What led to the emergence of reptiles. Evolutionary development of reptiles. The history of the emergence of ancient reptiles

Origin of reptiles

Origin of reptiles- one of the important questions in the theory of evolution, the process as a result of which the first animals belonging to the class Reptiles (Reptilia) appeared.

Varanus niloticus ornatus at London Zoo

Permian period

From the upper Permian deposits of North America, Western Europe, Russia, and China, remains of cotylosaurs are known ( Cotylosauria). In a number of ways, they are still very close to stegocephals. Their skull was in the form of a solid bone box with holes only for the eyes, nostrils and parietal organ, the cervical 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; in the shoulder girdle, a kleytrum was preserved - a skin bone characteristic of fish; 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 have become more mobile; their skeleton became lighter, but at the same time stronger. Reptiles used a more varied diet than amphibians. The technique of obtaining it has changed. In this regard, the structure of the limbs, the axial skeleton and the skull underwent significant changes. Most of the limbs became longer, the pelvis, acquiring stability, was attached to two or more sacral vertebrae. In the shoulder girdle, the "fish" bone of the kleytrum disappeared. The solid shell of the skull has undergone a partial reduction. In connection with the more differentiated muscles of the jaw apparatus in the temporal region of the skull, pits and bone bridges separating them appeared - arcs that served to attach a complex system of muscles.

synapsids

The main ancestral group that gave all the variety of modern and fossil reptiles were cotylosaurs, however, the further development of reptiles went in different ways.

Diapsides

The next group to separate from the cotylosaurs were the Diapsida. Their skull has two temporal cavities located above and below the postorbital bone. Diapsids at the end of the Paleozoic (Permian) gave extremely wide adaptive radiation to systematic groups and species, which are found both among extinct forms and among modern reptiles. Among the diapsids, there are two main groups of Lepidosauromorphs (Lepidosauromorpha) and Archosauromorphs (Archosauromorpha). The most primitive diapsids from the Lepidosaur group are the Eosuchia order ( Eosuchia) - were the ancestors of the order Beakheads, of which only one genus is currently preserved - tuatara.

At the end of the Permian, scaly ones (Squamata) separated from primitive diapsids, which became numerous in the Cretaceous period. Towards the end of the Cretaceous, snakes evolved from lizards.

Origin of archosaurs

Notes

Literature

Naumov N.P., Kartashev N.N. Part 2. Reptiles, birds, mammals // Vertebrate Zoology. - M .: Higher School, 1979. - S. 272.

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

It had tremendous consequences for all of nature. The origin of reptiles is one of the important questions in the theory of evolution, the process that resulted in the appearance of the first animals belonging to the class Reptiles (Reptilia). The first terrestrial vertebrates arose in the Devonian (more than 300 million years ago). These were shell-headed amphibians - stegocephals. They were closely associated with water bodies, since they bred only in water, lived near water. The development of spaces remote from water bodies required a significant restructuring of the organization: adaptation to protecting the body from drying out, to breathing atmospheric oxygen, efficient movement on a solid substrate, and the ability to reproduce outside water. These are the main prerequisites for the emergence of a qualitatively different group of animals - reptiles. These restructurings were quite complex, for example, it required the design of powerful lungs, 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 (cotilosaurs and turtles);

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

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

Anapsid group is the oldest branch of reptiles, which, in terms of the structure of the skull, have many features in common with fossil stegocephalians, since not only many of their early forms (cotilosaurs), 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 diverged directly from cotylosaurs. Already in the Triassic, this ancient group was fully developed and, thanks to its extreme specialization, has survived to the present, almost unchanged, although in the process of evolution, some groups of turtles several times switched from a terrestrial to aquatic lifestyle, due to which they almost lost their bone shields then bought them again.

synapsid group. Marine fossil reptiles - ichthyosaurs and plesiosaurs - separated from the group of cotilosaurs. 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 within a very wide range: some species were only half a meter long, 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 fusiform, the neck was not expressed, the head was elongated, the tail had a large fin, the limbs were in the form of short flippers, and the hind ones were much smaller than the front ones. The skin was bare, numerous sharp teeth (adapted to feeding on fish) sat in a common furrow, there was only one zygomatic arch, but of an extremely peculiar 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 understood, better known than others are lounginia - a small reptile resembling a lizard in physique, with relatively weak limbs that had the usual reptilian structure. Its primitive features are expressed mainly in the structure of the skull, the teeth are located both on the jaws and on the palate.

Now there are about 7,000 species of reptiles, that is, almost three times more than modern amphibians. Living reptiles are divided into 4 orders:

· scaly;

· Turtles;

· Crocodiles;

· Beakheads.

The most numerous squamous order (Squamata), which includes about 6,500 species, is the only now flourishing group of reptiles, widespread throughout the globe and constituting the bulk of the reptiles of our fauna. This order includes lizards, chameleons, amphisbaenas and snakes.

There are 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 - a shell in which their body is chained.

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

The only species of modern beakheads (Rhynchocephalia) - the tuatara has many extremely primitive features and has survived 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 also confirmed by the current ratio of the number of species of different classes of terrestrial vertebrates. If the share of amphibians and reptiles, which are the most dependent on the environmental temperature, on a global scale is quite high (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 vertebrates. 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 differ in the warmth of the climate, there are only 1.8 reptiles, 3.2% 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, in the CIS and Belarus, out of four orders of modern reptiles, only two (tortoises and scaly ones) live.

The Cretaceous period was marked by the collapse of the reptiles, the almost complete extinction of the dinosaurs. This phenomenon is a mystery to science: how did a huge, prosperous, ecologically niche army of reptiles, which included representatives from the smallest creatures to unimaginable giants, so suddenly died out, leaving only relatively small animals?

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

Two other orders - turtles and crocodiles - unite a relatively small number of species - about 200 and 23, respectively. And only one order - scaly, which includes lizards and snakes, can be assessed as flourishing in the current evolutionary era. This is a large and diverse group, numbering more than 6000 species.

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

The first vertebrates appeared on land in the Devonian. These were stegocephals, or shell-headed amphibians, the closest relatives of lobe-finned fish. Like the latter, they spent a significant part of their time in water bodies. However, during periodically recurring droughts, they could crawl out of drying up water bodies and spend some time on land in search of more favorable conditions.

Origin of reptiles . The ability to stay on land for longer and longer periods was determined by the favorable conditions of the subsequent Carboniferous period: the climate was humid, warm and even over most of what seemed to be a single mainland. But already at the end of the Carboniferous period, the conditions for existence on land changed. Enormous mountain-building processes, the movement of land areas relative to the poles of the Earth caused a change in climate and vegetation. In many areas of the Earth, the climate has become arid, continental. Annual rings on tree trunks indicate the difference in living conditions for the seasons of the year. The winters were apparently cold. The lush vegetation of horsetails and ferns associated with lakes and swamps has disappeared. Vast empty spaces appeared. Relatively dry-loving vegetation of conifers and cycads became increasingly prevalent.

The living conditions for stegocephalians became unfavorable. The dryness of the air environment made it difficult for them to stay on the surface of the earth for a long time, since their lung breathing was imperfect, and bare skin could not prevent the body from drying out. At the same time, the desert landscape in many areas did not provide breeding opportunities for stegocephalians, which laid their eggs in the water. Most stegocephalians died out before the onset of the Permian period. But at the same time, these environmental conditions caused the appearance of a number of new adaptive features in the most terrestrial of them.

The decisive adaptations that made it possible to live entirely on land were:

progressive development of the central nervous system, which ensures more perfect adaptive behavior of animals;
keratinization of the upper layer of the epidermis, and then the appearance of horny scales, which protected the body from drying out;
an increase in the amount of yolk in the egg and the appearance during its development of a number of shells that protect the embryo from drying out and at the same time provide the possibility of gas exchange.

Animals were able to live and breed on land. Naturally, other features of the organism appeared at the same time. Strengthened limbs, the skeleton became more durable. The lungs became more complex, now becoming the only respiratory organ.

reptile evolution

reptile evolution went very fast and furiously. Long before the end of the Permian period, they supplanted most of the stegocephalians. Having gained the opportunity to exist on land, the reptiles in the new environment faced new and extremely diverse conditions. The impact of such diverse living conditions and the lack of significant competition on land from other animals served as the main reason that led to the extremely rapid flowering of reptiles in subsequent times. They got the opportunity and at the same time were forced to adapt to the most diverse conditions of the terrestrial environment. Subsequently, many of them secondarily, to one degree or another, adapted to life in the water. Some have become aerial animals. The adaptive divergence of reptiles was striking. With good reason, the Mesozoic is considered the age of reptiles.

Primary reptiles

Kotilosaurs are the oldest reptiles known from the upper Carboniferous deposits.

According to a number of features, they are still very close to stegocephals. So, many had only one sacral vertebra; the cervical region is poorly developed, in the shoulder girdle there was a kleytrum - a skin bone characteristic of fish. The skull was in the form of a solid bone box with holes only for the eyes, nostrils and parietal organ (hence the name of this group - whole-cranial). The limbs were short and not specialized.

Among the generally few cotylosaurs, the most primitive are the Seymouria, found in the Permian deposits of North America, and closely related forms found on the Northern Dvina, also in the Permian deposits. These were medium-sized animals, no more than 0.5 m in size. Pareiasaurs (Pareiasaurus) reached large sizes, numerous remains of which were found by V.P. Amalitsky on the Northern Dvina. Their sizes reached 3 m. Most cotylosaurs were herbivorous, some fed on mollusks.

Kotilosaurs flourished in the Middle Permian. 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.

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

Below we consider the main groups of reptiles, a review of which should show the exceptional diversity of these animals, their adaptive specialization, and their likely relationship with living groups.

The first lizards (Prosauria) are one of the most primitive groups of reptiles, whose skull had two zygomatic arches. Teeth, like those of amphibians, sat not only on the bones of the jaws, but also on the palate. The vertebrae were amphicoelous, as in fish and lower amphibians. They look like large lizards. The most ancient representatives are known from the Permian deposits. In the Triassic, representatives of the Proboscis Heads (Rhynchocephalia) appear, one of the species of which, the tuatara (Sphenodon punctatus), has survived to this day in New Zealand.

Pseudosuchia (Pseudosuchia) probably originated from the same root with the first lizards. They first appear at the beginning of the Triassic. In general appearance and size, they were somewhat similar to lizards. The peculiar features of the organization were that the teeth sat in deep cells; the hind limbs were much more developed than the forelimbs, and in the majority they were the only ones used for walking. In this regard, the pelvis and lower parts of the skeleton of the hind limbs were elongated. Many led, apparently, an arboreal lifestyle. Such, for example, is Ornithosuchus.

Pseudosuchians are undoubtedly close to crocodiles, pterosaurs and dinosaurs, for the development of which they apparently served as the initial group. Finally, there is reason to believe that pseudosuchia gave rise to the ancestors of birds.

Crocodiles (Crocodilia) appear at the end of the Triassic. Jurassic crocodiles are significantly different from modern crocodiles in the absence of a true bony palate, and their internal nostrils opened between the palatine bones. The vertebrae were still amphicoelous. In the Cretaceous period, there were crocodiles of the modern type with a fully developed secondary bony palate and protruding vertebrae. Most lived in fresh water, but real marine species are also known among the Jurassic forms.

Winged lizards (Pterosauria) represent one of the remarkable examples of Mesozoic reptile specialization. These were flying animals of a very peculiar structure. Wings served as a flight instrument, representing a fold of skin stretched between the sides of the body and a very long fourth finger of the forelimbs. The wide sternum had a well-developed keel, like in birds, the bones of the skull fused early, many bones were pneumatic. Jaws extended into a beak had teeth in some species. The length of the tail and the shape of the wings varied. Some (Rhamphorhynchus) had long, narrow wings and a long tail; they flew, apparently, in a gliding flight, often planning. Others (pterodactyls) had a very short tail and wide wings; their flight was often rowing. Judging by the fact that the remains of pterosaurs were found in the sediments of salty reservoirs, they were inhabitants of the coasts. They fed on fish and, apparently, were similar in behavior to gulls and terns. Sizes ranged from a few centimeters to a meter or more. Pterosaurs reached their peak in the Jurassic. Individual species are also known from Cretaceous deposits.

Dinosaurs (Dinosauria) - the next, last branch of pseudosuchia, the species of which lived from the beginning of the Triassic to the end of the Cretaceous. This is the most numerous and diverse group of reptiles. Among the dinosaurs were small animals with a body length of less than a meter and giants up to almost 30 m in length. Some of them walked only on their hind legs, others on all four legs. The general external appearance of the body was also very diverse, but in all of them the head was relatively small, and the spinal cord in the sacral region formed a local expansion, the volume of which exceeded the volume of the brain.

Dinosaurs at the very beginning of their separation from pseudosuchians were divided into two branches, the development of which proceeded in parallel. A characteristic feature of them was the structural features of the pelvic girdle, in connection with which these groups are called ornithischian and saurischian.

Lizards were originally relatively small predatory animals, moving in leaps only on their hind legs, while the front legs served to grasp food. A long tail also served as a support. Subsequently, large herbivorous forms appeared that walked on all four legs. These include the largest vertebrates ever to have lived on land. So, brontosaurus had a body length of about 20 m, and diplodocus - up to 26 m. Most of the giant lizards, apparently, were semi-aquatic animals and fed on succulent aquatic vegetation.

The ornithischians got their name in connection with the elongated pelvis, similar to the pelvis of birds. Initially, they moved on one elongated hind legs, but later species had both pairs of limbs commensurately developed and walked on four legs. By the nature of their diet, ornithischians were exclusively herbivores. Among them, we will mention the Iguanodons, which walked only on their hind legs and reached 9 m in height. Their skin was without a bone shell. Triceratops was outwardly very similar to a rhinoceros, usually had a small horn at the end of the muzzle and two long horns above the eyes. Its length reached 8 m. Stegosaurus was characterized by a disproportionately small head and two rows of high bone plates located on the back. Its length was about 5 m.

Dinosaurs were distributed almost all over the globe and lived in extremely diverse living conditions. They inhabited deserts, forests, swamps. Some (for example, trachodonts) led a semi-aquatic lifestyle. There is no doubt that in the Mesozoic, dinosaurs were the dominant group of reptiles on land. They appeared in the Triassic and reached their greatest prosperity in the Cretaceous. By the end of this period, dinosaurs became extinct.

Scaly (Squamata). The history of this detachment, the most numerous at present, is the least clear.

Lizards apparently appeared as early as the Upper Jurassic, but only in the Cretaceous period is a relative diversity of this suborder observed. Snakes evolved later than all other reptiles. They appeared only towards the end of the Cretaceous, no doubt as a side trunk of lizards. The real heyday of the scaly came only in the Tertiary, when most groups of reptiles died out.

Turtles (Chelonia) represent one of the oldest reptile corpses, apparently descended directly from cotylosaurs. Their ancestor is considered to be the Permian Eunotosaurus. This is a small lizard-like animal with short and very wide ribs, forming a kind of dorsal shield. They did not have a ventral shield. There were teeth. In the Triassic, real turtles with a developed real shell appear (for example, Triassochelys).

However, their head and limbs could not yet be fully retracted into the shell. A horny cover was developed on the jaws, but at the same time there were teeth on the palate. Mesozoic tortoises were originally terrestrial and apparently burrowing animals. Only later did some groups switch to an aquatic way of life and, in this regard, partially lost their bone and horn shell.

For all the time from the Triassic to the present day, turtles have retained all the main features of their organization. They survived all the trials that killed most reptiles, and are now flourishing to the same extent as in the Mesozoic.

Ichthyosaurs (Ichthyosauria) are reptiles most fully adapted to life in the water. In the nature of the Mesozoic, they occupied the same place that cetaceans now occupy. Their convergent resemblance to dolphins is striking. They had a spindle-shaped body, an elongated snout and a large two-bladed fin. Paired limbs were 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. The appearance of ichthyosaurs dates back to the Triassic. They became extinct during the Cretaceous period. Genetic relationships with other reptiles have not been elucidated.

Plesiosaurs (Plesiosauria) - the second group of Mesozoic marine reptiles with other adaptive organizational features. Ichthyosaurs swam, wavy bending the body and especially its tail, their fins served to control. Plesiosaurs had a wide and flat body with a relatively underdeveloped tail. Powerful flippers served as a swimming tool. Unlike ichthyosaurs, they had a well-developed neck carrying a small head. Body sizes from 50 cm to 15 m. Apparently, the way of life was also different. In any case, some species inhabited coastal waters. They ate fish and shellfish.

Plesiosaurs appeared at the beginning of the Triassic. They became extinct at the end of the Cretaceous.

Animals (Theromorpha) are of great interest as a group that gave rise to mammals.

Animal - one of the most ancient groups of reptiles. Its appearance dates back to the end of the Carboniferous, and in Perm they were already numerous and varied. Animals survived their heyday long before the first dinosaurs appeared, and cotilosaurs were their immediate relatives. Primitive animal-like animals allocated to the order Pelycosaurus (Pelycosauria) were still very close to cotylosaurs. So, they had biconcave vertebrae and well-preserved abdominal ribs. However, their teeth sat in the alveoli, and in the temporal region of the skull there was a lateral cavity, not characteristic of any other group of reptiles. In appearance, they looked like lizards and were small in size - 1-2 m. In some, differentiation of teeth was outlined, although to a small extent (for example, in Sphenacodon).

In the Middle Permian, pelycosaurs were replaced by more highly organized mammal-toothed animals (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 the dentary. The position of the limbs also changed. The elbow moved back, and the knee forward, and as a result, the limbs began to take a position under the body, and not on the sides of it, as in other reptiles. The skeleton has many features in common with mammals.

Among the numerous Permian animal-like reptiles were very diverse in appearance and way of life. Many were predators. Such, for example, is foreigners (Inostrancevia aiexandrovi), found by the expedition of V.P. Amalitsky in the deposits of the Permian period on the Northern Dvina. Others ate vegetable or mixed foods. These unspecialized species are closest to mammals. Among them, it is necessary to point out the cynognathus (Cynognathus), which had many progressive features of organization.

Animal-toothed were numerous even in the Triassic, but with the appearance of predatory dinosaurs, they disappeared.

From the above review of the phylogeny of reptiles, it can be seen that the vast majority of large systematic groups of them (orders) died out before the beginning of the Cenozoic era, and modern reptiles represent only miserable remnants of the Mesozoic fauna.

The reason for this grandiose phenomenon is understandable only in the most general terms. It is noteworthy that most of the Mesozoic reptiles were extremely specialized animals. The success of their existence depended on the presence of very peculiar, narrowly defined living conditions. One must think that one-sided specialization was one of the prerequisites for the disappearance of most Mesozoic reptiles.

It has been established that, although the extinction of individual groups of reptiles was observed throughout the Mesozoic and the end of the Paleozoic, it was especially pronounced at the end of the Mesozoic, namely at the end of the Cretaceous period. At this time, in a relatively short period of time, the vast majority of Mesozoic reptiles died out. If the name of the Mesozoic as the age of reptiles is true, then it is no less justified to call the end of this era the age of the great extinction. Along with what has been said, it has been established that particularly significant changes in climates and landscapes were observed during the Cretaceous. This was due to significant redistributions of land and sea and movements of the earth's crust, which led to huge mountain-building phenomena, known in geology as the "Alpine stage of mountain building." Violations of the existing living conditions in this regard were very significant. They consist not only in climate change, the orography of the Earth and other conditions of dead nature. Suffice it to point out that in the middle of the Cretaceous the Mesozoic flora of conifers, cycads and others was replaced by plants of a new type, namely angiosperms. Naturally, all this could not but affect the success of the existence of all animals, and one-sidedly specialized in the first place.

Finally, it must be taken into account that by the end of the Mesozoic, incomparably more highly organized birds and mammals, which played a very important role in the struggle for existence between groups of terrestrial animals, received more and more development.

Terrestrial vertebrates arose in the Devonian. These were armored amphibians, or stegocephalians. They were closely associated with water bodies, since they bred only in water, lived near water bodies, where there was terrestrial vegetation. The development of spaces remote from water bodies required a significant restructuring of the organization: adaptation to protecting the body from drying out, breathing atmospheric oxygen, walking on a solid substrate, the ability to breed out of water, and, of course, improving forms of behavior. These are the basic prerequisites for the emergence of a qualitatively different new group of animals. All these features took shape in reptiles.

To this it must be added that by the end of the Carboniferous, strong changes in the natural situation occurred, which led to the emergence of a more diverse climate on the planet, the development of more diverse vegetation, its distribution in territories remote from water bodies, and, in this regard, to the wide distribution of tracheal-breathing arthropods, t .e. possible food items also spread to the watershed areas of the land.

The evolution of reptiles was very fast and violent. Long before the end of the Permian period of the Paleozoic, they replaced most of the stegocephalians. Having gained the opportunity to exist on land, the reptiles in the new environment faced new and extremely diverse conditions. The versatility of this diversity and the lack of significant competition on land from other animals were the main reasons for the flowering of reptiles in subsequent times. Mesozoic reptiles are primarily land animals. Many of them are secondary in one way or another.

adapted to life in the water. Some mastered the air environment. The adaptive divergence of reptiles was striking. With good reason, the Mesozoic is considered the age of reptiles.

early reptiles. The oldest reptiles are known from the upper Permian deposits of North America, Western Europe, Russia, and China. They are called cotilosaurs. According to a number of features, they are still very close to stegocephals. Their skull was in the form of a solid bone box with holes only for the eyes, nostrils and parietal organ, the cervical spine was poorly formed, the sacrum had only one vertebra; in the shoulder girdle, a kleytrum was preserved - a skin bone characteristic of fish; 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 three-meter herbivorous pareiasaurs (Pareiasaurus).

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

In the Middle Permian, cotilosaurs flourished. 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 the very diverse living conditions that they encountered during reproduction and settlement. Most of the groups have acquired greater mobility; their skeleton became lighter, but at the same time stronger. Reptiles used a more varied diet than amphibians. The technique of obtaining it has changed. In this regard, the structure of the limbs, the axial skeleton and the skull underwent significant changes. Most of the limbs became longer, the pelvis, acquiring stability, was attached to two or more sacral vertebrae. In the shoulder girdle, the kleytrum bone disappeared. The solid shell of the skull has undergone a partial reduction. In connection with the more differentiated muscles of the jaw apparatus in the temporal region of the skull, pits and bone bridges separating them appeared - arcs that served to attach a complex system of muscles.

In the formation of the appearance of ancient reptiles and in the assessment of their subsequent fate, the characteristic of their skull is essential.

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

The primitiveness of stegocephalians ("whole-cranial") and early reptiles was expressed in the structure of the skull by the absence of any depressions in it, except for the ocular and olfactory ones. This feature is reflected in the name Anapsida. The temporal region of the reptiles of this group was covered with bones. Turtles (now Testudines, or Chelonia) became probable descendants of this direction; they have a continuous bone cover behind their eye sockets. Tortoises known from the Lower Triassic of the Mesozoic reveal similarities with the current forms. Their fossil remains are confined to the territory of Germany. The skull, teeth, shell structure of ancient turtles are extremely close to modern ones. The ancestor of turtles is considered to be the Permian eunotosaurus(Eunotosaurus) - a small lizard-like animal with short and very wide ribs, forming a kind of dorsal shield (Fig. 115). He did not have a ventral shield. There were teeth. Mesozoic tortoises were originally terrestrial and apparently burrowing animals. Only later did some groups switch to an aquatic way of life, and in connection with this, many of them partially lost their bone and horn shell.

From the Triassic to the present day, turtles have retained the main features of their organization. They survived all the trials that killed most of the reptiles, and are now flourishing as well as in the Mesozoic.

The current crypto-necked and side-necked tortoises to a greater extent preserve the primary appearance of the Triassic land tortoises. Marine and soft-skinned appeared in the late Mesozoic.

All other reptiles, both ancient and modern, acquired one or two temporal cavities in the structure of the skull. One, lower, temporal cavity had synapsid. One superior temporal cavity is noted in two groups: paranoid and euryantsid. And finally, two depressions had diapsid. The evolutionary fate of these groups is different. The first to depart from the ancestral trunk synapsids(Synapsida) - reptiles with lower temporal cavities, limited by the zygomatic, squamous and postorbital 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 consecutive orders: pelycosaurs(Pelicosauria) and therapsids(Therapsida). They are also called bestial(Theromorpha). The animal-like survived the period of their heyday long before the first dinosaurs appeared, the cotylosaurs were their immediate relatives. In particular, pelycosaurs(Pelicosauria) were still very close to cotilosaurs. Their remains have been found in North America and Europe. Outwardly, 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, it was planned, albeit to a small extent, differentiation of teeth.

In the Middle Permian, pelycosaurs were replaced by more highly organized animal-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 the dentary. Position

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

Numerous Permian animal-toothed reptiles were very diverse in appearance and lifestyle. Many were predators. Perhaps this was found by the expedition of V.P. Amalitsky in the deposits of the Permian period on the Northern Dvina foreigners(Inostrancevia alexandrovi, Fig. 116). Others ate vegetable or mixed foods. These unspecialized species are closest to mammals. Among them, one should point out cynognathus(Cynognathus), which had many progressive features of the organization.

Animal-toothed were numerous even in the early Triassic, but with the appearance of predatory dinosaurs, they disappeared. Curious materials given in Table 6 testify to a sharp reduction in the diversity of animal-like animals during the Triassic. Animals are of great interest as a group that gave rise to mammals.

Rice. 116. Animal-toothed:
1 - foreigners, Upper Perm (animal restoration), 2 - cynognathus skull

Table 6

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

Period	bestial	Sauropsids
Upper Triassic Middle Triassic Lower Triassic Upper Perm	17 23 36 170	8 29 20 15

The next group to separate 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 extremely wide adaptive radiation to systematic groups and species, which are found both among extinct forms and among modern reptiles. Among the diapsid, two main groups (infra - classes) have been outlined: infraclass Lepidosauromorphs(Lepidosauromorpha) and infraclass archosauromorphs(Archosauromorpha).

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

Who are lepidosauromorphs? This ancient infraclass includes the living tuatara, lizards, snakes, chameleons and their extinct ancestors.

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

Lizards, snakes and chameleons now make up a wide variety of the squamous order (Squamata). Lizards are one of the oldest advanced groups of reptiles, their remains are known from. upper perm. Scientists discover many similarities between lizards and sphenodons. Their limbs are widely spaced and the body moves, wavy curving the spinal column. It is curious that among the common features of their morphological similarity is the presence of an intertarsal joint. Snakes only appear 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 have ever lived on Earth. Among them - crocodiles, pterosaurs, 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 crocodiles in the absence of a true bony palate. Their internal nostrils opened between the palatine bones. The vertebrae were still amphicoelous. Crocodiles of the modern type with a fully developed secondary bony palate and procoelous vertebrae descended from ancient archosaurs - pseudosuchians. They have been known since the Cretaceous (about 200 million years ago). Most lived in fresh water, but real marine species are also known among the Jurassic forms.

Winged lizards, or pterosaurs(Pterosauria), represent one of the remarkable examples of Mesozoic reptile specialization. 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 in birds; the bones of the skull 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 planning. Other's ( pterodactyls) the tail was very short, and the wings were wide; their flight was often rowing (Fig. 117). Judging by the fact that the remains of pterosaurs were found in the sediments of salty reservoirs, they were inhabitants of the coasts. They fed

fish and behavior, apparently, were close to gulls and terns. Sizes ranged from a few centimeters to a meter or more.

The largest 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 every continent except Antarctica.

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

Dinosaurs(Dinosauria) are known in the fossil record from the middle Triassic. This is the most numerous and diverse group of reptiles that have ever lived on land. Among the dinosaurs 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, the dinosaurs were divided into two branches, the development of which proceeded in parallel. Their characteristic feature was the structure of the pelvic girdle, in connection with which these groups are called lizard and ornithischian.

lizards(Saurischia) were originally relatively small predatory animals, moving in leaps only on their hind legs, while the front legs served to grasp food. A long tail also served as a 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, apparently, were semi-aquatic animals and fed on succulent aquatic vegetation.

Ornithischians(Ornithischia) got its name in connection with the elongated pelvis, similar to the pelvis of birds. Initially, they moved on one elongated hind legs, but later species had both proportionally developed pairs of limbs and walked on four legs. By the nature of their diet, ornithischians were exclusively herbivores. Among them - iguanodon, walking on its hind legs and reaching a height of 9 m. Triceratops outwardly it was very similar to a rhinoceros, usually had a small horn at the end of the muzzle and two long horns above the eyes. Its length reached 8 m. Stegosaurus 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 almost all over the globe and lived in extremely diverse environments. They inhabited deserts, forests, 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 died out.

Finally, it is necessary to recall another group of reptiles, in the skull of which there was only one upper temporal cavity. This was characteristic of parapsid and euryapsid. It has been suggested that they evolved from the diapsids by the loss of the lower depression. 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 the water turned out to be more profitable in terms of the availability of food sources and a small number of predators.

ichthyosaurs(Ichthyosauria) occupied in the Mesozoic the same place that is now occupied by cetaceans. They swam, wavy bending the body, especially its tail, their fins served to 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, they became extinct at the end of the Cretaceous.

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

they had a well-developed neck, carrying a small head. Their appearance resembled pinnipeds. Body sizes from 50 cm to 15 m. The way of life 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.

From the above brief review of the phylogeny of reptiles, it can be seen that the vast majority of large systematic groups (orders) died out before the beginning of the Cenozoic era, and modern reptiles are only miserable 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 highly 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 certain groups of reptiles occurred throughout the Mesozoic, this manifested itself at the end of the Cretaceous. At this time, in a relatively short period of time, most of the Mesozoic reptiles died out. If it is fair to call the Mesozoic the age of the 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 of the earth's crust, which led to huge mountain-building phenomena, known in geology as the Alpine stage of mountain building. It is believed that at that 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 in changing the physical state of the Earth and other conditions of inanimate nature. 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 are not excluded. Naturally, all this could not but affect the success of the existence of all animals and specialized ones in the first place.

Finally, it must be taken into account that by the end of the Mesozoic, incomparably more highly organized birds and mammals, which played an important role in the struggle for existence between groups of terrestrial animals, received more and more development.

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