SURPRISINGLY HIGH BIOTIC DIVERSITY IN A NEOPROTEROZOIC SHALE

We are examining the palynological contents of a presumed ancient lake, the ~1 Ga Nonesuch Shale (Mid-Continent Rift System) from the Upper Peninsula of Michigan, USA. Acid macerations of seven of fifteen samples from well WPB-4 and nine of nineteen samples from well PC-1 are dominated by palynomorphs, which are placed into more than 50 morphological categories corresponding to what would be species-level taxa in Paleozoic palynology. This is a surprising result, because globally, assemblages of early Neoproterozoic acritarchs have a species richness equivalent maximum of around 10 taxa. Together with reports of microfossils from the Torridon and Sleat Groups in the NW Scottish Highlands, it now appears that non-marine communities of the Early Neoproterozoic are considerably more diverse than their marine counterparts. The Nonesuch assemblages are dominated by sphaeromorph acritarchs, some of which are placed into four morphospecies of Leiosphaeridia, but which also include new species of discoidal and aperturate forms. Previously named presumed marine taxa include Valeria lophostriata, a possible Dictyosphaera, Archaeoellipsoides and Germinosphaera. A distinctive non-marine acritarch, Moyeria, previously proposed to be a preserved euglenoid pellicle, occurs in both wells. Cyanobacteria are not common. Primitive branched and stalked protoctists have been found. Some individual vesicular specimens are quite large, a tendency that is seen elsewhere in the Neoproterozoic. This suggests that coenocytic growth may have preceded complex multicellularity in biological evolution as mechanism to increase organism size. High species richness in the Nonesuch microbiota supports our previous observation that allopatry in non-marine habitats was an important driver of speciation during early eukaryotic evolution.