Article: Geographical, environmental and intrinsic biotic controls on Phanerozoic marine diversification
The Paleobiology Database now includes enough data on fossil collections to produce useful time series of geographical and environmental variables in addition to a robust global Phanerozoic marine diversity curve. The curve is produced by a new ‘shareholder quorum’ method of sampling standardization that removes biases but avoids overcompensating for them by imposing entirely uniform data quotas. It involves drawing fossil collections until the taxa that have been sampled at least once (the ‘shareholders’) have a summed total of frequencies (i.e. coverage) that meets a target (the ‘quorum’). Coverage of each interval’s entire data set is estimated prior to subsampling using a variant of a standard index, Good’s u. This variant employs counts of occurrences of taxa described in only one publication instead of taxa found in only one collection. Each taxon’s frequency within an interval is multiplied by the interval’s index value, which limits the maximum possible sampling level and thereby creates the need for subsampling. Analyses focus on a global diversity curve and curves for northern, southern and ‘tropical’ (30°N to 30°S) palaeolatitudinal belts. Tropical genus richness is remarkably static, so most large shifts in the curve reflect trends at higher latitudes. Changes in diversity are analysed as a function of standing diversity; the number, spacing and palaeolatitudinal position of sampled geographical cells; the mean onshore–offshore position of cells; and proportions of cells from carbonate, onshore and reefal environments. Redundancy among the variables is eliminated by performing a principal components analysis of each data set and using the axis scores in multiple regressions. The key factors are standing diversity and the dominance of onshore environments such as reefs. These factors combine to produce logistic growth patterns with slowly changing equilibrium values. There is no evidence of unregulated exponential growth across any long stretch of the Phanerozoic, and in particular there was no large Cenozoic radiation beyond the Eocene. The end-Ordovician, Permo–Triassic and Cretaceous–Palaeogene mass extinctions had relatively short-term albeit severe effects. However, reef collapse was involved in these events and also may have caused large, longer term global diversity decreases in the mid-Devonian and across the Triassic/Jurassic boundary. Conversely, the expansion of reef ecosystems may explain newly recognized major radiations in the mid-Permian and mid-Jurassic. Reef ecosystems are particularly vulnerable to current environmental disturbances such as ocean acidification, and their decimation might prolong the recovery from today’s mass extinction by millions or even tens of millions of years.