Distribution of Ediacaran acanthomorphic acritarchs in the lower Doushantuo Formation of the Yangtze Gorges area, South China: Evolutionary and stratigraphic implications

Abstract

As part of the most common eukaryotic fossils in early Ediacaran strata, acanthomorphic acritarchs are crucial for understanding the biostratigraphy and evolutionary dynamics of marine eukaryotes after the Marinoan global glaciation. Abundant and diverse acanthomorphic acritarchs have been reported from the Ediacaran Doushantuo Formation in the Yangtze Gorges area, South China, and different biozonation schemes have been proposed. However, these previously published biozones were based on biostratigraphic data compiled from different stratigraphic sections, with little effort to test whether these biozones can be consistently recognized at different sections, despite the fact that regional consistency is critical to a robust biostratigraphic framework. Also, the evolutionary pattern of the Doushantuo acritarchs has not been documented in detail, especially for acritarchs from the lowermost Doushantuo Formation, largely due to the perceived low taxonomic richness after the Marinoan global glaciation.In this study, we report silicified acanthomorphic acritarchs and other eukaryotic microfossils from chert nodules in the lower part (Member II) of the Doushantuo Formation at the Jiulongwan, Jinguadun, and Wuzhishan sections in the Yangtze Gorges area. From a total of 1547 acanthomorphic acritarch specimens found, 24 genera and 69 species have been identified, including two new genera (Annularidens gen. nov. and Crassimembrana gen. nov.), six new species (Annularidens inconditus gen. et sp. nov., Bispinosphaera vacua sp. nov., Crassimembrana crispans gen. et sp. nov., C. multitunica gen. et sp. nov., Distosphaera jinguadunensis sp. nov., and Mengeosphaera matryoshkaformis sp. nov.), five unnamed species (Annularidens sp., Mengeosphaera sp. 1, Mengeosphaera sp. 2, Sinosphaera sp., and Variomargosphaeridium sp.), and three possible new forms tentatively placed in open nomenclatures (Crassimembrana cf. C. crispans, Tanarium cf. T. conoideum, and Weissiella cf. W. grandistella). The high-resolution fossil occurrence and abundance data from the three sections reveal both spatial variations in acritarch composition, and stratigraphic similarities in acanthomorph range and abundance, indicating that acanthomorph taxa can be applied to biostratigraphic subdivision and correlation only if their stratigraphic records are thoroughly investigated at least at the regional scale. The FADs (first appearance data) of acanthomorph species such as Appendisphaera grandis, Appendisphaera heliaca n. comb., Ericiasphaera fibrilla, and Tianzhushania spinosa, as well as the relative abundance variations of certain taxa, are recognized as promising biostratigraphic markers in lower Doushantuo Formation. The new data also reveal a rapid diversification of marine eukaryotes, with taxonomic richness of acanthomorphs increasing to notable levels within probably ten million years after the termination of the Marinoan glaciation, followed by continuing increase in both taxonomic richness and evenness afterwards.