The Quanjishan Charnia assemblage from the northern Qaidam Basin, Tibetan Plateau, and implications for palaeoecology and taphonomy of Ediacaran fronds

Abstract

The late Ediacaran Quanjishan macrofossil assemblage of northwest China is one of the oldest fossil assemblages yet discovered in the Tibetan Plateau. It contains numerous specimens of the iconic and long-ranging soft-bodied rangeomorph Charnia, and the terminal Ediacaran candidate index fossil Shaanxilithes, preserved in siliciclastic rocks of the Zhoujieshan Formation from the Quanjishan area, northern Qaidam Basin, Tibetan Plateau. Here, we present a detailed description of Charnia fossils recently excavated from the Quanjishan assemblage, including specimens of Charnia masoni and Charnia gracilis, which are distinguished by the shape of their first-order branches and their divergence angle from the axial midline. Most Quanjishan Charnia specimens are small individuals (some are left in open nomenclature), some of which are comparable to juvenile fronds from other global Ediacaran localities, and are interpreted here as juvenile specimens. Sedimentary characteristics suggest that the Quanjishan Charnia lived in marine depositional environments somewhere between shoreface and offshore transition, and under low-to-moderate energy hydrodynamic conditions, which supports the suggestion that Charnia preferred calmer habitats. Three-dimensional preservation exhibiting both surfaces of the frondose fossils is recognized in the Quanjishan assemblage, providing a new taphonomic window into three-dimensional preservation of Ediacaran soft-bodied organisms. The Quanjishan assemblage extends the palaeogeographic distribution of Charnia as well as rangeomorphs, and emphasizes that Charnia is one of the longest-ranging genera among the Ediacaran macrobiota.