Australia separated from Gondwana 99 Ma, and initially remained warm and humid with rainforest vegetation. Inland Australia had systems of rivers and lakes with abundant wildlife. Fossil birds, platypus, frogs and snakes are present from this period. From 30 Ma there was a period of global cooling, and from 15 Ma the Antarctic ice sheet formed. Sand deserts and large inland salt lakes formed within the last 5 Ma. Climatic oscillation during the Pleistocene over the last million years led to repeated phases of glaciation with lower sea levels that linked Australia to New Guinea, and warmer interglacial periods with higher sea levels.