ONSHORE MIGRATION OF A DEEP-WATER BRACHIOPOD FAUNA FROM THE LOWER ORDOVICIAN TONGGAO FORMATION, JIANGNAN SLOPE, SOUTHEASTERN GUIZHOU PROVINCE, SOUTH CHINA
Brachiopods from the lower Tonggao Formation (early Arenig, late Early Ordovician) of Sandu, southeastern Guizhou Province, represent a deep-water benthic shell fauna from the Jiangnan Slope facies of the South China paleoplate. The fauna contains 16 brachiopod genera and species, with orthides, lingulates, and pentamerides being the common groups. Concurrent graptolites confine the brachiopod fauna to the earliest Arenig, within the Tetragraptus approximatus and the Acrograptus filiformis biozones. The first appearance datum (FAD) of several brachiopod genera, such as Paralenorthis, Protoskenidioides, Nereidella, and Progonambonites, in the Sandu slope facies is one or two graptolitic biozones lower than their FAD in shallower-water facies on the Yangtze Platform. Later in the Arenig, these genera became major components of the Sinorthis Fauna that flourished over much of the Yangtze Platform, suggesting a migration of the benthic shelly fauna from slope to platform facies during the Arenig biodiversification in South China. Compared to the platform brachiopod fauna, the deep-water brachiopod fauna of the Sandu area has a lower level of richness, diversity, and community organization. Three brachiopod associations are recognized: the Paralenorthis-Nereidella, the Palaeoglossa longa, and the Lingulella-Protoskenidioides associations. An upsection decrease in shell size, faunal richness, and species diversity in the Tonggao Formation indicates a deteriorating environment towards hypersalinity in the Sandu area, resulting in the ultimate disappearance of the deep-water brachiopod fauna.