TEMPORAL AND SPATIAL PATTERNS IN THE EVOLUTION OF WITHIN-COMMUNITY PLANT DIVERSITY

The number of taxa coexisting in a local area (alpha diversity) is a fundamental attribute of any ecosystem. Previous work has suggested dramatic increases in the alpha diversity of terrestrial plants during the Devonian-Carboniferous interval (concurrent with the radiation of several pteridophyte groups and the earliest seed plants) and Cretaceous-Paleogene interval (concurrent with the radiation of angiosperms). However, direct comparison of alpha diversities estimated from fossil assemblages can result in deeply flawed interpretations if taphonomic biases and sampling issues are not considered. Here we analyze species lists drawn from over 6000 fossil plant localities compiled in the Paleobiology Database (www.paleodb.org) from primary literature, museum collections and personal field work and vetted by specialists. Analyses were restricted to North American and European macrofloras preserved in non-marine fluvial, deltaic, and lacustrine environments where specimens were collected from geographically well-constrained (e.g., <10 x 10m) and stratigraphically well-constrained (e.g., a bed or horizon) sites. The paleolatitude of each site was derived from modern coordinates and estimated plate rotations. The number of species and morphospecies was tabulated for each collection and, because the paleobotanical record mainly consists of isolated organs, several approaches to estimating alpha diversity are compared. Our results indicate that standardization of the number of localities per unit time is critical. While the range of diversities increased during the two intervals (driven by an increase in the maximum in both instances), the pattern is much more complicated when mean, sampling-standardized, alpha diversity is considered. Preliminary results indicate that the intervals of increasing mean alpha diversity were not driven by greater sampling of low-paleolatitude floras, perhaps because the locus of peak diversity shifts from equatorial to mid-latitudes as climate shifts from ice-house to green-house intervals. Additional work will explore if latitudinal diversity gradients varied over geologic time.