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Charles Darwin coined the phrase "living fossil" to describe the ginkgo tree, whose distinctive wedge-shaped leaves are nearly identical to those of fossilized ginkgos from the Triassic Period 240 million years ago. Today scientists have identified hundreds of other living things that have persisted in an almost unchanged form for millions of years, including giant sequoia trees, millipedes, armadillos, crocodilians, and even some bacteria. Among living fossil fish, the coelacanth is the most famous, but there are many others. Perhaps even more than other kinds of living fossils, these ancient fish, whose kind have swum the seas for more than 450 million years, give scientists a window into what the Earth was like an incomprehensibly long time ago. As the ancestors of all vertebrates, they also provide important clues about the evolution of many animals, even humans. In this overview, learn about some of the living fossil fish that have escaped the limelight. |
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Bichir are the only living fossil fish commonly kept as pets. Bichir are bottom-dwelling freshwater fish native to western Africa. They are among the most primitive of the ray-finned fish, the dominant group of modern fish, which arose about 395 million years ago. A bichir's long, narrow body is commonly two to three feet long and covered with thick, diamond-shaped scales made of a shiny enamel-like substance called ganoine. Such scales were also present in the earliest ray-finned fish and are different from those of other living fish, except gar. Like many of the other living fossil fish, the bichir is especially adapted to life in dry environments. Instead of a regular swim bladder it has a pair of modified "lungs," like those of the lungfish, which enable it to survive out of water for several hours. |
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Bowfin are normally ravenous eaters but they can go without eating longer than any other fish—for nearly a year if necessary—because of their low metabolisms. There is only one living species of bowfin, which is also called the dogfish, mudfish, or grindle. This uniquely American freshwater fish is found in the Mississippi River basin, the Great Lakes, and other small bodies of water east of the Great Lakes. It is a fierce fighter with sharp teeth that is known to eat fish of all kinds as well as frogs, snakes, turtles, and even small mammals. It also sometimes cannibalizes other bowfin. Bowfin do not make good eating but are considered good game fish. Bowfin can use their swim bladders, which most other fish use as a kind of flotation device, as a lung, allowing them to survive out of water for up to a day. In oxygen-poor water, bowfin will often gulp surface air in order to breathe. Paleontologists have discovered fossilized bowfin from North America, South America, Europe, Africa, and Asia, the earliest of which dates to the Jurassic Era, which began 213 million years ago. |
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Gar eggs are lethally toxic to humans and other warm-blooded vertebrates. Gar (named for the Anglo-Saxon word for "spear") are freshwater fish found in the warmer rivers and lakes of the southern U.S., Central America, Mexico, and the West Indies. They are highly predaceous, with long jaws and large, sharp teeth. They will attack any fish in their path, lying perfectly still until they sense potential prey, then striking and taking a "victory lap" or two around their kill. Fishermen consider them a nuisance because they destroy many fish, sometimes without bothering to eat them; they are also inedible. Gar sport a fierce-looking armature of plate-like scales shaped like diamonds, which are made of inorganic salt, like those of many now-extinct fish. Like other primitive fish, the gar has a swim bladder connected to its esophagus, which can be used for breathing. |
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The hagfish is the only vertebrate whose body fluids are the same concentration as the surrounding seawater. It is easy to see how this mud-dwelling, eel-like fish named for its hideous appearance has been overshadowed by the comparatively comely coelacanth. Also known as slime eels, hagfish are indigenous to both the Atlantic and Pacific oceans, where they live at depths of 100 to 3,150 feet. Eyeless, jawless, and dependent on their sense of smell to find food, hagfish invade feeble or dead fish by entering through their mouths and settling in their stomachs, where they secrete a slimy substance that protects them from the digestive juices of their hosts. They then eat their hosts from the inside out. Because hagfish are soft-bodied, it has been difficult for scientists to find and interpret evidence of their existence in the fossil record. They believe, however, that this vertebrate's nearly identical ancestor arose more than 550 million years ago, in the Precambrian Era. |
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King Henry I of England died in Normandy in 1135 after gorging himself on his favorite food: boiled lampreys. Like hagfish, lampreys belong to the jawless fish, the most primitive of all fish. Adult lampreys have a notochord, a backbone-like structure that more-evolved vertebrates lose after the embryonic stage. Lampreys live in both freshwater and marine environments worldwide, usually migrating from saltwater to freshwater to spawn. Some sea lamprey populations have become landlocked and represent serious pests to fishing industries. Adult lampreys parasitically attach themselves to other fish, sucking out their body fluids after scratching a hole with their sandpapery tongues. Lampreys have an anticoagulant in their saliva that keeps the blood of their host fluid, and some lampreys eat flesh as well. Their unwilling hosts may eventually die. |
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The lungfish is the only living fossil fish to have a rock band named after it. The first living lungfish was discovered in the 1830s, almost a century before the coelacanth appeared. Like the coelacanth, this fish was previously thought to be extinct, because it was known only from fossilized Devonian-Era remains almost 450 million years old. The "lungs" of the lungfish are similar to those of land animals. They are modified swim bladders, which most fish use for buoyancy while swimming. Lungfish use theirs to absorb and conserve oxygen from air. Modern lungfish, which are found in rivers and swamps in Africa, South America, and Australia, can remain alive out of water by using their swim bladders like lungs. They are so adapted to living out of water that some species will drown if they are deprived of air. Scientists believe lungfish are the closest living relatives of tetrapods—four-legged animals—with which they share a number of important characteristics, including tooth enamel; the arrangement of some skull bones; a separation of pulmonary-system blood flow from blood flow throughout the rest of the body; and four similarly sized limbs that have the same position and structure as tetrapod legs. |
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Spanish explorer Hernando de Soto discovered the paddlefish while exploring the Mississippi River in the 16th century. There are only two species of paddlefish, which are defined by their large, paddle-shaped snouts. One is found in the Mississippi and the other in China's Yangtze River. Scientists once thought that these fish used their paddles to dig in muddy river bottoms or as a means of providing balance in river currents. The paddle, though, is used to gather sensory data, such as smells, tastes, and the electrical fields given off by other fish and prey. Oddly enough, adult paddlefish missing all or part of their paddle have no trouble surviving. Paddlefish are large, reaching up to 15 feet in length as adults. They feed on crustaceans, and humans feed on them. North American paddlefish used to reach weights of 300 pounds, but the species has been overfished and now 100-pound paddlefish are considered large. Unlike most modern fish, paddlefish have skins with few scales, skeletons made almost wholly out of cartilage, and upturned tail fins like those of sturgeon and sharks. |
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The sturgeon is the largest freshwater fish in the world. There are 24 species of sturgeon worldwide, the most famous of which is the beluga sturgeon of the Caspian Sea, prized for its caviar. Found in fresh and salt waters in the Northern Hemisphere, the sturgeon is a bizarre-looking creature whose retractable mouth can be projected like a miniature elephant trunk from the underside of its head and whose body is armored with rows of thick plates. An adult Pacific sturgeon can weigh 400 pounds and reach more than 20 feet in length. These fish can live well over 100 years.
In addition to their armored appearance, sturgeon have several characteristics
that are not found in modern fish and that mark them as ancient. Though most
modern bony fish have symmetrical tails (the top and bottom lobes are the same
size), sturgeon have asymmetric tails like those of a shark. They also have
notochords, lack scales, and, like sharks, have spiral valves in their guts
instead of intestines.
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Lexi Krock is assistant editor of NOVA online. 
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Bichir
