CONICET | Buscador de Institutos y Recursos Humanos

We found several new and historical fossil localities in mid-Cretaceous littoral and continental sediments in northern Austral Basin, Santa Cruz Province, Southern Patagonia. Four localities comprise a new flora at the highest levels of Piedra Clavada Formation (Albian), where ferns and conifers are the megaflora dominant. The upper fossiliferous level in this unit shows an unusual diversity of thirteen angiosperms leaf forms. The age of Piedra Clavada Formation based on ammonoids and pollen is lower Albian. The above unit, the Mata Amarilla Formation, has been dated by zircons in 96.23 ± 0.71My, constraining the unit to the Cenomanian. Fifteen leaf angiosperm morphotypes has been recorded in several fossiliferous levels, few of them found across the Albian-Cenomanian. Podocarps have dominated the forest canopy, and petrified trees preserved in life position in a huge forced regression surface at the Mata Amarilla Formation allow a more complete reconstruction of the forest structure. Taxodiaceae and Araucariaceae were also recorded on higher fossiliferous levels. Angiosperms seem to occupy the forest understory and over banks with mostly small flowers and winged seeds. Several insect injuries in leaves and woods were also noticed. The uppermost littoral-marine unit in the area, La Anita Formation, hosts a complete different megaflora, although the precise age is ambiguous (Coniacian?-Campanian?). Angiosperms are abundant within the lowermost section, but their diversity decreases. A lower Maastrichtian flora was recorded in the continental Cachorro Formation, although the palyno- and megaflora were deposited in a freshwater pond. Most leaf forms correspond to simple broad leaves of entire margin physiognomy correlated with climate interpretations of hydromorphic paleosols (gleysols) previously recorded for the Mata Amarilla Formation. Growth ring analyses on petrified trunks also agree with the paleosols interpretation of a marked seasonality by water supply. Tree ferns, cycads and probable palms also support frost free environments at these middle-high latitudes in southern Patagonia during the mid-Cretaceous. These data strengthen previous hypothesis based on palynofloras and establish a floristic change characterized by a rapid diversification of angiosperms during plantfavorable temperatures and humidity in the Albian-Cenomanian. The study of these floras and the age confirmation, state down that angiosperm diversification and dominance occur early in time at high latitudes in Patagonia.