How did the sandworms/sandtrout appear on Arrakis?
I'm going to ask a more pointed version of this question:
Has anyone found any references that explicitly state exactly where the Sandworms came from? Did either Frank or Brian give any clues? Even the Dune Encyclopedia doesn't have an answer (I looked at my copy just now)
In Children of Dune, Leto II says that they were "introduced" to Arrakis, but he never says where they came from.