Fósseis de pteridófitas das ilhas King George e Nelson: afinidade taxonômica e importância no contexto deposicional e do clima nas áreas setentrionais da península antártica

Abstract

Pteridophyta fossils from Antarctica known since the end of Paleozoic Era. During the Mesozoic they become more abundant in areas of the King George and Nelson islands, object of this study, located at the Antarctic Peninsula. On King George Island the volcanic rocks dominates the successions and the fossil assemblages concentrates in levels originated by ash flow and fall processes, or for its reworking, in a time span that includes the end of Cretaceous and Paleogene. There, the pteridophytic remains dominate at southern areas (Fildes Peninsula) and in the older Cretaceous levels. This older assemblages (Campanian / Maastrichtian) shows many and well preserved ferns, accompanied by araucaria and podocarp conifers, and primitive angiosperms dominated by Nothofagus. In the Upper Paleocene and Eocene successions, although the unchanged general composition, the flora shows more modern character with the appearance of many new conifer types mainly within the Podocarpaceae, and the angiosperms remains are represented by leaf morphotypes comparable to those found in extant families (mainly Sapindaceae, Aquifoliaceae, Lauraceae, Myrtaceae, and once again, abundant Nothofagaceae). On Nelson island, is find a single known fossiliferous locality (Rip Point), presented for the first time in this study. The fossil plant deposition brings the attention of its correlation to the older levels from the neighbouring King George island. The plant fossils at Rip Point concentrate in two levels of tuffitic rocks, between lava and conglomerate deposits, the lower one exclusively by its macrofloristic remains and restricted to fern frond fragments, exhibiting morphologies comparable to Coniopteris (Brongniart) Harris, a genus largely confined to the Mesozoic. The well preserved impressions and morphology of pinnae and pinnules allows the description of a new species, C. antarctica. Instead, the pollen analysis made in the same beds showed that the associated vegetation contains rare conifers representatives (podocarps and araucaria) and primitive angiosperms that include ancestral type of pollen grains from Nothofagus. This fossil assemblage, besides supports the late Campanian and /or basal Maastrichtian age to the Point Rip basal levels, allows inferring a wet and temperate climatic condition to the northern areas of Antarctic Peninsula during this time interval. Its composition also help to confirm the role of ancient Antarctic biomes in the origin of the vegetation that grow today in the high and middle latitudes, under oceanic climates, of the South Hemisphere.