Yeah. I don’t pay for pocket cast.

Start with long arms and walk on all fours. Evolve to walk bipedal and have shorter arms. The dinosaurs you’re thinking of are just in the process of evolving and might have eventually evolved to have no arms unless those little shorties are advantage somehow. (Holding on during mating, evolving towards wing development) Some modern snakes still have little nubbins where their legs used to be.