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Competitive displacement of one taxon by another in the fossil record may be indicated when: (1) an inverse correlation in diversity and, particularly, relative abundance can be demonstrated between the two groups through time; (2) aspects of their paleobiology suggest utilization of common resources; and (3) it can be shown that the two taxa evolved in allopatry prior to their sympatric association. Data from recent collections of Paleocene and Eocene mammals in the Western Interior of North America show marked inverse correlations both of generic diversity and relative abundance between multituberculates and rodents. The largest diminution in multituberculate diversity occurred in the latest Paleocene, near the Tiffanian-Clarkforkian boundary, not in the early Eocene as suggested previously. Reconstruction of diets, die1 activity patterns, locomotor habits, and body sizes of multituberculates and rodents suggests that both groups potentially utilized similar resources. The hypothesis that competitive exclusion may have played a role in the decline of multituberculates is strengthened by recent evidence that rodents evolved in Asia, immigrating to North America in latest Paleocene time. Evidence in support of alternative hypotheses employed to account for the decline and eventual extinction of multituberculates is wanting.

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