The first fishes are thought to have emerged some 518 million years ago during the Cambrian Period of Earth’s history. Today, there more than 30,000 species of fishes found in the fresh and salt waters of the world. Living species range from the primitive, jawless lampreys and hagfishes through the cartilaginous sharks, skates, and rays to the abundant and diverse bony fishes.
Fishes range in adult length from less than 10 mm (0.4 inch) to more than 20 meters (60 feet) and in weight from about 1.5 grams (less than 0.06 ounce) to many thousands of kilograms. Some live in shallow thermal springs at temperatures slightly above 42 °C (100 °F), others in cold Arctic seas a few degrees below 0 °C (32 °F) or in cold deep waters more than 4,000 meters (13,100 feet) beneath the ocean surface.
Fish reproduction methods vary, but most fishes lay a large number of small eggs that are fertilized and scattered outside of the body. The eggs of pelagic (open ocean) fishes usually remain suspended in the open water, while many shore and freshwater fishes lay eggs on the bottom or among plants. The mortality of the young and especially of the eggs is very high, and often only a few individuals grow to maturity out of hundreds, thousands, and in some cases millions of eggs laid.
Amphibians evolved from fully aquatic tetrapods—(which were essentially “limbed fish”) who descended from lobe-finned fish—sometime between the Early Devonian Period (which began 419 million years ago) to the Early Pennsylvanian Subperiod (which began 323 million years ago). The name amphibian, derived from the Greek amphibious meaning “living a double life,” reflects this dual life strategy—though some species are permanent land dwellers, while other species have a completely aquatic mode of existence.
There are three living groups of amphibians (caecilians, salamanders, and anurans [frogs and toads]) that, collectively, make up more than 7,300 amphibian species. One similar tendency among amphibians has been the evolution of direct development, in which the aquatic egg and free-swimming larval stages are eliminated. Development occurs fully within the egg capsule, and juveniles hatch as miniatures of the adult body form. Most species of lungless salamanders (family Plethodontidae), the largest salamander family, some caecilians, and many species of anurans have direct development. In addition, numerous caecilians and a few species of anurans and salamanders give birth to live young.
Frogs and toads display a wide variety of life histories. Some deposit eggs on vegetation above streams or ponds; upon hatching, the tadpoles drop into the water where they continue to develop throughout their larval stage. Some species create foam nests for their eggs in aquatic (watery), terrestrial (land-based), or arboreal (tree-based) habitats; after hatching, tadpoles usually develop in water. Other species deposit their eggs on land and transport them to water, while marsupial frogs are so called because they carry their eggs in a pouch on their backs. A few species lack a pouch and the tadpoles are exposed on the back; in some species, the female deposits her tadpoles in a pond as soon as they emerge from eggs.
Reptiles are air-breathing vertebrates. They have internal fertilization, amniotic development (in which the embryo develops within a set of protective extra-embryonic membranes—the amnion, chorion, and allantois), and epidermal scales covering part or all of their body. The major groups of living reptiles—the turtles, tuataras, lizards and snakes, and crocodiles account for over 8,700 species. Reptiles evolved from amphibians during the first part of the Pennsylvanian subperiod (323 million to 299 million years ago) and retained many amphibian structural characteristics. While most reptiles feed on other organisms, a few are herbivorous (e.g., tortoises). As cold-blooded animals, reptiles tend to be limited to temperate and tropical areas, but, where they occur, they are relatively common; however, they are not as large or conspicuous as birds and mammals. Most reptiles are terrestrial, but a few are aquatic. They move about by creeping or swimming in a fashion similar to amphibians. Some reptiles, however, can lift the body from the ground and run rapidly either in a quadrupedal or bipedal fashion. Reptiles lay relatively large, shelled eggs. In a few instances, the eggs and young are cared for by the female; in others, the young are born alive.
Birds make up any of the 9,600 living species unique in having feathers, the major characteristic that distinguishes them from all other animals. They are warm-blooded vertebrates more related to reptiles than to mammals. They have a four-chambered heart (as do mammals), forelimbs modified into wings (a trait shared with bats), a hard-shelled egg, and keen vision. Their sense of smell is not highly developed, and their auditory range is limited.
Although most are capable of flight, others are sedentary, and some are flightless. In a manner similar to their relatively close relatives the reptiles, birds lay shelled eggs. The young are usually cared for in a nest until they are capable of flight and self-feeding, but some birds hatch in a well-developed state that allows them to begin feeding immediately or even take flight. (Nesting activities similar to those of some birds are seen in the crocodilians.)
The origin of birds, feathers, and avian flight have long been hotly debated; the evolution of birds from reptilian ancestors is universally accepted, however. The diversity of theropod dinosaurs (a diverse group of carnivorous “lizard-hipped” dinosaurs), some with feathers, has greatly expanded our perspective of the evolution and early diversification of birds. While it is known that the critical period in avian evolution and flight took place during the Early Cretaceous (145.5 million to 99.6 million years ago), there is evidence that feathers on theropods emerged much earlier, possibly during the Triassic and Jurassic Periods (some 252 million to 145 million years ago)
There are approximately 5,000 species of mammals living today. Mammals differ from other vertebrate animals in that their young are nourished with milk from special mammary glands of the mother. Mammals are distinguished by several other unique features. Hair is a typical mammalian feature, although in many whales it has disappeared except in the fetal stage. The mammalian lower jaw is hinged directly to the skull, instead of through a separate bone (the quadrate) as in all other vertebrates. A chain of three tiny bones transmits sound waves across the middle ear. A muscular diaphragm separates the heart and the lungs from the abdominal cavity. Mature red blood cells (erythrocytes) in all mammals lack a nucleus; all other vertebrates have nucleated red blood cells. The oldest known animals classified as mammals evolved near the boundary of the Triassic and Jurassic Periods, some 200 million years ago.
This group of vertebrates ranges in size from tiny shrews or small bats weighing only a few grams to the largest known animals, the whales. Most mammals are terrestrial, feeding on both animal and vegetable matter, but a few are partially aquatic or entirely so, as in the case of the whales or porpoises. Mammals move about in a great variety of ways: burrowing, bipedal or tetrapedal (four-legged) running, flying, or swimming. Reproduction usually involves the young developing inside the uterus, where nutritive materials are made available through an allantoic placenta or, in a few cases, a yolk sac. In placental mammals, young have a longer developmental period within the uterus. In marsupials, the relatively undeveloped young are carried in a pouch, where they attach themselves to their mother’s nipple until they become fully developed. Monotreme mammals (that is, the platypus and echidna) differ from other mammals in that they lay eggs which hatch.
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