The order Squamata includes 95 percent of the living reptiles. Here we find the Gila Monster, the geckos, the rattlesnakes and the beautifully-patterned corn snake. the order is divided into two suborders: Sauria, the lizards, and Ophidia. the snakes.
Evolution. The Sauria are the more primitive of the two groups of squamates. They appeared about 200 million years ago during the middle and late Triassic, with realtively unspecialized forms. Later saurians, from the Jurassic and Cretaceous periods, were already quite similar to modern forms.
During the Triassic period the squamates evolved from the eosuchians through transitional forms such as Prolacerta. in this genus the skull has partially lost the lower arch, or bar, resulting in a single, fully enclosed temporal opening high up on the cheek. Thus the condition in lizards (where there is only one upper opening found with a single cheek bar beneath it, and the rest of the cheek is open) could have resulted from a dropping out of the second arch that encloses the lower opening in primitive diapsids.
One group of aquatic lizards that lived only in the Cretaceous period included extremely specialized representatives of a well known family of modern lizards, the varanids or monitors. these were the mosasaurs, giant marine reptiles of the Cretaceous seas. They were "sea monsters" with long bodies that reached up to 30 feet or more in length. They were well adapted to life in the seas. They had long deep tails that they moved back and forth in a sculling motion to propel themselves through the water. Their limbs, too, were specialized, they were transformed into paddles and had short upper bones and an increased number of joints in the digits, or fingers.
Recent discoveries of Triassic saurians also demonstrate and amazing range of adaptation, even in those early days. Two kinds of flying lizards have been found. These had greatly elongated ribs, which supported a gliding membrane on each side of the body. Interestingly enough, some modern lizards have this same adaptation for gliding. These animals lived in southeast Asia and belong to the genus Draco. However, they bear no direct relationship to the ancient flying lizards, which still had a primitive skull structure.
The Ophidia appeared much later and are the most recent and most specialized reptiles. The first known snake was found in Cretaceous sediments dating back about 90 million years. Our knowledge of the evolution of snakes still contains many gaps.
Ecology. There are roughly 5, 700 species of lizards and snakes, most of which live in tropical and equatorial zones, some species, usually smaller, also inhabit the temperate zone; there are even smaller varieties with a highly-developed capacity to absorb heat and take in food. These are active for only part of the year. The variation in size between animals in one zone and another points up and interesting difference between reptiles and warm-blooded animals. The mammals and birds tend to have larger species in cold areas. This is an adaption for exposing less surface area compared to hte total body weight to hte cold these animals lose less body heat than small ones would.
Reptile species grow larger in the tropics because only in the tropics does a large reptile absorb enough heat to reach its best temperature. The cold areas have small reptiles because their bodies have to absorb the maximum amount of heat in proportion to their weight. These reptiles solve the cold problem by burrowing underground and conserving heat when the temperature falls.
As a group, squamates live in water, on land, an in trees. Only true flight is beyond their capabilities, although there are species in each suborder that can glide. They have achieved success in their evolutionary expansion to different habitats, although they have never become the supreme animal in any of them. The mammals, which in almost every way are better organized, are usually ecologically dominant. They have greater mobility and greater speed for finding prey and escaping dangers, and far more highly developed brains. Apart from mammals, birds have the advantage of living in a third dimension. They have conquered the air, which no modern reptiles have been able to do.
The Squamata have a great many rivals among the higher vertebrates. The struggle for life goes on without cease, and the squamates try in many ways to assure the survival of their various species. These survival techniques include the poison in their hollow fangs, their capacity for camouflage, the speed with which they can disappear into the water, sand, or earth, and their relatively large numbers of offspring, which, despite predators that destroy many of them, are sufficient to ensure the next generation.
Although squamates are not dominant in any environment, they are well distributed throughout the world. They live in equatorial, tropical, and temperate forests; on savanas, prairies, steppes, and sandy and rocky deserts: and in mountain ravines swampland, rivers, lakes, and along seashores. Within each of these particular environments they may specialize further. Some choose to live in the tops of the tallest trees, while others live in thickets of underbrush. Still others hide in sandy or damp ground that is never reached by the light of day.
Differences and Similarities. Most people fancy that they would have little difficulty in telling a lizard from a snake. They tend to assume, therefore, that the two suborders are completely distinct. But comparative anatomy shows that the Sauria and the Ophidia are very closely related. in fact it is difficult to find a clear definition to distinguish the two suborders.
There are, however, a few unmistakable elements that allow us to classify a species in one group ar the other on sight. There are cases where these definitions are not sufficient, because characters of both groups may exist at the same time. This happens in transitional forms, which in every sector of biology serve to remind us that the living world cannot be broken up into fixed categories; it is a dynamic whole consisting of beings in every stage of evolution.
The Squamata include reptiles with long bodies. They have four, two, or no limbs at all. The limbs, when they exist, may have five or fewer digits, or fingers. In many cases not only are the limbs missing but also the entire hip and shoulder girdles. In other cases there are traces of these structures, which are made of cartilage rather than of bone as were the original structures.
If the whole living world is in continuous evolution, one may ask: in what sense are the squamates evolving? Are they in the process of losing their legs or of developing them? The answer is simple. The squamates have descended from other four-legged reptiles. Therefore they are in the process not of developing legs, but of losing them. This tendency may seem absurd at first, since a first principle of evolution is to increase the level of an animal's organization, not to simplify it. Thus the squamates seem to be acting in a paradoxical way. However, the squamates that long, long ago began a process of losing their legs were not actually regressing. Rather they were adapting for a specialized way of life, and this is a way of life that has proved very successful for these animals.
Squamates are not armored like some other reptile like some other reptiles. They are covered with scales rather than thick armor like turtles, or impenetrable skin like crocodiles. Therefore they have to develop other methods of defense. Of all defense methods used by animals, running away is one of the safest. A small plains-dwelling reptile needs a certain amount of agility because it has to flee through dense thickets and thorny bushes. In such situations legs can be more of a hindrance than a help as they may get in the way of an escape that is necessary for survival. Escape is easier for a thin; smooth body that can slither through the bushes without having anything sticking out to catch on branches or on a thorn. This is only one of the theories to account for some of the squamates loss of legs. Another theory snakes, are descended from burrowing animals. Limbs can get in the way of a burrowing animal, too.
At any rate it is clear that the loss of legs has been a fully successful and useful adaptation and makes up for the fact that these animals do not have the protective armor and other means of defense found in turtles and crocodiles.
Snakes have no forelegs and never any true hind legs, although the remnants of these limbs may persist inside their bodies. Some lizards also tend toward a loss of limbs, and it is possible to find saurians with every gradation between forms with well developed legs and very long animals with tiny legs or none at all. Even among lizards with legs of normal size, these legs are not usually as big as they were in the primitive reptiles.
Another characteristic that all squamates share is that their bodies are covered with scales. These scales give the animals a certain amount of protection while retaining the flexibility they need to move. Also, their skulls are made up of thin bones that are joined so that the whole skull and jaws can expand to take in large pieces of food. this characteristic is especially pronounced in snakes.
Snakes can stick their tongues far out and their jaws are attached very loosely. Some lizards have progressed in this direction. Snakes have no movable eyelid; instead each eye is covered by a clear scale, derived from the lower eyelid. Such a structure has also evolved in certain lizards.
In both lizards and snakes it is almost impossible to distinguish the males from the females without examining their internal anatomy. Squamate males differ from all other vertebrates by having paired capulatory organs, the hemipenes, only one of which is ever led to the outside during mating. In certain lizards (agamids and iguanids) there is a dorsal crest that is more developed in the male. The coloration, too, is sometimes more gaudy in male lizards that in the females.
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Evolution. The Sauria are the more primitive of the two groups of squamates. They appeared about 200 million years ago during the middle and late Triassic, with realtively unspecialized forms. Later saurians, from the Jurassic and Cretaceous periods, were already quite similar to modern forms.
During the Triassic period the squamates evolved from the eosuchians through transitional forms such as Prolacerta. in this genus the skull has partially lost the lower arch, or bar, resulting in a single, fully enclosed temporal opening high up on the cheek. Thus the condition in lizards (where there is only one upper opening found with a single cheek bar beneath it, and the rest of the cheek is open) could have resulted from a dropping out of the second arch that encloses the lower opening in primitive diapsids.
One group of aquatic lizards that lived only in the Cretaceous period included extremely specialized representatives of a well known family of modern lizards, the varanids or monitors. these were the mosasaurs, giant marine reptiles of the Cretaceous seas. They were "sea monsters" with long bodies that reached up to 30 feet or more in length. They were well adapted to life in the seas. They had long deep tails that they moved back and forth in a sculling motion to propel themselves through the water. Their limbs, too, were specialized, they were transformed into paddles and had short upper bones and an increased number of joints in the digits, or fingers.
Recent discoveries of Triassic saurians also demonstrate and amazing range of adaptation, even in those early days. Two kinds of flying lizards have been found. These had greatly elongated ribs, which supported a gliding membrane on each side of the body. Interestingly enough, some modern lizards have this same adaptation for gliding. These animals lived in southeast Asia and belong to the genus Draco. However, they bear no direct relationship to the ancient flying lizards, which still had a primitive skull structure.
The Ophidia appeared much later and are the most recent and most specialized reptiles. The first known snake was found in Cretaceous sediments dating back about 90 million years. Our knowledge of the evolution of snakes still contains many gaps.
Ecology. There are roughly 5, 700 species of lizards and snakes, most of which live in tropical and equatorial zones, some species, usually smaller, also inhabit the temperate zone; there are even smaller varieties with a highly-developed capacity to absorb heat and take in food. These are active for only part of the year. The variation in size between animals in one zone and another points up and interesting difference between reptiles and warm-blooded animals. The mammals and birds tend to have larger species in cold areas. This is an adaption for exposing less surface area compared to hte total body weight to hte cold these animals lose less body heat than small ones would.
Reptile species grow larger in the tropics because only in the tropics does a large reptile absorb enough heat to reach its best temperature. The cold areas have small reptiles because their bodies have to absorb the maximum amount of heat in proportion to their weight. These reptiles solve the cold problem by burrowing underground and conserving heat when the temperature falls.
As a group, squamates live in water, on land, an in trees. Only true flight is beyond their capabilities, although there are species in each suborder that can glide. They have achieved success in their evolutionary expansion to different habitats, although they have never become the supreme animal in any of them. The mammals, which in almost every way are better organized, are usually ecologically dominant. They have greater mobility and greater speed for finding prey and escaping dangers, and far more highly developed brains. Apart from mammals, birds have the advantage of living in a third dimension. They have conquered the air, which no modern reptiles have been able to do.
The Squamata have a great many rivals among the higher vertebrates. The struggle for life goes on without cease, and the squamates try in many ways to assure the survival of their various species. These survival techniques include the poison in their hollow fangs, their capacity for camouflage, the speed with which they can disappear into the water, sand, or earth, and their relatively large numbers of offspring, which, despite predators that destroy many of them, are sufficient to ensure the next generation.
Although squamates are not dominant in any environment, they are well distributed throughout the world. They live in equatorial, tropical, and temperate forests; on savanas, prairies, steppes, and sandy and rocky deserts: and in mountain ravines swampland, rivers, lakes, and along seashores. Within each of these particular environments they may specialize further. Some choose to live in the tops of the tallest trees, while others live in thickets of underbrush. Still others hide in sandy or damp ground that is never reached by the light of day.
Differences and Similarities. Most people fancy that they would have little difficulty in telling a lizard from a snake. They tend to assume, therefore, that the two suborders are completely distinct. But comparative anatomy shows that the Sauria and the Ophidia are very closely related. in fact it is difficult to find a clear definition to distinguish the two suborders.
There are, however, a few unmistakable elements that allow us to classify a species in one group ar the other on sight. There are cases where these definitions are not sufficient, because characters of both groups may exist at the same time. This happens in transitional forms, which in every sector of biology serve to remind us that the living world cannot be broken up into fixed categories; it is a dynamic whole consisting of beings in every stage of evolution.
The Squamata include reptiles with long bodies. They have four, two, or no limbs at all. The limbs, when they exist, may have five or fewer digits, or fingers. In many cases not only are the limbs missing but also the entire hip and shoulder girdles. In other cases there are traces of these structures, which are made of cartilage rather than of bone as were the original structures.
If the whole living world is in continuous evolution, one may ask: in what sense are the squamates evolving? Are they in the process of losing their legs or of developing them? The answer is simple. The squamates have descended from other four-legged reptiles. Therefore they are in the process not of developing legs, but of losing them. This tendency may seem absurd at first, since a first principle of evolution is to increase the level of an animal's organization, not to simplify it. Thus the squamates seem to be acting in a paradoxical way. However, the squamates that long, long ago began a process of losing their legs were not actually regressing. Rather they were adapting for a specialized way of life, and this is a way of life that has proved very successful for these animals.
Squamates are not armored like some other reptile like some other reptiles. They are covered with scales rather than thick armor like turtles, or impenetrable skin like crocodiles. Therefore they have to develop other methods of defense. Of all defense methods used by animals, running away is one of the safest. A small plains-dwelling reptile needs a certain amount of agility because it has to flee through dense thickets and thorny bushes. In such situations legs can be more of a hindrance than a help as they may get in the way of an escape that is necessary for survival. Escape is easier for a thin; smooth body that can slither through the bushes without having anything sticking out to catch on branches or on a thorn. This is only one of the theories to account for some of the squamates loss of legs. Another theory snakes, are descended from burrowing animals. Limbs can get in the way of a burrowing animal, too.
At any rate it is clear that the loss of legs has been a fully successful and useful adaptation and makes up for the fact that these animals do not have the protective armor and other means of defense found in turtles and crocodiles.
Snakes have no forelegs and never any true hind legs, although the remnants of these limbs may persist inside their bodies. Some lizards also tend toward a loss of limbs, and it is possible to find saurians with every gradation between forms with well developed legs and very long animals with tiny legs or none at all. Even among lizards with legs of normal size, these legs are not usually as big as they were in the primitive reptiles.
Another characteristic that all squamates share is that their bodies are covered with scales. These scales give the animals a certain amount of protection while retaining the flexibility they need to move. Also, their skulls are made up of thin bones that are joined so that the whole skull and jaws can expand to take in large pieces of food. this characteristic is especially pronounced in snakes.
Snakes can stick their tongues far out and their jaws are attached very loosely. Some lizards have progressed in this direction. Snakes have no movable eyelid; instead each eye is covered by a clear scale, derived from the lower eyelid. Such a structure has also evolved in certain lizards.
In both lizards and snakes it is almost impossible to distinguish the males from the females without examining their internal anatomy. Squamate males differ from all other vertebrates by having paired capulatory organs, the hemipenes, only one of which is ever led to the outside during mating. In certain lizards (agamids and iguanids) there is a dorsal crest that is more developed in the male. The coloration, too, is sometimes more gaudy in male lizards that in the females.
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