Abstract

The discovery of terrestrial fossil assemblages at several locations in the Transantarctic Mountains is transforming our understanding of the late Cenozoic environment of Antarctica. The most southerly fossil assemblage is at lat. 85.1°S, about 500 km from the South Pole. The environment was an active glacial margin in which plants, insects and freshwater mollusks inhabited sand and gravel bars and small lakes on an outwash plain. Initially the deposits were assigned a Pliocene age (3.5 Ma) but a mid- to early Miocene age is more probable (c. 14 – 25 Ma) based on correlation of fossil pollen from the deposits with 39Ar/40Ar dated pollen assemblages from the McMurdo Dry Valleys. Within the McMurdo Dry Valleys, the oldest fossiliferous beds are at least 19.76 Ma based on the 39Ar/40Ar age of a volcanic ash bed interbedded within a valley fill of diamictites, paleosols and lacustrine deposits. The valley floor during the non-glacial phases had poorly-drained soils and the extensive development of mossy mires. Wood and leaves of Nothofagus are abundant in lacustrine deposits. The silts of shallow fluvial channels contain abundant megaspores and spiky leaves of the aquatic lycopod Isoetes (Quillwort). The youngest fossiliferous beds within the Dry Valleys are 14.07 Ma. The fossils are mostly those of freshwater organisms including numerous species of diatoms and an ostracod species in which the soft anatomy is preserved. The base of the lake is marked by a moss bed with exceptionally well-preserved stems and leaves of the extant species Drepanocladus longifolius. Pollen evidence from marine cores in the Ross Sea basin suggests that tundra existed from the Oligocene to the Mid-Miocene. Fossil evidence from the Dry Valleys locations indicates organisms with complex life histories persisted in Antarctica until c. 14 Ma. At 14 Ma there was a shift in glacial regimes from wet- to cold-based, marking a profound and abrupt climatic shift likely causing widespread extinction. It seems probable that at least some of the mid-Miocene fossils had ancestors that evolved in Antarctica during the Paleogene or earlier. An important consequence of these studies is that the Cenozoic climate of Antarctica was warm enough until the mid-Miocene to support vascular plants and insects.