Holocene aquatic ferns (Salviniaceae) from the eastern Chaco Region, Northeastern Argentina

CONTRERAS, SILVINA A.; ROBLEDO, JUAN M.

Madrid

Congreso; 2nd Palaeontological Virtual Congress; 2020

The Chaco region is a large subtropical plainand one of the major biogeographic and morphostructuralareas of South-America. EasternChaco is the wettest sector of the Chaco regionand has heterogeneous environments. Vegetationresults of Quaternary alluvial plain dynamicsincluding the intensive migration of the Chacomain rivers. Paleobotanical records do notattest significant floristic changes in the regionduring the last 10,000 years. Fern macrofossils(vegetative part impressions) and microfossils(spores) belonging to the group of heterosporousaquatic ferns (Salviniaceae) were collected fromtwo sites (Formosa Province, Argentina) whichinclude sediments of the Fidelidad Formation(Late Holocene - Bermejo River natural levees,Villa Escolar). Impressions correspond to SalviniaSég. The floating leaves are orbicular to ellipticalin shape, with smooth margin and both theapex as the base are rounded. The base bearsa slightly developed keel. Venation pattern is dichotomous.These veins are anastomosed formingareolas. Generally, four tubercles (trichomebases) per areola are observed. The microfossilscorrespond to massulae and spores of AzollaLam. The massulae are rounded, oval or kidneyshaped and have septate glochidia. Sporesare trilete and spheroidal. Aquatic ferns are significantpaleoenvironmental indicators of openfresh water or wetlands, while Azolla and Salviniarepresent floating plants. This evidence suggeststhat the basal sediments of the FidelidadFormation formed under environments similarto those currently observed in the region.