The appearance of reptiles. Origin and evolution of reptiles

Origin of reptiles

The remains of the most ancient reptiles are known from the Upper Carboniferous period (Upper Carboniferous; age approximately 300 million years). However, their separation from amphibian ancestors should have begun earlier, apparently in the Middle Carboniferous (320 million years), when forms, apparently more terrestrial, separated from primitive embolomeric stegocephalians - anthracosaurs similar to Diplovertebron. Like their ancestors, they were still associated with wet biotopes and bodies of water, fed on small aquatic and terrestrial invertebrates, but had greater mobility and a somewhat larger brain; perhaps they have already begun to become keratinized.

In the Middle Carboniferous, a new branch arose from similar forms - Seymourioraorpha. Their remains were found in the Upper Carboniferous - Lower Permian. They occupy a transitional position between amphibians and reptiles, having undoubted reptilian features; some paleontologists classify them as amphibians. The structure of their vertebrae provided greater flexibility and at the same time strength of the spine; there has been a transformation of the first two cervical vertebrae into the atlas and epistropheus. For terrestrial animals, this created important advantages in orientation, hunting for moving prey, and protection from enemies. The skeleton of the limbs and their girdles was completely ossified; there were long bony ribs, but not yet closed into the chest. The limbs, stronger than those of stegocephals, lifted the body above the ground. The skull had an occipital condyle; Some forms retained gill arches. Seymuria, Kotlassia (found on the Northern Dvina), like other seymuriomorphs, were still associated with reservoirs; it is believed that they may still have had aquatic larvae.

It is not yet clear when the inherent pattern of reproduction and development of eggs in the air environment, characteristic of amniotes, took shape. It can be assumed that this happened in the Carboniferous during the formation of cotylosaurs - Cotylosauria. Among them were small lizard-like forms that apparently fed on various invertebrates, and large (up to 3 m long) massive herbivorous pareiasaurs such as the Severodvinsk scutosaurus. Some cotylosaurs led a semi-aquatic lifestyle, inhabiting humid biotopes, while others, apparently, became true terrestrial inhabitants.

The warm and humid climate of the Carboniferous was favorable for amphibians. At the end of the Carboniferous - beginning of the Permian, intense mountain building (uplift of the mountains of the Urals, Carpathians, Caucasus, Asia and America - the Hercynian cycle) was accompanied by dismemberment of the relief, increased zonal contrasts (cooling in high latitudes), a decrease in the area of wet biotopes and an increase in the proportion of dry biotopes. This contributed to the emergence of terrestrial vertebrates.

The main ancestral group that gave rise to all the diversity of fossils and modern reptiles were the above-mentioned cotylosaurs. Having reached their peak in the Permian, they, however, became extinct by the middle of the Triassic, apparently under the influence of competitors - various progressive groups of reptiles that separated from them. In the Permian, turtles separated from cotylosaurs - Chelonia - their only direct descendants that have survived to this day. In the first turtles, such as the Permian Eunotosaurus, the sharply expanded ribs do not yet form a continuous dorsal shell. Seymuriomorphs, cotylosaurs and turtles are grouped into the subclass Anapsida.

Apparently, in the Upper Carboniferous, two subclasses of reptiles evolved from cotylosaurs, which again switched to an aquatic lifestyle:

Order of mesosaurs.

Order of ichthyosaurs.

The subclass of synaptosaurs, Synaptosauria, includes two orders. order protorosaurs - Protorosauria order sauropterygia - Sauropterygia These include nothosaurs and plesiosaurs.

Proganosaurs and synaptosaurs went extinct without leaving descendants.

In the Permian, a large branch of diapsid reptiles separated from cotylosaurs, in the skull of which two temporal pits formed; this group subsequently split into two subclasses: the lepidosaur subclass and the archosaur subclass.

The most primitive diapsids are the order of eosuchians - Eosuchia of the subclass Lepidosauria - small (up to 0.5 m), lizard-like reptiles; had amphicoelous vertebrae and small teeth on the jaws and palatine bones; went extinct at the beginning of the Triassic. In the Permian, beak-headed animals, Rhynchocephalia, separated from some eosuchians, distinguished by large temporal pits, a small beak at the end of the upper jaws and hook-shaped processes on the ribs. Beakheads became extinct at the end of the Jurassic, but one species - New Zealand tuateria- has survived to this day.

At the end of the Permian, squamate - Squamata (lizards), became numerous and diverse in the Cretaceous, separated from primitive diapsids (possibly directly from eosuchians). At the end of this period, snakes evolved from lizards. The heyday of squamates occurred in the Cenozoic era; they make up the vast majority of living reptiles.

The most diverse in forms and ecological specialization in the Mesozoic era was the subclass of archosaurs Archosauria. Archosaurs inhabited land, bodies of water, and conquered the air. The original group of archosaurs were thecodonts - Thecodontia (or pseudosuchians), which separated from the eosuchians, apparently in the Upper Permian and reached their peak in the Triassic. They looked like lizards ranging from 15 cm to 3-5 m in length, most led a terrestrial lifestyle; the hind limbs were usually longer than the forelimbs. Some of the thecodonts (ornithosuchians) probably climbed branches and led an arboreal lifestyle; Apparently, the class of birds later evolved from them. Another part of the thecodonts switched to a semi-aquatic lifestyle; From them, at the end of the Triassic, crocodiles arose - Crocodilia, which formed many different forms in the Jurassic - Cretaceous.

In the mid-Triassic, the thecodonts gave rise to flying dinosaurs, or pterosaurs, Pterosauria; Pterosaurs were widespread and numerous during the Jurassic and Cretaceous periods; completely died out, leaving no descendants, by the end of the Cretaceous. The extinction may have been facilitated by competition with the increasingly numerous birds at that time. It should be emphasized that pterosaurs and birds are completely independent branches of evolution, the ancestral forms of which were different families of the thecodont order.

In the Upper Triassic, two more groups separated from the carnivores that moved primarily on the hind limbs of pseudosuchians (thecodonts): saurischian dinosaurs - Saurischia and ornithischian dinosaurs - Ornithischia. Saurischian and ornithischian dinosaurs differed in the details of the structure of the pelvis. Both groups developed in parallel; in the Jurassic and Cretaceous periods they gave an extraordinary variety of species, ranging in size from rabbits to giants weighing 30-50 tons; lived on land and coastal shallow waters. By the end of the Cretaceous period, both groups became extinct, leaving no descendants.

Finally, the last branch of reptiles - the subclass animal-like, or synapsids - Theromorpha or Synapsida, was almost the first to separate from the general trunk of reptiles. They separated from the primitive Carboniferous cotylosaurs, which apparently inhabited wet biotopes and still retained many amphibious features (skin rich in glands, the structure of the limbs, etc.). Synapsids began a special line of reptilian development. Already in the Upper Carboniferous and Permian, various forms arose, united in the order of pelycosaurs - Pelycosauria. They had amphicoelous vertebrae, a skull with a poorly developed one fossa and one occipital condyle, there were teeth on the palatine bones, and there were abdominal ribs. In appearance they resembled lizards, their length did not exceed 1 m; only single species reached 3-4 m in length. Among them were true predators and herbivorous forms; many led a terrestrial lifestyle, but there were semi-aquatic and aquatic forms. By the end of the Permian, pelycosaurs became extinct, but before that the beast-toothed reptiles - therapsids - Therapsida separated from them. The adaptive radiation of the latter occurred in the Upper Permian - Triassic, with continuously increasing competition from progressive reptiles- especially archosaurs. Therapsid sizes varied widely: from a mouse to a large rhinoceros. Among them were herbivores - Moschops - and large predators with powerful fangs - Inostrancevia (skull length 50 cm; Fig. 5), etc. Some small forms had, like rodents, large incisors and, apparently, led a burrowing lifestyle . By the end of the Triassic - the beginning of the Jurassic, diverse and well-armed archosaurs completely replaced the beast-toothed therapsids. But already in the Triassic, some group of small species, probably inhabiting damp, densely overgrown biotopes and capable of digging shelters, gradually acquired the features of a more progressive organization and gave rise to mammals.

Thus, as a result of adaptive radiation, already at the end of the Permian - beginning of the Triassic, a diverse fauna of reptiles (approximately 13-15 orders) emerged, displacing most groups of amphibians. The flourishing of reptiles was ensured by a number of aromorphoses, which affected all organ systems and ensured increased mobility, intensified metabolism, greater resistance to a number of environmental factors (dryness in the first place), some complication of behavior and better survival of offspring. The formation of temporal pits was accompanied by an increase in the mass of the chewing muscles, which, along with other transformations, made it possible to expand the range of food used, especially plant foods. Reptiles not only widely mastered the land, populating a variety of habitats, but returned to the water and rose into the air. Throughout the Mesozoic era - for more than 150 million years - they occupied a dominant position in almost all terrestrial and many aquatic biotopes. At the same time, the composition of the fauna changed all the time: ancient groups died out, replaced by more specialized young forms.

By the end of the Cretaceous period, two new classes of warm-blooded vertebrates had already formed - mammals and birds. The specialized groups of large reptiles that survived until this time could not adapt to changing living conditions. In addition, increasing competition with smaller but active birds and mammals played an active role in their extinction. These classes, having acquired warm-bloodedness, steadily high level metabolism and more challenging behavior, increased in number and importance in communities. They quickly and efficiently adapted to life in changing landscapes, quickly mastered new habitats, intensively used new food, and had an increasing competitive effect on more inert reptiles. The modern Cenozoic era began, in which birds and mammals occupied a dominant position, and among the reptiles only relatively small and mobile scaly ones (lizards and snakes), well-protected turtles and a small group of aquatic archosaurs - crocodiles - were preserved.

Fossil reptiles are of exceptional interest, since they belong to numerous groups that once dominated the globe. Ancient groups of this class gave rise not only to modern reptiles, but also to birds and mammals. The oldest reptiles belonging to the order of cotylosaurs, or whole-skulls (Cotylosauria), from the subclass of anapsids, are already known from the upper Carboniferous deposits, but only in the Permian period did they achieve significant development, and in the Triassic they already became extinct. Cotylosaurs were massive animals with thick, five-toed legs and body lengths ranging from several tens of centimeters to several meters. The skull was covered with a solid shell of dermal bones with openings only for the nostrils, eyes and parietal organ. This structure of the skull, as well as many other features, indicate the extreme closeness of cotylosaurs to primitive stegocephalians, which undoubtedly were their ancestors. The most primitive of the so far known anapsids, and therefore of reptiles in general, is the Lower Permian Seymouria. This relatively small (up to 0.5 m in length) reptile had a number of features characteristic of amphibians: the neck was almost not pronounced, the long sharp teeth still retained a primitive structure, there was only one sacral vertebra, and the bones of the skull showed remarkable similarities even in details with the cranial cover of stegocephali. Fossil remains of seymuriomorphic reptiles were found in the area former USSR(Kotlasia and others), made it possible for Soviet paleontologists to identify them systematic position as representatives of a special subclass of batrachosaurs (Batrachosauria), occupying an intermediate position between amphibians and cotylosaurs. Cotylosaurs are a very diverse group. The most major representatives Its members are the clumsy herbivorous pareiasaurs (Pareiasaurus), reaching 2-3 m in length. Their skeletons were later found in South Africa and here on the Northern Dvina. Cotylosaurs were the original group that gave rise to all other major groups of reptiles. Evolution mainly proceeded along the path of the emergence of more mobile forms: the limbs began to lengthen, at least two vertebrae took part in the formation of the sacrum, the entire skeleton, while maintaining its strength, became lighter, in particular, the initially solid bone shell of the skull began to be reduced by the appearance of temporal pits, which not only lightened the skull, but, most importantly, helped to strengthen the muscles that compress the jaws, since if a hole is formed in the bone plate to which the muscles are attached, the muscle, when contracting, can protrude somewhat into this hole. The reduction of the cranial shell proceeded in two main ways: by the formation of one temporal fossa, limited below by the zygomatic arch, and by the formation of two temporal fossae, resulting in the formation of two zygomatic arches. Thus, all reptiles can be divided into three groups: 1) anapsids - with a solid cranial shell (cotylosaurs and turtles); 2) synapsids - with one zygomatic arch (animal-like, plesiosaurs and, possibly, ichthyosaurs) and 3) diapsids - with two arches (all other reptiles). The first and second groups each contain one subclass, the latter is divided into a number of subclasses and many orders. The anapsid group is the oldest branch of reptiles, which have many common features in their skull structure with fossil stegocephalians, since not only many of their early forms (cotylosaurs), but even some modern ones (some turtles) have a solid cranial shell. Turtles are the only living representatives of this ancient group of reptiles. They apparently separated directly from the cotylosaurs. Already in the Triassic, this ancient group was fully formed and, thanks to its extreme specialization, has survived to the present day, almost unchanged, although in the process of evolution, some groups of turtles switched several times from a terrestrial lifestyle to an aquatic one, and therefore they almost lost their bony shields , then acquired them again. From the group of cotylosaurs, marine fossil reptiles separated - ichthyosaurs and plesiosaurs, which, together with other rarer forms, formed two independent subclasses: Ichthyopterygia and Synaptosauria. Plesiosaurs (Plesiosauria), related to synaptosaurs, were marine reptiles. They had a wide, barrel-shaped, flattened body, two pairs of powerful limbs, modified into swimming fins, very long neck , ending in a small head, and a short tail. The skin was bare. Numerous sharp teeth sat in separate cells. The sizes of these animals varied over a very wide range: some species had only half a meter in length, but there were also giants that reached 15 m. A characteristic feature of their skeleton was the underdevelopment of the dorsal parts of the limb girdles (scapula, ilium) and the exceptional thickness of the abdominal girdles (coracoid) , abdominal process of the scapula, pubic and ischial bones), as well as abdominal ribs. All this indicates an exceptionally strong development of the muscles that move the flippers, which served only for rowing and could not support the body out of the water. Although within the subclass of synaptosaurs the transition from terrestrial to aquatic forms has been restored quite clearly, the origin of the group as a whole is still largely unclear. While plesiosaurs, having adapted to aquatic life, still retained the appearance of terrestrial animals, ichthyosaurs (Ichthyosauria), belonging to ichthyopterygians, acquired similarities with fish and dolphins. The body of ichthyosaurs was spindle-shaped, the neck was not pronounced, the head was elongated, the tail had a large fin, and the limbs were in the form of short flippers, with the hind ones being much smaller than the front ones. The skin was bare, numerous sharp teeth (adapted to feeding on fish) sat in a common groove, there was only one zygomatic arch, but of an extremely unique structure. The sizes varied from 1 to 13 m. The diapsid group includes two subclasses: lepidosaurs and archosaurs. The earliest (Upper Permian) and most primitive group of lepidosaurs is the order Eosuchia. They are still very poorly studied, the best known is lounginia - a small reptile, reminiscent of a lizard in body, with relatively weak limbs that had the usual reptilian structure. Its primitive features are expressed mainly in the structure of the skull; teeth are located both on the jaws and on the palate. The first beaked animals (Rhynchocephalia) have been known since the Early Triassic. Some of them were extremely close to the modern hatteria. Beakheads differ from Eosuchians in the presence of a horny beak and in the fact that their teeth are attached to the bone, while the jaw teeth of Eosuchians sat in separate cells. According to the last feature, beakheads are even more primitive than eosuchians and, therefore, should have descended from some as yet undiscovered primitive forms of the latter group. Squamata, namely lizards, are known only from the very end of the Jurassic. Mosasauria (Mosasauria) apparently separated from the main trunk of squamate lizards already at the beginning of the Cretaceous. These were sea reptiles that had a long serpentine body and two pairs of limbs modified into flippers. Some representatives of this order reached a length of 15 m. At the end of the Cretaceous they died out without a trace. Somewhat later than the mosasaurs (end of the Cretaceous), a new branch separated from lizards - snakes. In all likelihood, a large progressive branch of archosaurs (Archosauria) originated from the Eosuchia - namely the Pseudosuchia, which subsequently split into three main branches - aquatic (crocodiles), terrestrial (dinosaurs) and airborne (winged lizards). Along with the two typical temporal arches, the most characteristic feature of this group was the tendency to transition to “bipedalism,” that is, movement on only the hind limbs. True, some of the most primitive archosaurs only began to change in this direction, and their descendants took a different path, and representatives of a number of groups returned to moving on four limbs for the second time. But in the latter case past history left a mark on the structure of their pelvis and the hind limbs themselves. Pseudosuchia first appeared only at the beginning of the Triassic. The early forms were small animals, but with relatively long hind legs, which, apparently, served them alone for movement. The teeth, which were present only on the jaws, sat in separate cells, and bone plates were almost always located in several rows along the back. These small forms, typical representatives of which are ornithosuchians, and apparently leading the arboreal life of Scleromochlus, were very numerous and gave rise not only to branches that flourished later - in the Jurassic and Cretaceous, but also to a number of highly specialized groups that became extinct without a trace. in the Triassic. Finally, pseudosuchians, in particular, if not Ornithosuchus itself, then forms close to it, could be the ancestors of birds. Crocodiles (Crocodylia) are very close to some Triassic pseudosuchians, such as Belodon, or Phytosaurus. Starting from the Jurassic, real crocodiles appeared, but the modern type of crocodiles was finally developed only during the Cretaceous period. On this long path of evolution, we can trace step by step how a characteristic feature of crocodiles - the secondary palate - developed. At first, only horizontal processes appeared on the maxillary and palatine bones, then these palatine processes converged, and even later they were joined by the palatine processes of the pterygoid bones, and simultaneously with this process the nostrils moved forward, and the secondary choanae moved backward. Dinosaurs (Dinosauria) are the most numerous and diverse group of reptiles that have ever lived. These included small forms, the size of a cat and smaller, and giants, reaching almost 30 m in length and 40-50 tons in weight, light and massive, agile and clumsy, predatory and herbivorous, devoid of scales and covered with a bony shell with various outgrowths. Many of them ran galloping on one hind limb, leaning on the tail, while others moved on all four. Dinosaurs' heads were usually relatively small, while the cavity of the cranium was very tiny. But the spinal canal in the sacral area was very wide, which indicates a local expansion of the spinal cord. Dinosaurs were divided into two large groups- lizard and ornithischians, which arose completely independently from pseudosuchians. Their differences lie mainly in the structure of the hind limb girdle. Lizard hippies (Saurischia), family ties which with pseudosuchia there is no doubt, were originally only predatory. Subsequently, although most forms continued to remain carnivorous, some turned into herbivores. The predators, although they reached enormous sizes (up to 10 m in length), had a relatively light build and a powerful skull with sharp teeth. Their forelimbs, which apparently served only for grasping prey, were greatly reduced, and the animal had to move by jumping on its hind limbs and leaning on its tail. A typical representative of such forms is Ceratosaurus. In contrast to predatory herbivorous forms, they moved on both pairs of limbs, which were almost equal in length and ended in five fingers, apparently covered with horny formations like hooves. These included the largest four-legged animals that ever lived on the globe, such as the Brontosaurus, which reached over 20 m in length and probably 30 tons in weight, and Diplodocus. The latter was slimmer and, undoubtedly, much lighter, but it was superior to the Brontosaurus in length, which in one specimen exceeded 26 m; finally, the lumbering Brachiosaurus, about 24 m long, must have weighed about 50 tons. Although hollow bones lightened the weight of these animals, it is still difficult to believe that such giants could move freely on land. Apparently, they led only a semi-terrestrial life and, like modern hippopotamuses, most spent time in the water. This is indicated by their very weak teeth, suitable for eating only soft aquatic vegetation, and the fact that, for example, Diplodocus's nostrils and eyes were moved upward, so that the animal could see and breathe with only part of its head out of the water. Ornithischia, which had a girdle of hind limbs extremely similar to a bird's, never reached such enormous sizes. But they were even more diverse. Most of these animals returned to moving on four legs for the second time and usually had a well-developed shell, sometimes complicated by various kinds of outgrowths in the form of horns, spines, etc. All of them remained herbivorous from the very beginning to the end, and the majority retained only their back teeth, while the front of the jaws was apparently covered with a horny beak. Iguanodons, stegosaurs and triceratops can be mentioned as characteristic representatives of various groups of ornithischians. Iguanodons, reaching 5-9 m in height, ran on their hind legs alone and were deprived of a shell, but the first finger of their forelimbs was a bone spike that could serve as a good weapon of defense. Stegosaurus had a tiny head, a double row of tall triangular bony plates on its back, and several sharp spines perched on its tail. Triceratops looked like a rhinoceros: at the end of its snout there was a large horn, in addition, a pair of horns rose above the eyes, and along the rear, extended edge of the skull there were numerous pointed processes. Pterodactyls (Pterosauria), like birds and bats, were true flying animals. Their forelimbs were real wings, but of an extremely unique structure: not only the forearm, but also the metacarpal bones fused to each other were greatly elongated, the first three fingers had a normal structure and size, the fifth was absent, while the fourth reached extreme length and between them and a thin flying membrane was stretched along the sides of the body. The jaws were extended, some forms had teeth, others had a toothless beak. Pterodactyls exhibit a number of common features with birds: fused thoracic vertebrae, a large sternum with a keel, a complex sacrum, hollow bones, a sutureless skull, and large eyes. The winged lizards apparently ate fish and probably lived on coastal rocks, since, judging by the structure of their hind limbs, they could not rise from a flat surface. Pterodactyls include quite diverse forms: a relatively primitive group of rhamphorhynchus, which had a long tail, and pterodactyls themselves with a rudimentary tail. The sizes ranged from the size of a sparrow to a giant pteranodon, whose wingspan reached 7 m. The group of synapsids constitutes an independent subclass of reptiles, as a special side branch that separated from the ancient cotylosaurs. They are characterized by strengthening of the jaw apparatus by the formation of a kind of temporal cavity for very powerful jaw muscles and progressive differentiation of the dental system - heterodontism, or heterodonty. This connects them with the highest class of vertebrates - mammals. Animal-like (Theromorpha) is a group whose primitive representatives were still very close to cotylosaurs. Their difference lies mainly in the presence of a zygomatic arch and a lighter build. Animal-like animals appeared at the end of the Carboniferous period, and starting from the Lower Permian they became very numerous and during this entire period, together with cotylosaurs, they were almost the only representatives of their class. Despite all their diversity, all beast-like animals were strictly terrestrial animals, moving exclusively with the help of both pairs of limbs. The most primitive representatives of pelycosaurs (for example, Varanops) had small sizes and in appearance they should have resembled lizards. However, their teeth, although homogeneous, were already sitting in separate cells. The mammals (Therapsida), which replaced the pelycosaurs from the Middle Permian, united extremely diverse animals, many of which were highly specialized. In later forms, the parietal foramen disappeared, the teeth differentiated into incisors, canines and molars, a secondary palate was formed, one condyle was divided into two, the dentary bone increased greatly, while the other bones of the lower jaw decreased. The reasons for the extinction of ancient reptiles are still not entirely clear. The most plausible explanation for this phenomenon is the following. In the process of struggle for existence, individual forms became more and more adapted to certain environmental conditions and became more and more specialized. Such specialization is extremely useful, but only as long as the conditions to which the organism has adapted continue to exist. Once they change, such animals find themselves in worse conditions than the less specialized forms that supplant them in the struggle for existence. In addition, in the struggle for existence, some groups may acquire properties that increase their overall vital activity. In contrast to narrow adaptation, or idioadaptation, this phenomenon is called aromorphosis. For example, warm-bloodedness made it possible for organisms that acquired this property to be less dependent on climate compared to animals with variable body temperature. During the long Mesozoic era, there were only minor changes in landscapes and climate, and therefore reptiles became more and more specialized and flourished. But at the end of this era earth's surface began to undergo such huge mountain-building processes and associated climate change that most reptiles could not survive them and died out without a trace by the end of the Mesozoic, which was called the era of the great extinction. However, it would be a mistake to explain this process solely by physical and geographical reasons. An equally important role was played by the struggle for existence with other animals, namely with birds and mammals, which, thanks to their warm-bloodedness and highly developed brain, turned out to be better adapted to these challenges. external phenomena and emerged victorious in the struggle of life.

Literature

1. Vorontsova M. A., Liozner L. D., Markelova I. V., Puhelskaya E. Ch. Triton and axolotl. M., 1952.

2. Gurtovoy N. N., Matveev B. S., Dzerzhinsky F. Ya. Practical zootomy of vertebrates.

3. Amphibians, reptiles. M., 1978. Terentyev P.V. Frog. M., 1950.

Everything about everything. Volume 5 Likum Arkady

When did the first reptiles appear?

The first reptiles walked the Earth about 300,000,000 years ago. At that time, the largest animals on land were amphibians. But they laid eggs in the water. The first reptiles resembled amphibians, but they already laid eggs on land. Their offspring had lungs and legs and could breathe air. They roamed the wet ground of forests and could feed on insects. Later the reptiles became bigger and stronger. They reminded appearance lizards and turtles.

There were also reptiles with short tails, thick legs and large heads. One species of early reptile had a very great importance because of their descendants, who also looked like lizards, but moved on their hind legs. From these creatures a new type of reptile evolved. Some of them had wings. Others fledged and became warm-blooded. This is how birds arose. Some reptiles gave rise to crocodiles and the first dinosaurs.

At one time, reptiles were the main animals on Earth. But over the course of millions of years, many of the ancient types of reptiles became extinct. There are many theories explaining why this happened. The main reason is seen in the fact that changes in conditions and climate that have occurred on Earth have made the existence of these animals impossible. The swamps dried up, and reptiles could not live on land. Food for them has disappeared. The climate has become seasonal, varying from summer heat until the winter frost. Most reptiles were unable to adapt to these changes, so they became extinct.

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The appearance of reptiles on Earth is the greatest event in evolution.

It had enormous consequences for all of nature. The origin of reptiles is one of the important questions in the theory of evolution, the process as a result of which the first animals belonging to the class Reptilia appeared. The first terrestrial vertebrates arose in the Devonian (more than 300 million years ago). These were armored-headed amphibians - stegocephalians. They were closely associated with bodies of water, since they reproduced only in water and lived near water. The development of spaces remote from bodies of water required a significant restructuring of the organization: adaptation to protecting the body from desiccation, breathing atmospheric oxygen, efficient movement on solid substrate, and the ability to reproduce outside of water. These are the main prerequisites for the emergence of a qualitatively different new group of animals - reptiles. These changes were quite complex; for example, it required the development of powerful lungs and a change in the nature of the skin.

Carboniferous period

Seymouria

All reptiles can be divided into three groups:

1) anapsids - with a solid cranial shell (cotylosaurs and turtles);

2) synapsids - with one zygomatic arch (animal-like, plesiosaurs and, possibly, ichthyosaurs) and

3) diapsids - with two arches (all other reptiles).

Anapsid group is the oldest branch of reptiles that have many common features in their skull structure with fossil stegocephalians, since not only many of their early forms (cotylosaurs), but even some modern ones (some turtles) have a solid cranial shell. Turtles are the only living representatives of this ancient group of reptiles. They apparently separated directly from the cotylosaurs. Already in the Triassic, this ancient group was fully formed and, thanks to its extreme specialization, has survived to the present day, almost unchanged, although in the process of evolution, some groups of turtles switched several times from a terrestrial lifestyle to an aquatic one, and therefore they almost lost their bony shields , then acquired them again.

Synapsid group. Marine fossil reptiles - ichthyosaurs and plesiosaurs - separated from the group of cotylosaurs. Plesiosaurs (Plesiosauria), related to synaptosaurs, were marine reptiles. They had a wide, barrel-shaped, flattened body, two pairs of powerful limbs modified into swimming flippers, a very long neck ending in a small head, and a short tail. The skin was bare. Numerous sharp teeth sat in separate cells. The sizes of these animals varied over a very wide range: some species were only half a meter in length, but there were also giants that reached 15 m. while plesiosaurs, having adapted to aquatic life, still retained the appearance of terrestrial animals, ichthyosaurs (Ichthyosauria), belonging to ichthyopterygians, acquired similarities with fish and dolphins. The body of ichthyosaurs was spindle-shaped, the neck was not pronounced, the head was elongated, the tail had a large fin, and the limbs were in the form of short flippers, with the hind ones being much smaller than the front ones. The skin was bare, numerous sharp teeth (adapted to feeding on fish) sat in a common groove, there was only one zygomatic arch, but of an extremely unique structure. The sizes varied from 1 to 13 m.

Diapsid group includes two subclasses: lepidosaurs and archosaurs. The earliest (Upper Permian) and most primitive group of lepidosaurs is the order Eosuchia. They are still very poorly studied; the best known is lounginia - a small reptile with a lizard-like physique, with relatively weak limbs that had the usual reptilian structure. Its primitive features are expressed mainly in the structure of the skull; teeth are located both on the jaws and on the palate.

There are now about 7,000 species of reptiles, almost three times as many as modern amphibians. Living reptiles are divided into 4 orders:

· Scaly;

· Turtles;

· Crocodiles;

· Beakheads.

The most numerous order of squamates (Squamata), including about 6,500 species, is the only currently thriving group of reptiles, widespread throughout the world. to the globe and making up the bulk of the reptiles of our fauna. This order includes lizards, chameleons, amphisbaenas and snakes.

Much fewer turtles(Chelonia) - about 230 species, represented in the animal world of our country by several species. This is a very ancient group of reptiles that has survived to this day thanks to a kind of protective device - the shell in which their body is encased.

Crocodiles (Crocodylia), of which about 20 species are known, inhabit continental and coastal waters of the tropics. They are direct descendants of ancient, highly organized reptiles of the Mesozoic.

The only species of modern rhynchocephalia, the tuateria has many extremely primitive features and is preserved only in New Zealand and on the adjacent small islands.

Reptiles have lost their dominant position on the planet mainly due to competition with birds and mammals against the background of a general cooling, which is confirmed by the current ratio of the number of species of different classes of terrestrial vertebrates. If the share of amphibians and reptiles that are most dependent on environmental temperature is quite high on a planetary scale (10.5 and 29.7%), then in the CIS, where the area of warm regions is relatively small, they are only 2.6 and 11.0% .

Reptiles, or reptiles, of Belarus represent the northern “outpost” of this diverse class of vertebrate animals. Of the more than 6,500 species of reptiles now living on our planet, only 7 are represented in the republic.

In Belarus, which does not have a warm climate, there are only 1.8% of reptiles and 3.2% of amphibians. It is important to note that the decrease in the proportion of amphibians and reptiles in the fauna of northern latitudes occurs against the background of a decrease in the total number of species of terrestrial vertebrates. Moreover, out of four orders of modern reptiles, only two (turtles and squamates) live in the CIS and Belarus.

The Cretaceous period was marked by the collapse of reptiles and the almost complete extinction of dinosaurs. This phenomenon poses a mystery to science: how does a huge, prosperous, all-occupying ecological niches an army of reptiles that ranged from the tiniest creatures to unimaginable giants, so suddenly went extinct, leaving only relatively small animals?

It was these groups at the beginning of modern Cenozoic era took a dominant position in the animal world. And among reptiles, out of 16-17 orders that existed during their heyday, only 4 survived. Of these, one is represented by a single primitive species - hatteria, preserved only on two dozen islands near New Zealand.

The other two orders - turtles and crocodiles - unite a relatively small number of species - about 200 and 23, respectively. And only one order - squamates, which includes lizards and snakes, can be assessed as thriving in the current evolutionary era. This is a large and diverse group, numbering more than 6,000 species.

Reptiles are distributed throughout the globe, except Antarctica, but extremely unevenly. If their fauna is most diverse in the tropics (in some regions there are 150-200 species), then only a few species penetrate into high latitudes (in Western Europe only 12).

). They lived near bodies of water and were closely associated with them, since they reproduced only in water. The development of spaces remote from bodies of water required a significant restructuring of the organization: adaptation to protecting the body from desiccation, breathing atmospheric oxygen, efficient movement on solid substrate, and the ability to reproduce outside of water. These are the main prerequisites for the emergence of a qualitatively different new group of animals - reptiles. These changes were quite complex; for example, it required the development of powerful lungs and a change in the nature of the skin.

Carboniferous period

The remains of the most ancient reptiles are known from the Upper Carboniferous (about 300 million years ago). It is assumed that the separation from amphibian ancestors should have begun, apparently, in the Middle Carboniferous (320 million years), when from anthracosaurs like Diplovertebron, forms became isolated, apparently better adapted to the terrestrial way of life. From such forms a new branch arises - seymuriomorphs ( Seymouriomorpha), the remains of which were found in the Upper Carboniferous - Middle Permian. Some paleontologists classify these animals as amphibians.

Permian period

From the upper Permian deposits of North America, Western Europe, Russia and China know the remains of cotylosaurs ( Cotylosauria). In a number of characteristics they are still very close to stegocephals. Their skull was in the form of a solid bone box with openings only for the eyes, nostrils and parietal organ. cervical region the spine was poorly formed (although there is a structure of the first two vertebrae characteristic of modern reptiles - atlanta And epistrophy), the sacrum had from 2 to 5 vertebrae; the cleithrum, a skin bone characteristic of fish, was preserved in the shoulder girdle; the limbs were short and widely spaced.

The further evolution of reptiles was determined by their variability due to the influence of various living conditions that they encountered during reproduction and settlement. Most groups became more mobile; their skeleton became lighter, but at the same time stronger. Reptiles consumed a more varied diet than amphibians. The technique of its extraction has changed. In this regard, the structure of the limbs, axial skeleton and skull underwent significant changes. For the majority, the limbs became longer, and the pelvis, gaining stability, was attached to two or more sacral vertebrae. The “fish” bone, the cleithrum, has disappeared from the shoulder girdle. The solid shell of the skull has undergone partial reduction. In connection with the more differentiated muscles of the jaw apparatus, pits and bone bridges separating them appeared in the temporal region of the skull - arches that served to attach a complex system of muscles.

Synapsids

The main ancestral group that gave rise to all the diversity of modern and fossil reptiles were cotylosaurs, but the further development of reptiles followed different paths.

Diapsids

The next group to split off from the cotylosaurs were the diapsids ( Diapsida). Their skull has two temporal cavities, located above and below the postorbital bone. Diapsids at the end of the Paleozoic (Permian) gave an extremely broad adaptive radiation to systematic groups and species, which are found both among extinct forms and among living reptiles. Among diapsids, two main groups emerged: Lepidosauromorphs ( Lepidosauromorpha) and Archosauromorphs ( Archosauromorpha). The most primitive diapsids from the group of Lepidosaurs are the order Eosuchia ( Eosuchia) - were the ancestors of the Beak-headed order, from which only one genus is currently preserved - hatteria.

At the end of the Permian, squamosids separated from primitive diapsids ( Squamata), which became numerous during the Cretaceous period. By the end of the Cretaceous period, snakes evolved from lizards.

Origin of archosaurs

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Notes

Literature

Naumov N. P., Kartashev N. N. Part 2. Reptiles, birds, mammals // Zoology of vertebrates. - M.: graduate School, 1979. - P. 272.

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Evolutionary processes

Factors of evolution

Population genetics

Origin of life

Historical concepts

Modern theories

Evolution of taxa

Excerpt characterizing the Origin of Reptiles

I remember how literally a few weeks before that terrible day, my grandfather and I were sitting in the garden and “listening” to the sunset. For some reason, grandfather was quiet and sad, but this sadness was very warm and bright, and even somehow deeply kind... Now I understand that he already knew then that he would be leaving very soon... But, unfortunately, not I knew this.
“Someday, after many, many years... when I’m no longer next to you, you will also look at the sunset, listen to the trees... and maybe sometimes remember your old grandfather,” grandfather’s voice gurgled like a quiet stream. – Life is very dear and beautiful, baby, even if at times it may seem cruel and unfair to you... Whatever happens to you, remember: you have the most important thing - your honor and your human dignity, which no one can take away from you, and no one can drop them except you... Keep it, baby, and don’t let anyone break you, and everything else in life can be replenished...
He rocked me like a little child in his dry and always warm arms. And it was so amazingly calm that I was afraid to breathe, so as not to accidentally frighten away this wonderful moment, when the soul warms up and rests, when the whole world seems huge and so extraordinarily kind... when suddenly the meaning of his words dawned on me!!!
I jumped up like a disheveled chicken, choking with indignation, and, as luck would have it, unable to find in my “rebellious” head the words that were so necessary at that moment. It was so offensive and completely unfair!.. Well, why on such a wonderful evening did he suddenly need to start talking about that sad and inevitable thing that (even I already understood) would sooner or later have to happen?! My heart did not want to listen to this and did not want to accept such “horror”. And it was completely natural - after all, all of us, even children, so do not want to admit this sad fact to ourselves that we pretend that it will never happen. Maybe with someone, somewhere, sometime, but not with us... and never...
Naturally, all the charm of our wonderful evening disappeared somewhere and I no longer wanted to dream about anything else. Life again made me understand that, no matter how hard we try, not so many of us are truly given the right to have control in this world... The death of my grandfather truly turned my whole life upside down in the literal sense of the word. He died in my children's arms when I was only six years old. It happened early on a sunny morning, when everything around seemed so happy, affectionate and kind. In the garden, the first awakened birds joyfully called to each other, cheerfully passing each other last news. The rosy-cheeked dawn, softened by the last morning's sleep, was just opening her eyes, washed with morning dew. The air was fragrant with the amazingly “delicious” smells of a summer riot of flowers.
Life was so pure and beautiful!.. And it was absolutely impossible to imagine that trouble could suddenly mercilessly burst into such a fabulously wonderful world. She simply had no right to do this!!! But it is not in vain that it is said that trouble always comes uninvited and never asks permission to enter. So this morning she came to us without knocking, and playfully destroyed my seemingly well-protected, affectionate and sunny children’s world, leaving only unbearable pain and the terrible, cold emptiness of the first loss in my life...
That morning, my grandfather and I, as usual, were going to go to our favorite forest to buy strawberries, which I loved very much. I was calmly waiting for him on the street, when suddenly it seemed to me that a piercing icy wind blew from somewhere and a huge black shadow fell on the ground. It became very scary and lonely... There was no one in the house except my grandfather at that moment, and I decided to go see if something had happened to him.
Grandfather was lying on his bed very pale and for some reason I immediately realized that he was dying. I rushed to him, hugged him and started shaking him, trying at all costs to bring him back. Then she started screaming, calling for help. It was very strange - for some reason no one heard me or came, although I knew that everyone was somewhere nearby and should hear me for sure. I didn’t yet understand that it was my soul screaming...
I had an eerie feeling that time had stopped and we were both outside of it at that moment. It was as if someone had placed us both in a glass ball in which there was neither life nor time... And then I felt all the hairs on my head stand on end. I will never forget this feeling, even if I live a hundred years!.. I saw a transparent luminous essence that came out of my grandfather’s body and, swimming up to me, began to gently flow into me... At first I was very scared, but immediately felt a soothing warmth and for some reason I realized that nothing bad could happen to me. The essence flowed in a luminous stream, flowing easily and softly into me, and became smaller and smaller, as if “melting” little by little... And I felt my body huge, vibrating and unusually light, almost “flying”.
It was a moment of merging with something extraordinarily significant, comprehensive, something incredibly important to me. And then there was a terrible, all-consuming pain of loss... Which washed over like a black wave, sweeping away any attempt I made to resist it... I cried so much during the funeral that my parents began to fear that I would get sick. The pain completely took over my childish heart and did not want to let go. The world seemed frighteningly cold and empty... I couldn’t come to terms with the fact that my grandfather would now be buried and I would never see him again!.. I was angry with him for leaving me, and angry with myself for not being able to save. Life was cruel and unfair. And I hated her for having to bury him. This is probably why these were the first and last funerals that I attended in my entire life. later life…

Afterwards, I couldn’t come to my senses for a very long time, I became withdrawn, and spent a lot of time alone, which saddened all my family to the core. But, little by little, life took its toll. And, after some time, I slowly began to emerge from that deeply isolated state into which I had plunged myself, and from which it turned out to be very, very difficult... My patient and loving parents tried to help me as best they could. But for all their efforts, they did not know that I was truly no longer alone - that, after all my experiences, an even more unusual and fantastic world suddenly opened up to me than the one in which I had already lived for some time. . A world that surpassed any imaginable fantasy in its beauty, and which (again!) was given to me with its extraordinary essence by my grandfather. This was even more amazing than everything that happened to me before. But for some reason this time I didn’t want to share it with anyone...
Days passed by. In my Everyday life I was an absolutely normal six-year-old child who had my own joys and sorrows, desires and sorrows and such unfulfillable rainbow childhood dreams... I chased pigeons, loved going to the river with my parents, played children's badminton with friends, helped, to the best of my ability , mother and grandmother in the garden, read my favorite books, learned to play the piano. In other words, I lived the most normal life ordinary life all small children. The only trouble was that by that time I already had two Lives... It was as if I lived in two completely different worlds: the first was ours ordinary world, in which we all live every day, and the second was my own “hidden” world, in which only my soul lived. It became more and more difficult for me to understand why what was happening to me was not happening to any of my friends?
I began to notice more often that the more I shared my “incredible” stories with someone from my environment, the more often they felt a strange alienation and childish wariness. It hurt and it made me very sad. Children are curious, but they don't like the unknown. They always try as quickly as possible with their childish minds to get to the bottom of what is happening, acting on the principle: “what is it and what do they eat it with?”... And if they cannot understand it, it becomes “alien” for their everyday environment and is very quickly fades into oblivion. This is how I started to become a little “alien”...

Land vertebrates arose in the Devonian. These were armored amphibians, or stegocephali. They were closely associated with bodies of water, since they reproduced only in water and lived near bodies of water, where there was terrestrial vegetation. The development of spaces remote from bodies of water required a significant restructuring of the organization: adaptation to protecting the body from desiccation, breathing atmospheric oxygen, walking on solid substrate, the ability to reproduce outside of water and, of course, improving forms of behavior. These are the main prerequisites for the emergence of a qualitatively different new group of animals. All of the above traits took shape in reptiles.

To this we must add that by the end of the Carboniferous there were strong changes in the natural environment, which led to the emergence of a more diverse climate on the planet, the development of more diverse vegetation, its distribution in areas remote from water bodies, and in this regard to the wide spread of tracheal-breathing arthropods, i.e. .e. possible feeding objects also spread to the watershed areas of the land.

The evolution of reptiles proceeded very quickly and violently. Long before the end of the Permian period of the Paleozoic, they displaced most of the stegocephalians. Having gained the opportunity to exist on land, reptiles in a new environment encountered new and extremely diverse conditions. The versatility of this diversity and the absence of significant competition on land from other animals were the main reasons for the flourishing of reptiles in subsequent times. Mesozoic reptiles are primarily terrestrial animals. Many of them are secondary in one way or another

adapted to life in water. Some have mastered air environment. The adaptive divergence of reptiles was astonishing. The Mesozoic is rightly considered the age of reptiles.

Early reptiles. The oldest reptiles known from the upper Permian deposits of North America, Western Europe, Russia and China. They are called cotylosaurs. In a number of characteristics they are still very close to stegocephalians. Their skull was in the form of a solid bone box with openings only for the eyes, nostrils and parietal organ, the cervical spine was poorly formed, the sacrum had only one vertebra; the cleithrum, a skin bone characteristic of fish, was preserved in the shoulder girdle; the limbs were short and widely spaced.

Cotylosaurs turned out to be very interesting objects, numerous remains of which were found by V.P. Amalitsky in the Permian deposits of Eastern Europe, on the Northern Dvina. Among them are the three-meter herbivorous pareiasaurs (Pareiasaurus).

It is possible that cotylosaurs were descendants of Carboniferous stegocephalians - embolomeres.

In the Middle Permian, cotylosaurs reached their peak. But only a few survived until the end of the Permian, and in the Triassic this group disappeared, giving way to more highly organized and specialized groups of reptiles that developed from various orders of cotylosaurs (Fig. 114).

The further evolution of reptiles was determined by their variability due to the influence of very diverse living conditions that they encountered during reproduction and settlement. Most groups acquired greater mobility; their skeleton became lighter, but at the same time stronger. Reptiles consumed a more varied diet than amphibians. The technique of its extraction has changed. In this regard, the structure of the limbs, axial skeleton and skull underwent significant changes. For the majority, the limbs became longer, and the pelvis, gaining stability, was attached to two or more sacral vertebrae. The cleithrum bone has disappeared in the shoulder girdle. The solid shell of the skull has undergone partial reduction. In connection with the more differentiated muscles of the jaw apparatus, pits and bone bridges separating them appeared in the temporal region of the skull - arches that served to attach a complex system of muscles.

The main groups of reptiles are discussed below, a review of which should show the exceptional diversity of these animals, their adaptive specialization and probable relationship with living groups.

In the formation of the appearance of ancient reptiles and in assessing their subsequent fate, the characteristics of their skull are essential.

Rice. 114. Cotylosaurs (1, 2, 3) and pseudosuchia (4):
1 - pareiasaurus (Upper Permian), skeleton; 2 - pareiasaurus, restoration of the animal; 3 - Seymouria; 4 – pseudosuchia

The primitiveness of stegocephalians ("whole-skull") and early reptiles was expressed in the structure of the skull by the absence of any cavities in it, except for the ocular and olfactory ones. This feature is reflected in the name Anapsida. The temporal region of reptiles of this group was covered with bones. Turtles (now Testudines, or Chelonia) became probable descendants of this trend; they retain a continuous bony cover behind their eye sockets. Similarities with current forms are found in turtles known from the Lower Triassic of the Mesozoic. Their fossil remains are confined to the territory of Germany. The skull, teeth, and shell structure of ancient turtles are extremely similar to modern ones. The ancestor of turtles is considered to be the Permian Eunotosaurus(Eunotosaurus) is a small lizard-like animal with short and very wide ribs that form something like a dorsal shield (Fig. 115). He did not have an abdominal shield. There were teeth. Mesozoic turtles were originally land-dwelling and apparently burrowing animals. Only later did some groups switch to an aquatic lifestyle and, as a result, many of them partially lost their bony and horny shells.

From the Triassic to the present day, turtles have retained the main features of their organization. They survived all the trials that killed off most reptiles, and are just as thriving today as they were in the Mesozoic.

Today's hidden-necked and side-necked ones retain their primary appearance to a greater extent. land turtles Triassic Marine and soft-skinned animals appeared in the late Mesozoic.

All other reptiles, both ancient and modern, acquired one or two temporal cavities in the structure of the skull. They had one, lower, temporal cavity synapsid. One superior temporal cavity is noted in two groups: paranoid and euryansid. And finally, two depressions had diapsid. The evolutionary fate of these groups is different. The first to move away from the ancestral trunk synapsids(Synapsida) - reptiles with lower temporal cavities, bounded by the zygomatic, squamosal and post-orbital bones. Already in the Late Carboniferous, this group of the first amniotes became the most numerous. In the fossil record they are represented by two successively existing orders: pelycosaurs(Pelicosauria) and therapsids(Therapsida). They are also called bestial(Theromorpha). Animal-like animals experienced their heyday long before the first dinosaurs appeared; cotylosaurs were their direct relatives. In particular, pelycosaurs(Pelicosauria) were still very close to cotylosaurs. Their remains were found in North America and in Europe. In appearance they looked like lizards and were small in size - 1-2 m, had biconcave vertebrae and well-preserved abdominal ribs. However, their teeth sat in the alveoli. In some, teeth differentiation was evident, albeit to a small extent.

In the Middle Permian, pelycosaurs were replaced by more highly organized ones. beast-toothed(Theriodontia). Their teeth were clearly differentiated, and a secondary bony palate appeared. The single occipital condyle split into two. The lower jaw was mainly represented by dentary bone. Position

limbs also changed. The elbow moved back and the knee moved forward, and as a result the limbs began to occupy a position under the body, and not on the sides of it, as in other reptiles. The skeleton appeared to have many features in common with mammals.

Numerous Permian beast-toothed reptiles were very diverse in appearance and lifestyle. Many were predators. Perhaps this was the one found by the expedition of V.P. Amalitsky in the sediments of the Permian period on the Northern Dvina inostranzevia(Inostrancevia alexandrovi, Fig. 116). Others ate a plant-based or mixed diet. These unspecialized species are closest to mammals. Among them it is necessary to point out Cynognathus(Cynognathus), which had many progressive organizational features.

Animal-toothed animals were numerous in the Early Triassic, but with the appearance predatory dinosaurs they disappeared. Interesting materials presented in Table 6 indicate a sharp reduction in the diversity of animal-like animals throughout the Triassic. Animal-like animals are of great interest as the group that gave rise to mammals.

Rice. 116. Animal-toothed:
1 - Inostracevia, Upper Permian (restoration of an animal), 2 - skull of Cynognathus

Table 6

The relationship between the genera of beast-like and sauropsid (lizard-like reptiles) at the end of the Paleozoic - beginning of the Mesozoic
(P Robinson, 1977)

Period	Bestial	Sauropsid
Upper Triassic Middle Triassic Lower Triassic Upper Perm	17 23 36 170	8 29 20 15

The next group to split off from the anapsid cotylosaurs were diapsid(Diapsida). Their skull has two temporal cavities, located above and below the postorbital bone. Diapsids at the end of the Paleozoic (Permian) gave an extremely broad adaptive radiation to systematic groups and species, which are found both among extinct forms and among living reptiles. Among the diapsids, two main groups (infra-classes) have emerged: infra-class Lepidosauromorphs(Lepidosauromorpha) and infraclass Archosauromorphs(Archosauromorpha).

Paleontologists do not have accurate information to say which of them is older and younger in terms of time of appearance, but their evolutionary fate is different.

Who are lepidosauromorphs? This ancient infraclass unites living hatteria, lizards, snakes, chameleons and their extinct ancestors.

Hatteria, or Sphenodon(Sphenodon punctatus), now living on small islands off the coast of New Zealand, is a descendant of the proto-lizards, or wedge-toothed ones, quite common in the mid-Mesozoic (superorder Prosauria, or Lepidontidae). They are characterized by many wedge-shaped teeth sitting on the jaw bones and on the palate, like amphibians, and amphicoelous vertebrae.

Lizards, snakes and chameleons now make up the wide variety of the order Squamata. Lizards are one of the oldest advanced groups of reptiles, their remains are known from. Upper Permian Scientists have discovered many similarities between lizards and Sphenodon. Their limbs are widely spaced and the body moves, curving the spinal column in waves. Interestingly, among their common morphological similarities is the presence of an intertarsal joint. Snakes appear only in chalk. Chameleons are a specialized group of a later era - the Cenozoic (Paleocene, Miocene).

Now about the fate of archosauromorphs. Archosaurs are considered the most amazing of all reptiles that ever lived on Earth. Among them are crocodiles, pterosaurs, and dinosaurs. Crocodiles are the only archosaurs that have survived to this day.

Crocodiles(Crocodylia) appear at the end of the Triassic. Jurassic crocodiles are significantly different from modern ones in the absence of a true bony palate. Their internal nostrils opened between the palatine bones. The vertebrae were still amphicoelous. Modern crocodiles with a fully developed secondary bony palate and procoelous vertebrae descended from ancient archosaurs - pseudosuchians. They are known from the Cretaceous (about 200 million years ago). Most lived in fresh water bodies, but true marine species are also known among the Jurassic forms.

Winged lizards, or pterosaurs(Pterosauria), represent one of the remarkable examples of specialization Mesozoic reptiles. These were flying animals of a very peculiar structure. Their wings were folds of skin stretched between the sides of the body and the very long fourth finger of the forelimbs. The wide sternum had a well-developed keel, like that of birds; the skull bones fused early; many bones were pneumatic. The jaws extended into a beak bore teeth. The length of the tail and the shape of the wings varied. Some ( Rhamphorhynchus) had long narrow wings and a long tail; they apparently flew in a gliding flight, often gliding. Other's ( pterodactyls) the tail was very short and the wings were wide; their flight was more often rowing (Fig. 117). Judging by the fact that the remains of pterosaurs were found in the sediments of salt water bodies, these were inhabitants of the coasts. They ate

fish and behavior, apparently, were close to gulls and terns. The sizes varied from a few centimeters to a meter or more.

The largest among flying vertebrates belong to the Late Cretaceous winged lizards. These are pteranodons. Their estimated wingspan is 7-12 m, body weight is about 65 kg. They are found on all continents except Antarctica.

Paleontologists suggest a gradual decline in the evolution of this group, which coincided with the appearance of birds.

Dinosaurs(Dinosauria) are known in the fossil record from the mid-Triassic. They are the largest and most diverse group of reptiles ever to live on land. Among the dinosaurs there were small animals, with a body length of less than a meter, and giants up to almost 30 m long. Some of them walked only on their hind legs, others - on all four. The general appearance was also very diverse, but in all of them the head was small relative to the body, and the spinal cord in the sacral region formed a local expansion, the volume of which exceeded the volume of the brain (Fig. 118).

At the very beginning of their formation, dinosaurs were divided into two branches, the development of which proceeded in parallel. Characteristic feature their structure was the pelvic girdle, which is why these groups are called lizard and ornithischian.

Lizard-pelvic(Saurischia) were originally relatively small predatory animals that moved in leaps only on their hind legs, while the front legs served for grasping food. The long tail also served for support. Subsequently, large herbivorous forms appeared that walked on all four legs. These included the largest vertebrates that ever lived on land: brontosaurus had a body length of about 20 m, diplodocus- up to 26 m. Most of the giant lizards were apparently semi-aquatic animals and fed on lush aquatic vegetation.

Ornithischian(Ornithischia) got their name due to their elongated pelvis, similar to the pelvis of birds. Initially, they moved on only elongated hind legs, but later species had both proportionately developed pairs of limbs and walked on four legs. By the nature of their diet, ornithischians were exclusively herbivorous animals. Among them - iguanodon, walking on its hind legs and reaching a height of 9 m. Triceratops in appearance it was very similar to a rhinoceros, usually possessing a small horn at the end of its muzzle and two long horns above the eyes. Its length reached 8 m. Stegosaurus was distinguished by a disproportionately small head and two rows of high bone plates located on the back. Its body length was about 5 m.

Rice. 118. Dinosaurs:
1 - iguanodon; 2 - brontosaurus; 3 - diplodocus; 4 - triceratops; 5 - stegosaurus; 6 – ceratosaurus

Dinosaurs were distributed throughout almost the entire globe and lived in extremely diverse environments. They inhabited deserts, forests, and swamps. Some led a semi-aquatic lifestyle. There is no doubt that in the Mesozoic this group of reptiles was dominant on land. Dinosaurs reached their greatest prosperity during the Cretaceous, and by the end of this period they became extinct.

Finally, it is necessary to recall another group of reptiles in whose skull there was only one superior temporal cavity. This was typical for parapsids and euryapsids. It has been suggested that they evolved from diapsids by losing the lower cavity. In the fossil record they were represented by two groups: ichthyosaurs(Ichthyosauria) and plesiosaurs(Plesiosauria). Throughout the Mesozoic, from the Early Triassic to the Cretaceous, they dominated marine biocenoses. As noted by R. Carroll (1993), reptiles became secondary aquatic whenever life in water turned out to be more advantageous in terms of the availability of food sources and a small number of predators.

Ichthyosaurs(Ichthyosauria) occupied in the Mesozoic the same place that cetaceans now occupy. They swam, bending their body in waves, especially its tail part, their fins served for control. Their convergent resemblance to dolphins is striking: a spindle-shaped body, an elongated snout and a large two-lobed fin (Fig. 119). Their paired limbs turned into flippers, while the hind limbs and pelvis were underdeveloped. The phalanges of the fingers were elongated, and the number of fingers in some reached 8. The skin was bare. Body sizes varied from 1 to 14 m. Ichthyosaurs lived only in water and ate fish, partly invertebrates. It was established that they were viviparous. Ichthyosaurs appeared in the Triassic and went extinct at the end of the Cretaceous.

Plesiosaurs(Plesiosauria) had other than ichthyosaurs, adaptive features in connection with life at sea: a wide and flat body with a relatively underdeveloped tail. Powerful flippers served as swimming tools. Unlike ichthyosaurs,

They had a well-developed neck carrying a small head. Their appearance resembled pinnipeds. Body sizes range from 50 cm to 15 m. The lifestyle was also different. In any case, some species inhabited coastal waters. They ate fish and shellfish. Having appeared at the beginning of the Triassic, plesiosaurs, like ichthyosaurs, became extinct at the end of the Cretaceous period.

From the above brief overview The phylogeny of reptiles shows that the vast majority of large systematic groups (orders) died out before the beginning of the Cenozoic era and modern reptiles are only pitiful remnants of the richest Mesozoic reptile fauna. The reason for this grandiose phenomenon is understandable only in the most general terms. Most Mesozoic reptiles were extremely specialized animals. The success of their existence depended on the presence of very peculiar living conditions. One must think that one-sided deep specialization was one of the prerequisites for their disappearance.

It has been established that although the extinction of individual groups of reptiles occurred throughout the Mesozoic, this became apparent at the end of the Cretaceous period. At this time, in a relatively short period of time, most Mesozoic reptiles became extinct. If it is fair to call the Mesozoic the age of reptiles, then it is no less justified to call the end of this era the age of the great extinction. It should be taken into account that significant changes in climate and landscapes occurred during the Cretaceous. This coincided with significant redistributions of land and sea and movements earth's crust, which led to enormous mountain-building phenomena, known in geology as the Alpine stage of mountain building. It is believed that at this time a large cosmic body passed near the Earth. Violations of the existing living conditions in this regard were very significant. However, they consist not only of changes physical condition Earth and other inanimate conditions. In the middle of the Cretaceous period, the Mesozoic flora of conifers, cycads and other plants was replaced by representatives of a new type of flora, namely angiosperms. Genetic changes in the nature of the reptiles themselves cannot be ruled out. Naturally, all this could not but affect the success of the existence of all animals and specialized ones in the first place.

Finally, we must take into account that by the end of the Mesozoic, incomparably more highly organized birds and mammals, which played important role in the struggle for existence between groups of land animals.

Figure 120 gives a general diagram of the phylogeny of reptiles.