Perspectives in community dynamics from geo-historical records of terrestrial mammals

Samples of ancient communities reconstructed using the mammalian fossil record beyond the range of Carbon-14 dating usually represent 10³ yrs or more because: 1) geochronological dating limits the temporal resolution of time-successive samples, and 2) taphonomic processes average vertebrate remains - thus species composition - over variable amounts of ecological time.  Mammalian fossil assemblages provide ecological evidence via taxonomic composition, abundance measures, and proportions of different diet, locomotion and body size categories. Successive fossil samples, each representing thousands of years and spatial areas of <10⁰ – 10⁴ km, can document millions of years of ecological history through continuous rock sequences.  These sample sets are used to examine macro-ecological transformations in community structure in relation to independently documented environmental variables including physical substrate, vegetation, climate, and continental connections.  Two examples of late Cenozoic mammalian community dynamics illustrate unique perspectives offered by deep-time fossil assemblages.  The Siwalik sequence of Pakistan between 10.5 and 5.5 Ma provides tests of alternative models of community evolution, i.e., environmental sorting vs. demographic dominance, during a period of sustained environmental change.  Data from 40 herbivore lineages show local extinction of frugivores and browsers, whereas immigrants were mainly mixed feeders—a pattern consistent with environmental sorting in response to climate change and the decline of forested habitats.  In the Pliocene Omo sequence of East Africa, herbivore abundances on an aggrading river floodplain show hundreds of thousands of years of relative stability followed by 100-kyr cycles of instability. Records from different East African basins document geographic variation in the structure of contemporaneous mammalian communities.  These examples of community dynamics at 10⁴ – 10⁶ yr resolution reveal patterns of persistence, adaptation within lineages, immigration and extinction in response to changes in the physical environment as well as regionally variable rates of transformation in guild composition and diversity.