BODY SIZE DISTRIBUTION OF MARINE BIVALVES THROUGH THE LATE TRIASSIC MASS EXTINCTION EVENT

Significant fluctuations in the body size of marine animals are commonly recorded through past extinction events. These include a temporary size reduction in surviving taxa in the immediate aftermath (the Lilliput effect) followed by a size increase during the subsequent recovery. In this study, I calculated the geometric mean size of marine bivalves sampled from three sections in the United Kingdom that span the Late Triassic mass extinction event (Rhaetian to Hettangian). Size-frequency distributions for each stratigraphic unit were compiled, and Kernel density estimations were used to estimate the modality of each distribution. Coefficients of variation were calculated, and a Kruskal-Wallis test was performed to detect significant changes in maximum and minimum sizes through time. Directional trends in those genera with the most extensive stratigraphic ranges were analyzed. Finally, a null model was generated to evaluate whether changes in body size were due to chance or some underlying mechanism. At the stratigraphic resolution of this study, the mean size, minimum size, and shape of the size-frequency distribution of the whole paleocommunity did not change significantly through the extinction event. However, species with mean sizes greater than 30mm disappeared during the extinction event. During the recovery, larger species re-appeared, and species that survived the extinction event record significant increases in the mean body size, but also show greater variance. Finally, the post-extinction increase in mean body size of bivalves is significantly greater than would be expected by chance. Through the extinction, bivalve paleocommunities do not record body-size reduction but apparently underwent selective pressure against the largest sized species. These results are consistent with studies of some other mass extinction events. However, is necessary to correlate changes in body size with environmental proxies to fully understand how external factors modulated body sizes under different extinction scenarios.