An island continent situated in the Southern Hemisphere and extending from 10 to 44°S and from 113 to 153°E. Australia's total area (2,941,526 mi² or 7,618,517 km² for the Australian mainland and 26,383 mi² or 68,332 km² for the island of Tasmania) is somewhat less than that of the United States. Bounded on the west by the Indian Ocean and on the east by the Pacific Ocean, Australia straddles the Tropic of Capricorn. Originally part of the ancient supercontinent of Gondwanaland, which began to disintegrate 200 million years ago, Australia broke away from what is now Antarctica about 70 million years ago and began its long drift northward to its present position. Australia's long-term isolation has contributed to the uniqueness of its flora and fauna. Australia possesses about 650 species of birds and 400 species of reptiles, a large proportion of which are endemic. Most interesting are its 255 species of mammals, in particular the marsupials (lacking placenta), which include the kangaroo, wallaby, and koala; and the monotremes (egg-laying), which include the platypus and spiny anteater. These animals appear to represent stages along the evolutionary path toward fully developed placental mammals.