EXTINCTION OF THE ANTARCTIC TERRESTRIAL BIOTA

Meyer Desert Formation fossils from the Oliver Bluffs at ~ lat. 85°S on the Upper Beardmore Glacier, Transantarctic Mountains, indicate that a tundra-like environment persisted until the Neogene. Plant and algal fossils include wood and leaves of Nothofagus, abundant seeds of Ranunculus (buttercups) and other herbaceous taxa, an entire cushion plant, moss stems and leaves, root casts, spores and pollen. Animals are represented by insects, ostracods, freshwater molluscs, and a fish. Insects include the skeletal parts of two species of listroderine weevils and the puparium and a leg of a cyclorraphan fly. Freshwater molluscs shells are those of a species of a fingernail clam, Pisidium and a lymnaeid gastropod. The fossils accumulated in an ice-marginal environment, adjacent to the terminus of the glacier, near the head of wide fiord. The landscape was sufficiently stable for a lake, ice-free for long enough during the summers, to support algae, benthic invertebrates and a species of fish. Mean summer temperature is estimated to have been about 5°C for at least two months of every year. Cooling of the climate and the growth of ice sheets, associated with the opening of the Drake Passage and the formation of the Circumpolar Current between 34 to 22 Ma, is the most probable cause for the initial decline in species diversity of plants and animals. The East Antarctic Ice Sheet was fully formed by 14 Ma and the West Antarctic Ice Sheet by 5 Ma. Increasingly the climate became more arid, and after 7.5 Ma in the Dry Valleys, a shift from wet-based to cold-based glaciations occurred. We speculate that ice-free areas, starved of moisture, became polar deserts resulting in the extinction of vascular plants, most cryptograms, arthropods other than a few species of mites and collembollans, and all aquatic organisms of the Neogene tundra biota.