EDIACARAN EXOSKELETAL MACROFOSSILS AND TRACES: EXAMPLES FROM THE GAOJIASHAN SECTION (SHAANXI), NORTHERN YANGTZE PLATFORM

The Gaojiashan Member of the Middle Dengying Formation (Upper Ediacaran, ca. 551-542 Ma) represents a regional siliciclastic interval within a dolostone-dominated sequence on the late Proterozoic Yangtze Platform and thus provides a unique taphonomic and facies window into a late Ediacaran low-latitude shallow-water environment. It yields biomat-related trace fossil beds and unusual preservation of metazoan exoskeletons.

The Gaojiashan biota consist of up to 15 cm long, tube-shaped and three-dimensionally preserved exoskeletons of early metazoans (Gaojiashania and Conotubus). They mainly occur in large numbers in distinct, storm-related event beds. Some display taphonomic features of biomat-mediated embedding processes, forming “Ediacaran death masks”. Conotubus tubes frequently occur in situ, their apical terminations stacked vertically in the sediment, thus indicating a sessile, partly infaunal life style. The majority of the specimens underwent early diagenetic pyritization and are weathered to iron oxides. Organic preservation of tubes in the form of carbonaceous films on sediment casts are rare in some mud- and dolostone intervals. In some beds, both tubular macrofossils are associated with mm-sized phosphatised vase-shaped fossils and algae (Protolagena).

Trace fossils belong to the simple bedding-parallel Helminthoidichnites-type ichnoguild and occur in distinct layers within a dolostone bed above the body-fossil horizons. They are always closely related to microbial mats, indicating that the unknown trace originators had a biomat-related lifestyle, possibly as undermat miners.