Fossil Unicellular Eukaryotes


Includes study of microscopic remains of single-celled organisms. We have touched on the difficult record of early life forms. Starting with the 1.6 Ga Ruyang Group, however, we encounter eukaryotic cell fossils of increasing diversity (Yin, 1997).

Schematic of typical eukaryote from The Gates of Academe


The key concept here is the stepwise acquisition, by endosymbiosis of the characters that we associate with the "typical" eukaryotic cell, Including: Thus, depicting their evolution as a strictly branching phylogeny is somewhat misleading. (See Palmer et al., 2004) Note their distribution in the major groups:

  • Numerous groups of eukaryotes actually lack some of the organelles typically associated with the group. E.G.:

    A phylogeny of significant groups from Adl et al., 2005, as summarized by Keeling et al., 2009 appears at right.

    Note: The phylogram to which this cladogram links superimposes the phylogeny of fossil-forming groups on to the stratigraphic record. Typically we could infer minimum divergence ages with some confidence. In the case of Eukaryotes, however, this is perilous as so few yield fossils. El Albani et al. 2014, for instance describe "colonial" organisms with features associated with colonies of eukaryotic cells in the 2.1 ga Francevillan B Formation of Gabon. These colonies were not organized into bodies in the manner of multicellular fungi, plants, and animals, but were akin to congregations of independent cells like slime molds.

    Hypotheses of the phylogeny or eukaryotes are subject to frequent revision, so we focus on descriptions of major fossil-forming groups, rather than the broad patterns. And yet a few words on large groups:

    Major microfossil-forming groups:


    Acritarch from Digermulen
    Acritarchs: (Proterozoic to... Ordovician? Devonian? Permian? Depends on whom you ask)

    Chitinozoan from Wikipedia
    Chitinozoa (Ordovician - Carboniferous):




    Choanoflagellates from M. Carr, 2008, PNAS
    Fungi have no significant fossil record, and we will deal with Metazoa in later lectures. First, however consider an opisthokontan group with no fossil record but which is, nevertheless important to the understanding of metazoan evolution:

    Choanoflagellates, the putative sister taxon to Metazoa, characterized by a collar of cilia surrouding a single flagellum. These free-living organisms are outwardly indistinguishable from the specialized choanocytes of sponges.

    Additional reading:

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