Hierarchies of ecologic changes and their roles in the Phanerozoic taxonomic and ecologic diversification history of the Class Bivalvia

Identification of ecologic diversification patterns and their correlation with relevant ecologic models is a major challenge in paleobiology. In part, this difficulty reflects that different evolutionary groups followed varying diversity trajectories through time; more specifically, the rate and timing of taxonomic and ecological diversification vary considerably when comparing the evolutionary histories of different groups. The pattern depends upon the nature and degree of ecologic and morphologic changes in the group – some of these changes are more novel (i.e., breakthroughs) as compared to other small-scale changes, which result in more modest modifications. In this study, we examine various models to explain the ecologic diversification displayed by marine Bivalvia, a group with an exceptional fossil record. We propose a hierarchical classification of ecologic changes (i.e., novelties and innovations) to explain the temporal trajectories of ecologic diversification within the context of the clade's taxonomic diversification history. Our results show a non-random evolution pattern in the Bivalvia through time characterized by three primary phases: 1) a rapid increase in ecologic diversification during the Cambrian and Ordovician, with comparatively low taxonomic diversity; 2) a slower but still increasing trend in ecologic diversity in the later part of the Paleozoic into the Mesozoic; and 3) a final phase of stasis in the Cenozoic, with only one novelty appearing in the Bivalvia accompanied by an increase in genera and species, occupying the existing ecologic modes of life. This overall pattern is suggestive of a late-filling model of evolution. The results suggest that the total number of prospective life modes – life modes that a clade could possibly exploit – did not remain time-constant but increased in conjunction with the evolution of novelties and innovations. Moreover, the data show that only a small portion of the prospective life modes were actually realized by the Class Bivalvia, and, in contrast to the commonly theorized pattern of continuous increase through the Phanerozoic, the proportion of realized life modes with respect to the prospective life modes did not increase.