LATITUDINAL DIVERSITY GRADIENTS AND THE CENOZOIC FOSSIL RECORD OF THE WESTERN ATLANTIC: CONSIDERATION OF SPATIAL SCALE AND SAMPLING ISSUES

There is widespread agreement that the biodiversity of modern oceans is highest in tropical regions of the world, and decreases in richness towards the poles, albeit with some exceptions. While evidence exists for latitudinal gradients since the Mesozoic, it has been suggested that these have increased in strength through the Cenozoic, perhaps associated with global climatic and tectonic processes. However, efforts to quantify the latitudinal diversity gradient in both the modern marine realm and fossil record are not straightforward, in particular because of variations in spatial sampling intensity, preservation of contemporaneous fossil faunas, and taxonomic consistency. The recent interest in the measurement of biodiversity at varying spatial scales provides an incentive to quantify the nature of these gradients while mitigating confounding biases.

We present an analysis of the relationship between alpha, beta and regional diversity, and temporal variation in strength of the diversity gradient within the Cenozoic fossil record of the western Atlantic region. We focus on Eocene to Pleistocene age faunas integrating abundance data sourced from museum-reposited stratigraphic collections and occurrence data archived in the Paleobiology Database (www.paleodb.org). A diversity gradient is difficult to observe in raw tabulations of regional diversity, because of spatial variations in sampling intensity. Sampling standardized analyses, however, demonstrate that the diversity of tropical regions (<30° N) is higher than that of temperate regions (>30° N) during the Miocene and particularly so during the Plio-Pleistocene. A pattern of increased alpha diversity in tropical regions is also observed from sampling standardized analyses of Miocene and Plio-Pleistocene abundance data. These analyses, conducted on multiple spatial scales with sampling standardization techniques, confirm the presence of Cenozoic latitudinal gradients and highlight the importance of mitigating biases and accounting for local palaeoenvironmental variations in making comparison of diversity along latitudinal clines.