AMMONITE MORPHOTYPES AS INDICATORS OF OXYGENATION IN A CRETACEOUS EPICONTINENTAL SEA

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 epic ontinental seas. Because some morphotypes appear to reflect a nektoben thic and others a pelagic habitat, the presence of specific morphotype s may be used to indicate benthic and water-column oxygenation, respec tively. At the same time, their absence may provide a valuable indicat or of dysaerobic to anaerobic conditions that may have promoted the de velopment of petroleum source rocks.