UC San Diego

The onset of the Phanerozoic (541 Ma to present) marks a transition point in the evolution of life on Earth. The evolutionary dynamics of this period and its linkages to the physical environment have long been the focus of paleontologists. In this dissertation, I investigate the role of changing oxygen concentrations on the long-term evolutionary trends in Phanerozoic oceans. In particular, I focus on the Mesozoic Marine Revolution, a period of rapid evolutionary transition towards increasingly active communities. In contrast to canonical model reconstructions of atmospheric oxygen during Phanerozoic which show little correlation to evolutionary trends in the marine fossil record, the data presented here suggest an important role for oxygen. I first construct a record of carbonate sediment color to track redox state at the sediment-water interface through time and demonstrate that, despite persistently high levels of atmospheric oxygen, benthic environments may have been poorly oxygenated until 200 million years ago. This record opens up the possibility that shallow marine communities faced selective pressures from oxygen-limited environments until about the time of the Mesozoic Marine Revolution. Building on the sedimentary evidence for a Mesozoic rise in ocean oxygenation, my second chapter builds a quantitative record of metabolic rates in marine communities using modern metabolic rates combined with molecular clock and fossil record ages. This record illustrates that pronounced shifts in metabolic demand occurred during the Mesozoic Marine Revolution from increasing body size and oxygen availability in shallow marine environments. I expand upon this finding by quantifying temperature and oxygen state spaces of living organisms to show that taxa from clades with Paleozoic origins occupy a lower temperature and dissolved oxygen space than clades evolved in the Mesozoic or later. This suggests temperature and oxygen conditions of the deep-sea may act as a barrier against Mesozoic clades and provide refugia for formerly shallow marine Paleozoic communities. In summary, the data presented from this thesis suggest that low oxygen concentrations acted as a limiting variable in the evolution of metabolically active fauna. The data also suggest Paleozoic deep-water communities experienced dissolved oxygen concentrations that varied significantly from equilibrium with the atmosphere.