Middle Ordovician shallow-water gastropods from southern Xizang (Tibet), China

Many new specimens (36 in total) of gastropods were collected from the Ordovician Alai Formation of the Chiatsun Group at Jiacun village, Nyalam County, southern Xizang (Tibet). These gastropods comprise five species assigned to two genera, including Maclurites parviumbilicatus Yu, M. cf. nyalamensis Yu, M. cf. xizangensis Yu, M. cf. subconicus Yu, and “Hormotoma” ordosensis Yu, forming a Maclurites-“Hormotoma” association. All the species are described in detail herein, and some of which are redefined. This fauna is of middle to late Darriwilian age, and palaeoecologically characterised by species of shallow-water within the photic zone. A comparison of the Maclurites community with the contemporary gastropod fauna from the Zhuozishan Formation exposed along the west margin of the North China Block shows a close palaeogeographical affinity. The occurrences of Ordovician gastropods from China are analyzed biogeographically and the results reveal that the peri-Gondwana gastropod faunal turnover event was initiated during the Middle to Late Ordovician transition. The coincident faunal turnover and the lithofacies changes in the Himalaya and western margin of North China suggest that tectonic activities and relative sea-level changes may account for the biogeographical affinities of some specific fossil groups in peri-Gondwanan regions.