ASSESSING ECOLOGICAL DOMINANCE FROM THE FOSSIL RECORD

Previous studies of the Phanerozoic fossil record have provided a wealth of knowledge concerning large-scale taxonomic diversity trends. However, the ecological context of these trends and the role of ecologically dominant clades are poorly understood. We propose an innovative approach that builds on this taxonomic foundation by assessing the ecological dominance of marine invertebrates. Here, dominant taxa are defined as abundant organisms that play a major role in community and ecosystem processes.

A literature review of 21 groups reveals several methods, largely controlled by the availability and variety of data sources, that can be used to assess their dominance history. A taxic approach can be employed when the studied clade is the most diverse group within a community. For example, rugose corals were both ecologically dominant and most diverse in the Devonian. Dominance in reef facies is recognized through the ecological role of each taxon. As the main framework builder, calcareous sponges were the most important, and thus ecologically dominant, component of reef communities in the Late Paleozoic and Late Triassic. Shells beds, easily identified in the rock record, are another approach used to assess dominance. For example, bivalve shell beds are extensive in the Early Triassic, a time when bivalves were one of the most abundant and dominant groups. Rock types named after a particular clade are another indication of ecological dominance, as illustrated by ‘graptolite facies’. Counts of abundance are also used to estimate dominance. For instance, brachiopods can be considered dominant in the Middle Triassic because they represent 90% of some marine communities. Any one or a combination of these methods can be applied to other taxa in an effort to define ecological dominance. Ultimately, production of a temporal history of dominance for marine invertebrates will allow for a better understanding of how ecological dominance has shaped evolutionary trends.