Temperate forests did in fact exist on Antarctica... as recently as three million years ago. That's during the Pliocene. The continents were largely in their present position; the panama isthmus rose during the Pliocene, which made the north Atlantic colder, but Berengia also formed which would have kept arctic waters out of the Pacific, making it relatively warmer.
Antarctica would have had a climate rather like Siberia; the coastal areas are taiga and cold-temperate forests, alpine and tundra further in, probably a cold desert in the middle of the continent. It's reasonable to guess that mammals would have been few and far between; like Australia, most of Antarctica's mammalian fauna would have been marsupials, and they have issues with cold temperatures - but maybe not, maybe there were some polar-adapted marsupials from the Cretaceous forward (the Cretaceous being when Antarctica took the pole)
Forward two point five million years; the world is hit by the ice age cycle. Glaciers creep down Antarctica's mountains. And keep going, and going, and going until the whole continent is glaciated; This is helped by axial tilt, for half the year there is no sun in Antarctica worth mention. What sun there is, is reflected by the ice; this results in a glacial feedback loop that preserves Antarctica's ice cap even as the rest of the world thaws, re-freezes, thaws again...
Antarctica's current coldness isn't the result of ocean currents, it's the result of Antarctica freezing over very recently (geologically), and being unable to un-freeze.
By the way? Screw dinosaurs. I want a time machine to go see those arctic marsupials. or the polar terror-birds... that place must have been crazy... and with the glaciation, probably all fossil records are being ground to dust. Fucking depressing in its way.
= new reply since forum marked as read
