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An examination of ammonite morphotypes through stratigraphic sections of the Greenhorn Cyclothem of the Cretaceous Western Interior suggests that they may be useful indicators of oxygenation within ancient epicontinental seas. Because some morphotypes appear to reflect a nektobenthic and others a pelagic habitat, the presence of specific morphotypes may be used to indicate benthic and water-column oxygenation, respectively. At the same time, their absence may provide a valuable indicator of dysaerobic to anaerobic conditions that may have promoted the development of petroleum source rocks. □Ammonites, anoxia, Cretaceous, Greenhorn Cyclothem, western Interior.