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.
Highlights
Acritarchs represent a polyphyletic group of resistant organicwalled eukaryotic unicellular organisms (Evitt, 1963), fundamental to our understanding of early eukaryotic evolution and biostratigraphy (Knoll, 2003; Grey, 2005)
Ediacaran acritarch biozones were first established in Australia (Grey, 2005; Grey and Calver., 2007), where an older one dominated by simple spheroidal acritarchs is referred to as the Ediacaran Leiosphere Palynoflora (ELP) zone, and a younger one, characterized by ornamented, spheroidal microfossils, is called the Ediacaran Complex Acritarch-dominated Palynoflora (ECAP) zone (Grey, 2005; Grey and Calver, 2007; Gaucher and Sprechmann, 2009)
Within the ECAP zone, larger ornamented acanthomorphs ranging in size from 100 to 700 μm are collectively known as the Doushantuo-Pertatatakaassemblage (Zang and Walter 1992; Grey, 2005), originally recognized in the
Summary
Acritarchs represent a polyphyletic group of resistant organicwalled eukaryotic unicellular organisms (Evitt, 1963), fundamental to our understanding of early eukaryotic evolution and biostratigraphy (Knoll, 2003; Grey, 2005) They dominate the Proterozoic fossil record of plankton from 2500 to 541 Ma, having successfully endured the “Boring Billion” and survived Neoproterozoic icehouse intervals and bolide impacts (Grey et al, 2003; Huntley et al, 2006; Moczydłowska, 2008). Attempts to extend these zones to other regions have met with limited success, but, in general, complex large acanthomorphs are largely limited to strata older than the Gaskiers glaciation at about 580 Ma (see discussion in Xiao and Narbonne, 2020)
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