Comparative paleoecology of Paleogene and Neogene mammalian faunas: body-size structure

Abstract

Abstract Species size is correlated with many aspects of life history, ecology, and behavior, which means that size changes within species, lineages, and faunas represent an important component of evolutionary paleoecology. Comparison of Paleogene mammalian faunas from the Bighorn, Clarks Fork, and Crazy Mountains basins of Wyoming and Montana with Neogene mammalian faunas from the Siwalik Group of northern Pakistan reveals similarities and differences in patterns of size change through intervals of 10 m.y. Two approaches to size change are presented. The first is to evaluate changes in the size distribution of faunas over three time intervals in each sequence. Rank-ordered size distributions, or cenograms, are used to depict faunal size structure for non-carnivorous species. The slopes and gaps in different regions of the size spectrum reflect conditions of vegetation and climate, by analogy with modern mammalian faunas (Legendre, 1986, 1989). For the Paleogene and Neogene faunas, subtle changes over time in size structure reflect changes in local vegetation and climate. The Paleogene cenograms suggest a habitat shift from mesic to humid forest, and the Neogene cenograms suggest a shift from open woodland to savannah scrub. These interpretations are supported by concurrent changes in trophic structure, faunal turnover, and in floral and geologic indicators. The second approach focuses on size change within species and lineages in several families of predominantly herbivorous species. For 60 Paleogene species and 39 Neogene species, change in average species size over successive biostratigraphic intervals is assessed by a criterion of doubling or halving of body mass relative to the preceding interval. New occurrences are compared to established species of the same genus and of the same family. In both records, size increases occur slightly more often than size decreases. The size distribution of groups changes more often through appearances of species of more than double or less than half the size of established species of the same group or by disappearances, rather than through rapid change of size within species. The pattern of change in median size and size range of contemporaneous species varies among families in both records. Three causes of evolutionary size change—climatic change, competition, and predation—are evaluated. In both records, climatic change and interspecific competition are considered the principle mechanisms for the observed changes.