A multivariate biochronological analysis of the North American mammal fossil record has established an independent ordering of appearance events for all described genera and species. This geochronologically confirmed hypothesis shows that first and last appearances in different parts of the continent are highly diachronous. Additional results show that (1) despite this diachrony, the new event sequence is compatible with a previously hypothesized sequence of immigrant first appearances; (2) immigrant genera are not particularly widespread, abundant, or synchronous in their appearances; (3) last appearances, large-mammal appearances, and species-level appearances are the most reliable; and (4) most of the observed diachrony appears to result from undersampling. Because diachrony has little effect on the overall multivariate analysis, and because the differences among classes of events are not extreme, joint analyses of all known appearance events are preferable to the a priori selection of particular appearance events as time indicators.

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