In general, a fish is a streamlined animal with a backbone that swims by undulations and breathes through gills. It has a cranium and a muscular tail with a tail fin. There are several different groups, the most important three being the sharks, rays, and chimaeras with cartilaginous skeletons (Class Chondrichthyes); the familiar ray-finned fishes with bony skeletons (Actinopterygii); and the lobefins and lungfishes that belong to the Sarcopterygii, which have pairs of fleshy fins or limbs with a series of internal bones, only one of which is attached to the shoulder girdle or pelvis. This last group is the one from which humans—indeed, all tetrapods (four-legged animals)—evolved.
The earliest recognizable bony fishes arose in the Ordovician period, more than 450 million years ago. Called ostracoderms, they lacked movable jaws but had a distinctive brain encased in cranial bones and often were armored with bony plates and scales. Other jawless fish, the still-existing lampreys and hagfishes, are about 300 million years old, from the late Carboniferous; they probably arose from a different group of ostracoderms than the jawed fishes.
The earliest fish with jaws probably appeared as early as the late Ordovician, 450 million years ago. Microscopic scraps of sharklike skin denticles have been found in the United States dating from that time and in Mongolia from the Silurian, about 420 million years ago. The most anatomically primitive shark fossils date from the Late Devonian, 360 million years ago. Even at this early stage, this class of animal had an elongate body, large triangular fins, an upturned tail, and a mouth filled with rows of teeth.
The placoderms were a formidable group of jawed fishes from the Late Devonian and were the largest vertebrates of their time, some of them reaching 26-33 feet in length. Nearly 200 fossil placoderm genera are known.
The earliest fossil lobefins are from China, Spitsbergen, Norway, and Canada and date from the Early Devonian, 400 million years ago. For a time, before bony fishes got into their stride, they were the dominant fishes of the Devonian. Lungfishes and coelacanths are the only finned relatives of land animals to have survived the major extinction at the end of the Permian, 251 million years ago. The CoelacanTHS include only two living saltwater species (Latime-ria chalumnae and L. menadoensis). Some of the more primitive coelacanths had a sharklike, asymmetrical tail with special muscles that allowed it to twitch. Coelacanths of the Early Cretaceous, 120 million years ago, such as Mawsonia (10 feet long), lived in brackish waters in Brazil and Africa.
Isolated scales from relatives of bony fishes have been obtained from the late Silurian of Russia and China, and fragmentary bones and teeth have been found in similar strata in Estonia. One of the most primitive bony species known is Cheirolepis, from the Middle Devonian, 380 million years ago. Its anatomy suggests it was a swift swimmer, and the pointed teeth in its large mouth indicate it was an efficient predator.
Almost half of all known species of vertebrates now alive are ray-finned bony fishes; the 23,681 species cataloged in 1994 by Joe Nelson are probably a vast underestimate, since isolated pools and streams in tropical forests can, over time, evolve new varieties. Sturgeons (Family Acipenseridae) belong to a primitive group called chondrosteans that are separate from the more advanced teleost fishes. Often listed as candidates for Freshwater Monsters, sturgeons have largely cartilaginous skeletons and live in the sea but travel a long distance up rivers and into lakes to breed. Teleost fishes fall into four major groupings: the Bonytongues (Os-teoglossomorpha); Eels and Tarpons (Elopo-morpha); Herrings and their relatives (Clupeo-morpha); and everything else, from salmon to minnows (Euteleostei).
The largest living fish is the Whale shark (Rhincodon typus), found in tropical oceans. In early 1919, an unverified 55-foot whale shark became wedged in a fish trap off Ban Ko Chik, Thailand. A huge individual known as Sapodilla Tom frequented the waters off Honduras for fifty years and was said to measure 60-70 feet in length. The largest official specimen measured 41 feet 6 inches and was caught in the Indian Ocean off Karachi, Pakistan, on November 11, 1949.
The largest carnivorous fish is the Great white shark (Carcharodon carcharias), which averages 14-15 feet in length. Outsize specimens grow to at least 20 feet.
The largest bony fish is the Ocean sunfish (Mola mola), which averages 6 feet from snout to tail and 8 feet in vertical length; an outsize specimen caught in 1908 off Sydney, Australia, measured 10 by 14 feet. The largest freshwater fish is the Giant catfish or Pa beuk (Pangasian-odon gigas), found in the Mekong River and its tributaries in Southeast Asia. A specimen 9 feet 10 inches long and weighing 533 pounds was caught in the Ban Mee Noi River, Thailand.
Of the twenty-three fishes in this list, seven appear to be unknown ray-finned fishes, eight appear to be sharks or rays, one might be a surviving placoderm, three are lungfishes or lobefins, two are of ambiguous provenance, and two others are probably not fishes at all.
Some Sea Monsters, Freshwater Monsters, and merbeings may also involve known or unknown species of fishes.
Beebe's Abyssal Fishes; Beebe's Manta; Black Fish (Venomous); Captan Hanna's Fish; Challenger Deep Flatfish; Clear Lake Catfish; CoeLACANTH (Unrecorded Populations); Dakuwaqa; Giant Cookiecutter Shark; Giant Lungfish; Giant Rat-Tail Giant Salmon; Glowing Mudskipper Ground Shark; Guara^a' Air-Breather; Japanese Hary Fish; Lake Sentani Shark; Lord of the Deep; Malpelo Monster; Manguruy^; Mcha-Mcha; Mongitore's Monstrous Fish; Sea Monk
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