An enigmatic origin for early animal biomineralization?

What is biomineralization and why is it important?

   Biomineralization is the process by which living organisms produce minerals, often resulting in hardened or stiffened tissues.  Animals biomineralize to create bones, teeth, and shells, to provide structural support, protect against predation, and maintain a healthy chemical balance.  However, when animals first acquired the ability to biomineralize remains shrouded in mystery.  Discoveries of Cloudinamorphs, an early worm-like animal with a carbonate shell, in the Mara Member of the Nama Group in southern Namibia, led to a prediction that Cloudina would be found in equivalent successions across the world.  A field excursion based on this prediction led to the Siberian Craton, where Cloudina were not found, but a groundbreaking discovery was made.  In a reef complex in the Siberian Chencha Formation, strange sponge-like fossils with biomineralized calcium carbonate shells were discovered.  This was a major discovery as these fossils may represent the oldest examples of sponge-like animals at approximately 574 million years old.

Top: Cloudina microfossils that were extracted from a
 microbial reefal core found in the Villarta Formation in
Spain (Alvaro et al., 2019)
Bottom:  Outcrop photo of the sponge-like bioclasts in the 
Siberian Chencha Formation reef complex (Kaufman, 2021)

   

So why is this important?

     Despite the significant geographical distance between these two formations, they, amongst several other formations elsewhere on the planet, host unique geochemical signatures that have left researchers perplexed for years, starting with the carbon isotopic signature of the rock units.  Aside from being the building block of all life forms, researchers use carbon isotopes, or atoms with the same number of protons but different number of neutrons, to reconstruct shifts in the composition of seawater throughout Earth history, correlate between rock units locally and globally, and construct age models. 

 

So how does this relate back to the discovery of the organisms?

    The formations in which these biomineralizing organisms were found, host one of the greatest negative carbon cycle anomalies in Earth history, known as the Shuram Excursion, named after the Shuram Formation in Oman.  The absence of a reliable age date for the duration of the event, the magnitude of the anomaly, the uncertainty surrounding the onset of the event, and potential for alteration in such ancient carbonate deposits makes this a controversial topic.  Recent geochemical data has been used to constrain the chemical state of the Shuram oceans to better understand the conditions in which early animals may have acquired the capability to biomineralize.  Weathering proxy data reveals enhanced weathering of the continents during this time period, likely due to tectonic rearrangement, which would have delivered nutrients, calcium ions, and sulfate to the oceans, stimulating microbial processes that would consume organic matter and sulfate to produce carbonate that would bind with the calcium ions to precipitate calcium carbonate.  The appearance of the biomineralized fossils towards the end of the Shuram suggests that the calcium levels were so high that the animals could not expel the excess calcium from their bodies at a fast enough rate so they had to get creative, and thus they started producing shells.  Building upon the weathering data, additional elemental data suggests that the oceans before the Shuram Excursion contained very little oxygen, or were anoxic, while during and after the event, the oceans may have reached near modern oxygen levels.  This may have been attributed to the actions of the sponge-like fossils which, during life, would have filtered massive amounts of water, clarifying the surface oceans, allowing for photosynthesizing organisms to ventilate the oceans.

Geochemical plot using carbon and uranium isotope data from several continents
that highlights the unique geochemical fingerprint that defines
the Shuram Excursion  (Li et al., 2020)

For more information please refer to:

Álvaro, J. J., Cortijo, I., Jensen, S., Martí Mus, M., & Palacios, T. (2020). Cloudina-microbial reef resilience to substrate instability in a Cadomian retro-arc basin of the Iberian Peninsula. Precambrian Research, 336, 105479. https://doi.org/10.1016/j.precamres.2019.105479

Cui, H., Kaufman, A. J., Xiao, S., Zhou, C., & Liu, X.-M. (2017). Was the ediacaran shuram excursion a globally synchronized early diagenetic event? insights from methane-derived authigenic carbonates in the uppermost Doushantuo Formation, South China. Chemical Geology, 450, 59–80. https://doi.org/10.1016/j.chemgeo.2016.12.010 

Li, Z., Cao, M., Loyd, S., Algeo, T., Wang, X., & Zhao, L. (2020). Transient and stepwise ocean oxygenation during the Ediacaran Shuram Excursion. Goldschmidt Abstracts. https://doi.org/10.46427/gold2020.1541

Rothman, D. H., Hayes, J. M., & Summons, R. E. (2003). Dynamics of the Neoproterozoic carbon cycle. Proceedings of the National Academy of Sciences, 100(14), 8124–8129. https://doi.org/10.1073/pnas.0832439100

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• Rachel A. Wood et al.
 

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Proterozoic Modular Biomineralized Metazoan from the Nama Group, Namibia.Science296,2383-2386(2002).DOI:10.1126/science.1071599 

Uriz, M.-J. (2006). Mineral skeletogenesis in sponges. Canadian Journal of Zoology, 84(2), 322–356. https://doi.org/10.1139/z06-032