JURASSIC MACROBENTHIC COMMUNITIES: BIOLOGICAL INTERACTIONS, MACRO-EVOLUTION, AND THE MESOZOIC MARINE REVOLUTION

Evaluating the relative importance of biotic versus abiotic factors in governing macroevolutionary patterns is a central question of paleobiology. Based on an extensive compilation of marine macrobenthic faunas, we analyzed patterns of global relative abundances and diversity of ecological groups to infer the role of biological interactions as driving evolutionary forces in the Jurassic. Specifically, we tested the hypothesis of escalation, which states that macroevolutionary patterns were controlled by an increasing pressure exerted by enemies on their victims. Associated with evidence of increasing levels of predation and biogenic sediment reworking (bulldozing), we find an increasing representation of predation- and disturbance-resistant groups during the course of the Jurassic. In particular, we observe increasing proportions of mobile organisms; a decline of vulnerable epifauna living freely on the substrate; and a trend towards infaunalization of the benthos. These trends were most pronounced in the paleotropics, i.e. the region where biological activity is thought to have been highest. The observation that these changes affected several biotic traits and occurred within independent clades argues against the overriding role of a single key adaptive innovation in causing shifts in ecological abundance. Changes in the abiotic environment cannot explain these faunal patterns owing to lacking cross-correlations with physico-chemical parameters such as global sea-level, climate, and sea-water chemistry. We conclude that in Jurassic marine benthic ecosystems, enemy-driven evolution, i.e. escalation, played a major role.