DISPARIFICATION BEFORE, DURING, OR AFTER DIVERSIFICATION? TESTING AN EARLY BURST HYPOTHESIS FOR FUNCTIONAL DIVERSITY IN EARLY PALEOZOIC BIVALVES

The Cambrian diversification of complex animals is generally seen as an exceptional burst of morphological and functional diversity. Bivalve mollusks originated as part of this Cambrian burst but did not diversify taxonomically until the Ordovician. We study the accumulation of new modes of life in Early Paleozoic bivalves relative to their taxonomic diversification, testing for an early burst in functional diversity using a disparity-diversity plot (Jablonski 2017, Evolutionary Biology 44:451) and a new Bayesian analysis of an early bivalve phylogeny (Carter et al. 2000, Evolutionary Biology of the Bivalvia, 47). We assigned 5 Cambrian and 177 Ordovician genera across 11 stages to functional groups and plotted disparity (number of functional groups) against log-transformed diversity (number of genera). For this time interval, rates of taxonomic and functional diversification were concordant (Jablonski’s Type 2 pattern). When analyzed in the context of standing diversity in late Permian and latest Cretaceous, functional diversity accumulated more slowly than taxonomic diversity, suggesting a shift from concordant rates of diversification and disparification to diversity accumulating within existing functional modes. This redundancy in certain functional groups has persisted through the evolution of the global bivalve fauna to today, although the relative ranking of diversity within functional groups has shifted over time. Revisiting a parsimony-based phylogeny of 67 early Paleozoic bivalve species (Carter et al. 2000), we used Bayesian methods to infer a chronogram, onto which we mapped composite functional group data and fitted alternative macroevolutionary models. The Early Burst model was not significantly supported, whereas the Lambda model–indicating a constant rate accumulation in functional diversity–received moderate support, agreeing with the Type 2 disparity-diversity pattern. More phylogenetic information would be valuable, but these preliminary results suggest that taxonomic and functional diversity of early Paleozoic bivalves increased at a comparable rate, with no early burst in their functional diversification. Such patterns may be characteristic of clades that originated in the Cambrian but lagged in diversification until the Ordovician Biodiversification Event.