A major new inventory of living marine Bivalvia (Mollusca) is based on 29 regional faunas. These again pick out strong latitudinal and longitudinal gradients in taxonomic diversity, but there are indications that the patterns are not so regular as previously thought. There are signs of asymmetry between the Northern and Southern Hemisphere latitudinal gradients, with the former tending to be more regular than the latter. Northern gradients are also characterized by a marked inflection at approximately 30°N, and the three Australian provinces seem to form a distinct “hotspot” in the Southern Hemisphere. The larger of the two tropical high-diversity foci (the southern China-Indonesia-NE Australia one) appears to be much more nearly arcuate in plan view than oval and is closely associated with the world's richest development of coral reefs.
A taxonomic and stratigraphic analysis reveals that the steepest latitudinal gradients are associated with the youngest bivalve clades. The most striking pattern is that shown by the heteroconchs, an essentially infaunal taxon that radiated extensively throughout the Cenozoic era. Steep gradients are also characteristic of the relatively young anomalodesmatan and arcoid clades and, somewhat surprisingly, the predominantly epifaunal pteriomorphs. Although the latter taxon falls within an older (i.e., “late Paleozoic–Jurassic”) group of clades, it is apparent that certain elements within it (and in particular the Pectinidae) radiated extensively in the latest Mesozoic–Cenozoic. A small but significant component of the later stages of the adaptive radiation of the Bivalvia comprised epifaunal taxa.
The presence of the steepest latitudinal gradients in the youngest clades provides further evidence that the Tropics have served as a major center of evolutionary innovation. Even though some sort of retraction mechanism cannot be completely ruled out, these gradients are most likely the product of primary radiations. Clade history can be an important determinant of contemporary large-scale biodiversity patterns. The markedly lower diversity of some bivalve clades, such as the heteroconchs, in the high-latitude and polar regions may simply reflect the fact that they are not yet fully established there. In a way that is reminiscent of the onshore-offshore radiation of certain benthic marine invertebrate taxa, it may take periods of tens or even hundreds of millions of years for bivalve clades to disseminate fully across the earth's surface.
The persistent spread of taxa from low- to high-latitude regions should perhaps come as no great surprise, as the tropical ocean is very much older than either of the polar ones. The late Cretaceous–Cenozoic evolutionary radiation of the Bivalvia was accompanied by a marked deterioration in global climates, and many new groups have yet to be fully assimilated into cool- and cold-water benthic ecosystems.