DIGGING IN: DIVERSITY, DISPARITY, AND THE REPEATED EVOLUTION OF THE BORING HABIT IN THE MARINE BIVALVIA

Phylogenetic niche conservatism holds that related species will tend to occur in similar habitats, and track their niches despite changes in their environment. But some species must, and demonstrably do, colonize new niches. One of the most striking examples of such shifts is the repeated evolution of the hard-substratum (rock and coral) boring habit in the marine Bivalvia from a variety of infaunal and epifaunal ancestors, which when mapped phylogenetically is seen to have independently evolved at least ten times post-Paleozoic. This radical ecological transition requires a number of morphological alterations, some of which are potentially maladaptive for non-boring life-strategies, impeding reversals. It has been hypothesized that the resurgence of corals to serve as substratum, and pressure from the rise of durophagous predators, promoted evolution of boring bivalves. To investigate the timing and magnitude of transitions to the boring habit in the Bivalvia, we use a phylomorphospace built from a database of 3D micro-CT shell scans, published molecular phylogenies, and occurrence data from the fossil record. Using this approach, we can evaluate the evolutionary consequences for disparity and diversification of colonizing the boring niche. In literally breaking new ground and escaping their various ancestral niches, boring lineages shift in morphospace, repeatedly but not universally converging on a cylindrical shape disadvantageous for most bivalve lifestyles, which may indicate boring to be an evolutionary dead-end. In no case does the species richness of a clade of borers exceed that of their non-boring sister group, indicating that this new niche, however advantageous in the short run, does not open opportunities for more prolific taxonomic diversification; however, their morphological disparity is higher on a per-genus basis. Some boring clades are actually impoverished relative to sisters, suggesting elevated extinction and/or damped speciation rates following the transition. However, many boring lineages are ancient and have survived extensive environmental changes and extinction events, including the end-Cretaceous extinction. New colonizations of the boring niche have arisen through geological time, and once established, despite low diversity, boring lineages persist.