CONTROLS ON EUKARYOTIC FOSSIL DIVERSITY IN THE TONIAN AND CRYOGENIAN PERIODS

The growing number of fossiliferous assemblages described from Proterozoic strata has allowed us to begin to test hypotheses about the relationships between environmental change and life. Particularly, Tonian and Cryogenian strata host evidence of global glaciations, or “Snowball Earth” events, changes in the redox state of oceans, an apparent diversification of microscopic eukaryotes, and the origin of the metazoa. However, integration of geological and paleontological records has been hampered by several factors that limit accurately defining and understanding fossil diversity in the Proterozoic rock record. Here, we quantitatively analyze the fossil record of eukaryotes from the Tonian and Cryogenian in an attempt to address these issues.

Our results show that Tonian within-assemblage diversity is higher than that of the preceding Mesoproterozoic and the succeeding Cryogenian. The majority of diversity in the Tonian comes from Laurentian assemblages, but this does not appear to bias general diversity metrics. However, smaller-scale patterns may be affected by the overwhelming input from Laurentian localities; for example, gaps in the early Tonian fossil record coincide with depositional hiatuses on Laurentia. More generally, paleontological trends in the Neoproterozoic are likely biased by the distribution of rock packages during specific intervals.

Another overwhelming pattern in our analysis is the appearance of multiple groups of resistant taxa in the late Tonian to early Cryogenian. This may be related to changing ecological dynamics in Neoproterozoic seas, particularly the chemical evolution of Neoproterozoic oceans and the advent and increase in protist-driven predation. In addition, the coeval rifting of Rodinia led to increased basin formation, preservation of rock packages, weathering, and phosphorous delivery to the ocean, which may have enabled the origination of mineralized tests.

Finally, we examine the cause of low fossil diversity in the Cryogenian, and examine whether this is a true biological signal, or a combination of sampling, lithological, and taphonomic biases. Determining the relative role of all of these factors is key to elucidating the effects of environmental change on biotic evolution.