DYNAMICS OF THE LATITUDINAL DIVERSITY GRADIENT: ANALYSES OF MARINE BIVALVES

The latitudinal diversity gradient is the most striking biodiversity pattern on Earth, with maximum richness of multicellular species and higher taxa near the equator and a decline towards the poles, but the dynamics that generate and maintain this remarkably pervasive trend remain poorly understood. The latitudinal diversity gradient is commonly viewed as the net product of in-situ origination and extinction, with the tropics serving as either a generator of biodiversity (the tropics-as-cradle hypothesis) or an accumulator of biodiversity (the tropics-as-museum hypothesis). However, a global analysis of Late Cenozoic marine bivalve genera challenges the assumption that species and higher taxa originated where they currently reside and indicates a more complex dynamic involving shifts in spatial distributions over time. Despite a strong sampling bias towards temperate latitudes, the fossil record indicates that bivalve genera tend to appear first in the tropics and expand outwards without losing tropical occupancy, while the high latitudes are primarily a diversity sink. We quantified the reliability of the fossil record of each individual bivalve family (N=105) according to the proportion of its living genera known to have a fossil record, and then excluded families with low values when we analyzed the spatial distributions of first occurrences of genera. When we exclude bivalve families in which >25% of the living genera lack a fossil record, a 2:1 ratio of tropical vs. high-latitude first occurrences is seen for both the late Miocene and the Pliocene; this result is not sensitive to the cutoff applied. The tropics are both the primary diversity source and the primary accumulator, suggesting that the long-standing cradle vs museum debate hinges on a false dichotomy. The tropics are so rich today not only because they give rise to many new taxa, but because the geographic ranges of old, mostly widespread taxa overlap there with young and spatially restricted taxa. A clear picture of the dynamics shaping global diversity patterns requires the spatially explicit historical data provided by the fossil record; such approaches can provide a basis for modeling biotic responses to past and future climate changes.