The Mesozoic Marine Revolution

Abstract

The Mesozoic Marine Revolution (MMR) was a major evolutionary episode involving the large-scale restructuring of shallow-marine benthic communities and the rise to dominance of the Modern Evolutionary Fauna. Although the majority of studies published on the MMR have been based on the body-fossil record, the ichnologic record yields valuable insights into this evolutionary event, most notably regarding the degree of infaunalization, complexity of infaunal tiering structures, and predation intensity. The main groups of bioturbators involved in the MMR were crustaceans, bivalves, echinoids, and “worms,” whereas the most important bioeroders were sponges, gastropods, bivalves, echinoids, and “worms.”

The Triassic, encapsulated between two major mass extinctions, can be regarded as setting the stage for the MMR. The sparse ichnologic information available suggests that full recovery from the end-Permian mass extinction had taken place in equatorial carbonate settings by the Middle Triassic, although ichnofaunas show limited infaunalization and relatively simple tiering structures. However, a few key players, most notably some decapod crustaceans, were already dominant in carbonate settings. Jurassic ichnofaunas reveal a compositional turnover, signaling the rise to dominance of the Modern Evolutionary Fauna. This faunal turnover occurred by the Early Jurassic, as indicated by the taxonomic composition and increased diversity of bioturbation structures and the complexity of infaunal tiering. During the Cretaceous, a continuation of the trends established in the Jurassic was recorded. However, by the end of the Cretaceous, a modern-style benthic–pelagic coupling pattern was established, accompanied by an increase in global ichnodiversity. Rapid development of coccolithophores provided a new substrate (chalk) and an increasing flux of organic matter to the seafloor, enabling establishment of complex tiering structures, unknown in older deposits. The degree of macrobioerosion indicates an increasing participation of some players typical of modern communities. Paleogene bioturbation structures are similar to those of the late Mesozoic. Macrobioerosion styles also persisted across the Cretaceous–Paleogene boundary, albeit with an increased role for bioerosion by sponges and fishes later in the Paleogene. Predation pressures (drilling and durophagy) increased during the Eocene. Ichnofaunas in shallow-marine sediments record continued expansion of diversity during the Neogene, as well as more complex tiering structures. Predation pressure continued to rise, involving primarily marine mammals.

Although the MMR for the most part took place in shallow, fully marine settings, the trace-fossil record also provides evidence for its expression in the deep sea and marginal-marine, brackish-water settings. Irregular echinoids and decapod crustaceans apparently had migrated to the deep sea by the Late Jurassic, whereas most of the main players of the MMR in fully marine settings (e.g., decapod crustaceans, bivalves, worms) were also dominant in brackish-water settings since the beginning of this major evolutionary event. Trace-fossil data indicate that infaunalization predates an increase of predation pressures by approximately 50 Myr, suggesting a complex set of feedback mechanisms between predation and infaunalization rather than simple cause and effect between the two. Turnover in ichnofaunal composition and the increased infaunalization that took place during the MMR strongly supports the “bulldozing hypothesis” (the notion that biological disturbance increased through the Phanerozoic), indicating a dramatic rise in bioturbational sediment processing by elements of the Modern Evolutionary Fauna.