Prehistoric fish refers to early fish that are known only from fossil records. They are the earliest known vertebrates, and include the first and extinct fish that lived through the Cambrian to the Tertiary. The study of prehistoric fish is called paleoichthyology. A few living forms, such as the coelacanth are also referred to as prehistoric fish, or even living fossils, due to their current rarity and similarity to extinct forms. Fish which have become recently extinct are not usually referred to as prehistoric fish.
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The first fish and the first vertebrates, were the ostracoderms, which appeared in the Cambrian Period, about 510 million years ago, and became extinct near the end of the Devonian, about 377 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 (1 ft) long. The ostracoderms are placed in the class Agnatha along with the living jawless fishes, the lampreys and hagfishes, which are believed to be descended from the ostracoderms, as are all jawed fishes, or gnathostomes. Paired fins, or limbs, first evolved within this group.
The placoderms, a group of jawed fishes, appeared by the beginning of the Devonian, about 416 million years ago, and became extinct at the end of the Devonian or the beginning of the Mississippian (Carboniferous), about 360 million years ago. Recent studies suggest that the placoderms are a possibly paraphyletic group of basal gnathostomes, and the closest relatives of all living jawed vertebrates. Some Placoderms were small, flattened bottom-dwellers, such as antiarchs. However, many, particularly the arthrodires, were active midwater predators. Dunkleosteus was the largest and most famous of these. 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 region. This allowed the upper part of the head to be thrown back and, in arthrodires, this allowed them to take larger bites.
The acanthodians, or spiny sharks, appeared by the late Silurian, about 420 million years ago, and became extinct before the end of the Permian, about 250 million years ago. However, scales and teeth attributed to this group, as well as more derived gnathostomes such as Chondrichthyes and Osteichthyes, date from the Ordovician (~460 million years ago). Acanthodians were generally small shark-like fishes varying from toothless filter-feeders to toothed predators. They were once often classified as an order of the class Placodermi, another group of primitive fishes, but recent authorities tend to place the acanthodians nearer to or within the living gnathostomes.
Cartilaginous fishes, class Chondrichthyes, consisting of sharks, rays and chimaeras, appeared by about 395 million years ago, in the middle Devonian. The modern bony fishes, class Osteichthyes, appeared in the late Silurian or early Devonian, about 416 million years ago. Both the Osteichthyes and Chondrichthyes may have arisen from either the acanthodians or placodermi. A subclass of the Osteichthyes, the ray-finned fishes (Actinopterygii), have become the dominant group of fishes in the post-Paleozoic and modern world, with some 30,000 living species. However, another subclass of Osteichthyes, the Sarcopterygii, including lobe-finned fishes including coelacanths and lungfish) and tetrapods, was the most diverse group of bony fishes in the Devonian. Sarcopterygians are basally characterized by internal nostrils, lobe fins containing a robust internal skeleton, and cosmoid scales.
Groups of various prehistoric fishes include:
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