GLOBAL SPECIES RICHNESS RECORD AND BIOSTRATIGRAPHIC POTENTIAL OF EARLY TO MIDDLE NEOPROTEROZOIC EUKARYOTE FOSSILS

Several studies have addressed the record of eukaryotic species richness in the Precambrian. However, the scarcity of radiometric age constraints had dictated the use of coarse time bins (~100 Ma) and the omission of fossiliferous but poorly dated units, resulting in low resolution. Here we describe a new dataset of early to mid-Neoproterozoic paleontological, geochemical and radiometric data to which we applied the CONOP9 (constrained optimization) seriation algorithm—permitting inclusion of poorly dated units. CONOP has been applied widely to biostratigraphic problems throughout the geologic record, notably in resolving Paleozoic time-lines and extinction dynamics.

Here we apply this approach to a new compilation of geochemical data and taxonomically well-constrained organic-walled microfossil occurrences from more than 130 formations from 54 groups worldwide. From this dataset CONOP9 produced a high-resolution eukaryotic species richness record for the early to middle Neoproterozoic Era. We find an increase in species richness ~805 Ma, sustained high richness levels until a decrease ~770 Ma driven by a large number of acritarch extinctions, and then a sharp and short-lived increase ~760 Ma reflecting first appearances of vase-shaped microfossil taxa, and finally a sharp decrease in richness and extended nadir through the Sturtian and Marinoan glacial periods.

These results permitted assessment of fossil taxa that had previously been suggested as Neoproterozoic biostratigraphic index taxa; we find support for biostratigraphic use of the acritarch Cerebrosphaera buickii and for vase-shaped microfossils both as a group and as individual taxa. The section correlations derived from these results can be used to make testable predictions about which sections may record specific fossil taxa and carbon isotopic excursions such as the Bitter Springs stage and Islay isotopic anomaly.