THE EVE OF BIOMINERALIZATION

Animals require oxygen, and therefore the first appearance of metazoans during the later Ediacaran (580-543 million years ago) and calcified skeletons (~553 million years ago) has been linked to the widespread development of oxygenated oceanic conditions. However, the global nature of ocean redox chemistry through this period has been shown to be complex, with deeper water anoxia persisting in certain areas. Few studies have directly documented the nature of ocean chemistry at locations that coincide with palaeontological evidence for major biological innovation so precise environmental context for early animal evolution remains unclear. A spatial and temporal reconstruction of carbon isotope and ocean redox dynamics, as recorded in in the Zaris Sub-Basin of the Nama Group, Namibia (~553-549 Ma), demonstrates that deeper waters remained consistently anoxic and iron-rich (ferruginous). Ferruginous conditions also occurred in the shallowest waters in units interbedded with the oxic horizons where the first skeletal metazoans are preserved. These data suggest that late Ediacaran surface-water oxygenation was unstable. The rise of biomineralisation was probably due to escalating selective pressures and appeared in many clades independently through the Neoproterozoic-Cambrian interval: quantitative compilations show that the choice of biomineral was controlled by ecology and metabolic cost, where biominerals were selected according to their principal function (passive defence, active defence, or active predation) under conditions of changing sea water chemistry.