ATMOSPHERIC OXYGEN CONCENTRATION CONTROLS THE SIZE HISTORY OF FORAMINIFERS

Body size correlates with numerous physiological traits and thus influences organism fitness. However, long-term controls on size evolution remain poorly understood because few datasets spans sufficiently long intervals. One proposed controlling factor is variation in atmospheric oxygen, which is widely argued to have influenced size evolution in numerous taxa, notably gigantism in arthropods during the late Paleozoic. In this study, we compiled a comprehensive genus- and species-level size database of foraminifers (marine protists) to enable an extensive analysis of factors influencing size evolution. Foraminifers are an ideal study group because they are present in all Phanerozoic periods and have been diverse and abundant in shallow-marine habitats since Devonian time. We observe significant correlation between foraminiferan size and atmospheric oxygen concentration. Variation in atmospheric oxygen explains one-quarter to one-third of the variation in 95^th percentile and median size among taxa. Larger size is associated with higher oxygen concentrations, as predicted by simple physiological models based on changes in the ratio of surface area to volume. Because the oxygen content ocean waters is controlled in part by atmospheric pO₂, we interpret the association between foraminiferan size and pO₂ to result from a direct physiological effect of oxygen availability. Multiple regression analysis including six other predictors (pCO₂, sea level, number of named geological formations, δ¹⁸O, δ¹³C, ⁸⁷Sr/⁸⁶Sr) reveals that oxygen is the best predictor of size variation even after controlling for the effects of other variables. ⁸⁷Sr/⁸⁶Sr is the only other significant predictor of size in all models, and it explains substantially less variance in size than atmospheric oxygen concentration does. These findings suggest that oxygen availability is one of the most important global influences on foraminiferan size evolution and provides quantitative support for the hypothesis that widespread Permo-Carboniferous gigantism was enabled by high pO₂.