MICROMORPHIC MOLLUSCS FROM A CARBONIFEROUS BLACK SHALE, XINXU, PRC

We have collected a diverse micromorphic molluscan fauna from two localities in the upper part of the Luocheng Formation (Late Early Carboniferous) near Xinxu, Guangxi Province, Peoples Republic of China. The fauna occurs in black mudstones that have been interpreted previously as nearshore (possibly lagoonal) deposits on a broad carbonate platform. At one locality, the fauna is dominated both numerically and taxonomically by micromorphic gastropods, with lesser numbers of bivalves, echinoderms, brachiopods, cephalopods, and rostroconchs, most of which are much smaller than normal. Exquisite preservation characterizes this micromorphic fauna, which results in extremely well-preserved growth stages, especially in the molluscs. The large number of small fossils in most layers suggests alternating conditions between those that were favorable for reproduction and those that favored high mortality rates. At the second locality, numerous thin intervals are dominated by the tabulate coral Cladoconus, which is normal-sized, and formed layers a few centimeters thick across the sea floor. Adult specimens of trilobites, orthocone cephalopods, and echinoderms also are found at this locality, as well as micromorphic molluscs. At this locality, conditions apparently remained more favorable for normal-sized benthic fauna. The Xinxu localities are similar to localities previously reported from the Visean of Arkansas, both in terms of paleoecology and exquisite preservation of earliest growth stages of the gastropods and cephalopods. Comparable micromorphic molluscan faunas are known from other Late Paleozoic intervals from apparently similar and dissimilar (e.g., oolitic grainstones) facies.