Megalosaurus
Later it was realised that all theropods were bipedal. Today is known that the early proto-dinosaurs in the late Middle Triassic were also bipedal. Certainly Eoraptor, from 231.4 million years ago, was bipedal. That means the dinosaurs were bipedal from the start (primatively bipedal). Only the heavier sauropods and armoured dinosaurs became quadrupedal, standing on all four legs.