Reinvestigation of Ariadnaesporites varius Hall and Peake, 1968 emend. Hall, 1975

Ariadnaesporites varius Hall and Peake 1968 emend. Hall 1975 possesses all of the characteristics of the morphogenus, but its megaspores are differentiated from other species of Ariadnaesporites in possessing longer hair-like appendages that almost completely obscure their body and in possessing small ornamented bodies or “floats” enmeshed in those filamentous appendages. Here we reinvestigate the morphology and ultrastructure of A. varius megaspores and microspores, based on specimens collected by Hall from the Windrow Fm. (Cretaceous) of Minnesota, using scanning and transmission electron microscopy. New observations include: a two-layered structure of the megaspore exine, a granular structure of the perine of the floats, a microspore exine without a porous layer, a two-layered structure of microspore perine, and the differentiation of perine on proximal and distal poles of the microspores. Overall, A. varius displays a combination of traits consistent with previous inferences about its status as a transitional form in the lineage leading to Salviniaceae and to Azolla in particular. In recognition of the unique combination of characters that A. varius possesses, the species is transferred to the new morphogenus Hallisporites.