ORIGIN,DISPERSAL AND BIOGEOGRAPHIC AFFINITY OF THE MIDDLE-LATE ORDOVICIAN AND THE LLANDOVERY RUGOSE CORALS IN THE YANGTZE REGION
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Abstract:
The Middle Ordovician Llandovery rugose corals are abundant in the Yangtze region, especially in the Early Silurian. Altogether 123 genera of Rugosa,which contain 4 genera of Middle Ordovician,25 genera of Late Ordovician,and 94 genera of Llandovery respectively,have been reported from this region, of which 30 genera first appeared in the Yangtze region and then dispersed to Europe and North America and other adjacent regions. The following may represent the earliest occurrence age of the referred genera. Calostylis in the Middle Ordovician(Llandeilo)of southern Sichuan; Aphyllum and Cantrillia in the middle Ashgill of western Zhejiang; the streptelasmatids Briantelasma, Pycnactis and Tunguselasma in the late Ruddanian of northeastern Guizhou; the columnariids Ceriaster, Stauria, Amplexoides and Synamplexoides , and the cystiphyllids Maikottia, Rhizophyllum among others in the Middle Llandovery of northeastern Guizhou, South China. Based on these data, We may regard that the Yangtze region may have been one of the origin centers for the Ordovician and Silurian rugose corals. This paper deals with the Middle Ordovician to Llandovery rugose coral faunas in the Yangtze region, especially with their palaeobiogeographic affinities. The Middle Ordovician Rugosa of this region is characterized by the calostylids Calostylis and Yohophyllum . The Late Ordovician (mid Ashgill) rugosan fauna from the Sanjushan Formation of western Zhejiang Province contains some Australian elements (Hillophyllum, Bowanophyllum) , while the late Ashgill rugose fauna from the Guanyinqiao Bed in the Upper Yangtze region shows a high degree of similarity to that of North Europe, indicating that Yangtze and Europe have a close palaeobiogeographic affinity with each other. The Llandovery rugose fauna in the Yangtze region is much closer with that of Siberia, Kazakhstan and Australia.
