Simple sediment rheology explains the Ediacara biota preservation

Abstract

The soft-bodied Ediacara biota (571–541 million years ago) represents the oldest complex large organisms in the fossil record, providing a bridge between largely microbial ecosystems of the Precambrian and the animal-dominated world of the Phanerozoic, potentially holding clues about the early evolution of Metazoa. However, the nature of most Ediacaran organisms remains unresolved, partly due to their enigmatic non-actualistic preservation. Here, we show that Flinders-style fossilization of Ediacaran organisms was promoted by unusually prolonged conservation of organic matter, coupled with differences in rheological behaviour of the over- and underlying sediments. In contrast with accepted models, cementation of overlying sand was not critical for fossil preservation, which is supported by the absence of cement in unweathered White Sea specimens and observations of soft sediment deformation in South Australian specimens. The rheological model, confirmed by laboratory simulations, implies that Ediacaran fossils do not necessarily reflect the external shape of the organism, but rather the morphology of a soft external or internal organic ‘skeleton’. The rheological mechanism provides new constraints on biological interpretations of the Ediacara biota.

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Fig. 1: Styles of preservation of the Ediacara biota fossils.
Fig. 2: Evidence for the absence of early diagenetic cementation in the White Sea and South Australian Ediacaran sections.
Fig. 3: Taphonomic laboratory experiments testing the Rheological model.
Fig. 4: The Rheological model of preservation of Ediacaran macrofossils.
Fig. 5: Dickinsonia overlapping with a negative structure below a clay pebble within sandstone.
Fig. 6: Organically preserved Ediacaran fossils, and Dickinsonia transitional between Flinders- and Nama-style preservation.

Data availability

All materials are available within the main text and Supplementary Information files. Palaeontological specimens from South Australia are stored at the South Australian Museum. Specimens from the White Sea area are stored at the Borissiak Palaeontological Institute. Thin sections and higher-quality images are available from the corresponding authors on request.

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Acknowledgements

The study was funded by Australian Research Council grants DP160100607 and DP170100556 (to J.J.B.) and Russian Foundation for Basic Research project number 17-05-02212A (to I.B., A.K. and A.I). I.B. acknowledges an Australian Government Research Training Program stipend scholarship. The authors are grateful to A. Nagovitsyn, P. Rychkov, V. Rychkov, S. Rychkov, T. Rychkova and A. Makushkina for help in the field, L. Zaytseva for help with scanning electron microscopy imaging, J. M. Hope, J. Wurtzel, S. Eggins, A. Rummery and R. Kerr for providing materials for the taphonomic experiments, M.-A. Binnie and J. G. Gehling for providing access to the South Australian Museum collections, and N. J. Butterfield and A. G. Liu for helpful comments on this study.

Author information

I.B. designed the study, studied the collections and developed the model. A.K. and I.B. performed the thin-section and scanning electron microscopy analyses. I.B., A.K., A.I. and E.L. participated in the field expeditions. A.I. and E.L. provided samples from the Borissiak Paleontological Institute (RAS) collections. I.B. and J.J.B. designed the taphonomic laboratory experiments. I.B. and J.J.B. wrote the paper with contributions from all authors.

Correspondence to Ilya Bobrovskiy or Jochen J. Brocks.

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Bobrovskiy, I., Krasnova, A., Ivantsov, A. et al. Simple sediment rheology explains the Ediacara biota preservation. Nat Ecol Evol 3, 582–589 (2019). https://doi.org/10.1038/s41559-019-0820-7

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