Paleobotany of the uppermost Cretaceous Chorrillo Formation, Santa Cruz Province, Argentina: insights in a freshwater floral community

Abstract

Few latest Cretaceous fossil floras are known for South America, and in particular for the Austral-Magallanes Basin. Recent studies carried out in the Chorrillo Formation (Maastrichtian) revealed a diverse array of fossil elements, including continental invertebrates, vertebrates, palynomorphs, fossil woods and leaf impressions. In this work, we describe the megafloristic elements identified in the unit from two fossiliferous levels, along with a palynological sample obtained from one such level. The first Mesozoic Nymphaeaceae (water lilies) remains for Argentina are reported, consisting of fragmentary leaves and seeds of a minute Nuphar-grade plant. Several leaf impressions referred to dicotiledonean and monocotiledonean morphotypes are also described. Additionally, microscopic remains reveal a diverse palynological assemblage containing terrestrial and aquatic ferns, conifers and angiosperms. A community that inhabited low energy, probably paludal, freshwater environments was identified, based on the sedimentology of the bearing strata such as the presence of hydromorphic paleosols and poorly decomposed organic matter suggesting poor drainage and eventual reducing and anoxic conditions, as well as the presence of Salviniaceae (water ferns), Marsileaceae (water-clovers), Nymphaeaceae and Zygnemataceae (freshwater conjugate algaes). Palynological elements suggest similarities with Campanian to Paleocene Patagonian units, mostly located in the Austral-Magallanes, Cañadón Asfalto and Golfo the San Jorge basins. Finally, aquatic communities from other Maastrichtian units (e.g. La Colonia and Lago Colhué Huapi formations) are compared with the Chorrillo Formation presented herein, suggesting similarities in functional groups even though taxa involved differ partially.