of Sciences, 39 East Beijing Road, Nanjing 210008, China; and Key Laboratory of Deep-time Geography and Environment Reconstruction and Applications of Ministry of Natural Resources, Chengdu University of Technology, Chengdu 610059, China; Tatiana A. Shcherbanenko [Shcherbanenkota@ipgg.sbras.ru], Trofimuk Institute of Petroleum Geology and Geophysics, Siberian Branch, Russian Academy of Sciences, Novosibirsk, 630090 Russia; Murun Bat-Orgil [murunb@mas.ac.mn], Institute of Paleontology, Mongolian Academy of Sciences, Ulaanbaatar 15160, Mongolia; Ya. Ariunchimeg [ariunyarinpil@gmail.com], Institute of Paleontology, Mongolian Academy of Sciences, Ulaanbaatar 15160, Mongolia.