Notable among the biological changes of the middle part of the Maastrichtian Age was a major pulse of extinction among inoceramid bivalves. Analysis of the distribution of inoceramid shell fragments suggests that the population of burrowing organisms increased across the same stratigraphic interval that the bivalve population decreased. A global reorganization of ocean circulation that resulted in cooler, better oxygenated bottom waters is proposed as an explanation for the observed changes in the deep sea. Other observations regarding the mid-Maastrichtian are consistent with this hypothesis.

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