Doushantuo-Pertatataka—Like Acritarchs From the Late Ediacaran Bocaina Formation (Corumbá Group, Brazil)

Abstract

Acritarchs, a polyphyletic group of acid-resistant organic-walled microfossils, dominate the eukaryotic microfossil record in the Proterozoic (2500–541 Ma) yet exhibit significant reduction in diversity and size at the transition to the Phanerozoic (541–520 Ma). Despite the difficulty of tracing phylogenetic relationships among acritarchs, changes in their complexity and diversity through time have allowed their use in paleoecological and biostratigraphic schemes. The Doushantuo-Pertatataka Ediacaran acritarch assemblage, for example, is usually considered as restricted to the early Ediacaran between 635 and 580 Ma. But similar, diverse acritarchs have been recovered from younger rocks in Mongolia and Arctic Siberia and are now reported here from phosphatized horizons of the upper Bocaina Formation (ca. 555 Ma), Corumbá Group, SW Brazil. In the overlying black limestones and shales of the latest Ediacaran Tamengo Formation (542 Ma) acritarch diversity is low, but the skeletal metazoans Cloudina and Corumbella are abundant. The Bocaina acritarch assemblage shares forms referable to the genera Leiosphaeridia, Tanarium, Asseserium and Megasphaera with the Doushantuo-Pertatataka assemblage, but also includes specimens similar to the Phanerozoic genus Archaeodiscina in addition to two new complex acritarchs. The first is covered by rounded low conical bumps, similar to Eotylotopalla but differs in having a distinct opening suggestive of greater (multicellular?) complexity. The second, identified here as Morphotype 1, is a double-walled acanthomorph acritarch with scattered cylindrical processes between the walls. The contrast in acritarch diversity and abundance between the Bocaina and Tamengo formations is likely due in part to paleoenvironmental and taphonomic differences (absence of the phosphatization window in the latter), as well as to the appearance of both suspension-feeding skeletal metazoans (Cloudina and Corumbella). The occurrence of Doushantuo-Pertatataka acritarchs in SW Brazil, northern Mongolia, and Arctic Siberia extend the biostratigraphic range of this assemblage up to the terminal Ediacaran Cloudina biozone.

Sedimentary evolution and sequence stratigraphy of Ediacaran high‐grade phosphorite–dolomite–shale successions of the Bocaina Formation (Corumbá Group), Central Brazil: Implications for the Neoproterozoic phosphogenic event

“…(2020) and Morais et al . (2021). See text for further details on sequence stratigraphy organization, classification and references.…”

“…Bocaina Formation sediments are characterized by a high diversity of dolostone‐dominated facies, including grainstones, mudstones and microbialites, as well as chert and shale levels (Boggiani, 1998). Additionally, phosphatic facies are also present in the middle–upper portions of the unit, occurring as brecciated clasts or irregular beds within dolomitic facies in the Corumbá area (Freitas et al ., 2011), and distinctively as discontinuous up to 10 m thick phosphate‐rich beds with up to 35% P 2 O 5 in the Bodoquena Range area (Luz et al ., 1980; Boggiani et al ., 1993; Boggiani, 1998; Justo, 2000; Morais et al ., 2021). Importantly, recent palaeontological evidence indicates that these sediments are highly fossiliferous, providing diversified acritarchs, including putative metazoan embryos associated with the Doushantuo–Pertatataka biota (Morais et al ., 2021).…”

“…These transitional deposits are key to understanding the environmental dynamics affecting the development of Ediacaran ecosystems, because the advance of phosphate mineralization processes towards deeper settings is held as direct evidence for progressive oxygenation of deep marine settings during the Ediacaran (Pufahl & Hiatt, 2012; Cui et al ., 2016; Zhang et al ., 2019). This event also seems to share a causal link with the diversification and preservation of important Ediacaran biotas such as acanthomorphic acritarchs, putative metazoan embryos and algae from the Doushantou–Pertatataka assemblages (Chen et al ., 2000; Xiao & Knoll, 2000; Xiao et al ., 2014; Muscente et al ., 2015; Anderson et al ., 2017; Cunningham et al ., 2017; Morais et al ., 2021), suggesting a direct relationship between phosphogenesis, increased biotic diversity and the opening of the Ediacaran–Cambrian phosphatic taphonomic window (cf. Porter, 2004; Papineau, 2010; Muscente et al ., 2015; Laakso et al ., 2020).…”

“…Ongoing phosphate exploration in Bocaina Formation strata has provided recent access to continuous drill core sections and freshly opened mine pit exposures, revealing a spectacular array of well‐preserved phosphatic sediments associated with a wide range of depositional settings. In addition, beyond the preservation of impressive phosphatic facies, recent palaeontological analysis held in this sample set has shown that these sediments preserve important Ediacaran Doushantuo–Pertataka biotas, featuring the occurrence of diversified acritarchs and putative metazoan embryos (Morais et al ., 2021), placing the unit as an important asset to Ediacaran palaeobiological and palaeoenvironmental investigation.…”

Sedimentary and tectonic breccias at the base of the Ediacaran Tamengo Formation (Corumbá Group): a comparative study

“…Gaucher et al (2003) described organic-walled microfossils in the Cerradinho Formation, which are consistent with an Ediacaran age. The Bocaina Formation marks the onset of carbonate sedimentation, with stromatolitic dolomudstone facies and phosphorite layers (Boggiani et al 1993, Fontaneta 2012, with a Doushantuo-Pertatataka-like acritarch fossil assemblage (Morais et al 2021). The Tamengo Formation comprises breccias, grainstones and wackestones interbedded with shales, with Cloudina and Corumbella skeletonized fossils (Hahn et al 1982, Hahn and Pflug 1985, Adorno et al 2017, Becker-Kerber et al 2017, Zaine and Fairchild 1985, acritarchs (Zaine 1991, Gaucher et al 2003, macroalgae (Diniz et al 2021) and vendotaenids (Becker-Kerber et al 2021).…”