A morphological comparison of two cladopyxidacean dinoflagellates: the extant <i>Micracanthodinium setiferum</i> and the fossil <i>Cladopyxidium saeptum</i> (Dinophyceae, Gonyaulacales)

Among dinoflagellates, extant cladopyxidaceans may provide a missing link to better understand the first evolutionary transformations from ancestral configurations toward the more abundant and more derived patterns in Gonyaulacales and Peridiniales. A restudy of the extant, motile-defined <i>Micracanthodinium setiferum</i> using plankton samples from the Indian and Atlantic oceans and the Mediterranean Sea demonstrates that the correct plate formula is Po, Pt, X, 3′ + *4′, 4a, 7′′, 7C, 4S, ?6′′′, 0p, 2′′′′. A ventral pore is found between 1′, 3′ and *4′. A restudy of the extinct, fossil-defined <i>Cladopyxidium saeptum</i> from the upper Paleocene of Delaware (USA), demonstrated the presence of an identical tabulation. A ventral pore (= porichnion) was positioned between *1′ and 7′′. <i>Cladopyxidium</i> is morphologically closer to <i>Micracanthodinium</i> than to <i>Cladopyxis</i>. Since <i>Cladopyxidium</i> has been extinct since the middle Eocene, it is unlikely that <i>Micracanthodinium</i> and <i>Cladopyxidium</i> have a direct biological link; the close morphological similarity between them does, however, suggest an important phylogenetic relationship between them in the evolution of cladopyxidaceans.