Acanthomorphic acritarchs from the Ediacaran Doushantuo Formation at Zhangcunping in South China, with implications for the evolution of early Ediacaran eukaryotes

Abstract

Microfossil assemblages preserved in the Ediacaran Doushantuo Formation phosphorites (i.e., the Weng’an biota) and chert nodules in South China provide key evidence for the diversification of marine eukaryotes in the aftermath of Cryogenian global glaciation. Two biozones (i.e., the lower Tianzhushania spinosa biozone and the upper Hocosphaeridium anozos biozone) have been proposed on the basis of acanthomorphic acritarchs preserved in the Doushantuo Formation chert nodules in the Yangtze Gorges area. Strata containing the Weng’an biota at Weng’an have been correlated either with the upper acritarch biozone based mainly on chemostratigraphic data, or with the lower acritarch biozone according to the occurrence of Tianzhushania spinosa in both assemblages. These alternative correlations need to be resolved in order to improve our understanding of the evolutionary pattern of microscopic eukaryotes after the Marinoan glaciation. To address this problem, we carried out an integrated litho-, bio-, and chemostratigraphic study of the Doushantuo Formation in the Zhangcunping area, where the microfossil assemblage resembles the Weng’an biota at Weng’an in both taxonomic composition and biostratigraphic ranges. Our study supports the litho- and chemostratigraphic correlation of the Doushantuo Formation between the Zhangcunping and Yangtze Gorges areas. Thus, the Doushantuo Formation at Zhangcunping provides a link between the Weng’an and Yangtze Gorges areas. Together with published paleontological and stratigraphic data, the present study favors a correlation between the fossiliferous strata at Zhangcunping and the upper part of the T. spinosa biozone recognized in the Yangtze Gorges area. The strata yielding the Weng’an biota at Weng’an can be at least partially correlated with the T. spinosa biozone, supporting the idea that the Weng’an microfossil assemblage is biostratigraphically transitional between the lower and upper acanthomorphic assemblages. This correlation is consistent with the fact that several taxa that used to be regarded characteristic of the H. anozos biozone may actually range downsection into the T. spinosa biozone, thus supporting an earlier diversification of Doushantuo acanthomorphs. These results call for more detailed investigation on the taxonomic composition and biostratigraphic ranges of acanthomorphs in the T. spinosa biozone, and re-consideration of acanthomorphic biozonation of the Doushantuo Formation in the Yangtze Gorges area.