Sixty-six million years along the road of mammalian ecomorphological specialization.

The fossil record of the large terrestrial mammals of the North American Cenozoic has previously been quantitatively summarized in six sequential episodes of faunal associations-"evolutionary faunas"-that correspond well with previously proposed qualitative "Chronofaunas." Here, we investigate the ecological spectrum of these faunas by classifying their major taxonomic components into discrete ecomorphological categories of diet, locomotion, and body size. To specifically address the potential influence of long-term climatic shifts on the ecomorphological composition of these faunas, we analyze via contingency tables and detrended correspondence analyses the frequency distribution of ecomorph types within faunas. We show that each evolutionary fauna has a unique, nonrandom association of ecomorphs, and we identify a long-term trend toward greater ecomorphological specialization over successive faunas during the past 66 My. Major vegetation shifts induced by climatic changes appear to underlie the ecomorphological dynamics of these six temporal associations that summarize Cenozoic North American mammalian evolutionary history.