The first fishes, and indeed the first vertebrates, were the ostracoderms, which appeared in the Cambrian Period, about 510 million years ago, 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 (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.
The first fishes with jaws, 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 generally small sharklike fishes varying from toothless filter-feeders to toothed predators. They are often classified as an order of the class Placodermi, another group of primitive fishes, but recent authorities tend to place the acanthodians in a class by themselves (class Acanthodii) or even within the class of modern bony fishes, the Osteichthyes. It is commonly believed that the acanthodians and the modern bony fishes are related and that either the acanthodians gave rise to the modern bony fishes or that both groups share a common ancestor.
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 typically 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 region.
The cartilaginous-skeleton sharks and rays, class Chondrichthyes, which appeared about 370 million years ago in the middle Devonian, are generally believed to be descended from the bony-skeleton placoderms. The cartilaginous skeletons are considered to be a 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, for no fossil remains of modern bony fishes have been found in marine deposits older than Triassic time, about 230 million years ago. The Osteichthyes may have arisen 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 found among another group 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 arose 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 (or suborder) Rhipidistia crawled out of the water to become the first amphibians.
The classification of fishes is a subject of considerable debate. The living fishes are often divided into three different classes. Divisions within these classes, however--particularly within the bony fishes (Osteichthyes)--are much in dispute, with different names being applied to the same group and with a given group being regarded as either a subclass, order, or some other rank, depending upon the authority consulted. The following is a general classification; to help clarify terms that may be found in other sources, it also includes as many additional definitions as space allows.
Class Agnatha or Cephalaspidomorphi, the jawless fishes
Subclass (or order) Cyclostomata, the lampreys and hagfishes.
(In certain classifications, the lampreys and hagfishes are each considered separate superclasses: Cephalaspidomorphi and Pteraspidomorphi, respectively.)
Class Chondrichthyes, the cartilaginous-skeleton fishes
Subclass Holocephali, the chimaeras, or ratfishes
Subclass Elasmobranchii, the sharks, skates, and rays
Class Osteichthyes, the bony fishes
Subclass (or order) Crossopterygii, the coelacanth
Subclass (or order) Dipnoi or Dipneusti, the lungfishes
(In some classifications, the above two subclasses are treated as orders of a single subclass, the Choanichthyes or Sarcopterygii, the lobe-finned fishes.)
Subclass Actinopterygii, the ray-finned fishes
Infraclass (or superorder) Chondrostei, the primitive ray-finned bony fishes: sturgeons, paddlefish, and bichirs
(In some classifications, the bichirs are placed in a subclass of their own, the Brachiopterygii.)
Infraclass (or superorder) Holostei or Neopterygii, the intermediate ray-finned fishes: gars and the bowfin
(In certain classifications, the gars are treated as a separate superorder, the Ginglymodi. The term Ginglymodi also has been used to designate the gars as an order, but this term has been replaced at the ordinal level by the term Lepisosteiformes; orders are now indicated by the ending -formes.)
Infraclass (or superorder) Teleostei or Neopterygii, the advanced bony fishes: herring, salmon, perch.
Distribution - Anatomy - Circulation -
Body Temperature - Water Balance - Swimming - Gas Bladder
Lateral Line System - Reproduction
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