Paratabulation in the Jurassic Dinoflagellate genusNannocera topsisand a comparison with modern taxa

Abstract

Scanning electron microscope studies of specimens of Nannoceratopsis senex and N. gracilis from the Tmetoceras scissum zone (Lower Bajocian) of England permit us to establish for the first time an unmistakable paratabulation and the presence of a unique archeopyle type. The paratabulation formula for both species is ?pr, 5 ‘, 5 “, 4c, 4s, 4H; and the archeopyle is formed by the loss of the third cingular paraplate. Nannoceratopsis is shown to be remarkable in combining features heretofore considered diagnostic of two distinct groups of dinoflagellates, the Peridiniales and Dinophysiales, and suggests that the evolutionary linkage between them is not so remote as generally had been thought.