Very hard for me to evaluate without any clear idea of how important a fossil turn is. Obviously; completely useless outside of its set, but that's not a huge problem for an uncommon.

Skipping main phases rather than a whole turn is kinda interesting. You still get untap and combat. Could be relevant.

This feels like it's a build-around; but I'm not sure how "Make the opponent play normally, give up most of a turn" works for that. Should it do more positive things for you?

Or is this just an escape valve in case of "Ooops, fossil turns are way too good, here use this hoser"?

It's a fossil-hoser for control decks.

Thinking about it I may add something so that it can exist during a fossil turn.

I fear this may play out this way:

Huh. There's a flat hoser for fossil. That any deck can run. And there's no defence against it, except a counterspell. So many people will have 4 in their sideboard. So... fossil decks are doomed. I'd better avoid those cards.

It seems like a rather ineffective fossil hoser though. I mean, it's a -cost Moment of Silence, that also removes your main phase for one turn! And even Moment of Silence does not get played in normal control decks.