An examination of the distributions of ammonite shell morphotypes in the Western Interior Cretaceous Greenhorn Cyclothem suggests that the varied morphologies represented by this diverse group of organisms reflect mode of life and to some extent environmental conditions. Although most ammonites apparently ranged from shallow nearshore to deeper mid-basinal habitats in the Western Interior Seaway (maximum depth about 300 m), the distributions of certain morphotypes suggest depth-restriction. For example, compressed, disc-shaped, and heavily nodose forms generally indicate bottoms shallower than about 50 m. Also, structural and morphological shell features appear in some cases to facilitate relative depth interpretations. While pelagic ammonites were sensitive to salinity, temperature, and oxygenation within the water column, nektobenthic and benthic ones may be useful indicators of oxygenation closer to the substrate. Some morphotypes are represented solely by taxa restricted to specific thermal regimes, and their changing distributions may reflect migrations of major watermasses. The distribution patterns of most morphotypes suggest that post-mortem drift was generally not a major factor affecting ammonites of relatively shallow epicontinental seas.
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Richard J. Batt (1989) studied this question.