WHEN LIFE GOT HARD: AN ENVIRONMENTAL DRIVER FOR METAZOAN BIOMINERALIZATION

The hypothetical driver(s) for the onset of metazoan biomineralization include protection from predation, structural support, and calcium regulation. Insofar as Cloudina, the earliest shell forming metazoan, occurs in Ediacaran strata worldwide above the Shuram Excursion, the greatest negative carbon isotope anomaly in Earth history, it is critical to understand if these events were coupled. To this end, we discovered the tubular fossil in a newly-recognized unconformity-bounded limestone-dominated sequence immediately above the Mara Member in southern Namibia, which preserves evidence for the Shuram anomaly. Given its global distribution, stratigraphically cohesive patterns of carbon, sulfur, and strontium isotope change, and its preservation in a wide range of depositional lithofacies, the carbon isotope excursion may be understood in terms of the pulsed addition of ¹²C-rich alkalinity to the oceans. The driver of the Shuram Excursion and the onset of biomineralization is thus envisioned as tectonic in nature, and related to widespread Pan-African orogeny as East and West Gondwana were sutured. Erosion of uplifted Ediacaran terrains delivered thick piles of sediments to the oceans, as well as bio-limiting nutrients, especially N, P, and Fe, sulfate, and carbonate alkalinity released through silicate weathering. The growth of the oceanic Ca²⁺ reservoir most likely promoted animal biomineralization insofar as phosphate-based metabolism necessitates the exclusion of calcium from cells. Cellular levels of the cation are <1000x of those in the extracellular fluid, and the excess Ca²⁺ is either excreted to the environment or stored in bones and shells. All cells have active transport mechanisms to remove Ca²⁺ and import phosphate and magnesium in order to regulate important biological functions, including metabolism, protein confirmation, and signaling. In particular, locomotion through muscular contraction requires constant oscillations in the concentration of Ca²⁺ within the cells, so storage of the element as inert minerals outside of animal cells provides a distinct advantage, and is hypothesized to have precipitated from a sudden increase in oceanic Ca²⁺ abundance. In sum, the coupling of a major biological innovation with the end of a profound carbon isotope excursion supports the view that the Shuram Excursion was a tectonic and oceanographic phenomenon – as opposed to a global diagenetic conspiracy.