EUKARYOTIC SPECIES RICHNESS IN THE EARLY TO MIDDLE NEOPROTEROZOIC

Several studies have addressed eukaryotic species richness in the Proterozoic, each clearly indicating that during the Neoproterozoic, in particular, major changes occurred in Earth’s biota. The scarcity of radiometric age constraints for rocks of this interval, however, have necessitated use of coarse time bins and omission of fossiliferous but poorly dated units, resulting in low resolution eukaryotic richness trends. We present a new estimate of early to mid-Neoproterozoic eukaryotic richness developed by use of the CONOP seriation algorithm, which permits inclusion of poorly dated and un-dated units allowing for greater resolution. This objective approach was applied to a new compilation of taxonomically well-constrained organic-walled microfossil occurrences as well as geochemical, sedimentological and geochronological data from more than 160 formations from 58 groups in paleogeographically distant successions.

This new, high-resolution eukaryotic species richness estimate for ~1000 to 635 Ma indicates an increase in richness began ~800 Ma and continued towards a peak ~770 Ma when it declined with the losses of many long-lived taxa. The overall decline is punctuated by a sharp richness increase ~738 Ma due to the appearance of the iconic and short-lived Tonian vase-shaped microfossils (VSM) taxa. These VSM taxa were lost ~733 Ma and richness continued to decline until flat-lining well in advance of the ~720 Ma onset of the Cryogenian Snowball Earth glaciations. Eukaryotic species richness did not rebound until after the termination of the second Cryogenian glaciation when a new suite of acritarch taxa appeared in the Ediacaran Period.

In addition to providing insight into eukaryotic richness these analyses result in construction of a hypothetical global composite that includes all geochemical events (carbon isotopic excursions), sedimentological events (Snowball Earth glaciations) and taxonomic first and last appearances. From this hypothetical composite, estimates can be made for stratigraphic ranges of potential index taxa and for global correlation of sections. Estimates of section correlations place higher demands on local information and entails large uncertainties, but leads to testable predictions for new occurrences of geochemical, sedimentological and paleontological events.