RTD just likes his stupid jokes, is all. Like throwing in a line in DW that hints Jack is destined to spend several million years as a paraplegic trapped in a jar, and with a completely different personality. Ho ho ho. Very funny, Russell.

The great thing about the DW universe - and the writers have been saying this practically since the beginning - is that with all the time travel and self-contradiction, the fans get to write their own canon. Rex just got a temporary side effect of something, that's all. He shouldn't go getting used to it!