Dinoflagellate phylogeny revisited: reconciling morphological and molecular based phylogenies

Ultrastructural and molecular phylogenetic data suggest that dinoflagellate s diverged as a lineage possibly as early as the Precambrian. However, the fossil record is problematic before the Mesozoic. Prom the mid Triassic tho ugh, the fossil record of dinoflagellates is a rich source of information o n Mesozoic-Cenozoic dinoflagellates especially the gonyaulacoids and peridi nioids. From the sequence of appearance of species and tabulation types and the impression of early morphological experimentation and later stabilizat ion, the early Mesozoic radiation of dinoflagellates appears to be a real e volutionary event: indeed, dinoflagellate morphology as we know it today ma y originate in that event. This would explain why it is so difficult to int erpret earlier fossils as dinoflagellates. However, that the dinoflagellate lineage existed in some form in the pre-Mesozoic is supported by biogeoche mical data. early results of which indicate that certain early Paleozoic ac anthomorph acritarchs may belong to the lineage. A surprising degree of consistency is observed between ultrastructural (inc luding tabulational), coarse biochemical and molecular sequence data. For e xample. sequence data provided by small subunit (SSU) rRNA support the hypo thesis of progressive loss of histones within the dinoflagellates. Gymnodin ioids have long been considered to be polyphyletic but are thought of gener ally as forerunners to the strongly thecate groups such as gonyaulacoids an d peridinioids. In molecular trees they appear in both early-derived and la te-derived positions, but mostly the latter. SSU data clearly support the g onyaulacoid/peridinioid ordinal separation, as does the fossil record. Pror ocentroids are now thought to be the among the most derived dinoflagellates (and presumably the morphologically similar dinophysoids), but SSU sequenc es have so far failed to resolve the relationships of most gymnodinioids, p eridinioids and prorocentroids (the so-called GPP complex) to one another. However, they do suggest the origin of prorocentroids from peridinioids rat her than gonyaulacoids and that gymnodinioids probably had several origins.