In earlier and simpler times in North American geology, there was an extraordinarily large amount of unexplored ground per available geologist to study it. State and territorial geological surveys, which undertook the first comprehensive mapping programs in the nineteenth century, were at first able to make only the grossest descriptions of outcrops in order to complete their assignments to map an area. In these pioneer surveys, black shales stood out as very easily identifiable strata; characteristically colored, fine grained, with distinctive lamination, and well known to lack fossils found so commonly in adjacent strata. “Black shale, undivided” is a very familiar legend in older outcrop descriptions; the lack of comparability of black shales to other strata effectively prevented their division, for both conceptual and technical reasons. This is one of the many paradoxes we can recognize in relation to black shales; they are as difficult to interpret as they are easy to recognize.

The first compilations of Proterozoic eukaryote diversity, published in the 1980s showed a dramatic peak in the Tonian Period (1000–720 Ma), interpreted as the initial radiation of eukaryotes in the marine realm. Over the decades, new discoveries filled in the older part of the record and the peak diminished, but the idea of a Tonian radiation of eukaryotes has remained strong, and is now widely accepted as fact. We present a new diversity compilation based on 181 species and 713 species occurrences from 145 formations ranging in age from 1890 Ma to 720 Ma and find a significant increase in diversity in the Tonian. However, we also find that the number of eukaryotic species through time is highly correlated with the number of formations in our dataset (i.e. eukaryote-bearing formations) through time. This correlation is robust to interpretations of eukaryote affinity, bin size, and bin boundaries. We also find that within-assemblage diversity—a measure thought to circumvent sampling bias—is related to the number of eukaryote-bearing formations through time. 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 versus all fossiliferous formations changed through the Proterozoic, as might be expected if the correlation reflected an increase in eukaryote diversity driving an increase in the number of eukaryote-bearing formations. Although the correlation could reflect a common cause such as changes in sea level driving both diversification and an increase in sedimentary rock volume, we favor the explanation that the pattern of early eukaryote diversity is driven by variations in paleontological sampling.