Pump water over gill-like structure. Note lack of appendages. Strain food from water while swimming. In vertebrates, the notochord evolved into either a bony or cartilaginous structure, and enveloped the nerve chord, producing a spinal or vertebral column . The earliest vertebrates were the Agnatha , or jawless fishes.
Evidence suggests that they originated in fresh water, then colonized marine systems.
Armored habit was probably for protection from eurypterids, cephalopods. Lack of jaws probably restricted most agnatha to an epibenthic life, scavenging organic detritus. However, adaptations such modem lamprey's parasitic habit must also have occurred. A primitive group was the Antiarchs , a small, shelled creature with 2 boney appendages which may have been used for locomotion or digging. To be a successful predator you need more than just big jaws, however.