History of biomineralization in early metazoan clades and the taphonomic phosphatization window at the PC-C boundary
Zusammenfassung der Projektergebnisse
The Cambrian System is one of the last systems with numerous undefined stages and series, and here especially the traditional “Lower Cambrian” Series raises many open questions for an internationally applicable chronostratigraphic subdivision. The application of biostratigraphic tools faces the problems of scarcity of typical Palaeozoic faunas in the Terreneuvian, faunal provincialism, uncertain biological assignments, limited spatial distribution, and a partial facies-dependence of earliest faunas. Especially assemblages of small shelly fossils (SSFs), acritachs, archaeocyatha, and trilobites played an important role for the biostratigraphic subdivision of the traditional “Lower Cambrian”, whereas some species of SSFs have a great potential for representing possible index fossils. Some recent chemostratigraphic investigations have cast doubts on the general applicability of biostratigraphy and the SSFs in particular for chronostratigraphic purposes in the early Cambrian. Claims that all SSFs have worldwide diachronous FADs were mostly based on independent chemostratigraphic correlations of carbon isotopic curves. These claims were critically evaluated by testing both biostratigraphic and chemostratigraphic tools within this project. For South China and the Karatau-Naryn Terrain (Kazakhstan) we were able to define and revise the biozonation based on rich SSF associations, which well support regional correlation of strata. Some SSFs have global synchronous appearances and may serve as index fossils for defining early Cambrian Stages 2 & 3 and for redefining the Fortunian Stage. According to the ISCS decision the trace fossil Trichophycus pedum defines the Cambrian System and Fortunian Stage. However, the FAD of Trichophycus pedum is diachronous on many palaeocontinents. We rather propose to base the definition of Cambrian System and Fortunian Stage on the widely distributed protoconodonts Protohertzina anabarica and P. unguliformis. Based on a similarity study of Cambrian Stage 3 SSFs we recognize the existence of seven bioprovinces, that are likely influenced by palaeogeographic constrains, ocean currents, water temperature and salinity profiles. Our detailed biostratigraphic revision of early Cambrian strata of South China and Maly Karatau (Kazakhstan) also confirmed that the mostly investigated shallow water sequences are rich in hiati, sediment reworking, karstification that had some impact on the recognition of biozones and chemostratigraphic correlation. With our applied mixed sedimentological screening and extraction of SSF assemblages we were able to specify disturbed sedimentary intervals where alien SSFs were introduced to strata. By this method we were able to improve the biostratigraphic applicability of SSFs in South China and Maly Karatau. In particular, carbon isotope stratigraphy has been developed into a widely applied stratigraphic correlation tool during past decades. This method has the great advantage that also unfossiliferous sections or strata can be correlated. However, we also recognized that diverging isotopic trends exist in different regions of the early Cambrian world (South China, Maly Karatau Naryn Terrain, Tuva-Mongolia versus Siberia, Morocco) and within single Cambrian carbonate platforms, such as the Yangtze Platform. We interprete this as an indication that some shallow water sediments do not reflect global seawater signatures and some stratigraphic intervals are strongly affacted by sedimentary reworking and recycling of deposited carbon sourses. We conclude that chemostratigraphy based on C isotope trends in Cambrian strata requires biostratigraphic control. Skeletal remains of metazoans appear in the fossil record of Precambrian- Cambrian transiton not at once but in a three-fold pattern, with poriferan remains and cloudinids, likely related to cnidarians, being first in the late Ediacaran. In a second stage SSFs of chancelloriids, chaetognaths and different clades of the lophotrochozoans appeared in the Fortunian. Finally, skeletal remains of ecdysozoan clades, such as the trilobites, bradoriids, lobopodians and palaeoscolecids, and of echinoderms appeared in the Cambrian Stage 3. Particularly Fortunian strata contain sometimes large numbers of eggs, embryos and larvae of metazoans and some “soft-tissued” remains often related to stem-group cycloneuralians. Part of the phosphatized embryos and “soft-tissued” remains were studied in the project and different ontogenetic stages assigned to ontogenetic cycles. Combining the skeletal fossil record with those of “soft-tissued” remains and trace fossils we conclude that the skeletal fossil record does not fully reflect the true biodiversity of ecdysozoans in the early Cambrian. We hypothesize that the delayed appearance of the ecdysozoan skeletal remains may be related to genetic changes causing a thickening and specialisation in the cuticle development of all ecdysozoan clades thus increasing the potential for fossilization since Cambrian Stage 3. Considering the temporal distribution of the different biominerals in the skeletal record we did not find a strong sequential succession. We hypothesize that the presence of high bioproductivity and depositional rate in the tropical realm, fluctuating oxic-dysoxic depositional environments, presence of bacteria decomposing organic matter and high influx of embryos or other easily decayable tissue remains are crucial for the preservation of phosphatized Orsten-type remains.
Projektbezogene Publikationen (Auswahl)
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(2014) Early Cambrian palaeobiogeography of the Zhenba– Fangxian Block (South China): Independent terrane or part of the Yangtze Platform? Gondwana Research
Yang, B., Steiner, M. & Keupp, H.
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(2011) An armoured Cambrian lobopodian from China with arthropod-like appandages. Nature, 470, 526-530
Liu, J., Steiner, M., Dunlop, J. A., Keupp, H., Shu, D., Ou, Q., Han, J., Zhang, Z., & Zhang, X.
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(2012) A new species of Hallucigenia from the Cambrian Stage 4 Wulongqing Formation of Yunnan (South China) and the structure of sclerites in lobopodians. Bulletin of Geosciences, 87(1): 107-124
Steiner, M., Hu, S., Liu, J. & Keupp, H.
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(2012) Chronology of early Cambrian biomineralization, Geological Magazine 149(2): 221-251
Kouchinsky, A., Bengtson, S., Runnegar, B., Skovsted, C., Steiner, M. & Vendrasco, M.
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(2012) Early Cambrian eodiscoid trilobite Hupeidiscus orientalis from South China: ontogeny and implications for affinities of Mongolitubulus-like sclerites. Bulletin of Geosciences, 87(1): 159-169
Li, G., Steiner, M., Zhu, M. & Zhao, X.
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(2013) First report of a Meishucun-type early Cambrian (Stage 2) ichnofauna from the Maly Karatau area (SE Kazakhstan): Palaeoichnological, palaeoecological and palaeogeographical implications. Palaeogeography, Palaeoclimatology, Palaeoecology 392, 209-231
Weber, B., Steiner, M., Evseev, S., Yergaliev, G.
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(2014) Chert-hosted small shelly fossils: expanded tool of biostratigraphy in the Early Cambrian. GFF, 136(1), 303-308
Yang, B., Zhang, L., Danelian, T., Feng, Q. & Steiner, M.
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(2014) Terreneuvian small shelly faunas of East Yunnan (South China) and their biostratigraphic implications. Palaeogeography, Palaeoclimatology, Palaeoecology, 398: 28-58
Yang, B., Steiner, M., Li, G., Keupp, H.
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(2014) The developmental cycles of early Cambrian Olivooidae fam. nov. (?Cycloneuralia) from the Yangtze Platform (China). Palaeogeography, Palaeoclimatology, Palaeoecology, 398: 97-124
Steiner, M., Qian, Y., Li, G., Hagadorn, J.W., Zhu, M.
