THE OLDEST BRYOZOANS: NEW EVIDENCE FROM THELATE TREMADOCIAN (EARLY ORDOVICIAN) OF EAST YANGTZE GORGES IN CHINA

摘要

Previous reports of Cambrian bryozoans have proved not to be bryozoans. No pre-Ordovician bryozoans have been recognized. The oldest unequivocal bryozoans known from North America, Britain, and Russia are evidently of early Arenigian age. New bryozoans recently collected from the Fenxiang Formation in the Daping and Guanzhuangping sections, situated in the area east of the Yangtze Gorges, are described here, including one new genus, Orbiramus, and six new species, Nekhorosheviella nodulifera, N. semisphaerica, Orbiramus normalis, O. ovalis, O. minus, and Prophyllodictya prisca. These are assigned to the Trepostomida, apart from the last species which belongs to the Cryptostomida. The new bryozoans are from the conodont Paltodus deltifer deltifer Zone of the late Tremadocian age, the first three species possibly being present in the P. deltifer pristinus Subzone at the base. Therefore, they are the oldest bryozoans known from anywhere in the world. Extensive reefs resulting from a major regression in the late Tremadocian were dominated by bryozoans in the upper Fenxiang Formation. The bryozoans lived in a shoal environment and accumulated essentially in situ, showing no signs of significant transportation.