Fossil range: 320–0 Ma Carboniferous – Recent
Reptiles are animals in the (Linnaean) class Reptilia characterized by breathing air, a "cold-blooded" (poikilothermic) metabolism, laying tough-shelled amniotic eggs (or retaining the same membrane system in species with live birth), and skin usually covered in scales or scutes. They are tetrapods (either having four limbs or being descended from four-limbed ancestors) and lay amniotic eggs, in which the embryo is surrounded by a membrane called the amnion. Modern reptiles inhabit every continent with the exception of Antarctica, and four living orders are currently recognized:
The majority of reptile species are oviparous (egg-laying), although certain species of squamates are capable of giving live birth. This is achieved by either ovoviviparity (egg retention) or viviparity (birth of offspring without the development of calcified eggs). Many of the viviparous species feed their fetuses through various forms of placenta analogous to those of mammals, with some providing initial care for their hatchlings. Extant reptiles range in size from a tiny gecko, Sphaerodactylus ariasae, that grows to only 1.6 cm (0.6 in) to the saltwater crocodile, Crocodylus porosus, that may reach 6 m in length and weigh over 1,000 kg. The science dealing with reptiles is called herpetology.
The reptiles were from the outset of classification grouped with the amphibians. Linnaeus, working from species-poor Sweden, where the common adder and grass snake are often found hunting in water, included all reptiles and amphibians in class "III – Amphibia" in his Systema Naturae. The terms "reptile" and "amphibian" were largely interchangeable, "reptile" (from Latin repere, "to creep") being preferred by the French. Josephus Nicolaus Laurenti was the first to formally use the term "Reptilia" for an expanded selection of reptiles and amphibians basically similar to that of Linnaeus. Not until the beginning of the 19th century did it become clear that reptiles and amphibians are in fact quite different animals, and Pierre André Latreille erected the class Batracia (1825) for the latter, dividing the tetrapods into the four familiar classes of reptiles, amphibians, birds and mammals.
The British anatomist Thomas Henry Huxley made Latreille's definition popular, and together with Richard Owen expanded Reptilia to include the various fossil “Antediluvian monsters”, including the mammal-like (synapsid) Dicynodon he helped describe. This was not the only possible classification scheme: In the Hunterian lectures delivered at the Royal College of Surgeons in 1863, Huxley grouped the vertebrates into Mammals, Sauroids, and Ichthyoids (the latter containing the fishes and amphibians). He subsequently proposed the names of Sauropsida and Ichthyopsida for the two.
Around the end of the 19th century, the class Reptilia had come to include all the amniotes except birds and mammals. Thus reptiles were defined as the set of animals that includes the extant crocodiles, alligators, tuatara, lizards, snakes, amphisbaenians, and turtles, as well as fossil groups like dinosaurs, synapsids and the primitive pareiasaurs. This is still the usual definition of the term. However, in recent years, many taxonomists have begun to insist that taxa should be monophyletic, that is, groups should include all descendants of a particular form. The reptiles as defined above would be paraphyletic, since they exclude both birds and mammals, although these also evolved from the original reptile. Colin Tudge writes:
Mammals are a clade, and therefore the cladists are happy to acknowledge the traditional taxon Mammalia; and birds, too, are a clade, universally ascribed to the formal taxon Aves. Mammalia and Aves are, in fact, subclades within the grand clade of the Amniota. But the traditional class Reptilia is not a clade. It is just a section of the clade Amniota: the section that is left after the Mammalia and Aves have been hived off. It cannot be defined by synapomorphies, as is the proper way. It is instead defined by a combination of the features it has and the features it lacks: reptiles are the amniotes that lack fur or feathers. At best, the cladists suggest, we could say that the traditional Reptilia are 'non-avian, non-mammalian amniotes'.
The terms "Sauropsida" ("lizard faces") and "Theropsida" ("beast faces") were taken up again in 1916 by E.S. Goodrich to distinguish between lizards, birds, and their relatives on the one hand (Sauropsida) and mammals and their extinct relatives (Theropsida) on the other. Goodrich supported this division by the nature of the hearts and blood vessels in each group, and other features such as the structure of the forebrain. According to Goodrich, both lineages evolved from an earlier stem group, the Protosauria ("first lizards") which included some Paleozoic amphibians as well as early reptiles.
In 1956 D.M.S. Watson observed that the first two groups diverged very early in reptilian history, and so he divided Goodrich's Protosauria between them. He also reinterpreted the Sauropsida and Theropsida to exclude birds and mammals, respectively. Thus his Sauropsida included Procolophonia, Eosuchia, Millerosauria, Chelonia (turtles), Squamata (lizards and snakes), Rhynchocephalia, Crocodilia, "thecodonts" (paraphyletic basal Archosauria), non-avian dinosaurs, pterosaurs, ichthyosaurs, and sauropterygians.
This classification supplemented, but was never as popular as, the classification of the reptiles (according to Romer's classic Vertebrate Paleontology) into four subclasses according to the number and position of temporal fenestrae, openings in the sides of the skull behind the eyes. Those subclasses were:
All of the above except Synapsida and the earliest anapsid stem-amniotes are included within Sauropsida.
Classification to order level, after Benton, 2004.
The cladogram presented here illustrates the "family tree" of reptiles, and follows a simplified version of the relationships found by Laurin and Gauthier (1996), presented as part of the Tree of Life Web Project.
The origin of the reptiles lies about 320–310 million years ago, in the steaming swamps of the late Carboniferous period, when the first reptiles evolved from advanced reptiliomorph labyrinthodonts. The oldest trace of reptiles is a series of footprints from the fossil strata of Nova Scotia, dated to 315 million years ago. The tracks are attributed to Hylonomus, the oldest known reptile in the biological sense of the word. It was a small, lizard-like animal, about 20 to 30 cm (8–12 in) long, with numerous sharp teeth indicating an insectivorous diet. Other examples include Westlothiana (for the moment considered to be more closely related to amphibians than to amniotes) and Paleothyris, both of similar build and presumably similar habit. One of the best known early reptiles is Mesosaurus, a genus from the early Permian that had returned to water, feeding on fish. The earliest reptiles were largely overshadowed by bigger labyrinthodont amphibians such as Cochleosaurus, and remained a small, inconspicuous part of the fauna until after the small ice age at the end of the Carboniferous.
The first reptiles are categorized as anapsids, having a solid skull with holes for only nose, eyes, spinal cord, etc. Turtles are believed by some to be surviving anapsids, since they share this skull structure, but this point has become contentious lately, with some arguing that turtles reverted to this primitive state in order to improve their armor (see Parareptilia). Both sides cite strong evidence, and the conflict has yet to be resolved.
Very soon after the first reptiles appeared, they split into two branches. One branch, the Synapsida (including both "mammal-like reptiles"/"stem mammals" and modern, extant mammals such as humans), had two openings in the skull roof behind the eyes; the other branch, the Diapsida, possessed a pair of holes in their skulls behind the eyes, along with a second pair located higher on the skull. The function of the holes in both groups was to lighten the skull and give room for the jaw muscles to move, allowing for a more powerful bite. The diapsids and later anapsids are classed as the "true reptiles", the Sauropsida.
With the close of the Carboniferous, reptiles became the dominant tetrapod fauna. While the terrestrial reptiliomorph labyrinthodonts still existed, the synapsids evolved the first terrestrial megafauna (giant animals) in the form of pelycosaurs such as Edaphosaurus and the carnivorous Dimetrodon. In the mid-Permian period the climate turned dryer, resulting in a change of fauna: The primitive pelycosaurs were replaced by the more advanced therapsids.
The anapsid reptiles, whose massive skull roofs had no postorbital holes, continued and flourished throughout the Permian. The pareiasaurs reached giant proportions in the late Permian, eventually disappearing at the close of the period (the turtles being possible survivors).
Early in the period, the diapsid reptiles split into two main lineages, the archosaurs (forefathers of crocodiles and dinosaurs) and the lepidosaurs (predecessors of modern snakes, lizards, and tuataras). Both groups remained lizard-like and relatively small and inconspicuous during the Permian.
The close of the Permian saw the greatest mass extinction known (see the Permian–Triassic extinction event). Most of the earlier anapsid/synapsid megafauna disappeared, being replaced by the archosauromorph diapsids. The archosaurs were characterized by elongated hind legs and an erect pose, the early forms looking somewhat like long-legged crocodiles. The archosaurs became the dominant group during the Triassic period, developing into the well-known dinosaurs and pterosaurs, as well as crocodiles and phytosaurs. Some of the dinosaurs developed into the largest land animals ever to have lived, making the Mesozoic era popularly known as the "Age of Reptiles". The dinosaurs also developed smaller forms, including the feather-bearing smaller theropods. In the mid-Jurassic period, these gave rise to the first birds.
The lepidosauromorph diapsids may have been ancestral to the sea reptiles. These reptiles developed into the sauropterygians in the early Triassic and the ichthyosaurs during the Middle Triassic. The mosasaurs also evolved in the Mesozoic era, emerging during the Cretaceous period.
The therapsids came under increasing pressure from the dinosaurs in the early Mesozoic and developed into increasingly smaller and more nocturnal forms, the first mammals being the only survivors of the line by the late Jurassic.
The close of the Cretaceous period saw the demise of the Mesozoic era reptilian megafauna (see the Cretaceous–Tertiary extinction event). Of the large marine reptiles, only the sea turtles are left, and, of the dinosaurs, only the small feathered theropods survived in the form of birds. The major surviving reptilian line is the lepidosaurs, of which the snakes are currently the most numerous and widespread representatives. The end of the “Age of Reptiles” led into the “Age of Mammals”. Despite this, reptiles are still a major faunal component, particularly in tropical climates. There are about 8,200 extant species of reptiles (of which almost half are snakes), compared with 5,400 species of mammals (of which two-thirds are rodents and bats). The most numerous modern group with reptilian roots are the birds, with over 9,000 species.
Most reptiles have a three-chambered heart consisting of two atria, one variably-partitioned ventricle, and two aortas that lead to the systemic circulation. The degree of mixing of oxygenated and deoxygenated blood in the three-chambered heart varies depending on the species and physiological state. Under different conditions, deoxygenated blood can be shunted back to the body or oxygenated blood can be shunted back to the lungs. This variation in blood flow has been hypothesized to allow more effective thermoregulation and longer diving times for aquatic species, but has not been shown to be a fitness advantage.
There are some interesting exceptions to the general physiology. For instance, crocodilians have an anatomically four-chambered heart, but also have two systemic aortas and are therefore capable of bypassing only their pulmonary circulation. Also, some snake and lizard species (e.g., pythons and monitor lizards) have three-chambered hearts that become functionally 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. Because of this ridge, some of these squamates are capable of producing ventricular pressure differentials that are equivalent to those seen in mammalian and avian hearts.
All reptiles breathe using lungs. Aquatic turtles have developed more permeable skin, and some species have modified their cloaca to increase the area for gas exchange (Orenstein, 2001). Even with these adaptations, breathing is never fully accomplished without lungs. Lung ventilation is accomplished differently in each main reptile group. In squamates, the lungs are ventilated almost exclusively by the axial musculature. This is also the same musculature that is used during locomotion. Because of this constraint, most squamates are forced to hold their breath during intense runs. Some, however, have found a way around it. Varanids, and a few other lizard species, employ buccal pumping as a complement to their normal "axial breathing." This allows the animals to completely fill their lungs during intense locomotion, and thus remain aerobically active for a long time. Tegu lizards are known to possess a proto-diaphragm, which separates the pulmonary cavity from the visceral cavity. While not actually capable of movement, it does allow for greater lung inflation, by taking the weight of the viscera off the lungs (Klein et al., 2003). Crocodilians actually have a muscular diaphragm that is analogous to the mammalian diaphragm. The difference is that the muscles for the crocodilian diaphragm pull the pubis (part of the pelvis, which is movable in crocodilians) back, which brings the liver down, thus freeing space for the lungs to expand. This type of diaphragmatic setup has been referred to as the "hepatic piston."
How turtles and tortoises breathe has been the subject of much study. To date, only a few species have been studied thoroughly enough to get an idea of how turtles do it. The results indicate that turtles and tortoises have found a variety of solutions to this problem. The difficulty is that most turtle shells are rigid and do not allow for the type of expansion and contraction that other amniotes use to ventilate their lungs. Some turtles such as the Indian flapshell (Lissemys punctata) have a sheet of muscle that envelops the lungs. When it contracts, the turtle can exhale. When at rest, the turtle can retract the limbs into the body cavity and force air out of the lungs. When the turtle protracts its limbs, the pressure inside the lungs is reduced, and the turtle can suck air in. Turtle lungs are attached to the inside of the top of the shell (carapace), with the bottom of the lungs attached (via connective tissue) to the rest of the viscera. By using a series of special muscles (roughly equivalent to a diaphragm), turtles are capable of pushing their viscera up and down, resulting in effective respiration, since many of these muscles have attachment points in conjunction with their forelimbs (indeed, many of the muscles expand into the limb pockets during contraction). Breathing during locomotion has been studied in three species, and they show different patterns. Adult female green sea turtles do not breathe as they crutch along their nesting beaches. They hold their breath during terrestrial locomotion and breathe in bouts as they rest. North American box turtles breathe continuously during locomotion, and the ventilation cycle is not coordinated with the limb movements (Landberg et al., 2003). They are probably using their abdominal muscles to breathe during locomotion. The last species to have been studied is the red-eared slider, which also breathes during locomotion, but takes smaller breaths during locomotion than during small pauses between locomotor bouts, indicating that there may be mechanical interference between the limb movements and the breathing apparatus. Box turtles have also been observed to breathe while completely sealed up inside their shells (ibid.).
Most reptiles lack a secondary palate, meaning that they must hold their breath while swallowing. Crocodilians have evolved a bony secondary palate that allows them to continue breathing while remaining submerged (and protect their brains against damage by struggling prey). Skinks (family Scincidae) also have evolved a bony secondary palate, to varying degrees. Snakes took a different approach and extended their trachea instead. Their tracheal extension sticks out like a fleshy straw, and allows these animals to swallow large prey without suffering from asphyxiation.
Reptilian skin is covered in a horny epidermis, making it watertight and enabling reptiles to live on dry land, in contrast to amphibians. Compared to mammalian skin, that of reptiles is rather thin and lacks the thick dermal layer that produces leather in mammals. Exposed parts of reptiles are protected by scales or scutes, sometimes with a bony base, forming armor. In lepidosaurians such as lizards and snakes, the whole skin is covered in overlapping epidermal scales. Such scales were once thought to be typical of the class Reptilia as a whole, but are now known to occur only in lepidosaurians. The scales found in turtles and crocodiles are of dermal, rather than epidermal, origin and are properly termed scutes. In turtles, the body is hidden inside a hard shell composed of fused scutes.
Excretion is performed mainly by two small kidneys. In diapsids, uric acid is the main nitrogenous waste product; turtles, like mammals, excrete mainly urea. Unlike the kidneys of mammals and birds, reptile kidneys are unable to produce liquid urine more concentrated than their body fluid. This is because they lack a specialized structure called a loop of Henle, which is present in the nephrons of birds and mammals,. Because of this, many reptiles use the colon to aid in the reabsorption of water. Some are also able to take up water stored in the bladder. Excess salts are also excreted by nasal and lingual salt glands in some reptiles.
Most reptiles are carnivorous and have rather simple and comparatively short guts, meat being fairly simple to break down and digest. Digestion is slower than in mammals, reflecting their lower metabolism and their inability to divide and masticate their food. Being poikilotherms (with varying body temperature regulated by their environment), their energy requirement is about a fifth to a tenth of that of a mammal of the same size. Large reptiles like crocodiles and the large constrictors can live from a single large meal for months, digesting it slowly.
While modern reptiles are predominately carnivorous, during the early history of reptiles several groups produced a herbivorous megafauna: in the Paleozoic the pareiasaurs and the synapsid dicynodonts, and in the Mesozoic several lines of dinosaurs. Today the turtles are the only predominantly herbivorous reptile group, but several lines of agams and iguanas have evolved to live wholly or partly on plants.
Herbivorous reptiles face the same problems of mastication as herbivorous mammals but, lacking the complex teeth of mammals, many species swallow rocks and pebbles (so called gastroliths) to aid in digestion: The rocks are washed around in the stomach, helping to grind up plant matter. Fossil gastroliths have been found associated with sauropods. Sea turtles, crocodiles, and marine iguanas also use gastroliths as ballast, helping them to dive.
The reptilian nervous system contains the same basic part of the amphibian brain, but the reptile cerebrum and cerebellum are slightly larger. Most 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.
Reptiles are generally considered less intelligent than mammals and birds. The size of their brain relative to their body is much less than that of mammals, the encephalization quotient being about one tenth of that of mammals. Crocodiles have relatively larger brains and show a fairly complex social structure. Larger lizards like the monitors are known to exhibit complex behavior, including cooperation. The Komodo dragon is known to engage in play.
Most reptiles are diurnal animals. The vision is typically adapted to daylight conditions, with color vision and more advanced visual depth perception than in amphibians and most mammals. In some species, such as blind snakes, vision is reduced. Some snakes have extra sets of visual organs (in the loosest sense of the word) in the form of pits sensitive to infrared radiation (heat). Such heat-sensitive pits are particularly well developed in the pit vipers, but are also found in boas and pythons. These pits allow the snakes to sense the body heat of birds and mammals, enabling pit vipers to hunt rodents in the dark.
Most reptiles reproduce sexually, though some are capable of asexual reproduction. All reproductive activity occurs through the cloaca, the single 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 inside the body. In turtles and crocodilians, the male has a single median penis, while squamates, including snakes and lizards, possess a pair of hemipenes. Tuataras, however, lack copulatory organs, and so the male and female simply press their cloacas together as the male excretes sperm.
Most reptiles lay amniotic eggs covered with leathery or calcareous shells. An amnion, chorion, and allantois are present during embryonic life. There are no larval stages of development. Viviparity and ovoviviparity have evolved only in squamates, and many species, including all boas and most vipers, utilize this mode of reproduction. The degree of viviparity varies: some species simply retain the eggs until just before hatching, others provide maternal nourishment to supplement the yolk, and yet others lack any yolk and provide all nutrients via a structure similar to the mammalian placenta.
Asexual reproduction has been identified in squamates in six families of lizards and one snake. In some species of squamates, a population of females is able to produce a unisexual diploid clone of the mother. This form of asexual reproduction, called parthenogenesis, occurs in several species of gecko, and is particularly widespread in the teiids (especially Aspidocelis) and lacertids (Lacerta). In captivity, Komodo dragons (Varanidae) have reproduced by parthenogenesis.
Some reptiles exhibit temperature-dependent sex determination (TDSD), in which the incubation temperature determines whether a particular egg hatches as male or female. TDSD is most common in turtles and crocodiles, but also occurs in lizards and tuataras. To date, there has been no confirmation of whether TDSD occurs in snakes.
Many small reptiles such as snakes and lizards which live on the ground or in the water are vulnerable to being preyed on by all kinds of carnivorous animals. Thus avoidance is the most common form of defense in reptiles. At the first sign of danger, most snakes and lizards crawl away into the undergrowth, and turtles and crocodiles will plunge into water and sink out of sight.
Reptiles may also avoid confrontation through camouflage. Using a variety of grays, greens, and browns, these animals can blend remarkably well into the background of their natural environment.
If the danger arises so suddenly that flight may be harmful, then crocodiles, turtles, some lizards, and some snakes hiss loudly when confronted by an enemy. Rattlesnakes rapidly vibrate the tip of the tail, which is composed of a series of nested, hollow beads.
If all this does not deter an enemy, different species will adopt different defensive tactics.
Snakes use a complicated set of behaviors when attacked. Some will first elevate their head and spread out the skin of their neck in an effort to look bigger and more threatening. Failure of this may lead to other measures practiced particularly by cobras, vipers, and closely related species, who use venom to attack. The venom is modified saliva, delivered through fangs.
When a crocodile is concerned about its safety, it will gape to expose the teeth and yellow tongue. If this doesn't work, the crocodile gets a little more agitated and typically begins to make hissing sounds. After this, the crocodile starts to get serious, changing its posture dramatically to make itself look more intimidating. The body is inflated to increase apparent size. If absolutely necessary it may decide to attack an enemy.
Some species try and bite, some will use their heads as sledgehammers and literally smash an opponent, some will rush or swim toward the threat from a distance, even chasing them onto land or galloping after them.
Geckos, skinks, and other lizards that are captured by the tail will shed part of the tail structure through a process called autotomy and thus be able to flee. The detached tail will continue to wiggle, creating a deceptive sense of continued struggle and distracting the predator's attention from the fleeing prey animal. The animal can partially regenerate its tail over a period of weeks. The new section will contain cartilage rather than bone, and the skin may be distinctly discolored compared to the rest of the body.
Fossil range: Carboniferous – Recent
Reptiles are a group of vertebrate animals. The name "reptile" comes from Latin and means "one who creeps". All living reptile species are cold blooded, have scaly skin, and lay cleidoic eggs. They excrete uric acid (instead of urea), and have a cloaca. A cloaca is a shared opening for the intestines, urinary tract and reproductive ducts. Reptiles also share an arrangement of the heart and major blood vessels which is different from that of mammals.
Many important groups of reptiles are now extinct. The great marine reptiles of the Mesozoic era, the ichthyosaurs, plesiosaurs and mosasaurs, are extinct. We used to say the dinosaurs were extinct, but they survive in the form of their feathered descendents (birds). Ancient reptiles that do survive include the turtles, the crocodiles and the Tuatara, the lone survivor of its group. The great majority of present-day reptiles are snakes and lizards.
The study of reptiles is called herpetology.
Reptilia is an evolutionary grade rather than a clade. The main reason is that the term 'reptile' does not include birds, the descendents of theropod dinosaurs. Thus instead of Reptilia as a taxonomic class, today most experts use Class Sauropsida (which includes all reptiles and birds, living and extinct). Class Synapsida includes mammals and all their forebears. Reptile is still the usual informal term to describe living Sauropsids (except birds). Mammals are a genuine clade, and hence Mammalia remains the taxonomic term.
Since reptiles are not monophyletic, reclassifying them is one of the key aims of taxonomy. Some taxonomists, such as Benton, make Sauropsida and Synapsida as class-level taxa. The two groups split (probably) in the Lower Carboniferous, from stem-group Amniotes (the early tetrapods who laid cleidoic eggs).
Chameleons, a lizard
Diamondback turtle adult
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|Amphibian • Bird • Fish • Mammal • Reptile|