When the first time travel ship crash lands in the prehistoric era, the crew have 3 days to fix the ship before the asteroid that wiped out the dinosaurs strikes -- exactly where they are stranded. (36 words)
I think the concept is strong enough that you don't have to sweat the details about designating an MC. Who was the MC in Apollo 13? Wasn't it a team effort -- an ensemble movie?
It goes without saying that the captain of the time travel ship would be the default leader, ergo, the obvious main character when it comes time to cast the movie.
Ditto, imho, for gilding the lilly of this concept with a glaring character flaw. What were the character flaws of the crew of Apollo 13? As if they needed more problems and complications than they already had to heighten tension and suspense and invoke audience interest in and empathy for the characters.
Which, btw, is the primary dramatic purpose of a character flaw: to ratchet up tension, heighten suspense, to strengthen the audience's emotional bond to the character, intensify the emotional catharsis of the character's struggle and denouement (oxytocin moments!) by worrying even more that he/she will fail.
Of course, landing back in the time when the asteroid will wipe out the dinosaurs, paving the ecological landscape for the rise of mammals and eventually homo sapiens raises all kinds of technical and existential questions, but like Older Joe says in "Looper": I don't want to talk about time travel because if we start talking about it then we're going to be here all day talking about it, making diagrams with straws."