CRETACEOUS-PALEOGENE MOSQUITO FERNS (AZOLLA) FROM PATAGONIA (CHUBUT PROVINCE, ARGENTINA)

Azolla, the mosquito ferns, includes about seven extant species of floating, aquatic, heterosporous ferns found in freshwater habitats. Azolla first appears in Cretaceous sediments and has an extensive fossil record. The most commonly recognized fossils are dispersed megaspores and microspore massulae. Only about 12 distinct extinct species of sporophyte fossils that include vegetative organs (i.e., Azolla macrofossils with roots, stem, and/or leaves) have been described from Upper Cretaceous to Miocene sediments of North America, Europe, and Asia. Fossil Azolla sporophytes have now been discovered in sediments from the Upper Cretaceous (Campanian-Maastrichtian) La Colonia Formation and lower Paleocene (Danian) Salamanca Formation of Chubut Province, Argentina. The fossil material from Patagonia comprises both vegetative and reproductive structures, including roots, stems, leaves, and attached sori. Sporophytes from both formations are small plants possessing the features characteristic of modern Azolla: Stems branch alternately; leaves are simple, alternately arranged, and somewhat overlapping; and roots are unbranched. Microsporangiate and megasporangiate sori are known from the La Colonia Azolla, and microsporangiate sori are known from the Salamanca Azolla. The La Colonia Formation sporophytes likely produced an extinct species of dispersed Azolla spores found in the same sediments, suggesting that they represent a stem-group to modern Azolla. The fascicled roots of the Salamanca sporophytes suggest a relationship with the modern African species A. nilotica and/or the North American fossil species A. schopfii. Both spores and sporophytes of modern Azolla bear phylogenetically informative characteristics, so the affinities of the fossil Azolla species from Patagonia can potentially be tested phylogenetically. Presently, these Patagonian fossils constitute the only record of vegetative structures of Azolla from the Southern Hemisphere.