Here's a question from my son I've found interesting enough to ask here. There are plenty examples of species returning back to the water environment, like dolphins, sea lions, walruses, some snakes, crocodiles etc.
The question is - are there any evidences that in evolution of certain species there was return to the land twice, that is, they've came from sea, then evolve as land species, then, again, evolved to somewhat marine or fresh-water and, finally, turn into land species again?
UPD Chordate are the most interesting but any example, even plants, would be nice.