A MORE DETAILED LOOK AT SEPKOSKI'S EVOLUTIONARY FAUNAS: TWELVE FAUNAS

Sepkoski’s factor analytic description of the Phanerozoic marine fossil record is the classic depiction of taxonomic turnover in marine metazoa on geological time scales (Sepkoski 1981, Paleobiology). His analysis of the number of families within classes revealed that diversity history could be described simply by the successive dominance of three evolutionary faunas: the Cambrian, Paleozoic, and Modern. Here, we perform a similar analysis on his genus-level database using cluster analysis, but we split the ten most diverse classes in the dataset into subtaxa in order to observe additional details of taxonomic turnover. We also converted raw numbers of genera to proportions because we were most interested in changes in dominance (and because biases may affect raw counts of diversity). We identified twelve faunas from the resulting cluster analysis. These faunas were not simple subdivisions of Sepkoski’s faunas; for example, some bivalve subtaxa grouped with faunas that most resembled Sepkoski’s Modern fauna, but others grouped with faunas that most resembled his Paleozoic fauna. These faunas can also be related to the concept of Ecologic Evolutionary Units (EEU’s). Dominance shifted rapidly from fauna to fauna in the early Paleozoic and then slowed as rates of extinction and origination in the biosphere declined. For example, five of the twelve faunas reached their maximum proportional diversity by the Ordovician. As in Sepkoski’s original analysis, the Permian extinction is obvious because it decimated a major fauna (the Late Paleozoic fauna). However, in this analysis, the other mass extinctions are more apparent as well—they either truncated or permanently reduced the diversity of various faunas. Major radiations (Cambrian, Ordovician, Mesozoic-Cenozoic) correspond with the expansion of new faunas. We use this analysis to examine general changes in the ecological composition of the marine fauna by assigning higher taxa to broad ecological categories. For example, taxa that contain infaunal, predatory, and motile members increased in relative diversity through time.