Origin and evolution of reptiles. Brief characteristics of the main groups of fossil reptiles. Origin and evolution of modern reptiles Progressive evolution of reptiles

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 important issues 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 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 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 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 did a huge, thriving army of reptiles occupying all ecological niches, which had representatives from the tiniest creatures to unimaginable giants, so suddenly become extinct, 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 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).

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 the air. The adaptive divergence of reptiles was astonishing. WITH 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 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 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 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)

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 lizards, 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 the specialization of 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. A characteristic feature of them was the structure of the pelvic girdle, and therefore 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 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 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 different 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 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. Appearing at the beginning of the Triassic, plesiosaurs, like ichthyosaurs, became extinct at the end Cretaceous period.

From the above brief overview of the phylogeny of reptiles, it is clear that the vast majority of large systematic groups (orders) became extinct 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 unique 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 during the Cretaceous there were significant changes climate and landscapes. 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 in changes in the physical state of the Earth and other conditions 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 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 general scheme phylogeny of reptiles.

Reptiles have their origins in the Paleozoic, when during the Carboniferous period they separated from the ancient stegocephalic amphibians. The diverse evolution of reptiles, resulting in a complex picture of adaptations to different conditions existence, lasted a very long time: G. F. Osborne (1930) is inclined to determine the duration of this process at 15-20 million years.

Rice. 1. Skull and lower jaw Therocephalia: Scylacosaurns sclateri ( A) and Cynognathus cratero-notus ( IN) from Perm ( A) and Triassic (IN) South Africa. The first of the early Therocephalia, the second of Cynodontia.

1-praemaxillare; 2-septomaxiliare; 3-maxillare; 4-nasale; 5 - frontal; 6-lacrymale; 7-adlacrymale; 8-postfrontal; 9-postorbitale; 10 -parietale; 11 - jugale; it-squamosum; 13-quadratum; 1 4-day tale; 15 -angu-lare; 16-supraangulare; 17-articulare; 18-Inferior temporal fossa.

Due to various conditions, some of which are only difficult to account for due to the plasticity of organization, environmental influences and a number of other reasons, reptiles have undergone a complex evolution in the history of their development. They took possession of a diverse environment: land, water, air, and in the development of some groups, as we will see later, some returns of adaptations to the living environment that was once dominant for this group were observed (for example, in sea turtles).

Due to the large number and diversity of individuals, the taxonomy of extinct reptiles presents significant difficulties and lacks unity. Thus, F. Broili, E. Koken and M. Schlosser (1911) count 10 orders of extinct and recent reptiles, M. V. Pavlova (1929) -13, G. F. Osborne (1930) - 18, Abel (1924) -20.

Rice. 2. Thaumatosaurus victor, plesioSaurus, 3.44 m long from the Upper TriassicYuzhnoah Germany.

First of all, it should be noted that the differences between these “orders” are so striking and significant [it is enough to point out, for example, the calico-skulls (Cotylosauria), the helmet-skulls (Pelyco-sauria) or the ichthyosaurs and plesiosaurs] that for the taxonomy of recent animals it becomes the obvious inevitability of a sharper taxonomic differentiation. Many of the mentioned orders, in our opinion, are more correct and natural to consider as subclasses. True, in some systems a unifying grouping into subclasses is adopted based on the structure of the temporal fossa and arches (Anapsida, Diapsida, Syn,apsida and Parapsida). However, many quite compelling objections can be raised against the rationality of such a division.

The temporal region of the skull during the evolution of one group, for example, in turtles, has undergone such significant changes that based on purely external morphological features (without taking into account the picture evolutionary process) some of the turtles (modern marine ones with a solid wall of the temporal region) should be classified as Anapsida, others - as Synapsida. In systematic divisions, we are based primarily on specific, existing morphological characters, and not on speculative data from an evolutionary process that has not yet been fully identified. Therefore, varying even in smallgroup, the structure of the temporal region cannot serve as a criterion for establishing subclasses, as M. V. Pavlova does (1929), but only as a control auxiliary feature for analyzing the development process of diverse branches of reptiles.

Review of some subclasses and phylogenetic relationships with other vertebrates. The most primitive group is the subclass of cauldrons (Cotylosauria), distinguished by a quilted skull, awkward rather high five-fingered limbs, and amphicoelous vertebrae. The first representatives of this subclass, undoubtedly related to stegocephalic amphibians, appear already in the Upper Carboniferous deposits, reach a special peak in the Permian deposits and end their existence in the Triassic.

The best known representatives of this subclass are the Pareiasauras, which were first known in a significant number of forms from slates and sandstones of the Permian Karoo Formation (in southern Africa). In relatively recent times, many skulls and skeletons of pareiasaurs were discovered by Prof. V. P. Amalitsky on the Northern Dvina. These were large, massive forms. For example, the length of the skeleton of R. karpinskii reaches 2 m 45 cm, the length of the skull of this animal is 48 cm. Peculiar appearance had a labidosaurus (Labidosaurus hamatus), a small (up to 70 cm long), short-tailed animal from the Permian deposits of Texas.

Rice. 3. Reconstruction of the skeleton of Eunnotosaurus africanus from the Permian layers (reduced).

Helmet-skull reptiles (Pelyeosauria)

Belonged to Varanops from the Lower Permian of Texas. It was an agile long-tailed animal. Osborne is inclined to consider it the prototype of the wholea number of further reptiles: alligators, lizards, dinosaurs. Some extremely specialized forms belong to the mentioned subclass, for example, Dimetrodon gigas from the Permian deposits of Texas, a predatory reptile in which the upper processes of the dorsal vertebrae were extremely elongated. Between these processes, a fold of skin was probably stretched, giving the animal a completely unusual appearance.

Subclass of reptiles (Theromorpha)

Divided into at least three orders (Fig. 1), it is especially interesting for the structure of heterodoite teeth, differentiated into groups, canines and molars. Further it can be noted; development of the coronoid process on the lower jaw, the presence of a double condyle in the occipital region of the skull for articulation with the vertebrae.

Rice. 4. Thalassemys marina shell (Upper Jurassic).

Some species of beast-like animals reached significant sizes, for exampleMer, Inostrancevia alexandri, up to 3 m long. Many remains of several species of Theromorpha were obtained by the expedition of Prof. V. P. Amalitsky on the Northern Dvina.

Based on the location of the skeletal remains, it can be assumed that they weretrekked along the edges of the ancient riverbeddisappeared river. In addition to the findings of Severodvinsk reptiles, the closest relatives of these reptiles were found in the Permian layersNorth America and in the Karoo strata of southern Africa. These data show that the ancient Permian animal-like fauna was relatively uniform.

Rice. 5. Shell and skeleton of Archelon chyros (Upper Cretaceous, North America).

An extremely specialized subclass consisted of ichthyosaurs (Ichthyosauri a) - marine animals with a bare fusiform body, a narrow elongated snout, and reduced hind limbs; their forelimbs turned into long flippers. On the back there are sharp fins similar to the fins of sharks; tail with a bilobed shark-type fin. The skull has one pair of temporal arches; the jaws have a large number of sharp cone-shaped teeth.

Ichthyosaurs, as the history of their development shows, descended from terrestrial forms; Subsequently, species that had adapted to pelagic life again returned to the state of coastal existence, and females laid eggs on the sands near the shallows. Then a secondary process of adaptation took place, and these animals, which arose from the Triassic, ended their existence in the Cretaceous era in the form of true inhabitants of the open sea, and they developed an important adaptive feature - viviparity. Thanks to their ability to swim for long periods of time, ichthyosaurs made huge migrations.tions. Osborne (1930) determines the length of such journeys from the shores of Spitsbergen to the Antarctic zone.

Rice. 6. DiploclocTis carnegii - diplodocus from the Upper Jurassic of North America

Plesiosaurs constituted a unique subclass of marine animals.(Piesiosauria; Fig. 2), living from the Triassic to the Upper Cretaceous. They were distinguished by pasty limbs and differently developed dental apparatus, adapted for gnawing hard mollusk shells. The skull is characterized by only one pair of temporal holes, and the spine has weakly amphicoelous, almost platycoelous vertebrae. The length of the neck varied: in many species (Elasmosaurus) the neck reached an enormous length and contained up to 76 vertebrae. The ratio of the length of the neck to the length of the body, which reached 3 m, was 23:9. In other forms, such as the Cretaceous Brachauchenius, the neck was shortened and contained only 13 vertebrae. Body sizes varied greatly. Along with relatively small animals 1.5 m long (Plesiosaurus macrocephalus), there were giants 13 m long (Elasmosaurus).

We now turn to a brief overview of the evolution of turtles (Chelonia). Some authors consider the ancestor of Triassic turtles Placodus gigas, region gave with flat teeth, relatively small on the jaws and especially wide and large on the palate. there was no occipital condyle in the placodus skull, and the processes of the occipital bones entered into the corresponding depressions of the first cervical vertebra. All these distinctive features make Placodus stand out completely.

Apparently, the original form for turtles can be considered Eunnotosaurus africanus (Fig. 3) from the Permian layers of the Cape Colony of Africa. This wonderful reptile has 8 middle thoracic ribs that are extremely expanded, adjacent to each other at their edges and forming, as it were, a bony shield. Eunnotos aurus also has teeth on the jaws and palate; this animal led a life similar to that of land turtles.

Already in the Triassic, cryptonecks arose. Their evolution is full of deep interest. Probably, in Jurassic times, a group separated from terrestrial turtles, first adapting to life in the coastal zone, and then gradually moving to the open sea. In this regard, these turtles have a simplified dorsal armor, which, in addition, has become lighter due to the development of marginal notches; the ventral carapace lost its integrity and received a significant fontanelle in the middle part (in Thalassemys marina from Upper Jurassic deposits; Fig. 4). This process of armor reduction was greatly advanced in undoubted open-sea forms, such as the North American Upper Cretaceous Archelonis (Figure 5). IN high degree interesno, that in early Tertiary times a branch separated from these pelagic formsinhabitants of the coastal zone. They have a shell again becomes more array nym and is composed of small polygonal plates. These coastal creatures changed the littoral station for the second time to pelagic, which in its the turn caused a secondary reduction of the shell. In modern leatherbacks, a descendant of secondary migrants, the reduced carapace contains derivatives of the primary and secondary leg skeletons. But in any case, the shell of turtles that have again switched to life in the open sea is built on a different principle than that of ancient pelagic species. In 1803, Louis Dollo formulated the law of the irreversibility of the evolutionary process. According to this law, any branch of animals, having taken a certain direction in its specialization, can in no case go back along the same path. In the described case, we have, as it were, a repetition of the evolutionary process. However, it should be especially emphasized that although adaptations in turtles to the pelagic environment secondarily caused a number of corresponding changes in the animal’s body, yet the picture of evolution morphological characteristics was different in this case and did not follow the old path.

Higher the antiquity of lizard-eaters was indicated(Rhynchocephalia). Here in addition To history of this subclass, it can be indicated that the most ancient representatives(Palaeohatteria longicaudata) are known from the Lower Permian layers near Dresden and that this subclass has survived to this day in the person of a single recent representative.

Rice. 7. Brontosaurus excelsus (Lower Cretaceous, North America)

The subclass of crocodiles has its roots in the Triassic. The primary forms of crocodiles (for example, Scleromochlus taylori) were small in size,long tail, shortened sharp muzzle. In terms of distribution, the extinct species were confined to freshwater bodies of water, although purely pelagic species were also found (Jurassic Teleosauridae and Geosauridae).

From the Triassic to the Upper Cretaceous, representatives of the subclass of dinosaurs (Dinosauria) lived - a heterogeneous group, divided into several orders. Characterized by the presence of two pairs of temporal arches. They varied in size and appearance; some representatives reached the size of a domestic cat,others are of enormous length, more than 20 m. Giants, like the Brontosaurus excelsus, Fig. 7 or Diplodocus carnegii, Fig. 6, both from the Upper Jurassic, were distinguished by the enormous length of their neck and tail, were herbivorous and moved slowly on four limbs. Other species, such as the Jurassic North American ceratosaurus (Ceratosaurus nasicornis) or the tyrannosaurus (Tugapposaurus rex), were true predators. Guanodonts, huge herbivorous reptiles that walked on their massive hind limbs, also made up a unique group. The skeleton of the huge Trachodon amu rensis was found near Blagoveshchensk (on the Amur) and restored by Prof. N. A. Ryabinin. Finishing short review of this subclass, we mention the stegosaurs, characterized by the presence of large bony plates and spines located along the back and tail.

Rice. 8. Pterodactylus spectabilis (Jurassic)

Dinosaurs, so abundantly represented, became extinct without a trace. The reasons for the death of this group are largely unclear. It is possible that the factors of processes of deep, excessive specialization and increased growth played a role here (C. Depere,1915), which led to the body’s loss of plasticity and adaptability to living conditions that continued to gradually change. It is possible that there was also vital competition with other more adapted organisms.

A completely unique subclass was made up of Jurassic and Cretaceous winged lizards (Pterosauria), which included two orders: rhamphorhynchus and pterodactyls (Fig. 8). These reptiles have reached extreme specialization in their forelimbs with a very elongated fifth finger and the presence of real flight membranes on narrow, long, sharp wings. The tail varied in length; in some forms it was reduced. The skull was elongated, sometimes beak-shaped; teeth of thecodont type or were completely absent. Some forms were distinguished by a huge wingspan (in Pteranodon, up to 7 m). The paleontological history of the modern most species-rich subclass of squamates (Squamata) is relatively poorly studied. A reliable ancestor This group can be considered Permian Araeoscelis gracilis. (For the relationship diagram, see Fig. 9).

Rice. 9. Scheme of the evolutionary development of reptiles and related relationships of various groups.

The main subclasses of extinct and modern reptiles

Subclass 1. Cauldron-skulls - Cotylosauria (Permian-Triassic).

2. Helmetocranial-Pelycosauria (Permian-Triassic).

"3. Animal-like Theromorpha (Permian-Triassic).

» 4. Ichthyosaurs - Ichthyosauria (Triassic-Cretaceous).

"5. Plesiosaurs-Plesiosauria (Triassic-Upper Cretaceous).

"6. Lamellar tooth Placodontia (Triassic).

"7. Lizard-eaters-Rhynchocephalia (from the Lower Permian to the modern).

"8. Turtles-Chelonia (from Permian and Triassic to modern)

"9. Crocodiles-Crocodilia (from Triassic to modern).

"10. Dinosaurs-Dinosauria (from Triassic to Upper Cretaceous).

"eleven. Winged lizards-Pterosauria (Jurassic).

"12. Squamate-Squamata (from Permian to modern).

Article on the topic Evolution of reptiles

). 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, 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 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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Literature

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

Excerpt characterizing the Origin of Reptiles

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 fantasy world 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 his 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, with my mother and grandmother in the garden, I read my favorite books and 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 our 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”...

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 ability to remain on land for longer and longer periods of time was determined by favorable conditions for 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 dryness of the air made it difficult for them to remain on the surface of the earth for a long time, since their pulmonary respiration was imperfect, and their 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 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 varied conditions of the 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 the Northern Dvina, also in the 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 the exceptional diversity of 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) appeared, one of the species of which, the hatteria (Sphenodon punctatus), has survived 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 led, apparently, wood image life. 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. During the Cretaceous period, modern crocodiles existed 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 the specialization of 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 wings 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. Some 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 body, 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.

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 throughout 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. The 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 Perm animal-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 back in the Triassic, but with the appearance of predatory dinosaurs they disappeared.

From the above review of the phylogeny of reptiles, it is clear that the vast majority of large systematic groups of them (orders) became extinct 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 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, in a relatively short period of time, 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 of the 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.