NO EARLY BURST IN ECOLOGICAL DIVERSITY IN CLASS BIVALVIA: INTEGRATING TAXIC AND PHYLOGENETIC APPROACHES

Despite its mid-Cambrian origin, Class Bivalvia did not diversify taxonomically until the Ordovician. Their ecological diversification is less clear and might be expected to show the early burst seen in other Cambrian-originating groups. However, plotting disparity (number of functional groups) against log-transformed diversity (number of genera) of 5 Cambrian and 180 Ordovician genera across 11 stages (mid-Cambrian to end-Ordovician) shows a concordant rate of taxonomic and functional diversification during this period (Type 2 pattern per Jablonski 2017 Evol. Biol. 44:451). This result is robust to several tests, including sensitivity analyses that force a stepwise evolution of genera and functional groups, which rarely recover an early burst given the continued accumulation of genera within functional groups through the Ordovician. The early burst pattern receives even less support when including data from the late Permian and latest Cretaceous, where taxa tend to accumulate within existing functional groups, an evolutionary trend that continues to the present day in this clade.

Using a parsimony-based phylogeny of 67 early Paleozoic bivalve species (Carter et al. 2000, in Evolutionary Biology of the Bivalvia), we constructed a Bayesian chronogram labeled with functional data and fitted alternative macroevolutionary models. The Lambda model is the best supported, indicating a strong phylogenetic signal and slightly accelerating diversification rates, consistent with the Type 2 disparity-diversity pattern. We tested the robustness of our results by generating diversity-disparity plots assuming different evolutionary patterns in the Cambrian and performing sensitivity analyses on the model fitting procedure.

Thus, while bivalves arose during the early burst of body plans in the Cambrian, our analyses suggest that they accumulated ecological diversity at a steady per-taxon rate throughout the early Paleozoic, with no early burst in functional diversification. Such patterns may be characteristic of other clades established in the Cambrian but participated mainly in the Ordovician Diversification.