Trailer B is up.

It explains how there are dinosaurs in the time frame.

It also contains, in the closing scene of the trailer, a gag that's pretty funny, though it would be NSFW if it were done with humans.

Well, that does explain it, doesn't it. A "lost world" scenario. Hopefully, they can pull it off. Though I've yet to see the second, so maybe I should catch that one, first.

I initially just figured that it would something along the lines of a herd of dinosaurs frozen in ice/suspended animation, and brought back to life after they thawed in the mammals' period.

But yeah, the scenario they went with works too. If it worked for the X-Men, why not these guys?

I had thought that, too.