Forms and Regulation of the Paleozoic Marine Latitudinal Diversity Gradient

The pattern of taxonomic richness peaking at the equator and dwindling off towards the Earth’s poles is known as the latitudinal diversity gradient (LDG). It was among the first global biodiversity pattern to be discovered, but is also one that, to date, has resisted comprehensive explanation. The Paleozoic provides a unique perspective to study how the LDG has variated and its form regulated is in the deep time. Here a summary of genus-level global marine invertebrates fossil data was obtained from the Paleobiology Database and used to evaluate LDG patterns for each Paleozoic stage. After data cleaning, this dataset included 485,129 occurrences of 18,234 genera distributed across 85,259 fossil localities. Bias in the observed fossil record, as recorded by these data, was addressed via rarefaction, shareholder quorum subsampling and bootstrap approaches. Pattern analyses of LDG form across Paleozoic stages reveal seven main patterns. Location of latitudinal peak diversity relative to the paleoequator and the steepness of the LDG slopes across latitudes, represent the features that distinguish these seven LDG patterns across Paleozoic time. Although all observed LDG patterns are characterized by a low-latitude richness peak from 30°N to 30°S, they are not symmetrical about the paleoequator in most stages. Comparison of variation in LDG geometries with time-series variation in a suite of environmental proxies suggests that variation in tectonic plate configurations and aspects of global climate change are associated closely with the time-series of LDG variation. These associations suggest that the form of the LDG at different points in earth history was structured by secular changes in global climate states since tectonic configurations would be expected to affect organismal populations primarily through variation in environmental state. Moreover, our results suggest that the current LDG represents a form that is exceptionally balanced – and so somewhat atypical – when compared with LDG forms that existed over much of the Paleozoic.