Late Ediacaran cavity-dwelling filamentous microorganisms accommodated in a valve-like organism from the uppermost Dengying Formation in eastern Yunnan of South China

Abstract

The Ediacaran–Cambrian transition, one of the major turning points in Earth’s history, is characterized by an epoch-making change in biosphere. In South China, the late Ediacaran Dengying Formation (ca. 551–538 Ma) is an important stratigraphic unit yielding trace fossils, tubular skeletal fossils and some complex forms that represent possible bilateral animals. The fossil-bearing Ediacaran strata of eastern Yunnan is hitherto known from the Jiucheng Member containing the Jiangchuan Biota, together with the enigmatic cosmopolitan taxon Shaanxilithes. Here, we present for the first time cavity-dwelling filamentous microorganisms accommodated within a valve-like fossil from the upper Baiyanshao Member of the Dengying Formation at the Sujiawa Section in Huize County, eastern Yunnan of China. SEM scanning and Micro-CT reconstruction indicate that the well-preserved filaments were characterized by mycelial network structures with abundant branching pattern. Preservation of filamentous microorganisms might result from the decay process of soft parts that were originally accommodated and enclosed within the valve-like organism, and subsequently preserved by early diagenetic phosphatization, which obscures their original organic identity and chemical composition. This paper potentially presents the first fossil evidence of a saprophytic relationship between microorganisms as decomposers and a valve-like organism prior to the eve of the Cambrian Explosion of metazoan animals.