PHANEROZOIC TRENDS IN ECOLOGICAL DOMINANCE

The secular trends in Phanerozoic marine diversity, both at a global scale and within communities, are relatively well-understood and have been applied successfully to investigate patterns and processes involved in evolutionary radiations, mass extinctions, and biotic turnover in the marine realm. In contrast, changes in the numerical abundance and ecological dominance of groups within communities have largely been ignored in favor of the taxic approach. We have attempted to compile a temporal history of ecological dominance in the fossil record from reef facies, shell bed, rock type, and community abundance data. Groups can be subdivided, based on their overall abundance and duration of dominance, into those that were major (e.g. stromatoporoids, trilobites), minor (e.g. nautiloids, rudist bivalves), and trace dominants (e.g. calcareous sponges, inarticulated brachiopods). Overall trends in ecological dominance broadly, but not exactly, parallel previously-established taxonomic diversity changes. More detailed patterns of dominance are also apparent even with the sparse nature of existing data. Some groups, such as trilobites, became dominant early or at the beginning of their history and lost their dominance in a gradual or step-wise fashion. Other organisms, including ammonoids, existed for a considerable time as accessory components of marine communities before rising to dominance, only to lose their dominant position abruptly during a mass extinction. Scleractinian corals are an example of a group that enjoyed two periods of dominance separated by an interval during which they were less important. Understanding the specifics of how, when, and how many times a group gained and lost ecological dominance will help to elucidate the interdependent effects of extinctions, environmental change, and ecological interactions on Phanerozoic marine ecosystems.