Diversity analysis of the Early Ordovician Sinorthis Fauna (Brachiopoda) from the Meitan Formation of Zunyi, northern Guizhou, South China

Abstract

The lower Meitan Formation (Floian, upper Lower Ordovician) at the Dajiaosi section, Zunyi District, northern Guizhou Province of South China, contains a moderately rich and diverse Sinorthis Fauna, with 22 species attributable separately to 15 families and seven orders of brachiopods. The fauna can be differentiated into three associations: the Paralenorthis serica, the Sinorthis typica, and the Tarfaya intercalare associations. These occupied a relatively wide palaeoecological range from lower BA3 to upper BA2 settings and from silty to clay substrate conditions. In South China, the Early Ordovician brachiopod radiation was marked by the diversification of the orthide-dominated Sinorthis Fauna, which first appeared and diversified in the middle part (relatively deep water) of the Upper Yangtze Platform in the Didymograptellus eobifidus Biozone, but rapidly declined in the succeeding Corymbograptus deflexus and basal Azygograptus suecicus biozones. During the latest Floian and early Dapingian, it expanded into comparatively shallow-water settings in the onshore direction (Changning area, southern Sichuan), offshore carbonate platform (Yichang, Hubei), and areas adjacent to the submergent Qianzhong Arch (such as the Zunyi area). The first appearance datum of the Sinorthis Fauna in shallower-water settings generally postdates that in the deeper-water environment in the central Upper Yangtze Platform, probably as a result of the fauna tracking a favoured BA3 setting during a gradual marine transgression.