Evolution and paleontology ( reptile)
Tagged: animal, animals, bird, cat, fish, invertebrate, mammalHistorical development
Reptiles occupy an evolutionary position between amphibians, on the one hand, and the birds and mammals on the other, the last two classes having evolved from reptilian ancestors. Reptiles first appear in the fossil record of the Carboniferous Period, more than 280,000,000 years ago. By the Triassic, about 50,000,000 years later, they began to dominate the terrestrial life of the world and continued that dominance through the Mesozoic Era (65,000,000–225,000,000 years ago). Reptiles succeeded in adapting to deserts, swamps, forests, grasslands, rivers, lakes, and even the air and the seas. Coincident with the rise of mammals at the end of the Mesozoic, most reptilian groups became extinct. The big evolutionary step made by reptiles was the final emancipation from life in water, for, until that step was taken, vertebrates could not exploit all of the Earth’s surface. That breakthrough required two basic changes, the first of which took place in the skin. Modern amphibians have naked skins that lack horny scales, hair, and other protective devices. One small amphibian group, the caecilians, has small fishlike scales embedded in the skin; similar scales occurred in certain extinct amphibians. Because of their thinness and position, such amphibian scales are no protection against desiccation, one of the principal hazards of life for all animals, vertebrate and invertebrate. This susceptibility to drying out forces amphibians to remain in water or in very humid places, thus limiting their exploitation of the terrestrial environment. Reptiles evolved a different type of scale consisting of keratin (or horn) deposited in the outermost layer of the skin. This type of scale was in a position and of a thickness to prevent desiccation. The second basic change made by the reptiles was the development of the amniote egg. This development expanded the possibilities of exploiting terrestrial environments. The egg could be laid under rocks or logs, in holes in the ground, in deserts or forests in fact, almost anywhere except in water. The development of the reptilian egg had several other consequences. An egg that is enclosed by a shell must be fertilized before that shell is deposited, thus necessitating internal fertilization. The evolution of a land egg also increased the efficiency of the life cycle of terrestrial vertebrates. An amphibian, hatching from an aquatic egg, must develop and grow in water in a larval form, the tadpole. The history of a reptile, on the other hand, is one of development and growth of adult structures adapted for a terrestrial life. It need not develop gills or a lateral line system (series of sense organs), which are needed by the aquatic tadpole and which must be resorbed and reworked into other structures. This important change in the type of development was made possible by a great increase in the amount of yolk in the reptilian egg. The large amount of yolk also permitted the lengthening of the embryonic period, which in turn allowed the development of all structures that are necessary for successful existence on land. When a reptile hatches, it is ready to carry out all the activities of the adult (with the exception of reproduction), in the same environment and manner. After reptiles acquired scaly protection for their skin and an egg that did not have to be laid in water, they were free to move over most of the Earth’s surface. That freedom set the stage for the evolution of the many varied types of reptiles and, ultimately, for the evolution of birds and mammals.
Fossil distribution
What is known of the fossil record of reptiles shows that most of the major groups, or orders, were worldwide or nearly so at some time in their individual histories. Few orders are known from South America and Australia; the absence of most major groups from these areas is explained more likely on the basis of lack of preservation or lack of discovery of fossil beds than on the basis of a genuine absence of the animals throughout such a long interval as the Mesozoic. In the following discussion the names of present-day continents are used, though it should be understood that continental outlines in the past did not always coincide with those of today. Few orders of reptiles are known from Paleozoic times i.e., Carboniferous (280,000,000 – 345,000,000 years ago) and Permian (225,000,000 – 280,000,000 years ago). The stem reptiles, the Cotylosauria, have been found in Carboniferous deposits of eastern and western North America and western Europe and in Permian beds of what was formerly the Soviet Union and of Africa. Presumably they also lived in Asia during this interval of more than 100,000,000 years, but their remains have yet to be found there. In the same period the Pelycosauria lived in North America and Europe, where their fossils are well known, and possibly in Africa and Asia. The mammal-like Therapsida and perhaps other forms were fossilized in Africa and Europe. The probable ancestors of turtles appeared in Africa, and the first diapsids (reptiles having two-arched temporal structures) emerged in Africa and Europe in the Permian. By the Triassic (190,000,000 – 225,000,000 years ago), the earliest portion of the Mesozoic, the mammal-like reptiles had spread to all of the continents except Australia. Turtles were still in Africa and had spread at least as far as Europe. The Ichthyosauria were living in seas covering what is now western North America and western Europe and may have been much more widely distributed, considering their oceanic habitat. North America at that time was also the home of primitive diapsids (Thecodontia), phytosaurs (suborder Phytosauria), and the earliest dinosaurs (Saurischia) and crocodiles. Besides the mammal-like therapsids, Eurasia and Africa in the Triassic had phytosaurs, the first Rhynchocephalia, and the early dinosaurs. The major groups of reptiles, therefore, were essentially worldwide in distribution by the Triassic. The giant dinosaurs began their efflorescence in the Jurassic (136,000,000 – 190,000,000 years ago). The big carnivorous types, such as Allosaurus, roamed the landscapes of the major continents, presumably preying on even larger herbivorous dinosaurs (Apatosaurus, Diplodocus, etc.), whose remains have been found in North America, Europe, Africa, and Australia. Marine ichthyosaurs and plesiosaurs (order Sauropterygia) still swam in the shallow seas of both hemispheres. The ornithischian dinosaurs (Ornithischia), the ancestors of the duckbilled and horned dinosaurs, became widely distributed in the Jurassic. One group, the armoured stegosaurians (suborder Stegosauria), left fossils in North America, Europe, and Africa. The flying reptiles (Pterosauria) also made their appearance at least in Africa and Europe during the Jurassic. The culmination of dinosaur evolution occurred in the Cretaceous 65,000,000 – 136,000,000 years ago), when every part of the world had herbivorous ornithischian dinosaurs: the ankylosaurs (suborder Ankylosauria), medium-sized dinosaurs armoured with heavy plates and large spines, ranging from South America to Africa; the duck-billed dinosaurs (suborder Ornithopoda), ranging from North America to Africa; and the horned dinosaurs (suborder Ceratopsia) in North America and Asia. Everywhere they were preyed upon by the big carnivorous types, which culminated in North America in the gigantic carnivore Tyrannosaurus. The skies over North America, Africa, and Europe (and probably Asia and South America as well) were the province of the flying reptiles (Pterosauria). There were also inconspicuous groups that later inherited the reptilian world. Lizards (suborder Sauria) appeared in most continents, and snakes(suborder Serpentes) appeared in some places. Birds, having splintered off from reptilian ancestors in the Jurassic, became more numerous in the Cretaceous; mammals, which ultimately replaced most of the reptiles, were represented on most continents by small creatures. Through all this evolutionary activity, the conservative turtles continued their plodding evolutionary pace, changing but little, yet lasting through all.
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