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Abstract
The centric diatom order Thalassiosirales includes all diatoms with a fultoportula (strutted process), a feature now recognized as a synapomorphy for the lineage. Within Thalassiosirales, Thalassiosira is perhaps the most taxonomically and morphologically diverse genus, and at least two distinct morphological groups have been recognized within it. Group “A” Thalassiosira species, which include the type species, T. nordenskioeldii, have short inward and long outward extensions of the strutted processes and a labiate process on the valve mantle. Group “B” species have exceptionally long inward and reduced outward extensions of the strutted processes, and a labiate process on the valve face. We collected and cultured Thalassiosira ritscheri, which has a combination of group A and B characters. It has a labiate process on the valve face and reduced outward extensions of the strutted processes. We show for the first time that T. ritscheri has short inward, A-type extensions of the strutted processes. A phylogenetic interpretation of these conditions suggests a close relationship between T. ritscheri and the traditionally held group “B” species. Species diagnosed by the autapomorphic condition of a labiate process away from the valve mantle, including many group “B” Thalassiosira species, are transferred into Shionodiscus gen. nov.
