IS THE TONIAN RADIATION OF EUKARYOTES AN ARTIFACT OF SAMPLING?

Initial compilations of Proterozoic fossil diversity from the 1980s showed a dramatic peak during the mid-Tonian, ca. 800 Ma, inspiring the hypothesis that eukaryotes rose to dominance around this time. Though this diversity peak has diminished as new discoveries filled in the older part of the record, the idea of a mid-Tonian eukaryote radiation has remained strong, in part supported by additional discoveries of diverse microfossils and the first appearance of steranes biomarkers ca. 800 Ma. The puzzling mismatch between the first appearance of eukaryotes >1.65 Ga and their diversification more than 800 million years later, has been explained by changes in mid-Tonian redox state, nutrient supply (e.g. increased P), and ecology (e.g. the appearance of predators). We present a new diversity compilation based on 181 species from 145 formations ranging in age from ~1890 to 720 Ma and find a significant increase in diversity in the Tonian. However, we also find that the number of eukaryote species through time is highly correlated with the number of sampled formations through time. This correlation is robust to interpretations of eukaryote affinity, bin size, and bin boundaries. We also find that within-assemblage diversity through time—a measure thought to circumvent sampling bias—is related to the number of formations as well. Biomarkers show a similar pattern to body fossils, where the rise of eukaryotic biosignatures correlates with increased sampling. We find no evidence that the proportion of eukaryote-bearing vs. all fossiliferous formations (including only prokaryote-bearing formations) changed through the Proterozoic, as might be expected if an increase in eukaryote diversity were driving the increase in the number of eukaryote-bearing formations. This correlation could reflect a common cause, i.e. a hidden variable such as changes in sea level which drove both eukaryote diversification and an increase in sedimentary rock volume. However, we favor the hypothesis that the pattern of early eukaryote diversity is driven by variations in paleontological sampling through time. Though the Tonian radiation hypothesis remains plausible, the evidence cited in its favor rests on shaky grounds.