There are new facts about the evolution of fish. An ancient fish fossil has provided preserved muscle tissue. European and Australian scientists have examined a placoderm fossil 380 million years old. Early fish did not have moving jaws with joints. Their mouths were fixed partially open, though they could still move.
Jaw structure was presumed to be like sharks, because that was taken to be a living example. The fossil has changed this assumption. It shows a pronounced shoulder structure. From this girdle neck muscles attached to a dermal joint. So the head moved relative to the body but the jaw did not. Sharks do not have joints in the neck - their necks are flexible.
Another difference is musculature in the abdomen much like four-limbed vertebrates such as horses. Rather than being a evolutionary step for four-legged animals it was fully developed by placoderms, the first vertebrate with armoured plates and no teeth.