We still have much to learn about the evolution of taxonomic diversity gradients through geologic time. For example, have latitudinal gradients always been as steep as they are now, or is this a phenomenon linked to some form of Cenozoic global climatic differentiation? The fossil record offers potential to address this sort of problem, and this study reconstructs latitudinal diversity gradients for the last (Tithonian) stage of the Jurassic period using marine bivalves. At this time of relative global warmth, bivalves were cosmopolitan in their distribution and the commonest element within macrobenthic assemblages.
Analysis of 31 regional bivalve faunas demonstrates that Tithonian latitudinal gradients were present in both hemispheres, though on a much smaller magnitude than today. The record of the Northern Hemisphere gradient is more complete and shows a steep fall in values at the tropical/temperate boundary; the Southern Hemisphere gradient exhibits a more regular decline in diversity with increasing latitude.
Tithonian latitudinal gradients were underpinned by a tropical bivalve fauna that comprises almost equal numbers of epifaunal and infaunal taxa. The epifaunal component was dominated by three pteriomorph families, the Pectinidae, Limidae and Ostreidae, that may be regarded as a long-term component of tropical bivalve diversity. Of the mixture of older and newer “heteroconch” families that formed the bulk of the infaunal component, the latter radiated spectacularly through the Late Cretaceous and Cenozoic to dominate tropical bivalve faunas at the present day. This pulse of heteroconch diversification, which was a major cause of the steepening of the bivalve latitudinal gradient, provides important evidence that rates of speciation may be negatively correlated with latitude.
Nevertheless, we cannot exclude the possibility that elevated extinction rates in the highest latitudes also contributed to the marked steepening of bivalve latitudinal gradients over the last 150 Myr. Rates of extinction within epifaunal bivalve taxa appear to have been higher in these regions through the Cretaceous period, but this was largely before any significant global climatic deterioration. Infaunal bivalve clades have had differential success over this time period in the polar regions. Whereas, in comparison with the Tropics, heteroconchs are very much reduced in numbers today, the anomalodesmatans are much better represented, and the protobranchs have positively thrived. We are beginning to appreciate that low temperature per se may not be a primary cause of elevated rates of extinction. Food supply may be an equally important control on both rates of speciation and extinction; those bivalves that have been able to adapt to the extreme seasonality of food supply have flourished in the polar regions.