ORDINAL AND FAMILIAL DIVERSITY OF CENOZOIC MAMMALS

Abstract

SummaryVarious kinds of data concerning changes in total mammalian ordinal and familial diversity through the Cenozoic are graphed for the world and for each of the continents from which a mammalian fossil record is known. Ordinal and familial diversity increased rapidly through the Paleocene and Eocene to reach respective all‐time highs of 26 orders in the late Eocene and 116 families in the early Oligocene (bats and odontocete plus mys‐ticete whales excluded). The major continents suffered a decline in total ordinal and familial diversity in the latter half of the Oligocene with a secondary rejuvenation in the early Miocene. A remarkable uniformity in total number of orders and families per continent has obtained from the early Oligocene to Recent, and no apparent relationship exists between total diversity and continental size.A series of striking correlations exists between the times of first appearances of many mammalian and angiospermous orders and families. High numbers of first appearances (a kind of index to rates of evolutionary diversification) occurred in both mammals and flowering plants in the late Eocene to early Oligocene and in the earlier half of the Miocene. These specific times of high taxonomic diversification also correlate closely with the times of first appearance of “modern” (present at some time during the Quaternary) families of mammals and extinction of archaic families. Major mammalian faunal “turnovers” occurred in the Late Cretaceous, Eocene through early Oligocene, and early Miocene. Nevertheless, total familial and ordinal diversity of mammals on any given continent at any given time has remained remarkably stable. Some sort of faunal equilibrium is suggested.The dramatic taxonomic changes of the earlier parts of the Oligocene (i. e., high rates of appearance of new mammalian and plant taxa plus high rate of extinction of archaic mammals) correlate well with a general deterioration of the world's climate (lowering of mean annual temperature and decrease of equability) ar that time. Contemporaneous adjustments can be observed between climatic change, rates of evolutionary diversification in plants and mammals, and rates of extinction in mammals.