EVIDENCE OF SEXUAL CONJUGATION AND CROWN-GROUP STATUS IN EARLY PROTEROZOIC EUKARYOTES

Total-group eukaryotes have a deep Proterozoic history, but the biological attributes and precise phylogenetic status of early fossil representatives remain unresolved. Microfossils recovered from the early Mesoproterozoic Greyson Formation (lower Belt Supergroup) of Montana include diagnostically eukaryotic Dictyosphaera, Valeria and Tappania, substantially extending the early record of eukaryotes on Laurentia. Exceptionally well preserved populations of Tappania also yield a uniquely resolved view of this taxon’s underlying biology. Novel anatomical features include vesicles bearing multiple branching processes, compound branching processes, and a bi-layered wall structure in which the inner and outer walls exhibit independent differentiation. Examples of paired and fused Tappania in the Greyson recall similar associations in populations from India and Siberia, and are convincingly interpreted as an expression of sexual conjugation. Modern analogues for Tappania’s characteristic ‘neck-like extensions’ can be found in the conjugation papillae/tubes of sexually reproducing zygnematalean algae, as well as ‘shmoo’ formation in ascomycetous yeasts. The presence of such developmental complexity in Tappania substantially expands its documented range of derived eukaryotic features, bolstering its recognition as a sexually reproducing crown-group eukaryote, arguably the oldest on record.