A new silicified microfossil assemblage from the Ediacaran Dengying Formation in South Shaanxi, China

Abstract

A silicified fossil assemblage is newly recovered from the acid-macerated residuals of the siliceous dolomite in the late Ediacaran upper Dengying Formation, Tudimiao Section, Ningqiang, southern Shaanxi Province, China. It consists of a large number of tubular and spheroidal fossils. Of them, Cloudina and Sinotubulites can be well identified by the remaining “cone-in-cone” structure or transverse corrugations and longitudinal ridge and multi-layered outer wall, and several other tubular fossils of unknown affinities can also be recognized. However, most of the tubular fossils are generally preserved as steinkerns lacking detailed surface ornamentation and are difficult to distinguish. Morphologically, spheroidal fossils are highly diversified, comprising one-cell to various aggregated ones including two-cell, three-cell, four-cell, five-cell, or other irregular morphotypes. Therefore, three different preservational styles in the weakly biomineralized organisms may be presented in this area, referring to three tubular fossil assemblages in the Gaojiashan biota, i.e. from the lower to upper, a pyritized Conotubus-Gaojiashania in the siltstone-mudstone and calcified Cloudina in the limestone at the mid-upper Gaojiashan Member, Dengying Formation in the Gaojiashan section and at upper Gaojiashan and lower Beiwan members, Dengying Formation in the Shengchangba section, a phosphatized Cloudina-Sinotubulites association in the lower Beiwan Member, Dengying Formation in the Lijiagou section, and a silicified tubular and spheroidal fossil association at the mid-upper Beiwan Member, Dengying Formation in the Tudimiao section.This finding thus may have some broad implications, it not only opens a new taphonomic window for the late Ediacaran weakly biominerlizing organisms, encourages us to explore more extensively the once neglected cherty dolomite, but also considerably expands the geographical range and species richness of the terminal Ediacaran biosphere. More importantly, the finding also indicates that Cloudina having a worldwide distribution in different paleoecologies and biofacies has the greatest advantage of being the stratigraphic marker for correlation with precision to different paleocontinents.