Taxonomic rarity along latitudinal gradients: The distribution of fossil organisms through space and time

Rare taxa, defined by their low relative abundances within samples, share diagnostic characteristics such as specialization of life habit, small geographic range, and lower population density. Here, we investigate the relationships between geographic and numerical rarity through space and time using fossil molluscan occurrence and abundance data to gain a better understanding of the distribution of organisms within communities along a latitudinal gradient on an evolutionary time scale.The data used here are derived from two sources 1) abundance data from bulk samples collected from Neogene (24 million years ago to present) western Atlantic fossil assemblages and 2) occurrence data obtained from the Paleobiology Database (http://paleodb.org) for the same geographic range and temporal span. These data permit the investigation of processes that control the nature of latitudinal gradients (e.g., global climate and oceanography). This research provides the opportunity to investigate long-term ecological trends by observing the changing distributions of rare taxa, which may be particularly stenotypic and sensitive to environmental variability. Our previous research illustrated a strengthening of the latitudinal diversity gradient through time, with greater taxonomic richness and ecospace utilization at tropical latitudes. The present research identifies a clear pattern in the abundance distributions of taxa from high to low latitudes. The number of individuals represented by rare taxa varies systematically with latitude; fewer individuals belong to rare taxa within communities from temperate and polar regions. In general, common taxa are geographically widespread, while rare taxa are restricted in distribution, although there are interesting exceptions to this pattern that may be ecologically meaningful. Continued investigation into the relationships between numerical rarity, geographic range and paleoecology will contribute to an understanding of the biotic and abiotic factors affecting the distribution and structure of communities through time along a strengthening latitudinal gradient.