CONTROLS ON EUKARYOTIC FOSSIL DIVERSITY IN THE TONIAN AND CRYOGENIAN PERIODS
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.