THE ORIGIN OF ADAPTIVE ZONES: COMPARATIVE ECOLOGICAL DIVERSITY (RICHNESS AND DISPARITY) OF HIGHER TAXONOMIC CATEGORIES

Although taxonomic practice explicitly deals with phylogenetic relatedness, high taxonomic categories have long been construed as reflecting information about ecological identity. Here I address the basic question of whether high taxonomic levels (phyla and classes) during the Early and Middle Paleozoic were ecologically distinctive from one another, and the roles played by constraint, convergence, and shifting ecological habits. I also evaluate how similar these higher taxa are in terms of their ecological richness and breadth (disparity).

I address these issues using a database of the life habits of ~1700 genera that inhabited deep-subtidal settings during the Cambrian–Devonian. Life habits were operationalized using a quantifiable ecospace framework describing 44 basic autecological characteristics reliably inferred for fossil taxa. This standardized framework facilitates quantitative assessment of the ecological diversity of common Paleozoic phyla and classes, both in terms of life habit richness (number of unique character-state combinations) and disparity (mean Euclidean distance).

Ordination yields an abundantly inhabited empirical ecological landscape punctuated with peaks corresponding to dominant adaptive strategies. Ecological distributions were evaluated by superimposing major higher taxa (eight classes in arthropods, brachiopods, echinoderms, and mollusks) upon this landscape. Although most phyla and classes overlap to some extent, convergence is sparse and limited to few genera. Instead, the overriding pattern is one in which peaks are dominated by only one of the four phyla. Thus, early representatives of major animal taxa are broadly ecologically distinct from one another. This same pattern of ecological individuality is also apparent among constituent classes, although with more complicated degrees of overlap.

Notably, taxa are also distinct in terms of both measures of ecological diversity. After standardizing for differences in sample size using rarefaction, the ecologically most diverse phylum is the mollusks, while trilobites, bivalves, and gastropods are the most diverse classes. Although this result might be biased by the many studies of their functional morphology, such an artifact is unlikely because most classes fall along a linear and broadly continuous regression line.