Aquatic ferns (Salviniaceae) in the Holocene sediments from the Bermejo River, Eastern Chaco, Argentina

Abstract

The Gran Chaco Americano is a large subtropical plain and one of the major biogeographic and morphostructural territories of South-America. Eastern Chaco is the wettest sector of the Gran Chaco and has heterogeneous environments. It includes different types of woodlands and forests together with savannahs, grasslands and wetlands. These vegetation patterns have been proposed to be the result of the Quaternary alluvial plain dynamics and the intensive migration of the Chacoan main rivers. Fossil records-that show change-are scarce and most of them come from the Fidelidad Formation (late Holocene). This Formation is located on the Bermejo River natural levees near the Villa Escolar Town (Formosa Province, Argentina). Fern fossils discovered in this Formation correspond to heterosporous aquatic ferns (Salviniaceae). The impressions were assigned to Salvinia cf. minima Baker 1886. These correspond with orbicular to elliptical leaves, with a smooth margin, and both the apex and the base are rounded. The base bears a slightly developed petiole. The venation pattern is dichotomous with anastomoses forming areolas. Generally, four tubercles (trichome bases) per areola are observed. The massulae and spores correspond to Azolla cristata Kaulfuss 1824. The massulae are rounded, oval or kidney-shaped and have septate glochidia. Spores are trilete and spheroidal. The floating ferns Salvinia and Azolla are good indicators of open fresh-water or wetlands paleo-environments. Hence, the lower levels of the Fidelidad Formation are associated with mostly calm freshwater bodies such as rivers, streams or wetlands, which are closely related to the activity of the Bermejo River during the late Holocene. These aquatic fern fossils provide new insights into paleoenvironmental conditions and increase the paleontological richness of the Eastern Chaco.