Late Cenozoic evolution of the latitudinal diversity gradient

Aim: The Late Cenozoic flourishing of polar marine ecosystems, just when temperatures were reaching their lowest levels, has always seemed anomalous. Such an observation is coupled with an increasing volume of molecular phylogenetic evidence to indicate that some polar taxa radiated at exceptionally high evolutionary rates. The canonical latitudinal diversity gradient (LDG) may not be underpinned by a parallel gradient in evolutionary rates. This study provides a review of this critical question.Location: A variety of polar - tropical comparisons are made, with some emphasis on the Southern Ocean.Taxa: Both marine and terrestrial taxa are used, with some emphasis on marine Mollusca.Methods: High - low latitude comparisons of evolutionary rates over the last 66 Myr are made using two approaches: rates derived from molecular phylogenies, and rates derived from the fossil record. For the former of these data are taken from various marine and terrestrial taxa, and for the latter use is made of the excellent fossil record of marine Mollusca.Results: For approximately the first half of the Cenozoic era (i.e., 66 Ma onwards) there is strong palaeontological evidence to suggest that the global LDG developed in a conventional way, with higher rates of origination in the tropics than at the poles. However, there then appears to have been a marked change in the latter half of the Cenozoic when origination rates increased in the high-latitude and polar regions. The modern latitudinal diversity gradient may be a fossil feature that formed in the Early Cenozoic.Main Conclusions: As the extra-tropical regions expanded through the Cenozoic they provided increasingly more ecological opportunities for a wide variety of marine and terrestrial taxa. Substantially lower temperatures in the polar regions were clearly not an obstacle to spectacular evolutionary radiations such as that based on the krill ecosystem.