EARLY-MID ORDOVICIAN YANGTZEELLA (SYNTROPHIIDINA, BRACHIOPODA) AND ITS EVOLUTIONARY SIGNIFICANCE

Re-examination of newly collected topotype material confirms that the type species of Yangtzeella, Y. poloi, a widespread Early and Mid Ordovician syntrophiidine brachiopod in South China, has a finely costellate shell rather than being smooth as previously thought. Thus, the subgenus Yangtzeella (Vadimella) Nikitina et al., established on the basis of fine costellae, is invalidated. Among 15 species of Yangtzeella, five species are recognized as valid based on multivariate analyses: Y. poloi, Y. unsulcata, Y. songziensis, Y. kueiyangensis and Y. igori, among which the type species was the oldest known. Six are synonymized: Y. septata, Y. reticulata, Y. lensiformis, Y. depressa, Y. yichangensis and Y. minuta. Four are rejected from Yangtzeella: Y. extensa, Y. similior, Y. yohi and Y. poloi var. minor. Regional biostratigraphy indicates that Yangtzeella first appeared in a relatively deep-water setting on the Lower Yangtze Platform (South China palaeoplate) during late Tremadoc time (Scolopodus warendensis conodont biozone) and then expanded to the deeper Jiangnan Slope as well as to the shallower Upper Yangtze Platform. The genus experienced two episodes of heightened abundance and diversity on the Upper Yangtze Platform during late Dapingian and mid Darriwilian times, respectively. Outside South China, Yangtzeella occurs sporadically in a few microplates or terranes, such as Tarim, Chu-Ili (southern Kazakhstan) and Taurides (southern Turkey) during the Dapingian and Darriwilian. Worldwide, Yangtzeella became extinct by the end of the Darriwilian.