Ordovician trilobites from the lower part of the Dawan Formation (Floian–Dapingian) at Huanghuachang, Yichang, Hubei, southern China

Abstract

Twelve trilobite species assigned to 11 genera are described from the Lower Unit of the Dawan Formation at the Huanghuachang section of Yichang, Hubei, southern China, which has been ratified by the International Union of Geological Sciences (IUGS) as the Global Stratotype Section and Point (GSSP) for the base of the Middle Ordovician Series and the Third Stage (Dapingian) of the Ordovician System. Of these trilobites, five species, i.e., Carolinites genacinaca genacinaca, Ovalocephalus eoprimitivus, Rhombampyx chinensis, Taihungshania tachengssuensis, and Niobe (Niobella) obscura n. sp., occur in the lowermost part of the Dawan Formation, and are restricted to the upper part of the Oepikodus communis conodont biozone to the lower part of the Oepikodus evae Biozone. Two species, Liomegalaspides taningensis and Pseudocalymene transversa, are distributed almost throughout the Lower Unit of the Dawan Formation; two taxa, Annamitella sp. and Mioptychopyge sp., are recorded from the middle part and upper part of the Oepikodus evae Biozone, respectively; and two other species, Rhombampyx yii and Aulacopleura (Paraaulacopleura) dawanensis, appear in the upper part of the Lower Unit of the Dawan Formation, which is referred to the Middle Ordovician Microzarkodina flabellum-Baltoniodus triangularis Biozone. Agerina parva has a shorter range, across the Lower–Middle Ordovician boundary. The fauna is dominated by Liomegalaspides taningensis, which is assigned to the Liomegalaspides Biofacies, and indicates an inner-shelf environment.For regional chronostratigraphic correlation, the occurrence of the middle–late Floian (approximately Corymbograptus deflexus Zone) forms Carolinites genacinaca genacinaca and Taihungshania tachengssuensis in the fauna is of great significance: the former, a pelagic trilobite species, occurs extensively in North America, East Siberia, Novaya Zemlya, and Spitsbergen; the latter is widely distributed in the Yangtze Region, occurring in Sichuan, Guizhou, Yunnan, southern Shaanxi, and eastern Anhui as well as in Hubei.The trilobites of the fauna are mostly typical of the South China Plate and eastern Peri-Gondwana, but the presence of Agerina and Niobe (Niobella) suggests faunal links with coeval Baltoscandian assemblages, and the occurrence of Rhombampyx is indicative of a Laurentian affinity.