A shallow-water trilobite fauna, with low diversity (seven species of six genera), from the Koumenzi Formation (Katian) in the Qilian area, northeastern Qinghai Province, Northwest China, is recognised as the Pliomerina Association. Of these, Dulanaspis and Amphilichas are reported for the first time in the North Qilian Mountains. The shallow-water Pliomerina and relatively deep-water BirmanitesSinocybele associations were found to occur in the Qilian and Menyuan areas, respectively, during the Katian. They exhibit a distinctive ecological differentiation with water depth from inner shelf to outer shelf environments. The Pliomerina Association of the Qilian area clearly belongs to the Pliomerina and/or Sinocybele Province of the Proto-Tethys Archipelagic Ocean. Faunal evidence indicates that the palaeogeographical position of the North Qilian Mountains area was situated closer to the Kazakh terranes during the Katian, particularly to the Chu-Ili and Chingiz-Tarbagatai terranes. (c) 2025 Elsevier B.V. and Nanjing Institute of Geology and Palaeontology, CAS. All rights are reserved, including those for text and data mining, AI training, and similar technologies.