Antarctic Peninsula and South America (Patagonia) Paleogene terrestrial faunas and environments: biogeographic relationships

The Eocene of Seymour Island contains the only association of Cenozoic plants and land vertebrates known from anywhere in Antarctica and lies at about latitude 63° south. The late Early to latest Eocene La Meseta Formation fills an incised valley and comprises sediments representing deltaic, estuarine and very shallow marine environments. The Paleogene sequence in southern South America (Patagonia) and the Antarctic Peninsula reveals floristically distinct periods (late Paleocene, early and middle Eocene and latest Eocene), based largely on leaf assemblages. The late Paleocene Cross Valley flora (Seymour Island) contains ferns and other elements suggesting a much warmer climate than at this latitude today. The Middle Eocene Fossil Hill (South Shetland Islands) and the Rio Turbio (Santa Cruz Province, southern Patagonia) floras have a mixture of both Neotropical and Antarctic elements. The La Meseta paleoflora is distinctive in having a predominance of Antarctic taxa especially Nothofagus, podocarps, and araucarian conifers in the Eocene deciduous and evergreen forests. This suggests a cooling trend during the Eocene of Antarctica with mid- to late Eocene seasonal, cool-temperate, rainy climates and latitudinal and altitudinal gradients. The Seymour Island La Meseta Fauna (Cucullaea Allomember, middle Eocene) contains at least 10 mammal taxa, predominantly tiny marsupials (mostly endemic and new taxa). The endemism of these marsupials suggests the existence of some form of isolating barrier (climatic and/or geographic) during the Eocene. Faunal similarity between the La Meseta Fauna and the fauna assigned to the Riochican (late Paleocene) South American Land Mammal Age of Patagonia strongly suggests that the former derived from the latter. The occurrence on Seymour Island of sudamericids, that had become extinct in South America in the Paleocene, also indicates that isolation may have allowed extended survival of this Gondwanan group in the Eocene of Antarctica and the factors that caused their extinction did not affect this continent. Global warming and intercontinental dispersal have been major influences on the timing and magnitude of terrestrial biotic change in the late Paleocene and early Eocene epochs. The faunistic evidence indicates that the La Meseta mammalian fauna derived from late Paleocene/early Eocene Riochican/Vacan faunas. The dispersal and vicariance events may have occurred during the onset of the climatic optimum of the Cenozoic (late Paleocene–early Eocene) when major regressive events are recorded either in the northern Antarctic Peninsula and southernmost Patagonia (between 58.5 and 56.5 Ma). The absence of notoungulates in the La Meseta fauna is noteworthy. We speculate that the notoungulates could have passed into Antarctica during the latest part of the Paleocene when the environmental conditions were warmer, and then became extinct at the onset of the climatic deterioration during the early Eocene.