LATE PALEOZOIC FUSULINOIDEAN GIGANTISM DRIVEN BY ATMOSPHERIC HYPEROXIA

Numerous examples of evolution toward large size documented in the fossil record have led to widespread speculation regarding the underlying selection pressures. Although variation in atmospheric oxygen concentrations has been proposed to account for episodes of size increase in foraminifers, insects, fish, amphibians, and mammals, analyses linking gigantism to increases in atmospheric oxygen (hyperoxia) are inconclusive. Here, we directly test the hyperoxia hypothesis by examining the Late Paleozoic fossil record of fusulinoidean foraminifers, a dramatic example of protistan gigantism with some individuals exceeding 10 cm in length. Fusulinoideans are an ideal test because of their abundance and diversity as fossils and lack of a complex respiratory or circulatory system. Here, we assemble and examine comprehensive regional and global, species-level datasets containing 274 and 1818 species, respectively. A statistical model of size evolution forced by pO₂ is conclusively favored over alternative models. Moreover, the ratios of volume to surface area in the largest fusulinoideans are consistent in magnitude and trend with a mathematical model based on oxygen transport limitation. We further validate the hyperoxia-size model through an examination of modern foraminiferal species living along a measured gradient in oxygen concentration. These findings confirm long-standing speculation that hyperoxia was the driver of late Paleozoic gigantism and suggest that environmental change may impact size evolution through direct effects on physiology.