Instead, they started their evolutionary lives much closer to the seafloor, where they could suck up food before they evolved jaws and teeth. By around 400 million years ago, jawed fish dominated the oceans. Credit: Pierre Poirier / Canadian Museum of NatureHead and shouldersNorselaspis isn’t only challenging the timeline of jawed fish. Because of this, they now theorize the shoulders seen in tetrapods (four-limbed vertebrates) evolved as a new structure connected to the neck which separated the torso from the head. Most early jawless fishes had torsos leading continuously to the head, whereas jawed vertebrates evolved a neck and throat.