Developing some backbone
One of these animals (Acanthodii) were acquiring an internal bony skeleton and included the beginnings of a vertebral column running longitudinally through the body and encompassing the primitive notochord. These were the probable ancestor to the bony fish we know today and possessed a streamlined body, large lateral eyes and wide mouths with numerous teeth. Their heads are bony and their small scales are thick and hard, but unlike the placoderms they did not have armour. The numerous lateral fins of these animals are unique in that each has a thin membrane supported at its leading edge by a long stout spine.
Acanthodii (sometimes called spiny sharks) is a class of extinct fishes, having features of both bony fish (Osteichthyes) and cartilaginous fish (Chondrichthyes). They appeared in the early Silurian (430 mya) and lasted until the late Permian (250 mya). The earliest ancanthodians were marine, but during the Devonian, freshwater species became predominant. They are distinguished in two respects: they were the first known jawed vertebrates, and they had stout spines supporting their fins, fixed in place and non-movable (like a shark's dorsal fin).