Evolution of taxonomic diversity gradients in the marine realm: evidence from the composition of recent bivalve faunas

A major new inventory of living marine Bivalvia (Mollusca) is based on 29 r egional faunas. These again pick out strong latitudinal and longitudinal gr adients in taxonomic diversity, but there are indications that the patterns are not so regular as previously thought. There are signs of asymmetry bet ween the Northern and Southern Hemisphere latitudinal gradients, with the f ormer tending to be more regular than the latter. Northern gradients are al so characterized by a marked inflection at approximately 30 degrees N, and the three Australian provinces seem to form a distinct "hot-spot" in the So uthern Hemisphere. The larger of the two tropical high-diversity foci (the southern China-Indonesia-NE Australia one) appears to be much more nearly a rcuate in plan view than oval and is closely associated with the world's ri chest development of coral reefs. A taxonomic and stratigraphic analysis reveals that the steepest latitudina l gradients are associated with the youngest bivalve clades. The most strik ing pattern is that shown by the hetero-conchs, an essentially infaunal tax on that radiated extensively throughout the Cenozoic era. Steep gradients a re also characteristic of the relatively young anomalodesmatan and arcoid c lades and, somewhat surprisingly, the predominantly epifaunal pteriomorphs. Although the latter taxon falls within an older (i.e., "late Palcozoic-Jur assic") group of clades, it is apparent that certain elements within it (an d in particular the Pectinidae) radiated extensively in the latest Mesozoic -Cenozoic. A small but significant component of the later stages of the ada ptive radiation of the Bivalvia comprised epifaunal taxa. The presence of the steepest latitudinal gradients in the youngest clades p rovides further evidence that the Tropics have served as a major center of evolutionary innovation. Even though some sort of retraction mechanism cann ot be completely ruled out, these gradients are most likely the product of primary radiations. Clade history can be an important determinant of contem porary large-scale biodiversity patterns. The markedly lower diversity of s ome bivalve clades, such as the heteroconchs, in the high-latitude and pola r regions may simply reflect the fact that they are not yet fully establish ed 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 e ven hundreds of millions of years for bivalve clades to disseminate fully a cross the earth's surface. The persistent spread of taxa from low- to high-latitude regions should per haps come as no great surprise, as the tropical ocean is very much older th an either of the polar ones. The late Crc taceous-Cenozoic evolutionary rad iation of the Bivalvia was accompanied by a marked deterioration in global climates, and many new groups have pet to be fully assimilated into cool- a nd cold-water benthic ecosystems.