Out of the tropics, but how? Fossils, bridge species, and thermal ranges in the dynamics of the marine latitudinal diversity gradient

Abstract

Latitudinal diversity gradients are underlain by complex combinations of origination, extinction, and shifts in geographic distribution and therefore are best analyzed by integrating paleontological and neontological data. The fossil record of marine bivalves shows, in three successive late Cenozoic time slices, that most clades (operationally here, genera) tend to originate in the tropics and then expand out of the tropics (OTT) to higher latitudes while retaining their tropical presence. This OTT pattern is robust both to assumptions on the preservation potential of taxa and to taxonomic revisions of extant and fossil species. Range expansion of clades may occur via "bridge species," which violate climate-niche conservatism to bridge the tropical-temperate boundary in most OTT genera. Substantial time lags (∼5 Myr) between the origins of tropical clades and their entry into the temperate zone suggest that OTT events are rare on a per-clade basis. Clades with higher diversification rates within the tropics are the most likely to expand OTT and the most likely to produce multiple bridge species, suggesting that high speciation rates promote the OTT dynamic. Although expansion of thermal tolerances is key to the OTT dynamic, most latitudinally widespread species instead achieve their broad ranges by tracking widespread, spatially-uniform temperatures within the tropics (yielding, via the nonlinear relation between temperature and latitude, a pattern opposite to Rapoport's rule). This decoupling of range size and temperature tolerance may also explain the differing roles of species and clade ranges in buffering species from background and mass extinctions.biodiversity | biogeography | climate | macroecology | macroevolution T he latitudinal diversity gradient (LDG), meaning the decrease in the number of species and higher taxa from the equator to the poles, is as pervasive among marine organisms as it is on land (1, 2). Although the marine LDG is increasingly well documented, we are only beginning to understand the evolutionary and biogeographic dynamics of speciation, extinction, and distributional shifts that generate and maintain it. Here we evaluate these dynamics in marine bivalves, a group that not only parallels diversity patterns of the overall marine biota (1-3), but permits the integration of biogeographic, phylogenetic, and spatially explicit paleontological data (4, 5) and has thus become a model system for macroecological and macroevolutionary analysis. Extending our previous work, we show that clade origination in the tropics and range expansion out of the tropics [the OTT model (5)] are major factors in the origin and maintenance of the bivalve LDG. We reanalyze and update paleontological data on the OTT dynamic and present evidence that at least some bridge species, whose ranges cross the tropical/extratropical boundary, are important in the expansion of lineages along the LDG. Although bridge species violate niche conservatism with their expanded thermal ranges, we find that species with narrow th...

Convergence, divergence, and parallelism in marine biodiversity trends: Integrating present-day and fossil data

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Macroevolutionary consequences of profound climate change on niche evolution in marine molluscs over the past three million years

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