Sauropsida

The sauropsids (Sauropsida or also Reptilia sensu stricto) are a clade of amniotic vertebrates that belong most of the reptiles and all the birds. They have in common the possession of keratin epidermal scales and an amniotic egg virtually identical in all of them. They were very diverse in the Mesozoic, the time when dinosaurs, pterosaurs and ichthyosaurs arose. Today there are about 9,000 species of reptiles and almost 10,000 of birds.

The first to propose this relationship was Thomas Huxley in 1864, at the same time as proposing his theory that birds evolved from dinosaurs which is currently accepted among specialists.

Definition of reptile

In the early 21st century, when cladistics was beginning to be adopted, in which all groups are defined in such a way as to be monophyletic; that is, groups that include all the descendants of a particular ancestor. Reptiles, as historically defined, are paraphyletic, excluding both birds and mammals. These evolved respectively from dinosaurs and early therapsids, which were traditionally called mammalian reptiles. Birds are more closely related to crocodiles than crocodiles are to all other extant reptiles. Colin Tudge wrote:

The mammals are a clay and therefore the clods are happy to recognize the traditional taxon Mammalia; and the birds are also a clay, universally attached to the formal Aves taxon. Mammalia and Aves are, in fact, subclassed into the great clay of Amniota. But the traditional class Reptilia is not a nail. It is only a section of the Amniota clade: the section that remains after Mammalia and Aves have separated. It cannot be defined by synapomorphous, as is the correct form. Instead, it is defined by a combination of the characteristics it has and the characteristics that it lacks: reptiles are the amniotas of scales that lack hair or feathers. At best, the classists suggest, we could say that traditional reptiles are "non-aviary and mammal aniotas".

Despite early proposals to replace the paraphyletic Reptilia with a monophyletic Sauropsida, which includes birds, that term was never widely adopted or, when it was, not applied consistently.

When Sauropsida was used, it often had the same content or even the same definition as Reptilia. In 1988, Jacques Gauthier proposed a cladistic definition of Reptilia as a node-based monophyletic crown group containing turtles, lizards, snakes, crocodiles, and birds, their common ancestor and all their descendants. While Gauthier's definition was close to the modern consensus, it was nevertheless considered inadequate because the actual relationship of tortoises to other reptiles was not yet well understood at the time. Major revisions since then have included the reassignment of synapsids as non-reptilian and the classification of turtles as diapsids.

Other scientists proposed a variety of other definitions in the years following Gauthier's paper. The first such new definition, which attempted to adhere to the PhyloCode standards, was published by Modesto and Anderson in 2004. Modesto and Anderson reviewed the many earlier definitions and proposed a modified definition, which was intended to retain the more traditional content of the group while keeping it stable and monophyletic. They defined Reptilia as all amniotes closer to Lacerta agilis and Crocodylus niloticus than to Homo sapiens. This stem-based definition is equivalent to the definition of Sauropsida, which Modesto and Anderson synonymized as Reptilia, since the latter is the best known and most frequently used. However, unlike most previous definitions of Reptilia, Modesto and Anderson's definition includes birds, as they are in the clade that includes lizards and crocodiles.

Evolution

Origins

Sauropsids are one of the two major evolutionary branches of amniotes (the other major branch is the synapsid branch, which leads to mammals). They originated from primitive tetrapods in the Carboniferous period, diversifying during later periods.

Carboniferous

Hylonomus.

Sauropsids appeared in the early Late Carboniferous. They were descended from advanced reptiliomorphs, although the direct ancestry of these animals is not yet known. These reptiliomorphs evolved to originate the first amniotes, characterized by laying eggs on dry land, since thanks to their envelopes they could withstand the lack of water in the environment. The first amniotes soon separated into two evolutionary lineages: the synapsids (which later gave rise to mammals) and the sauropsids. Both groups differ in the number and arrangement of the fenestrae or temporal openings in the skull; synapsids had one and sauropsids two (diapsids) or none (anapsids).

Of the earliest sauropsids, only a few genera are known: Protothyris, Paleothyris, Cephalerpeton, Hylonomus and Petrolacosaurus. The most advanced was Petrolacosaurus, because it was one of the first diapsids, a group of sauropsids characterized, as has been said, by having two fenestrae in the skull.

Scutosaurus.

Permian

During the Permian, the mainland was dominated by synapsids. At the beginning of this period, seven out of ten land animals belonged to this group. Several species of anapsids stood out in this period, such as the pareiasaurs (large armored animals, such as Scutosaurus and Pareiasaurus), the mesosaurs or the milleretids, such as Milleretta i>. These sauropsids were not diapsids, but rather belonged to the less advanced group of anapsids. Among the Permian diapsids were Coelurosauravus, Hovasaurus or Youngina. An important group of diapsids appeared in the Permian, the archosauromorphs, a group that includes crocodiles, pterosaurs, and dinosaurs. However, in the Permian they did not have a relevant role.

Cymbospondylus.

Triassic

The Permian-Triassic mass extinction, which occurred at the end of the Permian, wiped out nearly all life on Earth. The few survivors quickly diversified. On land, synapsids continued to dominate, but archosaurs quickly took on a larger role. Among the diapsids appeared the choristoderos and the lepidosaurs; this group includes the squamous (lizards and snakes) and the sphenodonts (tuatara) that appeared in the mid-Triassic. Archosaurs diversified into several forms. Among the primitive archosaurs, Tanystropheus, rhynchosaurs and thecodonts, such as Euparkeria, ancestors of advanced archosaurs, stood out. Among the latter were the first crocodile tarsals (a group to which crocodiles belong, of which there was a great variety in the Triassic, and the first dinosaurs (ornithodirans) that appeared at the end of the Triassic. In the water, marine reptiles (ichthyosaurs and sauropterygians) and turtles made their first appearance.Finally, in the air, pterosaurs emerged, of uncertain origin.

Dimorphodon.

Jurassic

At the end of the Triassic there was a mass extinction that wiped out almost all crocodile tarsals, all early archosaurs, placodonts, and nothosaurs. On land, dinosaurs ruled. The Jurassic is also known as the golden age of dinosaurs, as they reached their greatest diversity and dimensions, with some exceeding 30 meters in length. The dinosaurs were divided into most of their suborders at the end of the Jurassic. Among the lizards appeared the infraorders that exist today. In the water, plesiosaurs reached large dimensions and began to replace ichthyosaurs, which was in decline. Crocodiles and turtles continued to be mostly freshwater. In the air, the pterosaurs dominated, although their hegemony in this medium would not prevail for long. At the end of the Jurassic, birds appeared, the only surviving group of dinosaurs today.

Velociraptor.

Cretaceous

In the Cretaceous, the dinosaurs continued their hegemony. New forms appeared that replaced the previous ones. Lizards evolved to give rise to snakes and amphisbenians. In the sea, few ichthyosaurs survived, and plesiosaurs were in steep decline. In their place appeared sea turtles and mosasaurs, a group of giant marine lizards. Some birds have adapted to an aquatic way of life. Late in the period, gigantic pterosaurs, the largest animals capable of flight, appeared in the air, although small forms were very rare at this time. The birds developed new flight techniques and diversified by having fewer competitors in the air. At the end of the Cretaceous, a mass extinction wiped out non-avian dinosaurs, pterosaurs, plesiosaurs, mosasaurs, and early birds.

Megalania.

Cenozoic

In the Cenozoic, after the extinction of the dinosaurs, the dominators of the planet were the mammals; the saurops were not as successful and were relegated to the background. The group of snakes evolved the most, although lizards, crocodiles and turtles also did. The tuataras, the last rhynchocephalians, did not diversify much. The birds, on the other hand, were successful, so much so that today there are almost 11,000 species.

Taxonomy

For years the term reptiles (Reptilia) was used to classify most of the members of the group. The classical concept of reptiles included three major lineages:

• Reptilia = Synapsida+Anapsida+Diapsida

But according to cladistic systematics, reptiles, in the classical sense, are a paraphyletic taxon because it does not include their descendants the birds (included in Diapsida) and Mammals (included in Synapsida). Therefore, according to this point of view, it is not valid. Furthermore, according to cladistics, Synapsida does not belong to the reptiles, which are therefore limited to Anapsida and Diapsida; to avoid confusion, the name Sauropsida is usually used, with the following meaning:

• Sauropsida = Anapsida+Diapsida

Therefore, Reptilia and Sauropsida are not synonymous, since they include different groups; sometimes the term Reptilia is used in the strict sense (Anapsida+Diapsida) and in this case both names are synonymous.

Now recent fossil analysis has suggested that Anapsida is a paraphyletic taxon made up of all reptiles that lack temporal fossa. According to these analyzes, the cotylosaurs or captorhinids are related to the origin of the diapsids and the turtles are related to the archosaurs. For this reason, Sauropsida is divided into two clades in a monophyletic way, Parareptilia (which contains most of the anapsids). and Eureptilia (containing diapsids, cotylosaurs, and turtles).

Phylogeny

Diagram showing the relationship between the cadistic and traditional classification of the Reptilia class. Reptiles are, in a classic sense, a paraphylactic taxon. Here it is excluded from the natural group Sauropsida to birds and includes non-mammal synapsies.

According to Tree of Life, the phylogenetic relationships of the aforementioned groups are as follows:

As can be seen, synapsids (which include modern mammals and a large number of fossil forms related to them known as mammalian "reptiles"), are not considered sauropsids (reptiles).

Regarding the internal phylogeny of saurops, if only current forms are taken into account, based on genetic analyzes (including protein sequences obtained from Tyrannosaurus rex and Brachylophosaurus canadensis) is as follows:

More detail with all extinct forms is as follows:

Testudines position

The grouping of turtles has historically been highly variable. Classically, turtles were considered related to the primitive reptiles of the Anapsida or Parareptilia, however several recent genetic studies and fossil evidence have placed turtles within the diapsids. All genetic studies have supported the diapsid position of tortoises generally as a sister group to archosaurs (crocodiles, birds, and relatives), while only one has been able to place tortoises as a sister group to lepidosaurs (lizards)., snakes and tuataras) leaving this possibility open.

Recent discoveries of Pappochelys, Eorhynchochelys and the discovery of Eunotosaurus specimens with diapsid skulls have confirmed that the Turtles are descendants of diapsid reptiles that lost their temporal fossae through shell development, and that the similarities with anapsids are a case of convergent evolution. Also some anapsids such as milleretids among others have temporal fossae and some diapsids such as sauropterygians or ichthyosaurs lost a temporal fossa, this indicates that the different cranial configurations are not a well-defined ancestral characteristic. Studies using a combination of genetic, morphological, and fossil data suggest that Sauropterygians and Sinosaurosphargis are extinct close relatives of turtles forming with them the Pantestudines clade. Morphological similarities between turtles and sauropterygians had been proposed long ago but not accepted at the time. Most genetic studies suggest including this clade in Archosauromorpha (Archelosauria Hypothesis) since the relationship between turtles and archosaurs has been proven. using various molecular methods such as ultraconserved elements, DNA sequence, mitochondrial DNA, microRNA and proteins. Others propose to classify this clade in Lepidosauromorpha (Ankylopoda Hypothesis) because turtles and sauropterygians share more morphological similarities with lepidosaurs than with archosaurs, in addition to being supported by a genetic analysis. However, none of these hypotheses is considered fully accepted and we will have to wait for the next studies to be able to define the position of the turtles well.

Anatomy and morphology

Unlike amphibians, sauropsids have keratinized (hard and dry) skin, usually covered in scales (birds also have feathers), and their eggs have amnion and a nearly impermeable shell. These characteristics allow them to live far from water and in some of the driest habitats in the world.

Skeleton

There are vast differences between the skeletons of all groups of sauropsids. They normally have a basic structure: the skull is attached to the vertebral column, the ribs and limbs are attached to the vertebral column in the trunk area, and the caudal vertebrae are located behind the pelvis. This basic body structure is altered depending on the groups of sauropsids. Crocodiles, lizards, and tuataras have a structure similar to that of the first saurops (head, trunk, limbs, and tail), while snakes and amphisbenians (as well as other extinct saurops) have lost their limbs as a result of adaptation to life. underground and in water. Turtles, placodonts, and other saurops have a defensive shell, some gliding saurops have extendable ribs, pterosaurs elongated the fourth toe on their forelimbs, dinosaurs' feet were vertical to the ground, and birds' tails have been shortened. forming the pygostyle.

Skull

Scheme of a traditional skull of an anathopedic.

Scheme of a traditional diapsy skull.

Scheme of an euriapsy skull now considered part of Diapsida.

The skull was a differentiating element between groups of amniotes, but is now no longer considered a well-defined ancestral feature because the different configurations have appeared by convergence in unrelated groups of amniotes. The subclass Anapsida is traditionally defined as amniotes without temporal fossae, although some anapsids such as mileretids and others have developed the synapsid configuration by evolutionary convergence but are not considered part of the Synapsida class, turtles were also considered anapsids but cladistic studies recent ones contradict their inclusion. The Diapsida subclass is mainly characterized by presenting two temporal fossae, although some aquatic diapsids such as ichthyosaurs and sauropterygians have lost an opening behind the skull developing the polyphyletic Euryapsid configuration, while turtles considered genetically diapsids lost both temporal fossa.

The sauropsid jaw is made up of several bones. In some groups, the jaw has gained flexibility and efficiency. An example of this case is the jaw of snakes. Other saurops, such as crocodiles, have a stronger jaw. In birds, the mandible has been transformed into the lower part of the beak.

Skin

Pez de arena (Scincus scincus). This lizard has a very smooth skin that helps you to move through the sand of the deserts in which you live.

The skin of sauropsids has several functions. It acts as a barrier and defense, and may play a role in concealment, mating, and locomotion. The epidermis is made up of keratin. The scales are made of a hard, thick layer of this material, often folded back to overlap each other. These scales are not like those of fish, but instead form a continuous sheet of skin. Scales vary greatly by species and group: lizards and snakes are usually smooth, with a few exceptions. Many other saurops, including extinct ones, have similar skin. Others, like crocodiles and turtles, have a hard shell of scales. Birds have transformed the scales into feathers, for the most part adapted to flight or swimming. The epidermis normally sheds from time to time, the best known case of which is the shedding of snakes.

The dermis does not participate in molting, although it does participate in the structure of the body. Together with the epidermis, it helps form the shell of turtles, and gives flexibility to the skin of legless lizards. Pigmentation cells are found in the dermis. Saurops skin has relatively few glands. Some examples are the musk glands of musk turtles and the venom glands of poisonous snakes.

Senses

Scheme of Jacobson's organ.

Geconid eye, with the pupil adapted to the dark.

The sense of sight is, without a doubt, the most important among those of the sauropsids. They use sight to find food and a partner, to know their territory, defend themselves or flee. Proof of the importance of sight is the coloration of many species: mottled or striped, contrasting coloration, and bright colors in feathers and scales. Saurops embryos possess a small third eye, or parietal eye, which is only preserved to adulthood in tuataras. This organ is under the skin, and is very sensitive to light. That of the tuataras has a retina and lens. Although other sauropsids have a parietal eye, it is not as developed as in tuataras, and it does not exist in birds. Snakes have the most developed sense of sight, as they are able to detect infrared radiation through receptors located near the nostrils. Many birds have exceptional eyesight, and are able to spot food or mates from a considerable distance. Crocodiles and other aquatic saurops have an extra eyelid that allows them to see underwater.

The sense of smell is also developed, especially in lizards and snakes. The chemical receptors are on the tongue, and they need to stick it out to detect odors. Jacobson's organ, located behind the mouth, detects substances picked up by the tongue. These sauropsids use it to detect prey. In this case, the nostrils are only used for breathing. Birds do not have this sense highly developed, except for some, such as New Zealand kiwis.

Although sauropsids have hearing, it is not highly developed. Lizards have a small auditory opening. Snakes don't have ears. Turtles have a middle and inner ear. Crocodiles use this sense more, and some communicate through sounds. Birds have it well developed, with a middle and inner ear, but they lack ears, so they have to turn their heads to hear the origin of the vibrations. Some, like the guácharos, use echolocation to guide themselves in the dark.

Touch is not very useful in most sauropsids, due to the presence of scales. Birds have Herbst's corpuscles on their tongues, which help them sense pressure changes.

Circulatory system

Scheme of the circulatory system of the sauropsides.

Most groups of living saurops have a heart with three chambers: two atria and one ventricle. The degree of mixing of oxygenated and deoxygenated blood in the ventricle depends on the species. This circulatory system consists of two arteries. However, some squamous can divide the ventricle slightly during diastole and completely in systole, thus acting like a four-chambered heart.

Crocodiles and birds have a four-chambered heart. Although crocodiles have this type of heart, their circulatory system consists of two arteries. Birds, on the other hand, have a circulatory system more similar to that of mammals.

Digestive system

Water snake (Nerodia sipedonfeeding on a siluriform fish.

The digestive system of saurops is very similar to that of synapsids, but it differs in how it works. No sauropsids chew food, so in many species the teeth have disappeared. The beaks and teeth of turtles and crocodiles are used to hold prey and/or tear food. When swallowing food in one piece, digestion is usually slower, except in the case of birds, which have a crop to soften the food. Many herbivorous saurops (current and extinct, such as pareiasaurs or sauropods) have a gizzard and swallow stones to crumble food in it. Crocodiles also swallow stones, but not for the same purpose. As for digestion, many large carnivorous sauropsids, not having an active metabolism, take a long time to digest food. This factor, together with the size of the prey, makes many carnivorous sauropsids capable of fasting for a long time.

Many saurops do not have powerful muscles in their necks and esophagus, so when drinking they must tilt their heads back to allow the liquid to flow down under gravity. Some birds, such as those of the order Columbiformes, have powerful muscles in the esophagus for drinking like mammals. Seabirds have special glands in their eyes to drink seawater.

Respiratory system

Scheme of the respiratory apparatus of the birds.

All sauropsids have pulmonary respiration. However, there are some variations regarding the different groups. Turtles cannot enlarge or contract their ribcage through the shell, so contraction and expansion of the lungs is accomplished by movement of abdominal muscles that function as a diaphragm, and they breathe by pumping movements of the pharynx. In sea turtles the breathing is cloacal. In snakes, the right lung is more elongated and voluminous and, therefore, fulfills a more important function than the left, which is atrophied or may even be absent. Some lizards and crocodiles have a primitive diaphragm. Birds ventilate their lungs through air sacs, and fresh air is obtained on both inhalation and exhalation. Instead of alveoli and diaphragm, they have small passages known as parabronchi, and the entire body functions as a bellows. They also have the syrinx in the trachea, which produces the sound of birds. Many aquatic saurops have a bony second palate, which allows them to breathe partially submerged, even with a mouth full of water.

Nervous system

The nervous system of saurops has the same basic structure as that of other amniotes. The brain and cerebellum are larger than those of amphibians, and those of birds are proportionally large. Saurops have 12 pairs of cranial nerves. In birds, the brain controls the movement of flight, and the cerebellum coordinates various body movements. The cerebral hemispheres control mating and behavior patterns.

In terms of intelligence, sauropsids have it highly developed. Komodo dragons are known to be capable of play, many monitor lizards can cooperate with each other, and crocodiles create complex social systems. Birds have highly developed intelligence. Parrots and crows are capable of developing complex problems. Many birds use tools and can learn. They are able to count, learn concepts and create complex societies. Their language is very complex, and some are capable of imitating the human voice, associating words with objects and concepts.

Excretory system

Sauropsids have a pair of kidneys, which extract nitrogenous waste. Sauropses expel uric acid to a large extent through the cloaca. These wastes are expelled together with the excrement of the digestive system. Although many sauropsids have a bladder and can hold liquids in it, birds do not. In many groups, uric acid is highly concentrated. This is due to the absence of the loop of Henle in their kidneys (although birds have it).

Many marine sauropsids have glands that help them extract sea salt accumulated in their bodies, such as the lacrimal glands of sea turtles. Birds are capable of sweating to control body temperature, although to a lesser extent than mammals, due to the complexity of their respiratory system.

Biology and behavior

Reptiles communicate in a variety of ways: sometimes communication is visual, as in many lizards; snakes communicate chemically through pheromones. Crocodiles and some lizards make sounds, such as bellows, grunts, and calls. Birds communicate through sounds such as squawks, clucks, and whistles. The communicative signals inform about the species, the sex and the reproductive capacity of an individual.

In terms of territoriality, the techniques to defend the territory are varied. Many male lizards defend their territory with rituals and displays, and agamids and iguanids enhance their body coloration. Rattlesnakes and vipers fight for the right to reproduce. The males of some turtles beat their shells to have reproductive priority. Some aquatic turtles defend their territory through aggressive behavior. Some geckos attack and devour the tail of their adversaries in fights over territory.

Courtship

Sauropsids have various courtship techniques. In snakes, the males often crawl above the female to get the mate to face the same direction. Some male snakes pin females by the neck with their jaws. Some lizards use a similar pattern, adding the tips of their tails in the mating position. Turtles Graptemis simonyi vibrate the sides of the face with the claws of their forelimbs. Tortoises of the genus Gopherus circle around their mate and beat their shell in order to mount them. In birds, courtship is more developed, and some species have plumes of feathers, unfolding tails or crests, and brightly colored limbs. Sometimes males fight for the right to mate and engage in exhibition fights.

Python molurus taking care of their eggs.

Parental care

Female sauropsids usually lay their eggs in nests dug in sand, humus, or in burrows. Some lizards and snakes retain embryos in oviducts and give birth to live young. This is not viviparism, but ovoviviparism. Parental care is rare in this group, but there are exceptions. Female North American skinks protect and moisten their eggs, and clean their newborn pups with their tongues. Indian pythons and cobras guard their eggs until they hatch. Rattlesnakes care for their young for the first week of life (or longer). Better known are the parental care of crocodiles. Both males and females of some species defend the nest from oophagous animals. When the young are born, they grunt high-pitched, and the female picks them up, carrying them to the water. The young are protected by their mother for the first two months. American alligator hatchlings remain near the nest for about 1-2 years. As the father defends the territory and kills any young male in his area, female crocodiles and alligators often defend their offspring from their father. Birds are an exception in this group. The vast majority provide parental care to their offspring. In many species both parents care for their young, while in some cases the young are cared for by a single parent, but in some species other members of a group may care for the young. The length of care varies greatly: talegals abandon their young upon hatching, while many seabirds care for their young for as long as a year and a half.

Chlamydosaurus kingii showing off her gorguera.

Defense

Sauropsids have various methods of defense. The bite is the most used method. Venomous snakes and lizards have developed this method, with the production of venom. Rattlesnakes move the rear end of their body to make sounds so they can alert their enemies. Other snakes blend in with their surroundings or pretend to be a species that they are not. Some snakes, like the collared snake, play dead. Lizards have more diverse methods of defense. Many blend in with the environment. Some, like American anoles, change color to blend in with their surroundings. Others surprise the enemy, like the Australian chlamydosaurus, which displays a ruff to scare its adversary, or the blue-tongued skinks, which show their tongues. Other lizards have hard or spiny scales, such as the moloc or armadillo lizard, which prevent them from being eaten. The basilisk has smooth scales that make it difficult to catch, and it escapes by running across the water. Monitor lizards and iguanas strike with their tails, and other lizards sacrifice their tails. The latter have bright colors on the tail, so that enemies attack it instead of the head. Horned lizards shoot spurts of foul-tasting blood from their eyes. Turtles have a shell to defend themselves. Many are able to lock themselves in it. Some turtles, like the wedge turtle, get into crevices and then inflate their shell so they can't get out of their place. Many birds have spurs on their feet or pointed beaks for defense, but they can also spread their wings to appear larger.

Chelonia mydasOne of the few sauropsides that migrate.

Migration

Sauropsids today don't usually migrate, but there are exceptions. The case of sea turtles is the best known. They travel thousands of kilometers from their feeding area to the nesting area. Some green turtles travel from the Brazilian coast to Ascension Island, 5,000 kilometers in the Atlantic, and leatherbacks go almost as far as the Arctic Ocean. Less spectacular are the migrations of prairie rattlesnakes, which travel up to 15 kilometers from their hibernation burrow in spring and return in autumn. There are marine crocodiles that travel around northern Australia and neighboring islands. Marine iguanas migrate hundreds of kilometers from feeding grounds to nesting grounds. Many species of birds migrate, usually to seek a warmer climate (as is the case with many European birds that migrate to Africa in winter or North American birds that migrate to South America). However, there are other species that migrate for breeding reasons. Some lead a nomadic lifestyle.

Henry, the longest tuatar (111 years), which can still be reproduced.

Longevity

The age of sauropsids, as in all animal groups, is varied. Some small lizards don't live much longer than a year, but boas and large pythons reach 30 years, Komodo dragons reach 40 years, and tuataras can live as long as a century. Tortoises are famous for their longevity, with some having lived for almost 2 centuries. Birds also vary greatly in longevity, with highs of 70 or 80 years for crows and parrots. Longevity depends on two factors: size (usually the larger the animal, the longer it lives) and metabolism (the less active the animal, the longer it lives). Specimens in captivity normally reach more age than the wild ones. Some long-lived saurops are famous for their age. This is the case of Henry, the longest-lived tuatara in the world (currently in a museum in Invercargill, New Zealand), Lonesome George (the last specimen of his subspecies, with a bleak future), the deceased turtle Harriet (who arrived at 175 years) and the longest-lived tortoise of all time, Tu'i Malila, who died at 192 in 1965.

Habitats and adaptations

Testudo hermanniA terrestrial sauropsy.

Terrestrial Saurops

Sauropsids adapted to a terrestrial environment have several adaptations. Lizards generally have long legs and well-developed hind toes and claws for pushing off the ground. Some can get up on two legs and run faster; they also have a long tail that serves as a counterweight when running. Snakes have other types of adaptations. They can move by bending the body and pushing it backwards. The scales are arranged so that the snake does not roll backwards. Other snakes move through sand or mud with a sinuous lateral movement, which consists of using a point of contact with the ground as leverage, then moving away from the ground to secure a new point of contact. Many terrestrial snakes are characterized by being long and narrow, with long tails, to move quickly. Tortoises have domed or stellate carapaces and thick feet with strong claws for better movement or digging. As for birds, the adaptations are similar between them. Some birds have lost the ability to fly (because they are very large or live without predators. Others, on the other hand, keep their wings, but they usually walk and run. These birds are often heavy, such as the peacock and the bustard.

Morelia viridisAn arboreal snake.

Arboreal Saurops

Like terrestrial reptiles, arboreal saurops have adaptations to their way of life. Lizards, for example, have sharp claws to cling to tree branches. Geckos cling to surfaces using padded blades on the soles of their feet. Chameleons have opposable fingers and a prehensile tail. Even without legs, snakes climb easily. There are some that even climb trees without bending their bodies, using the cracks in the bark as gripping points. Other tree snakes have a triangular cross section, giving the body more rigidity when extended without supports on the climb. Arboreal sauropsids tend to have their eyes forward to appreciate distances well. Other saurops with these habits can glide, such as flying geckos, lizards of the Draco genus, or gliding snakes. However, these animals do not really fly. Arboreal birds are characterized by the shape of their legs and their structure, adapted to firmly cling to branches, as is the case with toucans and parrots.

Up Crocodylus acutus and down Crocodylus intermedius Two aquatic neuropsy.

Aquatic Saurops

Many saurops can swim, but they are only considered aquatic if they spend much of their lives in the water. Many of these saurops are oviparous and have to go ashore to spawn, but some sea snakes are ovoviviparous and do not go to dry land. Another need of these animals is to breathe, but some have developed adaptations to stay underwater for long periods. Sea snakes have valves that close their nostrils, a tight mouth, and permeable skin that absorbs oxygen. Sea turtles can stay underwater for many hours. Semi-aquatic crocodiles and lizards have webbed feet and move sinuously. Sea snakes, crocodiles, and semi-aquatic lizards often have paddle-shaped tails that help them move. Sea turtles have paddle-shaped front legs to propel themselves in the water. Since the kidneys of sauropsids do not tolerate salt, marine sauropsids have salt-excreting glands. Sea turtles, for example, have modified tear glands to expel brine, and marine iguanas sneeze and expel salt. To adapt their senses to life in the water, crocodiles and aquatic snakes have high eyes and nostrils to stay immersed so they can see and breathe. Seabirds have many adaptations to this environment: some have waterproof plumage, others have lost the ability to fly in order to swim better, others have pouches in their beaks to catch fish, and many have cryptic coloration. They also have webbed toes. Freshwater birds also have this latter adaptation, in addition to typically long necks so they can catch their prey without submerging their bodies.

Blanus cinereusA typical underground amphisbene.

Subterranean Saurops

Since the head is always in contact with the ground and due to the lack of light, the eyes of subterranean sauropsids are atrophied and generally rudimentary. Likewise, most saurops that actually live underground lack legs, since although they can be useful for digging, they produce a lot of friction and take up space. The skull bones of many subterranean lizards and snakes are compactly fused and serve as a battering ram. Snakes in the Typhlopidae family have a sharp point on their tail that serves as an anchor when they claw their way across the ground with their smooth body. Amphisbaenians, which are almost entirely subterranean, have four types of burrowing heads: rounded, spade, keeled, and chisel heads.

Gallotia rooti, a kind of gender Gallotia, completely insular.

Insular Saurops

Insular sauropsids are considered to be those sauropsids descended from a continental or insular species that arrived on an island by means of a floating object. The most representative insular sauropsids are the lizards. Many live in and under driftwood that is washed up by the sea and carried to distant shores. Some geckos lay salt-tolerant eggs, are sticky after laying, and adhere to surfaces once dry. The main problem for a sauropsid arriving on an island is the absence of a second member of its species. Many times an individual arrives via log or raft, but this is a rare phenomenon and in almost all cases a second member of the species does not arrive while the main individual is still alive. However, some geckos are parthenogenetic, and males are not required for reproduction. Some parthenogenetic lizards of the genus Cnemidophorus need to be courted by other females in order to ovulate. Birds have less difficulty reaching the islands because of their ability to fly, although some, by the time they settle on the islands, lose the ability to fly. The insular sauropsids are characterized by being dwarf or giant: the jaraguas, the Cuban dwarf boa or the bee hummingbird present cases of dwarfism, while the Komodo dragon, the giant Galapagos tortoise, the giant Solomon skink, the lizard Iron giant or the extinct moa are giants by the standards of their group.

Apus apus, a bird very adapted to the flight.

Flying Saurops

Actually, the only saurops that can fly today are birds. The extinct pterosaurs also possessed the ability to fly. Although others are capable of gliding through the trees ("flying" geckos, gliding snakes), they are not considered to be volantes. Birds, like pterosaurs, have evolved many adaptations to flight. One of the most important is the modification of the skeleton. Both groups have hollow bones that carry less weight to lift when flying. Birds have also adapted their skeleton to flight by fusing together many of their bones. Thermal insulation is another adaptation. Flight is an action that requires a lot of energy, and hair and feathers help to conserve it. In addition, the feathers of birds have been diversified for various functions: while the down and the tectric feathers fulfill an insulating function, the shirts and the rudders help to move in the air. Another characteristic is the adaptation of the morphology of the bodies to gain speed and move better in this environment. Some predatory birds, such as hawks, need to be aerodynamic in order to gain airspeed and better catch their prey. Some birds have adapted so much to flight that their legs are atrophied, as in the case of swifts. Pterosaurs, on the other hand, developed a unique feature: the fourth toe of their forelimbs was elongated to develop a membrane between it and the hindlimbs in order to stay in the air.

Conservation

Spix's macaw, a parrot whose future is uncertain.

Sauropsids (with or without birds) are one of the most endangered groups of all vertebrates today. The causes for which species disappear are the same as in other groups: habitat fragmentation, poaching, excessive exploitation, pollution, predation by alien species, etc. Many turtles are killed by being run over or captured for use in cooking and traditional medicine. They also disappear due to hunting, the pet trade, and habitat fragmentation. Sea turtles are hunted for their meat and shell, and die from ingesting plastic bags. As for lizards, the most threatened are the insular species: Komodo dragons, iguanas from the Caribbean and Fiji, skinks and giant geckos..., and their greatest threat is, apart from habitat fragmentation, alien species. As for snakes, they are also threatened by the trade in their skin. Hunting is the biggest threat to crocodiles and alligators. The main problem with tuataras is the number of introduced alien species. As for birds, many die from hunting, introduced species, illegal trade, pollution, etc.

The Lone George was the last specimen of his species (Chelonoidis abingdonii). After dying in June 2012, the species became extinct.

However, all sauropsids except birds have little protection. The problem is in the popular image of this group. Ecological organizations do not adopt emblematic species, and many times species in this group are protected to save others. The fear that these animals instill in many people has a negative impact on their conservation. Birds are a different case and are given more protection.

According to the IUCN, in 2009 28% of non-bird sauropsids and 12% of birds are in danger of extinction. There are currently 1,667 non-bird saurops in danger of extinction, and more than 1,230 birds in the same situation.