Origin of reptiles. Evolutionary development of reptiles Origin of reptiles

), forms that apparently were more terrestrial became isolated. 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, such forms give rise to new branch- 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 (Fig. 3); 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.

Proganosaurs and synaptosaurs went extinct without leaving descendants.

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.

Late Devonian. These were armored-headed amphibians (the outdated name is stegocephals; now most of these animals are included in the labyrinthodonts). 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 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.

From the point of view of a progressive method of classification - cladistics, which considers the position of organisms from the point of view of their origin, and not the characteristics of their organization (in particular, the classic “reptilian” characteristics of crocodiles, such as cold-bloodedness and limbs located on the sides of the body, are secondary), reptiles are all developed amniotes, excluding taxa included in the clade synapsids and possibly anapsids.

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 - the 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 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, 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; 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, the pelvis, gaining stability, was attached to two or more sacral vertebrae. The “fishy” 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 for attachment complex system muscles.

Synapsids

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

Diapsids

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 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 have emerged: lepidosauromorphs (Lepidosauromorpha) and archosauromorphs (Archosauromorpha). The most primitive diapsids from the group of lepidosaurs - the order Eosuchia - were the ancestors of the order Beaked, from which only one genus is currently preserved - hatteria.

At the end of the Permian, squamate (Squamata) separated from the primitive diapsids, becoming numerous in the Cretaceous period. By the end Cretaceous period Snakes evolved from lizards.

Origin of archosaurs

see also

• Temporal arches

Notes

Literature

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

A transitional form is an organism with an intermediate state that necessarily exists during a gradual transition from one biological type of structure to another. Transitional forms are characterized by the presence of more ancient and primitive (in the sense of primary) traits than their later relatives, but, at the same time, the presence of more progressive (in the sense of later) traits than their ancestors. As a rule, when speaking about intermediate forms, they mean fossil species, although intermediate species do not necessarily have to die out. There are many known transitional forms, illustrating the origin of tetrapods from fish, reptiles from amphibians, birds from dinosaurs, mammals from theriodonts, cetaceans from terrestrial mammals, horses from a five-toed ancestor and humans from ancient hominids.

Reptiles, or reptiles (lat. Reptilia), are a class of predominantly terrestrial vertebrates, including modern turtles, crocodiles, beaked animals and squamates. Cladists are classified as reptiles and birds, since otherwise the former would be a paraphyletic group.

IN XVIII-XIX centuries together with amphibians, reptiles - cold-blooded terrestrial vertebrates - were united in a group. Traditionally, this group included various vertebrates, which, according to initial ideas, were similar in their organization to modern reptiles(for example, some synapsids - the ancestors of modern mammals). However, at present, questions about the physiology of many extinct groups of organisms remain open, and data on their genetic and evolutionary relationships do not support this kind of classification.

Many authors who adhere to traditional taxonomy believe that archosaurs (crocodiles, pterosaurs, dinosaurs, etc.) should be removed from the class of reptiles and combined into one class with birds, since birds are actually a specialized group of dinosaurs. About 10,885 species of non-avian reptiles are known in the world; 77 species live in Russia.

The largest land animals belonged to dinosaurs - representatives of ancient reptiles, currently represented only by birds. Reptiles flourished during the Mesozoic era, when they dominated the land, sea and air. At the end of the Cretaceous period, most reptiles became extinct. Modern non-avian reptiles are only scattered remnants of that world. However, ancient reptiles gave rise to the currently thriving group of animals - birds, and many of the adaptations that determined the evolutionary success of this group appeared in its archosaur ancestors, which were a specialized group of diapsids (warm-blooded, heat-insulating body cover - feathers, a developed brain, and etc).

Reptiles belong to the group Amniota, which unites them with birds and mammals in the group of true terrestrial vertebrates.

Transformation of the mucous, glandular skin of amphibians into a dry horny cover, protecting the body from drying out, and acquiring the ability to reproduce on landby laying eggs covered with dense shells was a major turning point in the life of terrestrial vertebrates. These changes gave them the opportunity to settle inland, previously inhabited by amphibians only along the shores of freshwater bodies, to new habitats and adapt to very diverse environmental conditions. Before us shining example a leap in evolution (aromorphosis), which subsequently caused a bright adaptive radiation. Modern turtles, tuataria, scaly reptiles and crocodiles are only the remnants of a once rich fauna. Fossil remains of reptiles show that the reptile fauna in Mesozoic era was extremely diverse, they populated all kinds of stations and dominated the globe.

The most ancient is the order of cotylosaurs (Cotylosauria), which are similar in skull structure to stegocephalians. They are separated in the Lower Carboniferous from embolomeric stegocephali. Currently, the most ancient cotylosaurs of the Seymouriamorpha group, which have such a great similarity with stegocephals that some paleontologists classify them as amphibians, are classified into a special subclass of Batrachosauria, intermediate between amphibians and reptiles.

By the beginning of the Permian period, cotilosaurs became extinct and were replaced by numerous descendants who occupied various stations. Turtles (Chelonia), which are the most ancient of modern reptiles, are taken directly from cotylosaurs in the Permian, so they are combined with cotylosaurs into the common subclass of anapsids (Anapsida). All other subclasses of reptiles are also derived from cotylosaurs as the original group. The central place is occupied by the subclass of archosaurs (Arhosauria), which unites thecodonts, or thecodontia, bird-hipped dinosaurs (Ornitischia), lizard-hipped dinosaurs (Saurischia), crocodiles (Crocodilia) and winged dinosaurs (Pterosauria). To the side of the archosaurs, reptiles branched off from the primary cotylosaurs and returned to an aquatic lifestyle for the second time: fish-like ichthyosaurs (Ichthyosauria) and mesosaurs (Mesosauria), classified as a special subclass of fish-footed animals (Ichthyopterygia), as well as plesiosaurs similar to pinnipeds (Plesiosauria), or lizard-footed animals ( Sauropterygii), and more primitive protorosaurs (Protorosauria). With the exception of crocodiles and turtles, this entire diverse fauna of reptiles became extinct by the beginning of the Tertiary era, replaced by higher vertebrates - birds and mammals.

Modern scaly lizards and snakes (Squamata) and hatteria (Rhynchocephalia), together with fossil eosuchians (Eosuchia), form a subclass of scaly reptiles (Lepidosauria).

Finally, even in the Upper Carboniferous, it branched special group beast-like lizards (Theromorpha), which gave rise to the ancestors of mammals. This group includes the orders pelycosauria (Pelycosauria) and therapsids, or beast-like animals (Therapsida), constituting a special subclass of synapsids (Synapsida).

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Origin of reptiles

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

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 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, 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; 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 “fishy” 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 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 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: Lepidosauromorpha and 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, squamate (Squamata) separated from the primitive diapsids, becoming numerous in the Cretaceous period. By the end of the Cretaceous period, snakes evolved from lizards.

Origin of archosaurs

see also

• Temporal arches

Notes

Literature

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

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

Origin of reptiles . The opportunity is more and more for a long time staying on land was determined by the favorable conditions of the subsequent Carboniferous period: the climate was humid, warm and even over most of what was apparently a single continent. But already at the end of the Carboniferous period, the conditions of existence on land changed. Enormous mountain-building processes and movements of land areas relative to the Earth's poles caused changes in climate and vegetation. In many areas of the Earth, the climate has become arid and continental. Tree rings on tree trunks indicate differences in living conditions between seasons. The winters were apparently cold. The lush vegetation of horsetails and ferns associated with lakes and swamps has disappeared. Vast desert spaces appeared. Relatively dry-loving vegetation of conifers and cycads became increasingly dominant.

Living conditions for stegocephals became unfavorable. The dry air made it difficult long stay them on the surface of the earth, since their pulmonary respiration 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 opportunities for the reproduction of stegocephals, which laid their eggs in water. Most stegocephalians became extinct before the Permian period. But at the same time, these environmental conditions caused the appearance of a number of new adaptive characteristics in the most land-dwelling of them.

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

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

Animals were given the opportunity to live and reproduce on land. Naturally, other features of the body appeared at the same time. The limbs became stronger, the skeleton became more durable. The lungs have become more complex, now becoming the only respiratory organ.

Evolution of reptiles

Evolution of reptiles It was going very fast and stormy. Long before the end of the Permian period, they displaced most of the stegocephalians. Having gained the opportunity to exist on land, reptiles in a new environment were faced with 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 flourishing of reptiles in subsequent times. They were given the opportunity and at the same time were forced to adapt to the most different conditions terrestrial environment. Subsequently, many of them again, to one degree or another, adapted to life in water. Some became air animals. The adaptive divergence of reptiles was astonishing. The Mesozoic is rightly considered the age of reptiles.

Primary reptiles

Cotylosaurs - ancient reptiles, known from the upper Carboniferous deposits.

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

Among the generally few cotylosaurs, the most primitive will be Seymouria, found in the Permian deposits of North America, and forms close to it found on Northern Dvina, also in Permian deposits. These were small animals, no more than 0.5 m in size. Large sizes reached the pareiasaurus (Pareiasaurus), numerous remains of which were found by V.P. Amalitskim on the Northern Dvina. Their sizes reached 3 m. Most cotylosaurs were herbivores, some fed on mollusks.

Cotylosaurs reached their peak 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 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 more durable. Reptiles used an increasingly varied diet. The technique of its extraction has changed. In this regard, the structure of the limbs, axial skeleton and skull underwent significant changes. In most, the limbs became longer, the pelvis 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 exceptional variety these animals, their adaptive specialization and probable relationship with living groups.

Protolizards (Prosauria) are one of the most primitive groups of reptiles, whose skull had two zygomatic arches. The teeth, like those of amphibians, sat not only on the jaw bones, but also on the palate. The vertebrae were amphicoelous, like those of fish and lower amphibians. Similar in appearance large lizards. The most ancient representatives are known from Permian deposits. In the Triassic, representatives of the proboscis (Rhynchocephalia) appear, one of the species of which is the hatteria ( Sphenodon punctatus) - preserved to this day in New Zealand.

Pseudosuchia probably originated from the same root as the proto-lizards. They first appear at the beginning of the Triassic. In general appearance and size they were partly 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 for 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 lengthened. Many apparently led an arboreal lifestyle. Such, for example, are 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 ones in the absence of a true bony palate, and their internal nostrils opened between the palatine bones. The vertebrae were still amphicoelous. There were crocodiles during the Cretaceous period modern type with a fully developed secondary bony palate and produral vertebrae. Most lived in fresh water bodies, but true marine species are also known among the Jurassic forms.

Winged lizards (Pterosauria) represent one of the remarkable examples of specialization Mesozoic reptiles. These were flying animals of a very peculiar structure. The instrument of flight was the wings, which were a fold 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, and many bones were pneumatic. In some species, the jaws extended into a beak had teeth. The length of the tail and the shape of the wings varied. Some (rhamphorhynchus) had long, narrow and a long tail; They apparently flew in a gliding flight, often gliding. 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 salt water bodies, these were inhabitants of the coasts. They ate fish and were apparently close in behavior to gulls and terns. The sizes varied from a few centimeters to a meter or more. Pterosaurs reached their greatest prosperity in the Jurassic. Selected species are also known from Cretaceous deposits.

Dinosaurs (Dinosauria) are the next, last branch of pseudosuchians, 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 there 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. It was very diverse and general appearance bodies, but all of them had relatively small heads, and spinal cord in the sacral region formed a local expansion, the volume of which exceeded the volume of the brain.

At the very beginning of their separation from pseudosuchians, dinosaurs 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, and therefore these groups are called ornithischian and lizard-pelvic.

Lizards were originally relatively small predatory animals that moved in leaps only on their hind legs, while the front legs were used for grasping food. The long tail also served for support. Subsequently, large herbivorous forms appeared that walked on all four legs. These include the largest vertebrates that ever lived on land. Thus, the 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 lush aquatic vegetation.

Ornithischians get their name due to their elongated pelvis, similar to the pelvis of birds. Initially, they walked on only elongated hind legs, but later species had both pairs of limbs proportionately developed and walked on four legs. By the nature of their diet, ornithischians were exclusively herbivorous animals. Among them, we 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 very similar in appearance to a rhinoceros, usually possessing a small horn at the end of its snout 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 its back. Its length was about 5 m.

Dinosaurs were distributed almost everywhere to the globe and lived in extremely diverse living conditions. They inhabited deserts, forests, and 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 currently largest detachment is the least clear.

Lizards apparently appeared in the Upper Jurassic, but only in the Cretaceous period is relative diversity of this suborder observed. Snakes evolved later than all other reptiles. They appeared only towards the end of the Cretaceous, undoubtedly as the side trunk of lizards. The real flourishing of squamates came only in Tertiary times, when most groups of reptiles became extinct.

Turtles (Chelonia) represent one of the oldest reptiles, 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 that form a kind of dorsal shield. They did not have an abdominal shield. There were teeth. In the Triassic, real turtles with developed real shells appeared (for example, Triassochelys).

However, their head and limbs could not yet be completely retracted into the shell. A horny sheath was developed on the jaws, but at the same time there were teeth on the palate. Mesozoic turtles were originally land-dwelling and apparently burrowing animals. Only later did some groups switch to an aquatic lifestyle and, as a result, partially lost their bony and horny shells.

Throughout the entire period from the Triassic to the present day, turtles have retained all the main features of their organization. They have survived all the challenges that killed off most reptiles and are thriving today as much as they were in the Mesozoic.

Ichthyosaurs (Ichthyosauria) are reptiles that are most fully adapted to life in 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-lobed fin. The 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 clarified.

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

Plesiosaurs appeared at the beginning of the Triassic. At the end of the Cretaceous period they became extinct.

The beast-like animals (Theromorpha) are of great interest as the group that gave rise to mammals.

Animal-like animals are one of the most ancient groups of reptiles. Its appearance dates back to the end of the Carboniferous, and in the Permian they were already numerous and diverse. Animal-like animals experienced their heyday long before the first dinosaurs appeared, and cotylosaurs were their direct relatives. Primitive animal-like animals, allocated to the order Pelycosauria (Pelycosauria), were still very close to cotylosaurs. Thus, 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 depression not characteristic of any other group of reptiles. In appearance they resembled lizards and were small in size - 1-2 m. In some, differentiation of teeth was evident, although to a small extent (for example, in Sphenacodon).

In the Middle Permian, pelycosaurs were replaced by more highly organized animals (Theriodontia). Their teeth were clearly differentiated, and a secondary bony palate appeared. The single occipital condyle split into two. Lower jaw was mainly represented by dentary bone. The position of the limbs also changed. The elbow moved back and the knee forward, and as a result the limbs began to occupy a position under the body, and not on its sides, as in other reptiles. The skeleton appeared to have many features in common with mammals.

Among the numerous Permian beast-like animals there were reptiles that were very diverse in appearance and lifestyle. Many were predators. Such, for example, is Inostrancevia aiexandrovi, found by the expedition of V.P. Amalitsky in Permian deposits on the Northern Dvina. Others ate a plant-based or mixed diet. These unspecialized species are closest to mammals. Among them, we must point out Cynognathus, which had many progressive organizational features.

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

From the above overview of reptile phylogeny, it is clear that the vast majority of large systematic groups their (orders) became extinct before the beginning Cenozoic era, and modern reptiles represent only pitiful remnants of the Mesozoic fauna.

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

It was 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, precisely at the end of the Cretaceous period. At this time, relatively short term the vast majority of Mesozoic reptiles became extinct. If it is true 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. Along with the above, it has been established that particularly significant changes in climate and landscapes were observed during the Cretaceous. This was associated 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”. Violations of the existing living conditions in this regard were very significant. They consist not only in changes in climate, orography of the Earth and other conditions of dead nature. It is enough to point out that in the middle of the Cretaceous period Mesozoic flora conifers, cycads and others were 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 the one-sidedly 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 a very important role in the struggle for existence between groups of terrestrial animals, began to develop more and more.