323-4 Analysis on the Origination Selectivity for Body Size of Bivalvia

Abstract:

Earth’s biodiversity is directly impacted by the gaining and loss of new and existing taxa. The cycle of origination and extinction has greatly changed the marine organisms that exist today. Much of the biodiversity is a function of taxonomic additivities, morphologies, and ecological modes of life. Much of the previous work in the cycle of origination and extinction focuses mainly on the extinction side of the equation; there are relatively few quantitative studies of the selectivity of origination. The overall goal of the study is to determine trends in origination selectivity patterns with respect to body size and motility in the molluscan class Bivalvia. 

This study uses nearly 2000 Phanerozoic marine bivalve genera with body sizes and stage-resolved stratigraphic ranges. We also incorporate the level of bivalve motility in our analyses. The primary analysis is a multiple logistic regression to assess the impact of body size and motility on the origination of new genera. Our results show that bivalves show nearly an order of magnitude increase in average body size between the Ordovician and Jurassic. This sustained increase is followed by a decrease through the Paleogene. Motility followed the Bambach and Bush (2007) ecospace framework. To simplify analyses, we converted coded motility as a binary variable: non-motile and motile. Motile genera include those who are fully motile and facultatively motile in the original ecospace framework. During most of the Paleozoic, the proportion of motile and non-motile genera is approximately equal. However, starting in the Pennsylvanian, the relative proportion of motile genera increases substantially. 

Our logistic regression analysis shows that over the Phanerozoic, new genera tend or originate at small body sizes, while there is no strong preference over newly originating genera being motile. Consistent with previous studies, an analysis of extinction selectivity shows that small genera also tend to morevulnerable to extinction, while motile genera are more likely to go extinct. These results suggest high turnover at small body size within bivalves. 

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