Early Cambrian biogeography and the diversification center of Cambrian bioradiation in the Palasian Ocean
Final Report Abstract
The palaeogeography of the Ediacaran to early Cambrian is not yet well constrained and there exist contradictionary reconstructions. In particular, smaller terrains and Proterozoic microcontinents are often not included into palaeogeographic models, because of the lack of reliable palaeomagnetic data. Thus, it is little surprising, that for the smaller Proterozoic/ early Palaeozoic blocks of the Central Asian Orogeny Belt (CAOB), one of the lagest orogenic belts on Earth, exist only vague hypotheses on their original palaeogeographic positions. Within our research project we studied the lithostratigraphy, biostratigraphy, Small Shelly Fossil (SSF) record, carbon isotope trends and provenances of detrital zircons of early Cambrian cover strata of some of the cratonal blocks (Zhabkhan, Tuva-Mongolia) and mobile belts (Khamsara-Tannuola, Dariv) of western Mongolia and the Republic of Tuva (Russia) in order to conclude on their lithotectonic and paleaobiogeographic affinities. For the Dzhabkhan Craton we document rich SSF assemblages from up to 1700m thick sediments spanning the terminal Ediacaran to Cambrian Stage 3 and define five biozones, which support a regional and partially global correlation of strata. Part of biozones, such as the Watsonella crosbyi Assemblage Zone and the Anabarites trisulcatus- Protohertzina anabarica Assemblage Zone are globally recognized. We document that a biozonation based on SSFs is feasible and the distribution of skeletal fossils in the early Cambrian strata follows an evolutionary pattern indicative of phylogenetic events in a chronostratigraphical frame. This allows the utility of SSFs for a chronostratigraphic subdivision of the early Cambrian. We show that carbon isotope stratigraphy on some shallow Cambrian carbonate platforms may have been impacted by aqua facies variations and conclude that a stratigraphic utilization of carbon isotope trends of Cambrian strata is only useful if detailed biostratigraphic correlations support the peak correlations. For the first time we document exceptionally well preserved phosphatized eggs, embryos and larvae from the Cambrian Stage 3 Salanygol Formation, as well as phosphatized cloudinids from the terminal Ediacaran Dzuun-Arts Formation. Preservation of ultrastructural details in the new cloudinid Zuunia chimidtsereni allow us to assign the fossil remains to housing tubes of annelids, representing some of the oldest metazoans on Earth. It is interpreted that the cloudinids were largely unmineralized during the Ediacaran and their common calcified preservation is due to early diagenetic decay processes. From the Telmen Complex of the transitional lithotectonic zone between Dzhabkhan Craton and Tuva-Mongolia Craton we for the first time document a unique Cambrian Stage 3 SSF assemblage with numerous tommotids and partially articulated eccentrothecids. Our cluster analysis of SSF distribution data for Cambrian Stage 3 indicates that all faunas of Proterozoic cratonal blocks of Mongolia were rather separated and had a specific fauna association. The fauna of the Dzhabkhan Craton and the Telmen Complex differ considerably from each other and from those of Tuva-Mongolia. Tuva-Mongolia indicates a high degree of similarity with the fauna of the surrounding mobile belts. In contrast to the Dzabkhan Craton, the fauna of the Telmen Complex also indicates biogeographic relationships toward the Kusnetzk Alatau mobile belt and to Siberia and Avalonia. Our provenance analysis of detrital zircons indicates as well that the Telmen Block had an independent evolution since the Ediacaran. The varying age peaks and topologies for the Zhabhkan, Tuva-Mongolia, Tarim, Yangtze and Siberia cratons speak for a connection within Rodinia and afterwards independent evolution during the Ediacaran and early Cambrian. We conclude that a combination of zircons provenance analysis from clastic cover strata and analysis of similarity in SSFs provides a strong tool to conclude on biogeographic and palaeogeographic relationships of Proterozoic blocks. Mongolian microcontinents did not form a single ribbon-continent during the early Cambrian but rather smaller islands dispersed within the Palasian Ocean. This dispersal of blocks developed following the breakup of Rodinia, where most blocks were in some proximity. The increasing dispersal of smaller Proterozoic blocks in the Palasian Ocean since the Ediacaran resulted in a diversification of ecospace, reflected by the strong diversity increase from SSF Assemblage 1 toward Assemblage 3. It can be assumed that a diversification in the ecospace in the result of breakup of Rodinia and independent evolution of microcontinets in the Palasian Ocean fuelled the speciation rate during the Cambrian bioradiation.
Publications
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(2016) Transitional Ediacaran-Cambrian small skeletal fossil assemblages from South China and Kazakhstan: Implications for chronostratigraphy and metazoan evolution. Precambrian Research, 285, 202-215
Yang, B., Steiner, M. Zhu, M., Li, G., Liu, J. & Liu, P.
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(2018) Microbial decay analysis challenges interpretation of putative organ systems in Cambrian fuxianhuiids. Proceedings of the Royal Society B: Biological Sciences, 285:20180051
Liu, J., Steiner, M. , Dunlop, J. A. & Shu, D.
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(2019) Early Cambrian Tommotiids of Khairkhan Section (Central Tyva). Paleontological Journal, 53:6, 575-582
Novozhilova, N.V., Steiner, M., Korovnikov, I.V. & Tokarev, D.A.
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(2020) Cambrian small skeletal fossil and carbon isotope records of the southern Huangling Anticline, Hubei (China) and implications for chemostratigraphy on the Yangtze Platform. Palaeogeography, Palaeoclimatology, Palaeoecology, 554 (2020) 109817
Steiner, M. ,Yang, B., Hohl, S., Zhang, L. & Chang, S.
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(2020) Ultrastructure of Ediacaran cloudinids suggests diverse taphonomic histories and affinities with non-biomineralized annelids. Scientific Reports, 10, 535
Yang, B., Steiner, M. , Schiffbauer, J.D., Selly, T. Wu, X., Zhang,C. & Liu, P.
