I'm no professional biologist like some of you guys, but the coincidence of marsupialism across so many otherwise quite different species has long led me to pre

I'm no professional biologist like some of you guys, but the coincidence of marsupialism across so many otherwise quite different species has long led me to presume that marsupial features were conferred across many species by a retrovirus... I actually thought that was the consensus. Mightn't this be a similar case?