Reptile

The reptiles (Reptilia, from the Latin reptĭlis -that crawls-), are a group of amniotic vertebrate animals equipped with keratin epidermal scales. It is a class proper to traditional taxonomy, but according to current cladistic systematics, it is a paraphyletic group, that is, it does not include all the descendants of the common ancestor (because it leaves out birds and mammals, for example). which has no phylogenetic value from the point of view of biology). Although a modern redefinition of Reptilia is the one that includes birds, but excludes synapsids, which led to mammals, since it has been shown that they do not they were reptiles.

The specialty of zoology that specifically studies reptiles is called herpetology.

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 cladists 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.

Evolutionary origin

HylonomusOne of the first reptiles of the Carboniferous.

Reptiles originated from the reptiliomorphs, a group of tetrapods possessing both amphibian and reptilian characteristics, in the steaming swamps of the Carboniferous period about 310-320 million years ago, most lineages diversifying during the Mesozoic. At the end of this era, several groups almost completely disappeared in the great Cretaceous-Tertiary mass extinction, about 65 million years ago.

The earliest known animal that may have been an amniote is Casineria (although it may have been a temnospondyl). A series of footprints from the fossil strata of Nova Scotia dated to 315 million years ago show typical reptilian toes and scale imprints. These footprints are attributed to Hylonomus, the unquestionably oldest known reptile. It was a small, lizard-like animal, about 20 to 30 centimeters (7.9 to 11.8 in) long, with numerous sharp teeth indicating an insectivorous diet.

Features

Skin and shedding

Reptile skin is covered by a horny epidermis, making it waterproof and allowing reptiles to live on dry land, unlike amphibians. Compared to mammalian skin, reptile skin is quite thin and lacks the thick dermal layer that produces leather in mammals. The exposed parts of reptiles are protected by scales, sometimes with a bony base (osteoderms), forming armor. In lepidosaurs, such as lizards and snakes, all skin is covered with overlapping epidermal scales. Such scales were once thought to be typical of the Reptilia class as a whole, but are now known to be typical of lepidosaurs. The scales found on turtles and crocodiles are of dermal, rather than epidermal, origin, and are correctly called scutes. In turtles, the body is covered in a hard shell composed of fused scutes.

Reptiles shed their skin through a process called ecdysis that occurs continuously throughout their lives. In particular, younger reptiles tend to shed once every 5-6 weeks, while adults shed 3-4 times a year. Younger reptiles shed more due to their rapid growth rate. Once full size, the molting frequency decreases dramatically. The ecdysis process involves forming a new layer of skin under the old one. Proteolytic enzymes and lymphatic fluid are secreted between the old and new layers of skin. Consequently, this lifts the old skin off the new allowing sloughing to occur. Snakes will shed from head to tail while lizards shed in an 'irregular pattern'. Dysecdysis is a common skin disease in snakes and lizards, it occurs when ecdysis fails or sheds. There are numerous reasons why molting fails and may be related to inadequate humidity or temperature, nutritional deficiencies, dehydration, and traumatic injuries. Nutritional deficiencies decrease proteolytic enzymes, while dehydration reduces lymphatic fluids to separate the skin layers. Traumatic injuries, on the other hand, form scars that will not allow new scales to form and interrupt the ecdysis process.

Vision

Most reptiles are diurnal animals. Vision is typically adapted to daylight conditions, with color vision and visual depth perception more advanced than in amphibians and most mammals. Reptiles usually have excellent vision, which allows them to detect shapes and movements at long distances. They often have only a few rod cells and have poor vision in low light. At the same time, they have cells called "double cones" which give them sharp color vision and allow them to see ultraviolet wavelengths. In some species, such as blind snakes, vision is reduced. Many lepidosaurs have a photosensory organ on top of their heads called the parietal eye, which is also called the third eye, pineal eye, or pineal gland. This "eye" it does not function in the same way as a normal eye, having only a rudimentary retina and lens and therefore cannot form images. However, it is sensitive to changes in light and darkness and can detect movement. Some snakes have additional sets of visual organs (in the broadest sense of the word) in the form of nostrils sensitive to infrared radiation (heat). These heat-sensitive pits are particularly well developed in pit vipers, but are also found in boas and pythons. These pits allow snakes to feel the body heat of birds and mammals, allowing vipers to hunt rodents in the dark. Most reptiles, including birds, possess a nictitating membrane, a translucent third eyelid that draws over the eye from the inner corner. In particular, it protects the surface of a crocodile's eyeball while allowing some degree of vision underwater. However, many scaly ones, such as lizards and snakes in particular, lack eyelids, which are replaced by a transparent scale. This is called shine, spectacle or cap. The shimmer is usually not visible except when the snake is molting, and it protects the eyes from dust and dirt.

Breathing

All reptiles breathe through lungs. Aquatic turtles have evolved a more permeable skin, and some species have modified their cloaca to increase the area for gas exchange. Even with these adaptations, breathing is never fully accomplished without the lungs. Lung ventilation is achieved differently in each major group of reptiles. In squamosos, the lungs are ventilated almost exclusively by the axial musculature. This is also the same musculature used during locomotion. Due to this restriction, most squamates are forced to hold their breath during intense runs. Some, however, have found a way around it. Monitor lizards and some other species of lizards use mouth pumping as a complement to their "axial breathing" normal. This allows the animals to completely fill their lungs during intense locomotion and thus remain aerobically active for a long time. Lizards are known to possess a proto-diaphragm, which separates the pulmonary cavity from the visceral cavity. While they are not actually capable of movement, it allows for further inflation of the lungs, by taking the weight of the viscera off the lungs. Crocodiles actually have a muscular diaphragm that is analogous to a mammalian diaphragm. The difference is that the crocodile's diaphragm muscles pull the pubis (part of the pelvis, which is mobile in crocodiles) backwards, which lowers the liver, thus freeing up space for the lungs to expand. This type of diaphragmatic configuration has been termed a "hepatic piston." The airways form a series of double tubular chambers within each lung. As you inhale and exhale, air moves through the airways in the same direction, thus creating a unidirectional airflow through the lungs. A similar system is found in birds, monitor lizards, and iguanas. Most reptiles lack a secondary palate, which means they must hold their breath when swallowing. Crocodiles have evolved a bony secondary palate that allows them to continue breathing while submerged (and protects their brains against damage from fighting prey). Skinks (family Scincidae) have also evolved a bony secondary palate, to varying degrees. The snakes took a different approach and instead extended their windpipe. Their tracheal extension sticks out like a meaty straw and allows these animals to swallow large prey without choking.

Digestion

Most reptiles are either insectivorous or carnivorous and have comparatively short and simple digestive tracts because meat is fairly simple to break down and digest. The diet consists of invertebrates and small vertebrates, in lepidosaurs, while crocodiles and giant snakes can consume larger vertebrates. Digestion is slower than in mammals and birds, reflecting their lower resting metabolism and inability to break down and chew food. Their poikilothermic metabolism has very low energy requirements, allowing large reptiles such as crocodiles and large constrictors to live on a single large meal for months, digesting it slowly. While modern reptiles are predominantly carnivorous, during the early history of reptiles, several groups produced some herbivorous megafauna: in the Paleozoic, the pareiasaurs; and in the Mesozoic several groups of dinosaurs. Today, tortoises are the only predominantly herbivorous group of reptiles, but several groups of agamas and iguanas have evolved to live wholly or partly by eating plants. Herbivorous reptiles face the same chewing problems as herbivorous mammals, but, lacking the complex mammalian teeth, many species swallow rocks and pebbles (so-called gastroliths) to aid in digestion: the rocks are washed into their stomachs., which helps to crush the plants. Fossil gastroliths associated with dinosaurs have been found, although it is disputed whether they actually functioned as a gastric mill in the latter. Saltwater crocodiles also use gastroliths as ballast, stabilizing them in the water or helping them dive. A dual role as a stabilizing ballast and aid in digestion has been suggested for gastroliths found in plesiosaurs.

Excretion

Excretion is carried out mainly by two small kidneys. In diapsids, uric acid is the main nitrogenous waste product; turtles, like mammals, primarily excrete urea. Unlike the kidneys of mammals and birds, the kidneys of reptiles cannot produce liquid urine more concentrated than body fluids. This is because they lack a specialized structure called the Henle loop, which is present in avian and mammalian nephrons. Because of this, many reptiles use the colon to aid in the reabsorption of water. Some can also absorb water stored in the bladder. Excess salts are also excreted by the nasal and lingual salt glands in some reptiles. In all reptiles, the urinogenital ducts and the anus open into an organ called the cloaca. In some reptiles, a ventral median wall in the cloaca may open into a urinary bladder, but not in all. It is present in all turtles, as well as in most lizards, and is absent in snakes and crocodiles. Many turtles and lizards have proportionately very large bladders. Charles Darwin noted that Galapagos tortoises had a bladder that could store up to 20% of their body weight. These adaptations are the result of environments like remote islands and deserts where water is very scarce. Other desert-dwelling reptiles have large bladders that can store a long-term reserve of water for several months and aid in osmoregulation. Turtles have two or more accessory urinary bladders, located lateral to the urinary bladder neck and dorsal to the pubis, occupying a significant portion of their body cavity. Their bladder is also usually bilobed with a left and right section. The right section is located under the liver, preventing large stones from remaining on that side, while the left section is more likely to have stones.

Playback

Reptiles generally reproduce sexually, although a few are capable of asexual reproduction by parthenogenesis. All reproductive activity occurs through the cloaca, the only exit/entrance at the base of the tail where waste is also eliminated. Most reptiles have copulatory organs, which are usually retracted or inverted and stored within the body. In turtles and crocodiles, the male has a single medium penis, while squamates, including snakes and lizards, possess a pair of hemipenes, only one of which is typically used in each session. However, tuataras lack a penis, so the male and female must simply press their cloacas together while the male discharges sperm. Most reptiles lay amniotic eggs covered with leathery or calcareous shells. An amnion, chorion, and allantois are present during embryonic development. The egg shell (1) protects the embryo (11) and prevents it from drying out, but is flexible to allow gas exchange. The chorion (6) helps in the exchange of gases between the inside and outside of the egg. It allows carbon dioxide to leave the egg and oxygen to enter the egg. Albumin (9) further protects the embryo and serves as a reservoir for water and protein. The allantois (8) is a sac that collects metabolic waste produced by the embryo. The amniotic sac (10) contains amniotic fluid (12) that protects and cushions the embryo. The amnion (5) aids in osmoregulation and serves as a saltwater reservoir. The yolk sac (2) that surrounds the yolk (3) contains nutrients rich in protein and fat that are absorbed by the embryo through the vessels (4) that allow the embryo to grow and be metabolized. The air space (7) provides oxygen to the embryo during hatching. This ensures that the embryo does not suffocate while it is hatching. There are no larval stages of development. Viviparity and ovoviviparity have evolved in many extinct clades of reptiles and in squamates. In the last group, many species, including all boas and most vipers, use this mode of reproduction. The degree of viviparity varies; some species simply retain the eggs until just before hatching, others provide maternal food to supplement the yolk, and still others are yolkless and provide all nutrients through a structure similar to the mammalian placenta. Asexual reproduction in squamates has been identified in six lizard families and one snake family. In some species of squamates, a population of females can produce a unisexual diploid clone of the mother. This form of asexual reproduction, called parthenogenesis, occurs in several species of lizards. In captivity, Komodo dragons have reproduced by parthenogenesis. Parthenogenetic species are held to be found among chameleons, agamids, xanthussids, and typflopids. Some reptiles exhibit temperature-dependent sex determination (TDSD), in which the incubation temperature determines whether a particular egg hatches as a male or female. TDSD is most common in turtles and crocodiles, but it also occurs in lizards and tuataras. To date, there has been no confirmation as to whether TDSD occurs in snakes.

Nervous system

The reptilian nervous system contains the same basic part of the amphibian brain, but the reptilian cerebrum and cerebellum are slightly larger. Most of the typical sense organs are well developed with certain exceptions, most notably the snake's lack of external ears (middle and inner ears are present). There are twelve pairs of cranial nerves. Due to their short cochlea, reptiles use electrical tuning to expand their range of audible frequencies.

Circulatory system

All lepidosaurs and turtles have a three-chambered heart consisting of two atria, a variably divided ventricle, and two aortas leading to the systemic circulation. The degree of mixing of oxygenated and deoxygenated blood in the three-chambered heart varies with species and physiological state. Under different conditions, deoxygenated blood can be diverted back to the body or oxygenated blood can be diverted back to the lungs. This variation in blood flow has been hypothesized to allow for more effective thermoregulation and longer dive times for aquatic species, but has not been shown to be a fitness advantage. For example, iguana hearts, like most squamous hearts, are composed of three chambers with two aorta and one ventricle, involuntary cardiac muscles. The main structures of the heart are the venous sinus, the pacemaker, the left atrium, the right atrium, the atrioventricular valve, the venous cavity, the arterial cavity, the pulmonary cavity, the muscular crest, the ventricular crest, the pulmonary veins, and the arches. paired aortics. Some species of squamates have three-chambered hearts that functionally convert to four-chambered hearts during contraction. This is made possible by a muscular ridge that subdivides the ventricle during ventricular diastole and completely divides it during ventricular systole. Due to this ridge, some of these squamosals are capable of producing ventricular pressure differentials that are equivalent to those seen in mammalian and avian hearts. Crocodiles have a four-chambered heart, similar to that of birds, but they also have two systemic aortas.

Metabolism

Modern reptiles exhibit some form of cold-bloodedness (ie, a mixture of poikilothermy, ectothermy, and bradymetabolism), so they have limited physiological means of maintaining constant body temperatures and often rely on external sources of heat. Due to a less stable core temperature than birds and mammals, reptile biochemistry requires enzymes capable of maintaining efficiency over a greater range of temperatures than warm-blooded animals. The optimal range of body temperature varies by species, but is usually lower than that of warm-blooded animals; for many lizards, it falls in the 24°-35°C (75°-95°F) range, while extreme heat-adapted species, such as the American desert iguana Dipsosaurus dorsalis, can have optimal physiological temperatures in mammals range, between 35° and 40°C (95° and 104°F). While the optimal temperature is often found when the animal is active, the low basal metabolism causes the body temperature to drop rapidly when the animal is inactive. As in all animals, the action of the reptilian muscles produces heat. In large reptiles, such as turtles, the low surface-to-volume ratio allows this metabolically produced heat to keep the animals warmer than their surroundings, even though they do not have a warm-blooded metabolism. This form of homeothermy is called gigantothermy; It has been suggested that it was common in large dinosaurs and other large extinct reptiles. The benefit of a low resting metabolism is that it requires much less fuel to maintain bodily functions. By utilizing the temperature variations in their environment, or by staying cool when they don't need to move, reptiles can save considerable amounts of energy compared to endothermic animals of the same size. A crocodile needs between one tenth and one fifth of the food required by a lion of the same weight and can live for half a year without eating. Lower food requirements and adaptive metabolisms allow reptiles to dominate animal life in regions where net calorie availability is too low to support large mammals and birds. Reptiles are generally assumed to be incapable of the sustained high-energy output necessary for long-distance pursuit or flight. A greater energy capacity could have been responsible for the evolution of warm-blooded birds and mammals. However, investigation of the correlations between active capacity and thermophysiology shows a weak relationship. Most extant reptiles are carnivorous with a sit-and-wait feeding strategy; It is not clear if the reptiles are cold-blooded due to their ecology. Energetic studies in some reptiles have shown active capacities equal to or greater than those of warm-blooded animals of similar size.

Distribution and habitat

Reptiles are present on almost the entire surface of the planet, with the exception of extremely cold areas near the poles. As they are cold-blooded animals, they still prefer relatively high temperatures, and their presence and diversity become more important near the tropics. Thus, the richest continents in reptiles are Asia, Africa and South America. Reptiles can adapt to very different habitats. They are very present in tropical forests, with a very high diversity of species, but they also inhabit deserts, where there are lizards and snakes that take refuge during the day and come out at night. In mountainous areas, lizards like to hide in piles of stones, and some snakes have specialized in high-altitude areas, such as the Orsini viper (Vipera ursinii) which is found in high altitudes. mountains of Europe at altitudes around 2,000 m 74. Some reptiles are said to be burrowers, spending part of their lives underground like amphibians. Reptiles have also colonized aquatic environments: crocodiles, certain turtles such as the European terrapin, and certain snakes such as the anaconda, water moccasin, and snakes are at home in freshwater rivers and lakes when sea turtles are present. in all the oceans of the world, and only come to land to reproduce. Sea snakes represent a higher level of adaptation, since most of them no longer return to land and have adopted an exclusively marine life cycle. Many species have arboreal habits, such as snakes or lizards. Some can move from tree to tree by "floating" such as flying dragons and, to a lesser extent, some snakes such as flying snakes.

Taxonomic rearrangement

Traditionally, reptiles were considered a class, just like mammals or birds, and were thought to be ancestors of these two. The classic taxon of reptiles included three large groups: synapsids, anapsids, and diapsids.

However, today it is known that birds have a common ancestor with modern reptiles, so both are classified within the Sauropsida clade because they are monophyletic. This clade, in principle, would include anapsids and diapsids. This clade can also be considered synonymous with Reptilia.

In this way, synapsids are left out of the group, hosting mammals and a large number of related fossil amniotes, formerly included within reptiles, such as dimetrodons. Non-mammalian synapsids were traditionally called mammalian reptiles.

On the other hand, according to recent fossil studies, anapsids are also a paraphyletic taxon, which is why reptiles or sauropsids are currently divided into two new monophyletic clades Parareptilia and Eureptilia. The classification, therefore, is as follows cladogram, based on Tree of Life and highly simplified.

Traditional classification

Lacerta agilisA modern diapsy.

Lycaenopsa synapside or "mamiferoid reptile".

BradysaurusAn anapsy.

Reptiles, in classical systematics, include the following extinct and extant orders:

• Subclase Anapsida† (P)
  • Order Mesosauria†
  • Order Millerosauria†
  • Order Procolophonomorpha†
  • Order Cotylosauria†
• Subclase Synapsida (now not considered reptile and from which mammals derive)
  • Therapsida Order (includes mammals)
  • Order Pelycosauria† (P)
• Subclass Diapsy
  • Order Thalattosauria†
  • Order Younginiformes†
  • Order Araeoscelida†
  • Order Avicephala†
  • Clado Pantestudines
    • Order Testudines (tortugas)
    • Superorden Sauropterygia†
      • Order Placodontia†
      • Order Plesiosauria†
      • Order Nothosauroidea†
  • Infraclase Ichthyosauromorpha†
    • Order Hupehsuchia†
    • Superorden Ichthyopterygia†
      • Order Ichthyosauria†
      • Order Grippidia†
  • Infraclase Lepidosauromorpha
    • Order Eolacertilia†
    • Superorden Lepidosauria
      • Squamata Order (lagarts and snakes)
      • Order Rhynchocephalia (traces and related fossil forms)
  • Infraclase Archosauromorpha
    • Order Choristodera†
    • Order Rhynchosauria†
    • Order Protorosauria†
    • Order Prolacertiforms†
    • Order Trilophosauria†
    • Archosauria Clado
      • Order Aetosauria†
      • Order Crocodilia (cocodiles and related fossil forms)
      • Order Rauisuchia†
      • Order Phytosauria†
      • Order Pterosauria†
      • Superorden Dinosauria
        • Order Saurischia (includes birds)
        • Order Ornithischia†