Cenozoic Molluscan Diversification: Dissecting the Global Pattern to Reveal the Biotic Response to Regional Paleoenvironmental Controls

The Cenozoic trajectory of global biodiversity is often portrayed as an interval of considerable diversification among marine invertebrates. These changes are contemporaneous with major paleogeographic events and abrupt climatic transitions. The Cenozoic is also of considerable interest to paleontologists because it links together evolutionary and macroecological processes observed in the modern marine environment with those inferred from deeper in the Phanerozoic fossil record.

An extensive dataset (http://paleodb.org) of fossil molluscan occurrences permits analysis of how spatial components of global biodiversity vary throughout the Cenozoic. On a global scale, sampling-standardized diversity appears to have increased only slightly during the course of the Eocene-Pleistocene, but shows small drops during the Early Oligocene and a slight decline since the Middle Miocene. However, there is considerable variation between major latitudinal zones (polar, temperate, and tropics) with respect to the timing and magnitude of Cenozoic diversity change. The tropics show significant diversification from the late Paleogene to the Pleistocene, while diversity of temperate zones in both hemispheres, although offset, declines through the Neogene. The Neogene divergence of these regional trajectories is concordant with the strengthening of the latitudinal gradient of diversity. Temporal changes in the shape of the gradient reflect not only increased tropical diversity but also a dramatic decline in the richness of high latitude faunas.

Preservational biases are likely to affect these patterns in both a temporal and spatial sense. Increased geographic sampling affects perceptions of provinciality and the compositional heterogeneity of the dataset, thus enhancing beta diversity. Lithification not only disguises temporal changes in community-level richness, but also blurs spatial differences in richness between tropical (carbonate-dominated) and temperate (silicicilastic-dominated) regions. True patterns of global diversification are a complex of varying regional diversity trends, biogeographic complexity, and changing alpha diversity.