COMPARATIVE FUNCTIONAL DIVERSITY OF MARINE ANIMALS: A NOVEL RANKING OF ECOLOGICAL INNOVATION

The animal kingdom displays a remarkable diversity of life habits. Yet some individual clades are ecologically depauperate whereas others are profligate. These relative ecological diversity rankings appear to occur in spite of a clade's duration or taxonomic diversity, proxies for evolutionary opportunity. However, a formal ranking has remained elusive. Here, I present a novel framework for measuring the ecological (functional) diversity of marine animals throughout their evolutionary history. Life habits for 4500 species spanning the Phanerozoic marine animal record were coded using a standardized 18-character ecospace framework. This core life-habit database can be deployed phylogenetically across the Paleobiology Database. Ecological diversity is measured for each major taxon using two numerically independent statistics: ecological richness, the number of unique life habits, and disparity, measured as mean Euclidean distance. Together, these two statistics jointly offer a measure of relative ecological diversity.

After standardizing for sampling heterogeneities, mollusks are unambiguously the ecologically most diverse animal phylum, followed by echinoderms, cnidarians, chordates, and arthropods, with sponges, brachiopods and bryozoans ecologically least diverse. Within classes, gastropods, bivalves, and echinoids are most diverse, with asteroids, lingulatans, and anthozoans ranking highly depending on analytical criteria. In contrast, most brachiopod and bryozoan classes rank poorly. Crinoids, cephalopods, and perhaps actinopterygians are ecologically rich but poorly disparate, whereas trilobites and malacostracans are more disparate than their richness predicts.

Rankings are robust to numerous sensitivity analyses. Exceptions include the gastropods, which are ecologically intermediate during the Paleozoic but assume consistent ecological dominance starting in the Mesozoic. Malacostracans and polychaetes assume substantially greater ecological diversity when including soft-bodied extant taxa, but never best the gastropods.

The consistency of these rankings across time and environmental milieu suggests that the differences are borne ultimately by inherent functional differences embedded deeply within phylogenetic constraints.