T Rex arms were actually really strong, just small compared to the rest of them.The quadrupeds would be no use so we have to look at the bipeds and flyers. Pterodactyl talons are way to small to lift those kind of blocks. And do you think a T-Rex could carry a hod with those little arms? Come on.
Obviously, it was aliens in association with the Rand Corporation.
Anyway, it's teamwork isn't it. Given a 4 legged dinosaur with a tiny brain, a two legged tiny armed dinosaur with a tiny brain, and a flying dinosaur (Pterodactyls aren't dinosaurs, but birds are so lets roll with it) with a tiny brain, we have all the attributes needed to be a success in the construction industry. Plus a few spare limbs.