Thecadinium is a morphologically heterogenous marine benthic genus. Its polyphyly has been discussed. After redefinition of the sensu stricto genus, sensu lato taxa now need reclassification. Heterotrophic, morphologically closely related species were studied in detail. Molecular phylogenetic data for three of the four known species (T. ornatum, T. acanthium, T. ovatum) and new morphological data were obtained, leading to an emended thecal plate pattern, including the presence of an apical pore complex and an additional hypothecal plate. The results confirm the close relationship of the species and justify the description of Carinadinium gen. nov., characterized by the tabulation APC 3/4' 1/0a 6″ 6c 5s 5‴ 2'‴, an epithecal plate of special morphology, an apical flange, a ventral pore, antapical appendages, a descending cingulum and lateral cell flattening. The genus can be separated into two sub-clades, one with a third precingular 'dimple'-plate, four apical and no anterior intercalary plates and the other with a 'multi-pimple'-plate as third precingular or its homolog plate, three apical and one anterior intercalary plate. Carinadinium is phylogenetically related to the planktonic genera Protoceratium, Pentaplacodinium, and Ceratocorys (family Protoceratiaceae), and clearly belongs into the order Gonyaulacales, but with uncertain family affiliation.
Keywords: Benthic; Ceratocorys; Pentaplacodinium; Protoceratium; Taxonomy; rRNA.
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