ICE AGES, ISOTOPES AND BIOLOGICAL TURNOVER IN THE PROTEROZOIC-CAMBRIAN

Very long periods of environmental stability are suggested by the carbon isotopic and palaeoclimatic record between c. 2000 and 1000 Ma. Such 'stasis' may have nurtured endosymbioses to the point at which eukaryotic organization and sexual reproduction became embedded in the genome. Dinoflagellate biomarkers plus protist-like microfossils and red algae appeared during this interval. This steady state world was repeatedly disprupted in the curious prelude to the 'Cambrian explosion'. Strontium, sulphur and carbon isotopes all attained maximal values at this time, and the latter show chaotic oscillations (in the mathematical sense) coincident with flips between extreme, low latitude glaciations and possible supergreenhouse conditions. Multicellularity in animals was an innovation associated with suspension feeding; this may have been favoured by widespread stratification and eutrophication in the Neoproterozoic. Episodic extinctions and founder effects associated with extreme climatic fluctuations could also have accelerated evolutionary rates, leading towards the new body plans we now know as the metazoan phyla. Some support for this view is provided by an extinction of acritarchs at c. 585 Ma, arguably coincident with the Moelv glaciation. New data will be put forward relating to events across the Precambrian-Cambrian boundary, involving a dramatic shift in habitats and preservational facies, coincident with a late phase of supercontinental assembly.