Global species richness record and biostratigraphic potential of early to middle Neoproterozoic eukaryote fossils

Abstract

Over the past several decades, a number of studies have addressed the record of eukaryotic species richness in the Proterozoic, each making quite clear that during the Neoproterozoic Era, in particular, tremendous changes occurred in Earth’s biota. The relative scarcity of radiometric age constraints for rocks of this interval, however, have necessitated the use of coarse time bins (∼100 Ma) and the omission of fossiliferous but poorly dated units, resulting in low resolution of eukaryotic richness trends. Here we present a new estimate of early to middle Neoproterozoic (Tonian and Cryogenian) eukaryotic richness developed by use of the CONOP seriation algorithm; this approach permits inclusion of poorly dated and un-dated units and allows for greater resolution.The CONOP (constrained optimization) algorithm operates by evolutionary ordination—considering evidence of stratigraphic order from all locations simultaneously and starting from a random ordinal sequence that improves by mutations retained or removed according to best-fit rules. This program has been applied successfully to biochronologic and biostratigraphic problems throughout the Phanerozoic geologic record. Here we apply this objective approach 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 60 groups in paleogeographically distant successions. From this dataset was developed a high-resolution eukaryotic species richness record for the early to middle Neoproterozoic Era (∼1000 to 635Ma).This new estimate of eukaryotic species richness indicates an increase in richness began ∼800Ma and continued towards a peak ∼770Ma when it declined with the losses of many long-lived acritarch taxa. The overall decline is punctuated by a sharp richness increase with a ∼738Ma peak due to the iconic and short-lived Tonian vase-shaped microfossil taxa such as Cycliocyrillium simplex. These VSM taxa were lost ∼733Ma and richness continued to decline until flat-lining well in advance of the ∼720Ma 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.Use of CONOP with this dataset also permitted assessment of fossil taxa that had previously been suggested as Neoproterozoic biostratigraphic index taxa. Our results provide particular support for biostratigraphic use of the acritarch Cerebrosphaera globosa (=C. buickii) and for species of vase-shaped microfossils.