Yogsothoth knorrus gen. n., sp. n. and Y. carteri sp. n. (Yogsothothidae fam. n., Haptista, Centroplasthelida), with Notes on Evolution and Systematics of Centrohelids.

Two closely related new species of centrohelid heliozoans with unusual morphology were studied with light and electron microscopy. Sequences of the 18S rRNA gene were also obtained and secondary structure of 18S rRNA molecule reconstructed. The cells, covered with inner siliceous plate scales formed colonies. The entire colony was surrounded with a thick layer of external scales. Inner scales were tabulate and had a patternless surface, except for the presence of an axial rib. Outer scales had a boat-like (Yogsothoth knorrus gen. nov., sp. nov.) or pot-like (Yogsothoth carteri sp. nov.) shape with an axial rib and numerous conical papillae on the scale surface. Analysis of 18S rRNA gene sequences robustly placed the new taxa within centrohelids, but not in any existing family. Scaled Yogsothoth represents a genetically divergent closest outgroup of Acanthocystida, branching after the supposedly primary non-scaled Marophrys, and together with acanthocystids, forming the novel taxon Panacanthocystida. Reconstruction of presumptive 18S rRNA secondary structure reveals interspecific differences in expansion segments 7 and 9 of Yogsothoth. Analysis of 18S rRNA secondary structure of other centrohelids allowed identification of length increases characteristic for Panacanthocystida location and reconstruction of 18S rRNA elongation in the course of the evolution of this group.