ED026-0057
Squids, Snails, and Shellfish: How Oxygen Affected Molluscan Body Size in the Ordovician
Thursday, 10 December 2020
Jonathan Chen1, Jackson Gosler1, Emma Klemperer1, Pedro Monarrez1,2, Michael Pimentel-Galvan1,2, Noel A Heim3 and Jonathan Payne1,2, (1)Stanford University, Stanford Earth Young Investigators, Stanford, CA, United States, (2)Stanford University, Department of Geological Sciences, Stanford, CA, United States, (3)Tufts University, Department of Earth and Ocean Sciences, Medford, MA, United States
Abstract:
The Great Ordovician Biodiversification Event (GOBE) was a major diversification event of marine fauna. The object of this study was to find the relationship between atmospheric O2 concentration (pO2) and mollusc body size to understand how body size of new molluscs compares to the body size of existing molluscs during the GOBE. This opportunity is significant as the biological importance and trends of body size are understudied, and the collective understanding of its relation to O2 levels is incomplete. We tested for the following correlations: 1) pO2 and body size of all genera; 2) pO2 and body size for mollusc genera (focusing on 3 classes: Cephalopoda, Gastropoda, and Bivalvia); 3) pO2 and the difference between the body size of genera originating during the Ordovician and the body size of all genera (repeated for both molluscs and all Ordovician genera). Our results show that the mean maximum size of molluscs increased significantly at the beginning of the GOBE, but by the Darriwilian, body size began to decrease. There is also a strong negative correlation (Pearson’s r: -0.91, p<0.01) between pO2 and mean mollusc body size. Upon investigating the difference between average maximum body size of molluscs that originated throughout the Ordovician and the average maximum body size of all genera, however, we found a strong positive correlation (Pearson’s r: 0.95, p=0.01).
The strong negative correlation between pO2 and mollusc body size might be explained by molluscs doing well in low pO2 environments, allowing them to outcompete other groups without the same adaptations when pO2 levels were low and attain larger body sizes. Despite this, they do ultimately benefit from higher pO2, explaining why new mollusc genera were larger than preexisting molluscs when there is more pO2 and vice versa. The opposite trends we observed between originating mollusc size and average mollusc size suggest that standing mean body size cannot be used to predict the body size of originating taxa. Our analysis of body size data of molluscs in the Ordovician revealed that molluscan body size negatively correlates with pO2 levels, yet the increase in the size of originating genera as opposed to all genera is positive, thus revealing that average body size of all genera does not reflect the body size of new genera.
