The first fishes, were the first vertebrates, the ostracoderms, which appeared in the Cambrian Period, about 510 million years ago were the first, and became extinct at the end of the Devonian, about 350 million years ago. Ostracoderms were jawless fishes found mainly in fresh water. They were covered with a bony armor or scales and were often less than 30 cm long.

The first fishes with jaws were the acanthodians, or spiny sharks, appeared in the late Silurian, about 410 million years ago, and became extinct before the end of the Permian, about 250 million years ago. Acanthodians were small sharklike fishes varying from toothless filter feeders to toothed predators.

The placoderms, another group of jawed fishes, appeared at the beginning of the Devonian, about 395 million years ago, and became extinct at the end of the Devonian or the beginning of the Mississippian (Carboniferous), about 345 million years ago. Placoderms were small, flattened bottom-dwellers. The upper jaw was firmly fused to the skull, but there was a hinge joint between the skull and the bony plating of the trunk area.

The cartilaginous-skeleton sharks and rays, class Chondrichthyes, which appeared about 370 million years ago in the middle Devonian, are generally believed to have come from the bony-skeleton placoderms. The cartilaginous skeletons are later development.

The modern bony fishes, class Osteichthyes, appeared in the late Silurian or early Devonian, about 395 million years ago. The early forms were freshwater fishes, there are no fossil remains of modern bony fishes older than Triassic time, about 230 million years ago. The Osteichthyes may have come from the acanthodians. A subclass of the Osteichthyes, the ray-finned fishes (subclass Actinopterygii), became and have remained the dominant group of fishes throughout the world. It was not the ray-finned fishes, however, that led to the evolution of the land vertebrates.

The ancestors of the land vertebrates are of bony fishes called the Choanichthyes or Sarcopterygii. Choanate fishes are characterized by internal nostrils, fleshy fins called lobe fins, and cosmoid scales. The choanate fishes appeared in the late Silurian or early Devonian, more than 390 million years ago, and possibly came from the acanthodians. The choanate fishes include a group known as the Crossopterygii, which has one living representative, the coelacanth Latimeria. During the Devonian Period some crossopterygian fishes of the order Rhipidistia crawled out of the water to become the first amphibians.

 

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