Taphonomy of Ediacaran Acritarchs From Australia: Significance for Taxonomy and Biostratigraphy

Abstract

A diverse assemblage of Australian Ediacaran (late Neoproterozoic) acritarchs from the Centralian Superbasin and Adelaide Rift Complex demonstrates a range of taphonomic degradation. Recognition of taphonomic variants is critical for taxonomic studies and biostratigraphic interpretation. Taphonomic features observed include compression features, folding and tearing of vesicle walls, pitting, perforation, abrasion, exfoliation, shrinking, twisting, splitting, curling, shredding, pyritization, particle entrapment, and thermal maturation effects. The physical and chemical structure of the vesicle wall is instrumental in determining the degree of taphonomic damage. Consistent associations allow identification of degradation series that incorporate previously described individual species and provide a framework for taxonomic revision. Taphonomic associations may also characterize taphofacies, providing an additional tool for basin analysis.

A Mineralized Alga and Acritarch Dominated Microbiota from the Tully Formation (Givetian) of Pennsylvania, USA

“…Rarely, as in Figure 9D, ovoid framboids can be found attached to the outer surface of a sphaeromorph. These are essentially identical to pyrite structures described by Grey and Willman [57] as bacterial infestation scars on the body wall in their Neoproterozoic acritarchs. Figure 9E shows an acanthomorphic specimen with a fine-grained outer covering, either incompletely formed, or partially ripped open.…”

“…The specimen in Figure 6D retains an outer halo of sparry calcite crystals, but most of its interior is filled with a solid mass of pyrite. It closely resembles the pyritized Ediacaran acritarch illustrated by Grey and Willman [57] ( Figure 4A), or the Devonian specimen of Tasmanites sinuosus illustrated by Winslow [56] (Plate 20, Figure 3). Only near the white arrows in Figure 6D is the body wall visible.…”

“…A illustrated by Winslow [56] which also contains pyrite globules in its interior [56] (Plate 20, Figure 7). Following Grey and Willman [57] such clusters may represent infestations of microbes concentrated on cell degradation products within the vesicle, including sub-cellular organelles and reproductive spores as suggested by Kaźmierczak and Kremer [26]. The specimen in Figure 6D retains an outer halo of sparry calcite crystals, but most of its interior is filled with a solid mass of pyrite.…”

“…Thus, muellerisphaerids are not a separate group of originally phosphatic spherical microorganisms, as conceived by Kozur [12]; they are actually originally organic-walled microorganisms that have been preserved through a process of rapid, post-mortem phosphatization [2,18]. Second, it is now clear that subcellular structures, which are often useful in classifying microscopic eukaryotes, can be preserved via a variety of different post-mortem preservation pathways including silicification [27] and pyritization [26,27,57]. Thus, some acritarchs with preserved internal bodies can be recognized as the sporulating vegetative cells of unicellular chlorococcalean green algae [26,27].…”

“…Organic walled microorganisms, like large organisms, are subject to post-mortem degradation that can modify, or obliterate, characters that are taxonomically significant [57] (and references therein), and thus hinder attempts to properly classify them. The Tully material has clearly undergone considerable post-mortem alteration.…”