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Department of Earth Sciences, University of Cambridge, Downing Street, Cambridge CB2 3EQ, UK (e-mail: njb1005{at}esc.cam.ac.uk)
Ecological and evolutionary principles are often context-dependent, particularly where the context is biologically defined. Organ-grade animals (eumetazoans) are particularly powerful contextual agents, with a unique capacity to drive escalatory co-evolution and build multi-tiered food-webs. The evolution of eumetazoans through the Ediacaran and early Cambrian fundamentally altered macroecological and macroevolutionary dynamics, including the structure and function of the marine carbon cycle. Pelagic eumetazoans can be held responsible for driving the evolution of relatively large eukaryotic phytoplankton, thereby shifting the system from a turbid, stratified, cyanobacteria-dominated stable state to the clear-water, well-oxygenated, algae-dominated condition typical of the Phanerozoic. Intermittent return to the pre-Ediacaran state during Phanerozoic extinctions and oceanic anoxic events suggests that the widespread anoxia detected in pre-Ediacaran deep-marine sequences may be a consequence of this alternate biological pump rather than a reflection of fundamentally lower levels of atmospheric oxygen. The transition between the pre- and post-Ediacaran states is also associated with the oldest commercially exploitable hydrocarbons, a possible by-product of invading animals and their top-down impact on the biological pump.
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