As far as anyone knows, there is no way to go back. You can hope to get lucky in the shop, through the Team Plaza rewards, transfer from the first game (if the vivosaur existed in that one anyway), or you can start a new game + and get back to that point in the story again.
jimengrocks posted... I missed a few fossils down in there and i want to go back there. But how??
You can't go back sadly, but thankfully postgame, you can still find their fossils scattered across different dig sites, For locations, look at the questions section and check out my question. If you're missing any that are one of the first 100, you can just send their fossils over from the first game.
jimengrocks posted... hmmm. Hey, is there diffrent dig sites where the fossils form sea floor cavern are?
Megalo, Cryo, Angato, and Centro can all be transferred from the original game. Archaeo can be found on the Bare Bones Brigade Base beach (just wear the head mask...). I haven't found an alternate location for Terata yet, sorry.
Please excuse my double post, but Vreeska posted on another forum that Terata could be found in the cave section of Hot Springs Heights. I suppose you could just wear a yellow mask, but keep in mind that they would be extremely rare.