It's a simple question... which fossil did you go for? I just reached this point and decided to go with Tyrunt, however from what I've seen it is possible that Tyrunt is the less useful Pokemon... but I guess that depends on whose hands the Pokemon is in...
I am regretting it, to be honest. Rock/Ice ends up being kind of a bad type combo, with Fighting, Grass, Gound, Steel, Rock and Water (!!) all being weaknesses...and Fighting and Steel being 4x damage. Yikes!
Used her for a while, but then I caught a Goomy and tossed her in the box since as T-bun pointed out, there were way too many popular weaknesses for her to be a viable party member ;p
I'm already having that kind of thought with Tyrunt... he's 4X weak to a lot of things. But he's so cool looking. I'm sure he'll leave my party soon enough.