Principal stages in evolution of precambrian organic world: Communication 2. The late proterozoic |
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| Authors: | V N Sergeev M A Semikhatov M A Fedonkin N G Vorob’eva |
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| Institution: | 1.Geological Institute, Russian Academy of Sciences,Moscow,Russia |
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| Abstract: | A new suggested model outlining the evolution of the organic world from the mid-Early Proterozoic (∼2.0 Ga) to the Early Cambrian
is based on data characterizing the relevant chert-embedded and compression-preserved organic-walled microbiotas, impressions
of soft-bodied multicellular organisms, and biomarkers. Critical analysis of overall paleontological data resulted in the
distinguishing of seven successive assemblages of Proterozoic micro- and macrofossils. Being of global geographic range, the
assemblages correspond to the major stages in evolution of the organic world and typify global units which are termed the
Labradorian (∼2.0–1.65 Ga), Anabarian (1.65–1.2 Ga), Turukhanian (1.2–1.03 Ga), Uchuromayan (1.03–0.85 Ga), Yuzhnouralian
(0.85–0.635 Ga), Amadeusian (0.635–0.56 Ga), and Belomorian (0.56–0.535 Ga). Characteristic of the Labradorian unit are microfossil
assemblages of the Gunflint type including remains of morphologically bizarre prokaryotic microorganisms: star-like Eoastrion, umbrella-shaped Kakabekia, dumbbell-shaped Xenothrix, and some others. Fine-grained siliciclastic deposits of the same age yield the oldest remains of millimeter-sized eukaryotes:
spherical to ribbon-like Chuaria and Tawuia. Microfossils prevailing in shallow-water carbonate facies of the Anabarian unit are akinetes of nostocalean cyanophyceae
Archaeoellipsoides and entophysalidacean cyanobacteria Eoentophysalis, whereas acanthomorphic acritarchs Tappania and Shuiyousphaeridium dominate the assemblages of open-shelf facies, where they are associated with the first-found rare macroscopic multicellular
fossils Horodyskia. The distinguishing feature of the next Turukhanian unit is the first occurrence of filamentous red alga Bangiomorpha and the stalked cyanobacterium Polybessurus. The Uchuromayan unit is characterized by the appearance and worldwide radiation of structurally complicated eukaryotic microorganisms,
primarily of acanthomorphic acritarchs Trachyhystrichosphaera and Prolatoforma, branching thalli of green algae Aimophyton, Palaeosiphonella, Palaeovaucheria and Proterocladus, and of spiral-cylindrical cyanobacteria Obruchevella. In the Yuzhnouralian unit is recorded the first occurrence of vase-shaped testate amoebas Melanocyrillium, Cycliocyrillium, and others, and of scale microfossils Characodictyon, Paleohexadictyon, etc. As distinct from the others, the Amadeusian unit characterizes the global expansion of acanthomorphic acritarchs of
complex structure (the Pertatataka-like assemblage of Tanarium, Cavaspina, Appendisphaera, and others) and associated remains of red algae and cyanobacteria Obruchevella. The terminal Belomorian unit marks the extinction of Pertatataka-type microfossils, the appearance of soft-bodied multicellular
organisms on different continents, and the origin of diverse skeletal fossils in the terminal phase. |
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