Thursday, 11 July 2019

Acritarchs from the Ediacaran of the Camaquã Basin in southernmost Brazil.

Acritarchs are unicellular Eukaryotic organisms (organisms with cells with a discrete nucleus) that appear in the fossil record from about 3200 million years ago until the end of the Permian, and possibly later (depending on what is classified as an Acritarch). They're affinities are unclear, and the group is probably paraphyletic (not all members sharing a common ancestry), though the majority are thought to have been unicellular planktonic Algae or the resting cysts of other unicellular organisms. The  Camaquã Basin of southernmost Brazil contains a sequence of Ediacaran (635-541 million year old) rocks laid down in both marine and lucastrine (lake) environments. These deposits are unusual for sediments of this age in that they contain trace fossils (very uncommon in Ediacaran-age deposits), as well as the microbial mats which are more-or-less ubiquitous in Ediacaran strata. To date no body-fossils have been described from these rocks.

In a paper published in the journal Scientific Reports on 8 July 2019, Ilana Lehn, Rodrigo Scalise Horodyski and Paulo Sérgio Gomes Paim, of the Geology Graduate Program at the Universidade do Vale do Rio dos Sinos, describe the presence of Acritarchs in depostits from three distinct geological units within the Ediacaran of the Camaquã Basin in southernmost Brazil.

The Maricá Group comprises fine-grained sandstones laid down in a shallow marine environment. The sediments are bound together with calcite, with Acritarchs trapped within microbial mats thought to be of Bacterial origin. Four types of Acritarchs were found to be present in these deposits, Leiosphaeridia sp., Tanarium sp., Lophosphaeridium sp., and Germinosphaera sp., all of which are fairly typical of Ediacaran shallow-marine assemblages.

Well-sorted, fine-grained sandstone with hummocky cross stratification from the Maricá Group. Lehn et al. (2019).

The Bom Jardim Group comprises mudstones and fine-grained sandstones laid down in a deep lucastrine environment, with aluvial (river) inputs and turbidite structures (structures left by underwater landslides) and occasional desication structures (indicating periods when the lake dried up). Despite being a very different environment to the Maricá Group, the Bom Jardim Group has an identical Acritarch assemblage, with all four general present in the Maricá Group also found in the Bom Jardim Group.

Desiccation cracks in hyperpycnal turbidites of the Bom Jardim Group. Lehn et al. (2019).

The third set of sediments looked at, the Santa Bárbara Group, again comprise lucastrine sediments, but from what appears to have been (on average) a much shallower lake, with far more frequent desiccation structures and less frequent turbidites. This third environment produced the same assemblage of Acritarchs as the other two.

 Fine-grained delta front facies from the Santa Bárbara Group. Lehn et al. (2019).

The Acritarchs of the Camaquã Basin provide an additional insight into life on part of the margin on the still-forming continent of Gondwana. It is a less diverse assemblage than known from other regions, with forms present that are found in both the Ediacaran Complex Acanthomorph Palynoflora and the Late Ediacaran Leiosphere Palynoflora. The presence of identical assemblages in both marine and lake deposits is interesting. As Acritarchs are essentially morphospecies (i.e. their classification is based upon similar shape rather than true biological affinity), it is possible that the marine and freshwater species are unrelated, but it seems somewhat unlikely that convergent evolution would have produced an identical range of forms in two geographically close but very different environments yet not anywhere else. An alternative possibility is this may represent the beginnings of colonisation of freshwater environments by single-celled Eukaryotes (something thought to have happened at about this time), with either the same species in both environments, or freshwater species that had not diverged notably from their marine relatives.

Acritarchs view under transmitted light microscopy. (A)–(C) Leiosphaeridia sp. from (A) Maricá (sample ULVG 12518), (B) Bom Jardim (sample ULVG 12495), and (C) Santa Bárbara (sample ULVG 12511) groups; (D,E) Lophosphaeridium sp. from (D) Maricá (sample ULVG 12518) and (E) Bom Jardim (sample 12495) groups; (F), Tanarium irregulare from Santa Bárbara Group (sample 12487); (G–J), Germinosphaera sp. from (G,H) Maricá (sample ULVG 12509), original opaque (G) and illuminated (H) image, (I) Bom Jardim (sample ULVG 12506), and (J) Santa Bárbara (sample ULVG 12515) groups.

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