The shapes of trait distributions give insight into evolutionary dynamics, and analyses of bivalve body size can uncover how genus body size, biogeography, and genus age interact to form different body-size distributions in time and space. Here I analyze body sizes of modern marine bivalve species belonging to genera with at least ten species, georeferenced occurrence data for each species, and the ages of all genera, in order to untangle these relationships. While medium- and large-bodied bivalves frequently sport either symmetric or left-skewed body-size distributions, small-bodied bivalve genera almost entirely possess symmetric distributions, significantly differing from larger genera. Genera most enriched in species restricted to the Southern Hemisphere are primarily small-bodied, while those enriched with species that inhabit both hemispheres tend to be large- or medium-bodied, and those enriched in species restricted to the Northern Hemisphere are more evenly distributed across the three size classes, slightly favoring the medium and small sizes. Surprisingly, genus age differs little between regions, size classes, and skew types, even when specific combinations of these three factors are considered. Thus, differences in distribution shape are evidently intrinsic to certain size classes and characteristics of each genus, rather than the signature of particularly old or young genera. Assuming differences are not the result of sampling, contrasts in competition or predation may lead to the different distribution shapes among size classes. Particularly small sizes may be fairly specialized, while medium- and large-bodied genera may be able to have some members reach small sizes for their genus, but not be able to reach particularly large sizes due to energy limitations from other medium and large genera. The contrasts in size occupancy by biogeography also suggest that there are different pressures across the globe, strongly favoring small sizes in the Southern Hemisphere in stark contrast with the tropics and Northern Hemisphere. These distributions may be shaped by different processes, perhaps with active trends sensu McShea particularly common in small-bodied genera, but both passive and active trends present in larger genera, and with these processes showing biogeographic variation.
Geological Society of America Abstracts with Programs. Vol. 55, No. 6
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