ABSTRACT
Mesozoic brachiopods have long afforded the classic case study in biotic replacements. Brachiopod workers have generally addressed the issue in ‘either/or’ terms, either clades are ‘ships that pass in the night’, or direct competition between two clades is a major determinant of taxonomic diversity and ecological dominance. Here I outline an alternative perspective hypothesizing pre-emptive exclusion as an indirect effect of the community structure of Mesozoic benthic communities. The effects of interactions among incumbent clades suppresses the radiation of other clades, such as articulate brachiopods, without direct competitive displacement ever taking place. Brachiopods appear to be generally excluded from the present-day tropics. For rhynchonellides, this pattern originated as early as the Jurassic, and did not change subsequently. In the Jurassic there is also little variability in the higher taxonomic composition of assemblages in low paleolatitudes. While this pattern and its ecological correlates are highly consistent with a provisional model of pre-emptive exclusion, diversity data alone are not sufficient to eliminate other possible mechanisms. I outline preliminary criteria drawing on several types of data to distinguish pre-emptive exclusion from other processes that could generate the observed latitudinal pattern.
