An Early Miocene coastal lagoon plant community from central Patagonia

Abstract

The lower Gaiman Fm. (20.5 Ma) cropping out at Isla Escondida (northeast Chubut) represents a land-sea transition depositional setting. Here, we present the plant fossil remains with preserved cuticles and palynomorphs (fungi, spores and pollen grains) recovered from these sediments. The megafossil assemblage includes several ribbon-like long parallel-veined leaves, with more or less perpendicular connection between the veins and sheathing base. They were assigned to the Alismatales, tentatively to the family Hydrocharitaceae due to the presence of paracytic stomata. It includes seagrasses with hydrophilous pollination that may have grown in confined or low-energy coastal lagoons. Among palynomorphs, we found several Gondwanan lineages including Nothofagaceae, Podocarpaceae, Araucariaceae, and ferns of the Lophosoriaceae. We also recovered specimens assigned to palms (Arecipites otagoensis (Couper) Mildenhall & Pocknall 1989), caesalpinoid (Margocolporites vanwijhei Germeraad et al. 1968), and mimosoid legumes (Margocolporites tenuireticulatus Barreda 1997), and Rubiaceae Gardenia (Canthiumidites bellus (Stover & Partridge) Mildenhall & Pocknall 1989). The typical coastal elements commonly found in salt-marshes are also detected such as Amaranthaceae (Chenopodipollis chenopodioceoides (Martin) Truswell et al. 1985), Poaceae (Graminidites sp.), Asteraceae Nassauvieae (Huanillipollis cabrerae Barreda & Palazzesi 2008), and Convolvulaceae Cressa (Tricolpites trioblatus Mildenhall & Pocknall 1989). The overall assemblage indicates a rich marginal-marine community developed under a frost-free climate, in agreement with previous sedimentological and paleontological studies that include mollusca, bryozoa and diatoms.