MIDDLE ORDOVICIAN APORTHOPHYLA BRACHIOPOD FAUNA FROM THE ROOF OF THE WORLD, SOUTHERN TIBET

A Darriwilian (late Middle Ordovician) brachiopod fauna from the Lower Formation of the Chiatsun Group at Jiacun, northern Nyalam, southern Tibet, consists of ten brachiopod species, forming a distinct Aporthophyla-Paralenorthis Association. Its taxonomic composition is typical of the Aporthophyla Fauna that occupied lower BA2 to upper BA3 benthic environments on sandy lime mud substrates. The occurrence of Paralenorthis in southern Tibet is confirmed for the first time, represented by P.costata sp. nov. Numerical analyses (PCA and CA) of 18 Darriwilian brachiopod faunas from ten palaeoplates or terranes indicate that: (1) the Aporthophyla Fauna was confined to a specific latitudinal belt although it had a wide lateral distribution from the large palaeocontinents of Gondwana to Laurentia; (2) the Saucrorthis Fauna, a typical late Middle Ordovician regional fauna, is limited to a much smaller area, marginal to the Gondwana supercontinent; (3) the strong provincialism persistent in the late Middle Ordovician contributed to increased gamma biodiversity during the Great Ordovician Biodiversification Event.