Did Alpha Diversity Increase during the Phanerozoic? Lifting the Veils of Taphonomic, Latitudinal, and Environmental Biases

We estimate the effects of three biases on the observed alpha diversity of paleocommunities from the Middle Paleozoic and Late Cenozoic. The first bias results from the preferential dissolution of aragonite relative to calcite; this bias can lower the relative abundance and preserved diversity of aragonitic taxa, potentially lowering the rarefied diversity of an entire fossil assemblage. We model the effects of this bias by analytically reinserting aragonitic specimens and taxa into Paleozoic assemblages that have been described in the literature. The aragonitic specimens are inserted using a wide range of reasonable assumptions about the original local paleocommunity composition. Although the dissolution bias is probably not as severe as has been argued by some, our analytical modeling indicates that the average Paleozoic assemblage may have lost up to 29% of its total diversity. The second bias results from the higher diversity of the tropics relative to temperate latitudes, but the Late Cenozoic collections we analyzed from the literature represent temperate assemblages whereas the Paleozoic collections were tropical in origin (the northward drift of North America and Europe through time caused this difference). On the basis of latitudinal diversity gradients in the Late Cenozoic, the diversity of the temperate Late Cenozoic samples should be at least doubled for an accurate comparison to the tropical Paleozoic samples. The third bias is environmental: our Late Cenozoic samples tend to come from more onshore, stressed habitats than the Paleozoic samples. In our study, this factor should reduce the apparent diversity of Late Cenozoic paleocommunities by about 9%. After correcting for these biases, standardized alpha diversity appears to increase by a factor of 3.0–3.7 from the Middle Paleozoic to the Late Cenozoic. Previous studies that did not correct for these biases suggested that alpha diversity increased by a factor of 2.5 times; the earlier studies produced approximately correct results because (by chance) the effects of the biases largely cancel out. In the “consensus” article on marine diversity history, an observed increase in alpha diversity was taken as powerful support for an increase in global diversity from the Paleozoic to the Cenozoic. Although we do not test all conflating factors, this study provides new rigor to this longstanding view on alpha diversity change in the Phanerozoic.