Paleoazolla, a new heterosporous fern from the Upper Cretaceous of Argentina.

Megaspore and microspore massulae of Paleoazolla patagonica gen. et sp. nov. are described from the Upper Cretaceous of La Colonia Formation, Chubut Province, Argentina. The new fern possesses megaspore complexes with three to four glochidiate floats attached directly to the megaspore; a columella-like structure appears to be absent. The megaspore wall consists of a two-layered exine that is smooth to irregularly perforate, a two-layered perine with a spongy, densely packed endoperine, and a loosely organized exoperine. Infrafilosum hairs cover the exoperine. Microspore massulae are irregular in size and shape and bear multibarbed glochidia that generally have anchor-shaped tips. A comparison with other azollaceous and salviniaceous genera, particularly Azolla, is provided, together with a discussion of some evolutionary trends within the family.