EXPLORING THE TAXONOMIC AFFINITIES OF NEOPROTEROZOIC AND PALEOZOIC ACRITARCHS

Acritarchs have historically been considered as phytoplankton because of their small size, paleoenvironmental distribution, and morphological similarity to extant cyst-forming algae. Morphological and ultrastructural comparisons to prasinophyte phycomata provide strong evidence for the green algal affinities of some Paleozoic taxa. Surprisingly, however, prasinophyte ultrastructure has yet to be demonstrated in Proterozoic microfossils. Dinoflagellates are commonly favored as the producers of process-bearing acritarchs in Paleozoic successions, but coeval bitumens contain only small concentrations of dinoflagellate-sourced biomarker molecules, leaving the generality of this interpretation in doubt. Animals have seldom been considered as acritarch progenitors, but a phylogenetically wide diversity of metazoans produce recalcitrant dormant egg cases that might accumulate in sediments. In particular, Ediacaran assemblages include acritarchs that are morphologically most similar to animal, and not algal, forms. Comparison of wall ultrastructure between modern invertebrate egg cysts and a diversity of Ediacaran acritarchs strengthens the hypothesis that large, process-bearing acritarchs in Ediacaran rocks record metazoans that contained a resting stage in their life cycles. Some Cambrian acritarchs also exhibit ultrastructures known best from animal egg cases. With this in mind, Ediacaran and Early Paleozoic acritarchs may record direct, and not just indirect (i.e. ecological), evidence of the radiation of animals in the world's oceans.