A new species of the unusual dinoflagellate cyst genus Geonettiade Verteuil and Norris, 1996a is here described from the Pliocene of the western North Atlantic and eastern England. Geonettia waltonensis new species is only the second species to be formally described for this genus, whose type, G. clineaede Verteuil and Norris, 1996a, has a range of Miocene through Pliocene. Geonettia is a gonyaulacalean, goniodomacean genus of the subfamily Pyrodinioideae and is closely related to EocladopyxisMorgenroth, 1966 and CapisocystaWarny and Wrenn, 1997, also found in the Cenozoic. However, Geonettia is the only known dinoflagellate cyst genus to have plates that dissociate extensively on both epi- and hypocyst during excystment. Geonettia waltonensis has this style of excystment, but its hypocystal tabulation is more akin to Capisocysta lataHead, 1998a than to G. clineae. Comparison of tabulation and other morphological features suggests that during the late Miocene, Capisocysta lata evolved from Geonettia waltonensis or a closely related species through failure of its epicystal plates to dissociate. Geonettia waltonensis probably did not evolve directly from G. clineae but may represent a separate lineage within Geonettia that arose during the Miocene.

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