CONSTRAINING THE ORIGINATION OF THE FRESHWATER DINOFLAGELLATE PERIDINIUM WISCONSINENSE EDDY FROM THE MARINE SCRIPPSIELLA LINEAGE USING PHYLOGENETIC AND BIOSTRATIGRAPHIC EVIDENCE

The dinoflagellate Peridinium wisconsinense Eddy has large distinctive spindle-shaped cellulosic thecae with an asymmetric plate pattern (4′, 3a, 7′′, ?s, 5′′′, 2′′′′) that are commonly reported from freshwater environments in North America. Proximocavate cysts corresponding to P. wisconsinense with pronounced apical and antapical horns that echo the morphology of the theca are also commonly reported in palynological studies due to their high preservation potential. The recently documented phylogenetic position of P. wisconsinense inferred from partial LSU rDNA sequences suggests that the marine genus Scrippsiella gave rise to a lineage adapted to freshwater environments. The “Chimonodinium clade” that includes Chimonodinium lomnickii (Wołoszyńska) Craveiro, Calado, Daugbjerg, Gert Hansen & Moestrup and P. wisconsinense is closely related to a sister clade of Peridinium aciculiferum Lemmermann and “Scrippsiella” hangoei (J. Schiller) J. Larsen, intermingled with a large number of Scrippsiella species (Luo et al., in review). It is distinct from other clades of freshwater dinoflagellates: the Peridinium sensu stricto clade that includes the type species of the genus Peridinium Ehrenberg (P. cinctum (O.F. Muller) Ehrenberg), the Peridiniopsis sensu stricto clade, and the diatom-harbouring clade recently identified by You et al. (in press). This suggests that several distinct marine – freshwater transitions occurred in the Phylum Dinoflagellata Bütschli.

The close relationship with many Scrippsiella species and other marine taxa that produce calcareous cysts implies that P. wisconsinense, C. lomnickii, and P. aciculiferum diverged from the brackish “S.” hangoei or a common ancestor (e.g., Pfiesteria; Logares et al., 2008) relatively recently. The report of cysts from nonmarine Miocene strata in Alaska as Geiselodinium tyonekensis sp. nov. by Engelhardt (1976) that are nearly identical to those formed by P. wisconsinense in culture (Wall and Dale, 1968; McCarthy et al., 2011) help to constrain the timing of this divergence.