The idea of a world being carried on the back of a turtle (and elephants) is not Terry's. It's much older than that and appears in Hindu mythology
For a long time we lived out of range of television (well there were two in town with anteneas as tall as the Eiffel tower) so i read alot as a child. My mother was religious, my father not but encouraged us to read about other religions to foster a tolerance and understanding of religion and race. (and be armed to argue). So many books! I remember reading about the world being carried by a turtle when I was about 9, long long before TP was publishing. Terry Pratchett isn't the only well read author who refers to myths, fables and historical anthropology in their work so its no suprise we see "Discworld" references in other works especially science fiction/fantasy that can slip into any age of man for a base.