Shifting sources of productivity in the coastal marine tropics during the Cenozoic era

Changes in the rates and sources of marine primary production over time are difficult to document owing to the absence of direct estimates of past productivity. Here, I use the maximum body sizes of the largest species in each of 23 tropical shallow-water marine molluscan guilds (groups of species with similar habits and trophic roles) to trace the relative importance of planktonic and benthic primary productivity from the Eocene (55 Ma) onwards. The largest members of guilds are least constrained in exploiting resources and therefore reflect the availability and accessibility of those resources most accurately. Maximum sizes of suspension-feeders and predators increased by a factor of 2.3 and 4.0, respectively, whereas those in four out of five herbivorous guilds declined. I interpret these patterns, which are discernible throughout the coastal tropics, to mean that primary production in the Eocene marine tropics was concentrated on the seafloor, as is the case today on offshore reefs and islands, and that the Miocene to the recent interval witnessed a dramatic increase in planktonic productivity along continental margins. The rise in planktonic fertility is best explained by an increase in nutrient supply from the land associated with intense global tectonic activity and more vigorous ocean mixing owing to cooling.