The Cold Cradle of Animal Life and Metazoan Colonization of Warm Carbonate
Basins During the Vendian-Cambrian Transition
Extended Abstract – Prato 2004

Mikhail A. Fedonkin1,2
1Paleontological Institute, Russian Academy of Sciences, 123 Profsoyuznaya, Moscow, Russia 117868
Email: mfedon@paleo.ru
2Research Fellow, School of Geosciences, Monash University, Melbourne, Victoria, 3800 Australia

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Running header text: Cold cradle of animal life

The Cold Cradle Hypothesis suggests that the origin of multicellular animals (Metazoa) took place in relatively cold marine environments, well outside the carbonate belt on Earth. Cold basins provided environmental advantages for early animals. The higher concentration of oxygen dissolved in cold water was a critical factor for aerobic metabolism. High primary production of predominantly eukaryotic phytoplankton, supported by a rich nutrient content due to active circulation of water, was a basic condition favouring the heterotrophic mode of feeding. The role of prokaryotic organisms in a cold ocean was (and still is) less significant than that of eukaryotes because, on average, their optimum temperature for growth of prokaryotes is higher than that of the eukaryotes. The highest diversity of Vendian metazoan assemblages occurs in siliciclastic (relatively cold) palaeobasins, for instance, the Avalon Peninsula of Newfoundland in Canada, the Flinders Range in South Australia, the White Sea region and Ural Mountains of Russia, and Podolia in the Ukraine. All of these metazoan assemblages manifest the global expansion of animals after the most severe Neoproterozoic glaciations, which took place about 600 Ma ago.
Carbonate palaeobasins of the Vendian (such as Arctic Siberia or China) or even the basins with an alternating regime of carbonate vs. siliciclastic sedimentation (such as perhaps Namibia) demonstrate lower diversity of metazoan fossils and only weak evidence of bioturbations. Metazoan colonization of these relatively warm basins was complicated by low oxygen content in seawater, relatively low eukaryotic primary production, and by more significant ecological role of the prokaryotic biota. In order to cope with such low oxygen content in warm seawater, early metazoans developed cooperative liaisons with endosymbiotic algae, which supplied their hosts with extra oxygen and organic nutrients such as sugars and amino acids.  The latter was an important contribution in the nutrient poor (oligotrophic) environment.  Mass biomineralization in many metazoan phyla has been related to the colonization of the carbonate basins by metazoans. Higher temperature would have increased the effectiveness of enzymes responsible for such biomineralization, as well as a decreased solubility of such biominerals, in particular the most common calcite (more stable in presence of oxygen). Metazoan colonization of carbonate basins terminated the three-billion-year history of the prokaryotic domination of the oceans. 

This study was supported by the Russian Fund for Basic Research (Grant 02-05-54658) and by a grant (1790.2004.5) of the President of the Russian Federation.