| Summary: | An impediment to understanding the origin and dynamics of the latitudinal diversity gradient (LDG) -- the most pervasive large-scale biotic pattern on Earth -- has been the tendency to focus narrowly on a single causal factor, when a more synthetic, integrative approach is needed. Using marine bivalves as a model system, and drawing on other systems where possible, we review paleobiologic and biogeographic support for two supposedly opposing views, that the LDG is shaped primarily by (a) local environmental factors that determine the number of species and higher taxa at a given latitude (in-situ hypotheses), or (b) the entry of lineages arising elsewhere into a focal region (spatial-dynamics hypotheses). Support for (a) includes the fit of present-day diversity trends in many clades to environmental factors such as temperature, and the correlation of extinction intensities in Pliocene bivalve faunas with net regional temperature changes. Support for (b) includes the age-frequency distribution of bivalve genera across latitudes, which is consistent with an out-of-the-tropics dynamic, as are the higher species diversities in temperate southeast Australia and southern Japan than in the tropical Caribbean. Thus, both in-situ and spatial-dynamics processes must shape the bivalve LDG, and are likely to operate in other groups as well. The relative strengths of the two processes may differ among groups showing similar LDGs, but dissecting their effects will require improved methods of integrating fossil data with molecular phylogenies. We highlight several potential research directions, and argue that many of the most dramatic biotic patterns, past and present, are likely to have been generated by diverse, mutually reinforcing drivers. JablonskiEtAlShapingLatDivGradientSupplTables1&2Table 1 - Geologic ages of extant marine bivalves, Arctic, Tropical, and Antarctic faunas, with supporting references; Table 2 - extant marine bivalve species in SE Japan and SE Australia, with supporting references
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