The first dinosaurs were two-legged reptiles with the same basic posture that most later dinosaurs kept with only minor changes, with the head and tail both lifted off of the ground with the animal balancing over its legs. Living dinosaurs - birds - still have this posture, although it is difficult to see exactly where a bird's body is because it is covered in long feathers. So, basically, an early dinosaur's body would look like a bird, but without the beak or the feathers.

The giant four-legged dinosaurs, the sauropods, went back to having four legs only after they began to get too big to balance on two legs.