A morphologically diverse assemblage of organic-walled fossils from the middle Neoproterozoic Svanbergfjellet Formation, Spitsbergen, is identified as a monospecific assemblage representing the Gongrosira-phase of a vaucheriacean xanthophyte alga. As such, it provides a range of additional criteria with which to identify fossil vaucheriaceans and confirms the identification of Palaeovaucheria in the Mesoproterozoic Lakhanda Formation. Pronounced taxonomic inflation, through the practice of form-taxonomy, suggests that overall estimates of eukaryotic diversity in the Proterozoic need to be adjusted downward. Combined with positive evidence for low levels of speciation and extended stasis, pre-Cambrian eukaryotes are seen to evolve at a fundamentally lower rate than their Phanerozoic counterparts. This slower turnover accounts for the “delayed” appearance of animals without appeal to external triggers or constraints. The Cambrian acceleration of evolutionary rates was a direct consequence of newly introduced animals, whereas the much slower overall rates of the Proterozoic imply an absence of earlier metazoans.