MID-MIOCENE EXTINCTION IN ANTARCTICA

In recent years, terrestrial Neogene fossil assemblages have been discovered in three localities in the Transantarctic Mountains (TAM).  The fossils are of algae, plants, freshwater molluscs, crustaceans, insects, and a fish.  Algae are represented by spores and by frustules of freshwater diatoms; plants by pollen, megaspores, wood, leaves, achenes, flowers, and include species of moss, grass, Isoetes (quillwort), Ranunculus (buttercup) and Nothofagus (southern beech).  The low species diversity and cushion growth forms indicate tundra vegetation.  The tundra was inhabited by a variety of invertebrates represented by fossils of beetles and flies, cypridoidean ostracods and freshwater gastropods and bivalves.  Taken together, the fossil assemblages require a minimum mean summer temperature (MST) of c. 4°C which is at least 20°C warmer than MST's in the TAM today.  Some variation in MST's must have existed between the localities because of differences in latitude (8°) and in original elevations.  Pollen evidence from the Cape Roberts borehole in the Ross Sea basin indicates that tundra vegetation existed in East Antarctica from the Oligocene into the Early Miocene.  We infer that a patchy tundra biota persisted in the TAM until the mid-Miocene (14 Ma) but deprived of summer heat and liquid water the majority of the organisms became extinct.  A small number of soil-dwelling organisms, mites, collembolans and nematodes may have survived the extinction.  Earlier interpretations allowed for the extinction of this terrestrial biota to have occurred as late as the Pliocene but a Miocene age now seems more probable.  In the Dry Valleys there is strong evidence for major climate change centered on 14 Ma during which time alpine glacial regimes switched from being wet- to cold-based (see Lewis et al. this volume).  It seems probable that at least some of the Miocene fossils, e.g. Nothofagus, were descended from ancestors which may have evolved in Antarctica during the Paleogene or even the late Cretaceous.  Taxa which disperse by wind or avian vectors, such as diatoms, ostracods and spore-bearing plants, may have continued to migrate to and to emigrate from Antarctica long after the continent became isolated from South America at c. 34 Ma.  Research supported by NSF grants: ANT 0230696 and ANT 0440761 to Ashworth and ANT 0440711 to Marchant