An enigmatic origin for early animal biomineralization?

What is biomineralization and why is it important?

 

A modern example of animal biomineralization is shown by
this conch shell. Mollusks use their shells to defend against
larger animals that may want a tasty snack as well as regulating
their internal chemistry (eBay).  

   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, protection from consumption, and maintain a healthy chemical balance (think about snails and their shells for example).  
However, when animals first acquired the ability to biomineralize remains shrouded in mystery.  Discoveries of an early worm-like animal with a carbonate shell, called Cloudina, in southern Namibia, led to a prediction that these organisms would be found in equivalent rock successions across the world.  A field excursion based on this prediction led to Siberia, where Cloudina were not found, but a groundbreaking discovery was made.  In a fossilized reef complex of an ancient rock 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 600 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 due to the data signifying an ocean with very different chemistry than in the modern ocean.  One element that researchers use to assess ocean chemistry is carbon.  Aside from being the building block of all life forms, researchers use carbon 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 rock formation in Oman where it was discovered.  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 rocks makes this a controversial topic.  Recent geochemical data has been used to constrain the chemical composition of the oceans roughly 600 million years ago to better understand the conditions in which early animals may have acquired the capability to biomineralize.  Elemental data reveals enhanced weathering of the continents during this time period, likely due to large-scale 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 prior to 600 million years ago contained very little oxygen, or were anoxic, while during and after the profound change in ocean chemistry, 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.

 

Orientation of the continents approximately 565 million years ago which is tremendously different than the modern orientation.  The names of the continents are very different than the modern names: Am, Amazonia; Ar, Arequipa; Aus, Australia; Az, Azania; Ba, Baltica; C, Congo; I, India; K, Kalahari; Lau, Laurentia; SC, South China; Si, Siberia; T, Tarim; WAC, West African Craton Each symbol represents known Shuram Excursion intervals (modified from Busch et al., 2022).

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

Busch, J. F., Hodgin, E. B., Ahm, A.-S. C., Husson, J. M., Macdonald, F. A., Bergmann, K. D., Higgins, J. A., & Strauss, J. V. (2022). Global and local drivers of the Ediacaran Shuram Carbon Isotope Excursion. Earth and Planetary Science Letters, 579, 117368. https://doi.org/10.1016/j.epsl.2022.117368

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