ECOLOGICAL DISPARITY OF DEEP-SUBTIDAL, SOFT-SUBSTRATE ASSEMBLAGES DURING THE CAMBRIAN THROUGH DEVONIAN

Well-preserved samples from deep-subtidal, soft-substrate assemblages during the Cambrian through Devonian are used to document long-term trends in ecological disparity. As defined here, ecological disparity measures the richness of functional types—or adaptive strategies—in an ecological unit. The disparity of individual taxa was operationalized using the ecospace framework, a functional classification of 42 autecological characters including aspects of body volume, mobility, feeding, and microhabitat, among others. Assemblage-wide disparity was measured using hutch richness (number of discreet combinations of functional types in an assemblage) and two measures of ecological dispersion (mean Euclidean distance and maximum range).

Taxonomically, these assemblages display a fourfold increase in average species richness, from eight during the Cambrian to thirty during the Silurian. This increase possibly was linked to increasing population density and decreasing evenness in individual assemblages. The trend in hutch richness generally parallels this trend, with little evidence for saturation in the number of hutches per assemblage. Although Cambrian assemblages tend to share similar numbers of species, their functional compositions vary dramatically. In contrast, assemblages after the Ordovician radiation maintain a constant functional composition, regardless of species richness. While this transition can be linked, in part, to the transition from primarily raptorial, trilobite-dominated biotas in the Cambrian to primarily filter-feeding, brachiopod-dominated biotas in the later Paleozoic, ecological disparity and taxonomical diversity describe largely different patterns. Cambrian assemblages tend to contain fewer numbers of hutches than later Paleozoic assemblages, even when standardized to a common number of species. Furthermore, taxa in individual Cambrian assemblages tend to be functionally more similar to one another than those in later assemblages. However, assemblages from all intervals are equally disparate when standardized to a common number of species. While these assemblages have changed appreciably in terms of the functional strategies of constituent taxa, their overall ecological organization has remained remarkably constant.