It has previously been thought that there was a steep Cretaceous and Cenozoic radiation of marine invertebrates. This pattern can be replicated with a new data set of fossil occurrences representing 3.5 million specimens, but only when older analytical protocols are used. Moreover, analyses that employ sampling standardization and more robust counting methods show a modest rise in diversity with no clear trend after the mid-Cretaceous. Globally, locally, and at both high and low latitudes, diversity was less than twice as high in the Neogene as in the mid-Paleozoic. The ratio of global to local richness has changed little, and a latitudinal diversity gradient was present in the early Paleozoic.

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...

Analyses of how environmental factors influence the biogeographic structure of biotas are essential for understanding the processes underlying global diversity patterns and for predicting large-scale biotic responses to global change. Here we show that the large-scale geographic structure of shallow-marine benthic faunas, defined by existing biogeographic schemes, can be predicted with 89-100% accuracy by a few readily available oceanographic variables; temperature alone can predict 53-99% of the present-day structure along coastlines. The same set of variables is also strongly correlated with spatial changes in species compositions of bivalves, a major component of the benthic marine biota, at the 1°grid-cell resolution. These analyses demonstrate the central role of coastal oceanography in structuring benthic marine biogeography and suggest that a few environmental variables may be sufficient to model the response of marine biogeographic structure to past and future changes in climate.bivalves | climate | macroecology | sea-surface temperature B iogeographic units (BUs), such as provinces or biomes, have been essential for understanding the macroecological and evolutionary processes underlying global biodiversity patterns (1-4) and are increasingly being incorporated in conservation planning (5-8). BUs are also becoming important from a global change perspective as components in models of biotic responses to global climate change in terrestrial settings (9). However, the factors that determine the biogeographic structure of benthic marine species in shallow-water habitats, where marine biodiversity is best documented and environmental changes are projected to be most severe (10-12), remain poorly understood, limiting our ability to model systems-level responses of marine biodiversity to environmental change. Here we use global datasets (Datasets S1, S2, S3, S4, and S5) to show that, in contrast to the more complex relationship between species richness and environment (13,14), the biogeographic structure of coastal and continental shelf habitats can be predicted using just a few oceanographic parameters. This robust first-order link between specific environmental factors and large-scale biotic patterns establishes the importance of climate and oceanography in structuring marine faunas and provides important insights into modeling past and future responses of marine biogeography to global change.We evaluate the correspondence between BUs and oceanographic variables using a model-fitting approach (Materials and Methods) to determine which oceanographic variables (individually or in combination) are most strongly correlated with biogeographic structure at both the ocean-basin and coastline scales (Table 1 and Tables S1 and S2). Our models focus on mean annual values and seasonal ranges of sea surface temperature, salinity, and productivity (hereafter TSP) because these variables have been previously hypothesized, separately or together, to affect taxonomic compositions and species richness in benthic marine system...

The first-order biodiversity pattern on Earth today and at least as far back as the Paleozoic is the latitudinal diversity gradient (LDG), a decrease in richness of species and higher taxa from the equator to the poles. LDGs are produced by geographic trends in origination, extinction, and dispersal over evolutionary timescales, so that analyses of static patterns will be insufficient to reveal underlying processes. The fossil record of marine bivalve genera, a model system for the analysis of biodiversity dynamics over large temporal and spatial scales, shows that an origination and range-expansion gradient plays a major role in generating the LDG. Peak origination rates and peak diversities fall within the tropics, with range expansion out of the tropics the predominant spatial dynamic thereafter. The origination-diversity link occurs even in a "contrarian" group whose diversity peaks at midlatitudes, an exception proving the rule that spatial variations in origination are key to latitudinal diversity patterns. Extinction rates are lower in polar latitudes (> or =60 degrees ) than in temperate zones and thus cannot create the observed gradient alone. They may, however, help to explain why origination and immigration are evidently damped in higher latitudes. We suggest that species require more resources in higher latitudes, for the seasonality of primary productivity increases by more than an order of magnitude from equatorial to polar regions. Higher-latitude species are generalists that, unlike potential immigrants, are adapted to garner the large share of resources required for incumbency in those regions. When resources are opened up by extinctions, lineages spread chiefly poleward and chiefly through speciation.