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

CONTRERAS, SILVINA A.; ROBLEDO, JUAN MANUEL

JOURNAL OF SOUTH AMERICAN EARTH SCIENCES

PERGAMON-ELSEVIER SCIENCE LTD

Lugar: Amsterdam; Año: 2021

0895-9811

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