Features of Primitive Fish
The ancestor of modern fish had already appeared in the Cambrian. It is called agnathous fish because it was small, mouth-opened and finless. This is the most primitive vertebrate.
In the Silurian, the primitive fish with upper and lower jaws appeared, that were the earliest placodermi and acanthodii. In the Devonian, they had evolved into Chondrichthyes and Osteichthyes, becoming the major animals in the ocean. Therefore, the Devonian is called the age of fish. In the Permian, placodermi and acanthodii had gone extinct successively. There were many species of Chondrichthyes before the Permian. They had gradually declined in the Triassic. Their only remaining modern species are sharks, rays and chimaeras. Osteichthyes became the dominant species in this competition, one of the most flourishing animals in the ocean. Some of the primitive Osteichthyes have evolved into Dipnoi and Crossopterygii, of which the latter has finally developed into Amphibian.