early ideas of a southern continent . . .
As you may know, it was well known in ancient times that the world was a globe, and philosophers even had a good idea of the size.
However, they still did not have a good grasp on many of the details, so some of them decided that there HAD to be a southern continent to balance the weight of the continents they knew about. That's the primary reason why they assumed there must be a southern continent prior to the discovery of Australia. Even in Dante's "Inferno" there is reference to a southern continent.
Can't help you with the maps, though. Many globes and maps of the time included this theoretical southern continent, as well as quite a few other oddities.