The spatial structure of biodiversity in the fossil record
Heim, Noel Alexander
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Global, continental, and regional scales of diversity are examined in the context of past global climate change. A null model of global diversity highlights the importance of migration as an evolutionary process that can inßuence not only global, but continental diversity as well. The model also highlights extinction as a scale-dependent process that has differing effects on diversity depending on the scale of observation. The model results are conÞrmed by a global-scale data set of Ordovician invertebrates. A continental scale examination of the latitudinal diversity gradient in the tropical regions of western Pangea (Laurentia and South America) during the Carboniferous further highlight the differences between continental and global diversity. Previous global-scale studies produced simple symmetrical latitudinal diversity gradients. The data for western Pangea suggest a more complex and potentially asymmetric gradient during the Carboniferous. The data do support, however, a previously recognized global shallowing of the latitudinal diversity gradient with the onset of Gondwanan glaciation in the middle Carboniferous. A regional Þeld study of middle Carboniferous paleocommunities was conducted in the southern Ozark highlands of Arkansas and Oklahoma. This study incorporated presence-absence and abundance information for brachiopod genera on the tropical shallow-water Arkoma Shelf. These data do not show the extinction event previously recognized at the global scale. Additionally, the Þeld data show remarkable stability in the overall diversity structure of the latest Mississippian (Chesterian) and earliest Pennsylvanian (Morrowan) paleocommunities. Additive diversity partitioning of taxonomic richness and evenness show no differences between the Chesterian and Morrowan in among-collection diversity, within-bed diversity, and among-bed diversity. Likewise the overall shape of the rank-abundance distribution is preserved, although the rank-order of taxonomic membership is not. These data suggest that tropical carbonate platforms were relatively stable and recovered quickly from major environmental perturbations, like a fall in sea level. They also suggest that strong interspeciÞc interactions were not persistent at the stage level. All of these analyses highlight the spatial complexities that are masked when diversity is measured at the global scale.
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