Placoderms were the most diverse and dominant vertebrates in the Devonian, the "Age of Fishes." They were found in aquatic ecosystems of all types, and were the dominant vertebrates in these ecosystems. However, they only existed on Earth for a geologically short time, about 50 million years.
According to Unification thought, the development of life on Earth is a step-by-step process, with earlier stages providing the foundation for subsequent stages. The placoderms were the earliest branch of jawed fishes. They may have provided the foundation for modern Chondrichthyes (cartilaginous fish) and Osteichthyes (bony fish). (They may also have shared a common ancestry with these modern forms, rather than be ancestral.) According to Unification thought, they would have disappeared when their purpose was fulfilled.