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<p>The diversification of acritarchs (organic-walled vesicular microfossils of unknown affinity), filamentous, and multicellular microorganisms, happened during a time of profound environmental, biological, and ecological change. The Mesoproterozoic to Neoproterozoic transition is a key interval, notably for eukaryotic organisms. New assemblages of organic-walled microfossils from the ca. 1230 Ma to 900 Ma lower Shaler Supergroup of Arctic Canada record an impressive diversity, including macroscopic compressions of Chuaria circularis and numerous unambiguous eukaryotic taxa. The index taxon Trachyhystrichosphaera aimika co-occurs with eukaryotic taxa hitherto reported only from earlier (Dictyosphaera, Gigantosphaeridium, Satka favosa – sensu Javaux and Knoll, 2016) or later (Microlepidopalla) time intervals. Five new taxa, comprising the spheroidal acritarchs with inner wall sculpture Nunatsiaquus cryptotorus n. gen., n. sp. and Daedalosphaera digitisigna n. gen., n. sp.; the acanthomorphs (spiny acritarchs) with regularly distributed processes Herisphaera arbovela n. gen., n. sp. and Herisphaera triangula n. sp.; and the process-bearing multicellular Ourasphaira giraldae n. gen., n. sp.; are reported, along with three unnamed forms. Collectively, these remarkable microfossils (63 taxa, including 25 eukaryotic forms) demonstrate the greatest diversity of eukaryotes ever recorded for this time interval.</p>