The carnivores were bipedal (walked on their back legs), though not as we do. Their body was more towards the horizontal, balanced at the back by their tail. Some were very large, like "Tyrannosaurus", "Allosaurus" and "Spinosaurus", but some were small, like "Compsognathus". It was the smaller sized meat-eaters that may have evolved into birds. The first fossil bird, "Archaeopteryx", had a skeleton which looked much like that of a dinosaur.