Origin of reptiles. Major subclasses of extinct and modern reptiles

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

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

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

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

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

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

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

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

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

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

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

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

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

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

The current crypto-cervical and side-necked ones retain their primary appearance to a greater extent. land turtles Triassic Marine and soft-skinned appeared in the late Mesozoic.

All other reptiles, both ancient and modern, acquired one or two temporal cavities in the structure of the skull. One, lower, temporal cavity had synapsid. One superior temporal cavity is noted in two groups: paranoid and euryantsid. And finally, two depressions had diapsid. The evolutionary fate of these groups is different. The first to depart from the ancestral trunk synapsids(Synapsida) - reptiles with lower temporal cavities, limited by the zygomatic, squamous and postorbital bones. Already in the Late Carboniferous, this group of the first amniotes became the most numerous. In the fossil record, they are represented by two consecutive orders: pelycosaurs(Pelicosauria) and therapsids(Therapsida). They are also called bestial(Theromorpha). The animal-like survived the period of their heyday long before the first dinosaurs appeared, the cotylosaurs were their immediate relatives. In particular, pelycosaurs(Pelicosauria) were still very close to cotilosaurs. Their remains have been found in North America and Europe. Outwardly, they looked like lizards and were small in size - 1-2 m, had biconcave vertebrae and well-preserved abdominal ribs. However, their teeth sat in the alveoli. In some, it was planned, albeit to a small extent, differentiation of teeth.

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

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

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

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

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

Table 6

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

The next group to separate from the anapsid cotylosaurs were diapsid(Diapsida). Their skull has two temporal cavities located above and below the postorbital bone. Diapsid at the end of the Paleozoic (Permian) gave an extremely wide adaptive radiation systematic groups and species that are found both among extinct forms and among current reptiles. Among the diapsid, two main groups (infra - classes) have been outlined: infraclass Lepidosauromorphs(Lepidosauromorpha) and infraclass archosauromorphs(Archosauromorpha).

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

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

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

Lizards, snakes and chameleons now make up a wide variety of the squamous order (Squamata). Lizards are one of the oldest advanced groups of reptiles, their remains are known from. upper perm. Scientists discover many similarities between lizards and sphenodons. Their limbs are widely spaced and the body moves, wavy curving the spinal column. It is curious that among the common features of their morphological similarity is the presence of an intertarsal joint. Snakes only appear in chalk. Chameleons are a specialized group of a later era - the Cenozoic (Paleocene, Miocene).

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

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

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

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

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

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

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

At the very beginning of their formation, the dinosaurs were divided into two branches, the development of which proceeded in parallel. characteristic feature they were the structure of the pelvic girdle, in connection with which these groups are called lizard and ornithischian.

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

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

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

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

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

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

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

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

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

It has been established that although the extinction individual groups reptiles occurred throughout the Mesozoic, this manifested itself at the end of the Cretaceous period. At this time, in a relatively short period of time, most of the Mesozoic reptiles died out. If it is fair to call the Mesozoic the age of the reptiles, then it is no less justified to call the end of this era the age of the great extinction. It should be taken into account that significant changes in climate and landscapes occurred during the Cretaceous. This coincided with significant redistributions of land and sea and movements earth's crust, which led to huge mountain-building phenomena, known in geology under the name of the Alpine stage of mountain building. It is believed that at that time a large cosmic body passed near the Earth. Violations of the existing living conditions in this regard were very significant. However, they are not only a change 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 are not excluded. Naturally, all this could not but affect the success of the existence of all animals and specialized ones in the first place.

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

Figure 120 is given general scheme phylogeny of reptiles.

), the forms, apparently possessing a greater terrestrial character, were isolated. Like their ancestors, they were still associated with wet biotopes and water bodies, fed on small aquatic and terrestrial invertebrates, but had greater mobility and somewhat big brain; perhaps they have already begun keratinization of the integument.

In the Middle Carboniferous, a new branch arises from similar forms - Seymouriomorph-Seymourioraorpha. Their remains have been 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 atlas and epistrophy. For land animals, this created important advantages in orientation, hunting for mobile 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 in chest. Stronger than those of stegocephalians, the limbs 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 water bodies; it is believed that they may have still had aquatic larvae.

Proganosaurs and synaptosaurus became extinct without descendants.

Thus, as a result of adaptive radiation, already at the end of the Permian - the beginning of the Triassic, a diverse fauna of reptiles (approximately 13-15 orders) was formed, displacing most groups of amphibians. The flowering of reptiles was ensured by a number of aromorphoses that affected all organ systems and ensured an increase in mobility, intensification of metabolism, greater resistance to a number of environmental factors (to dryness in the first place), some complication of behavior and better survival of offspring. The formation of the temporal pits was accompanied by an increase in the mass of chewing muscles, which, along with other transformations, made it possible to expand the range of feeds 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 entire Mesozoic era - for more than 150 million years - they dominated almost all terrestrial and many aquatic biotopes. At the same time, the composition of the fauna changed all the time: the ancient groups were dying out, being replaced by more specialized young forms.

Permian period

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

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

synapsids

The main ancestral group that gave all the diversity of modern and fossil reptiles was probably cotylosaurs, but the further development of reptiles went in different ways.

Diapsides

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

At the end of the Permian, squamates (Squamata) separated from primitive diapsids, which became numerous in

Origin of reptiles

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

Permian period

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

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

synapsids

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

Diapsides

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

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

Origin of archosaurs

see also

• Temporal arches

Notes

Literature

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

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

Rice. 1. Skull and lower jaw of Therocephalia: Scylacosaurns sclateri ( A) and Cynognathus crateronotus ( 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-nasal; 5-frontale; 6-lacrymale; 7-adlacrymale; 8-postfrontal; 9 postorbital; 10-parietale; 11-jugale; it-squamosum; 13-square; 1 4-den-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 the organization, the impact environment and a number of other reasons, reptiles have gone through 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 below, some returns of adaptations to that living environment, which was once dominant for a given group (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. So, F. Broili, E. Koken and M. Schlosser (1911) number 10 orders of extinct and recent reptiles, M. V. Pavlova (1929) -13, G. F. Osborn (1930) - 18, Abel (1924) -20.

Rice. 2. Thaumatosaurus victor, plesiosaurus, 3.44 m long from the Upper TriassicSouthNoah Germany.

First of all, it should be noted that the differences between these “orders” are so striking and significant [suffice it to point out, for example, the heifer cranial (Cotylosauria), the helmet cranial (Pelycosauria) or ichthyosaurs and plesiosaurs] that for the taxonomy of recent animals it becomes the obvious inevitability of a sharper taxonomic differentiation. Many of the above detachments, in our opinion, are more correct and natural to consider as subclasses. True, in some systems a unifying grouping into subclasses is accepted based on the structure of the temporal pits and arches (Anapsida, Diapsida, Syn,apsida and Parapsida). However, quite a few fairly weighty objections can be raised against the rationality of such a division.

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

An overview of some subclasses and phylogenetic relationships with other vertebrates. Most primitive group constitutes a subclass of cauldron-cranied (Cotylosauria), distinguished by a quilted skull, clumsy 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 flowering in the Permian deposits and end their existence in the Triassic.

Most well-known representatives of this subclass are the Pareiasauras, which in a significant number of forms were first known from the slates and sandstones of the Permian layers of the 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. They were large, massive forms. So, 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. appearance had a labidosaurus (Labidosaurus hamatus), a small (up to 70 cm long), short-tailed animal from the Permian deposits of Texas.

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

Helmet-headed reptiles (Pelyeosauria)

Belonged to Varanops from the Lower Permian deposits of Texas. It was a mobile long-tailed animal. Osborne is inclined to consider him the prototype of the wholea number of further reptiles: alligators, lizards, dinosaurs. Some extremely specialized forms belong to the subclass mentioned, 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 skin fold was probably stretched, giving the animal a completely unusual appearance.

Subclass of belief-like reptiles (Theromorpha)

Dividing into at least three orders (Fig. 1), it is especially interesting in terms of the structure of heterodoytic teeth differentiated into groups, canines and molars. Further it may 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 by the vertebrae.

Rice. 4. Armor of Thalassemys marina (Upper Jurassic).

Some animal-like species reached a significant size, for examplemeasures, 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.

According to the location of the remains of skeletons, it can be assumed that they are concentratedtraversed along the edges of the ancient channelvanished river. In addition to the finds of Severodvinsk reptiles, the closest relatives of these reptiles were found in the Permian layers.North America and in the Karoo layers South Africa. These data show that the ancient Permian animal-like fauna was relatively uniform.

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

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

Ichthyosaurs, as the history of their development shows, evolved from terrestrial forms; later on, the species, adapted to pelagic life, again returned to the station of a lowland existence, and the females laid their 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 Cretaceous as real inhabitants high seas, and they have developed an important adaptive feature - live birth. Thanks to the ability to swim for a long time, ichthyosaurs made huge migrations.tions. Osborn (1930) determines the length of such journeys from the coast of Svalbard to the Antarctic zone.

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

A peculiar subclass of marine animals corresponded to plesiosaurs(Piesiosauria; Fig. 2), who lived from the Triassic to the Upper Cretaceous. They were distinguished by pasty limbs, variously developed dentition, adapted to gnaw hard shells of mollusks. In the skull, only one pair of temporal holes is characteristic, in the spine, the presence of 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), giants 13 m long (Elasmosaurus) came across.

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

Apparently, Eunnotosaurus africanus (Fig. 3) from the Permian layers of the Cape colony of Africa can be considered the initial form for turtles. In this remarkable reptile, the 8 middle thoracic ribs are extremely widened, their edges adjoin each other and form, as it were, a bone shield. Eunnotos aurus also has teeth on the jaws and palate; this animal led a life similar to that of terrestrial turtles.

Already in the Triassic, cryptocervical ones arose. Their evolution is full of deep interest. Probably, in the Jurassic, a group separated from the terrestrial turtles, first adapted to life in the coastal zone, and then gradually moved to the open sea. In this regard, in these turtles, the dorsal armor became simpler, which, in addition, became lighter due to the development of marginal cuts; the ventral armor lost its integrity and received a significant fountain in the middle part (in Thalassemys marina from the Upper Jurassic deposits; Fig. 4). This process of armor reduction has advanced considerably in certain forms of the open sea, such as the North American Upper Cretaceous Archelonis (Fig. 5). In a high degreeresno, that in early Tertiary time, 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 dwellers changed the littoral station for the second time to pelagic, which in its turn caused a secondary reduction of the shell. In modern skin and pto ya, 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 on the high seas is built according to a different principle than that of the 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 case described, we have, as it were, a repetition of the evolutionary process. However, it should be emphasized that although adaptations in turtles to the pelagic environment for the second time caused a number of corresponding changes in the animal organism, yet the picture of evolution morphological features was in this case different and did not follow the old path.

Higher it was pointed to the antiquity of lizards(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. Primary forms of crocodiles (for example, Scleromochlus taylori) differed in small size,tail length, shortened with a sharp muzzle. In terms of distribution, the extinct ones were confined to freshwater bodies, although purely pelagic species (Jurassic Teleosauridae and Geosauridae) were also encountered.

From the Triassic to the Upper Cretaceous, representatives of the subclass of dinosaurs (Dinosauria) lived - a heterogeneous group that breaks up into several orders. They are 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 (Brontosaurus excelsus, Fig. 7) or diplodocus (Diplodocus carnegii, Fig. 6), both from the Upper Jurassic, were distinguished by an enormous neck and tail length, were herbivorous and moved slowly on four limbs. Other species, such as the Jurassic North American Ceratosaurus (Ceratosaurus nasicornis) or Tyrannosaurus Rex (Tugappo-saurus rex), were real predators. Guanodonts, huge herbivorous reptiles that walked on massive hind limbs, also made up a peculiar detachment. The skeleton of the huge Trachodon amurensis was found near Blagoveshchensk (on the Amur) and restored by prof. N. A. Ryabinin. Concluding our brief review of this subclass, let's mention the stegosaurs, which were characterized by the presence of large bony plates and spikes located along the back and tail.

Rice. 8. Pterodactylus spectabilis (Jurassic)

Dinosaurs, so abundantly represented, died out 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 growth growth played their role here (S. Depere,1915), which led to the loss of plasticity and adaptability to the life conditions that continued to gradually change. It is possible that there was also a vital competition with other more adapted organisms.

The Jurassic and Cretaceous winged lizards (Pterosauria), which comprised two orders: rhamphorhynchus and pterodactyls, constituted a completely peculiar subclass (Fig. 8). In these reptiles, forelimbs with a very elongated fifth finger and the presence of real flying membranes on narrow, long, sharp wings have reached extreme specialization. The tail varied in length; in some forms it was reduced. The skull was elongated, sometimes beak-shaped; teeth of thecodont type or 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 understood. Authentic ancestor this group can be considered the Permian Araeoscelis gracilis. (Relationship diagram see Fig. 9).

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

Major subclasses of extinct and modern reptiles

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

2. Helmet-Cranial-Pelycosauria (Permian-Triassic).

»3. Animal-Theromorpha (Permian-Triassic).

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

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

»6. Lamellar-toothed-Placodontia (Triassic).

»7. Lizards-Rhynchocephalia (from the Lower Permian to the present).

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

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

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

"eleven. Winged lizards - Pterosauria (Jurassic).

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

Article on the topic of reptile evolution