GLOBAL PHANEROZOIC DIVERSITY IS A FUNCTION OF GEOGRAPHIC SAMPLING INTENSITY

Variation in observed global generic richness over the Phanerozoic must be partly explained by changes in the number of fossils and their geographic spread over time. The influence of sampling intensity (i.e., the number of samples) has been well addressed, but the extent to which the geographic distribution of samples might influence biodiversity is comparatively unknown. To investigate this question, we create steady-state models of genus richness through time by resampling occurrence datasets of modern global biodiversity using spatially explicit sampling intensity defined by the paleogeography of occurrences from the Paleobiology Database (PBDB). Fossil occurrences are parsed into 48 ~11 myr time bins and spatially grouped into 812 equal-area grid cells. For each time slice, the number of taxa and occurrences are recorded for each cell, along with the cell’s latitudinal and longitudinal coordinates. Then, modern grid cells with data sourced from the Ocean Biodiversity Information Service (OBIS) are selected to match the geographic distribution of fossil cells. Occurrences are randomly drawn from these modern cells, so the number of occurrences, cells, and their geographic distribution is as close as possible to fossil record sampling. The total number of modern genera recovered is counted for each time bin, with 25 replicates. Using linear regression, we compare secular richness estimates from the PBDB to predictions of the steady-state models, which sample only modern biodiversity but use PBDB-dictated spatial coverage. Our steady-state models explain 50% of change in uncorrected fossil diversity but only about 25% of variation in sampling standardized estimates. Including additional structural variables (latitudinal range of occurrences, percent of PBDB occurrences from shallow environments) in models further increases their accuracy. A stepwise increase in diversity is observed at the Mesozoic-Cenozoic boundary and is best explained in models using a “Cenozoic or not” factor rather than continuous time. Models comparing the influence of structural variables in the PBDB consistently indicate the importance of geographic factors (e.g., the number of cells, latitudinal range of occurrences) over environmental ones (e.g., the proportion of occurrences from reefal or tropical environments).