Katian (Late Ordovician) trilobites of the North Qilian Mountains and their palaeogeographical implications for the Proto-Tethys Archipelagic Ocean (PTAO)

Trilobites from the middle Koumenzi Formation (Katian, Upper Ordovician) of the North Qilian Mountains, Menyuan, northeastern Qinghai Province are systematically documented for the first time. The fauna consists of five families, seven genera and seven species, amongst which one is new (Remopleurides zhangi sp. nov.), showing a close relationship to those of the Kazakh terranes (such as Chu-Ili terrane, Chingiz-Tarbagatai area and KNNTS (Karatau-Naryn and North-Tien Shan Microcontinents)), North China and Laurentia palaeoplates during the Katian (Late Ordovician). The cluster and Non-metric Multidimensional Scaling analyses of the Middle–Late Ordovician (late Darriwilian–Katian) trilobite faunas with 299 genera or subgenera from 46 horizons of 37 areas, provide valuable information for the palaeogeographical reconstruction of the Proto-Tethys Archipelagic Ocean (PTAO) of this interval. The Qilian terrane and adjacent areas are essential components of the PTAO, some of which include the Qilian terrane (QT), the North Qilian Mountains area (NQ), the Altun faulted terrane (AFT), the Hexi Corridor area (HX) and the East Qinling terrane (EQT). Their relative positions within the PTAO are inferred by the palaeobiogeography of trilobite faunas. Based on further discussions on the spatiotemporal distribution of those faunas, the Pliomerina and/or Sinocybele Province of the Middle–Late Ordovician (late Darriwilian–Katian) age is defined as a trilobite faunal province of the PTAO. Moreover, a distinct faunal subprovince, essentially comprised of the South China Palaeoplate and its neighbours (e.g. Tarim, Annamia, Sibuma, East Qinling, Turkestan-Alai and probably Talesh), might be surrounded by the equatorial cold-water tongue.