Reptiles have been subject to several conflictingtaxonomic definitions.[3] Inevolutionary taxonomy, reptiles are gathered together under theclassReptilia (/rɛpˈtɪliə/rep-TIL-ee-ə), which corresponds to common usage. Moderncladistic taxonomy regards that group asparaphyletic, sincegenetic andpaleontological evidence has determined thatcrocodilians are more closely related tobirds (class Aves), members ofDinosauria, than to other living reptiles, and thus birds are nested among reptiles from a phylogenetic perspective. Many cladistic systems therefore redefine Reptilia as aclade (monophyletic group) including birds, though the precise definition of this clade varies between authors.[4][3] A similar concept is cladeSauropsida, which refers to allamniotes more closely related to modern reptiles than tomammals.[4]
Reptiles are tetrapodvertebrates, creatures that either have four limbs or, like snakes, are descended from four-limbed ancestors. Unlikeamphibians, reptiles do not have an aquatic larval stage. Most reptiles areoviparous, although several species of squamates areviviparous, as were some extinct aquatic clades[7] – the fetus develops within the mother, using a(non-mammalian) placenta rather than contained in aneggshell. As amniotes, reptile eggs are surrounded by membranes for protection and transport, which adapt them to reproduction on dry land. Many of the viviparous species feed theirfetuses through various forms of placenta analogous to those ofmammals, with some providing initial care for their hatchlings.Extant reptiles range in size from a tiny gecko,Sphaerodactylus ariasae, which can grow up to 17 mm (0.7 in) to thesaltwater crocodile,Crocodylus porosus, which can reach over 6 m (19.7 ft) in length and weigh over 1,000 kg (2,200 lb).
In the 13th century, the category ofreptile was recognized in Europe as consisting of a miscellany of egg-laying creatures, including "snakes, various fantastic monsters, lizards, assorted amphibians, and worms", as recorded byBeauvais in hisMirror of Nature.[8]In the 18th century, the reptiles were, from the outset of classification, grouped with theamphibians.Linnaeus, working from species-poorSweden, where thecommon adder andgrass snake are often found hunting in water, included all reptiles and amphibians inclass"III – Amphibia" in hisSystema Naturæ.[9]The termsreptile andamphibian were largely interchangeable,reptile (from Latinrepere, 'to creep') being preferred by the French.[10]J.N. Laurenti was the first to formally use the termReptilia for an expanded selection of reptiles and amphibians basically similar to that of Linnaeus.[11] Today, the two groups are still commonly treated under the single headingherpetology.
It was not until the beginning of the 19th century that it became clear that reptiles and amphibians are, in fact, quite different animals, andP.A. Latreille erected the classBatracia (1825) for the latter, dividing thetetrapods into the four familiar classes of reptiles, amphibians, birds, and mammals.[12] The British anatomistT.H. Huxley made Latreille's definition popular and, together withRichard Owen, expanded Reptilia to include the various fossil "antediluvian monsters", includingdinosaurs and the mammal-like (synapsid)Dicynodon he helped describe. This was not the only possible classification scheme: In the Hunterian lectures delivered at theRoyal College of Surgeons in 1863, Huxley grouped the vertebrates intomammals, sauroids, and ichthyoids (the latter containing the fishes and amphibians). He subsequently proposed the names ofSauropsida andIchthyopsida for the latter two groups.[13] In 1866,Haeckel demonstrated that vertebrates could be divided based on their reproductive strategies, and that reptiles, birds, and mammals were united by theamniotic egg.
The termsSauropsida ("lizard faces") andTheropsida ("beast faces") were used again in 1916 byE.S. Goodrich to distinguish between lizards, birds, and their relatives on the one hand (Sauropsida) andmammals 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, Protosauria ("first lizards") in which he included some animals today consideredreptile-like amphibians, as well as early reptiles.[14]
In the late 19th century, a number of definitions of Reptilia were offered. The biological traits listed byLydekker in 1896, for example, include a singleoccipital condyle, a jaw joint formed by thequadrate andarticular bones, and certain characteristics of thevertebrae.[16] The animals singled out by these formulations, theamniotes other than the mammals and the birds, are still those considered reptiles today.[17]
The synapsid/sauropsid division supplemented another approach, one that split the reptiles into four subclasses based on the number and position oftemporal fenestrae, openings in the sides of the skull behind the eyes. This classification was initiated byHenry Fairfield Osborn and elaborated and made popular byRomer's classicVertebrate Paleontology.[18][19] Those four subclasses were:
Phylogenetic classifications group the traditional "mammal-like reptiles", like thisVaranodon, with other synapsids, not with extant reptiles
The composition of Euryapsida was uncertain.Ichthyosaurs were, at times, considered to have arisen independently of the other euryapsids, and given the older name Parapsida. Parapsida was later discarded as a group for the most part (ichthyosaurs being classified asincertae sedis or with Euryapsida). However, four (or three if Euryapsida is merged into Diapsida) subclasses remained more or less universal for non-specialist work throughout the 20th century. It has largely been abandoned by recent researchers: In particular, the anapsid condition has been found to occur so variably among unrelated groups that it is not now considered a useful distinction.[20]
By the early 21st century, vertebrate paleontologists were beginning to adoptphylogenetic taxonomy, in which all groups are defined in such a way as to bemonophyletic; that is, groups which include all descendants of a particular ancestor. The reptiles as historically defined areparaphyletic, since they exclude both birds and mammals. These respectively evolved from dinosaurs and from early therapsids, both of which were traditionally called "reptiles".[21] Birds are more closely related tocrocodilians than the latter are to the rest of extant reptiles.Colin Tudge wrote:
Mammals are aclade, and therefore thecladists are happy to acknowledge the traditional taxonMammalia; and birds, too, are a clade, universally ascribed to the formal taxonAves. 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 cladeAmniota: The section that is left after the Mammalia and Aves have been hived off. It cannot be defined bysynapomorphies, as is the proper way. Instead, it is 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'.[17]
Despite the early proposals for replacing the paraphyletic Reptilia with a monophyleticSauropsida, which includes birds, that term was never adopted widely or, when it was, was not applied consistently.[3]
When Sauropsida was used, it often had the same content or even the same definition as Reptilia. In 1988,Jacques Gauthier proposed acladistic definition of Reptilia as a monophyletic node-basedcrown group containing turtles, lizards and snakes, crocodilians, and birds, their common ancestor and all its descendants. While Gauthier's definition was close to the modern consensus, nonetheless, it became considered inadequate because the actual relationship of turtles to other reptiles was not yet well understood at this time.[3] Major revisions since have included the reassignment of synapsids as non-reptiles, and classification of turtles as diapsids.[3] Gauthier 1994 and Laurin and Reisz 1995's definition of Sauropsida defined the scope of the group as distinct and broader than that of Reptilia, encompassingMesosauridae as well as Reptiliasensu stricto.[4][22]
A variety of other definitions were proposed by other scientists in the years following Gauthier's paper. The first such new definition, which attempted to adhere to the standards of thePhyloCode, was published by Modesto and Anderson in 2004.[3] Modesto and Anderson reviewed the many previous definitions and proposed a modified definition, which they intended to retain most traditional content of the group while keeping it stable and monophyletic. They defined Reptilia as all amniotes closer toLacerta agilis andCrocodylus niloticus than toHomo sapiens. This stem-based definition is equivalent to the more common definition of Sauropsida, which Modesto and Anderson synonymized with Reptilia, since the latter is better known and more frequently used. Unlike most previous definitions of Reptilia, however, Modesto and Anderson's definition includes birds, as they are within the clade that includes both lizards and crocodiles.[3]
Thecladogram presented here illustrates the "family tree" of reptiles, and follows a simplified version of the relationships found by M.S. Lee, in 2013.[25] Allgenetic studies have supported the hypothesis that turtles are diapsids; some have placed turtles within Archosauromorpha,[25][26][27][28][29][30] though a few have recovered turtles as Lepidosauromorpha instead.[31] The cladogram below used a combination of genetic (molecular) and fossil (morphological) data to obtain its results.[25]
The placement of turtles has historically been highly variable. Classically, turtles were considered to be related to the primitive anapsid reptiles.[32] Molecular work has usually placed turtles within the diapsids. As of 2013, three turtle genomes have been sequenced.[33][needs update] The results place turtles as asister clade to thearchosaurs, the group that includes crocodilians, non-avian dinosaurs, and birds.[34] However, in their comparative analysis of the timing oforganogenesis, Werneburg and Sánchez-Villagra (2009) found support for the hypothesis that turtles belong to a separate clade withinSauropsida, outside thesaurian clade altogether.[35]
The origin of the reptiles lies about 310–320 million years ago, in the steaming swamps of the lateCarboniferous period, when the first reptiles evolved from advancedreptiliomorphs.[22][failed verification]
The oldest known animal that may have been anamniote isCasineria (though it may have been atemnospondyl).[36][37][38] A series of footprints from the fossil strata ofNova Scotia dated to315 Ma show typical reptilian toes and imprints of scales.[39] These tracks are now attributed toHylonomus,[40] historically widely regarded as the oldest known reptile, but whose placement in the group has been recently questioned.[41] It was a small, lizard-like animal, about 20 to 30 centimetres (7.9 to 11.8 in) long, with numerous sharp teeth indicating an insectivorous diet.[42] Other examples includeWestlothiana (for the moment considered areptiliomorph rather than a trueamniote)[43] andPaleothyris, both of similar build and presumably similar habit.
However,microsaurs have been at times considered true reptiles, so an earlier origin is possible.[44]
The earliest amniotes, including stem-reptiles (those amniotes closer to modern reptiles than to mammals), were largely overshadowed by larger stem-tetrapods, such asCochleosaurus, and remained a small, inconspicuous part of the fauna until theCarboniferous Rainforest Collapse.[45] This sudden collapse affected several large groups. Primitive tetrapods were particularly devastated, while stem-reptiles fared better, being ecologically adapted to the drier conditions that followed. Primitive tetrapods, like modern amphibians, need to return to water to lay eggs; in contrast, amniotes, like modern reptiles – whose eggs possess a shell that allows them to be laid on land – were better adapted to the new conditions. Amniotes acquired new niches at a faster rate than before the collapse and at a much faster rate than primitive tetrapods. They acquired new feeding strategies including herbivory and carnivory, previously only having been insectivores and piscivores.[45] From this point forward, reptiles dominated communities and had a greater diversity than primitive tetrapods, setting the stage for the Mesozoic (known as the Age of Reptiles).[46] One of the best known early stem-reptiles isMesosaurus, a genus from theEarly Permian that had returned to water, feeding on fish.
A 2021 examination of reptile diversity in the Carboniferous and the Permian suggests a much higher degree of diversity than previously thought, comparable or even exceeding that of synapsids. Thus, the "First Age of Reptiles" was proposed.[44]
It was traditionally assumed that the first reptiles retained ananapsid skull inherited from their ancestors.[47] This type of skull has askull roof with only holes for the nostrils, eyes and apineal eye.[32] The discoveries ofsynapsid-like openings (see below) in the skull roof of the skulls of several members ofParareptilia (the clade containing most of the amniotes traditionally referred to as "anapsids"), includinglanthanosuchoids,millerettids,bolosaurids, somenycteroleterids, someprocolophonoids and at least somemesosaurs[48][49][50] made it more ambiguous and it is currently uncertain whether the ancestral amniote had an anapsid-like or synapsid-like skull.[50] These animals are traditionally referred to as "anapsids", and form aparaphyletic basic stock from which other groups evolved.[3] Very shortly after the first amniotes appeared, a lineage calledSynapsida split off; this group was characterized by a temporal opening in the skull behind each eye giving room for the jaw muscle to move. These are the "mammal-like amniotes", or stem-mammals, that later gave rise to the truemammals.[51] Soon after, another group evolved a similar trait, this time with a double opening behind each eye, earning them the nameDiapsida ("two arches").[47] The function of the holes in these groups was to lighten the skull and give room for the jaw muscles to move, allowing for a more powerful bite.[32]
Turtles have been traditionally believed to be surviving parareptiles, on the basis of their anapsid skull structure, which was assumed to be primitive trait.[52] The rationale for this classification has been disputed, with some arguing that turtles are diapsids that evolved anapsid skulls, improving their armor.[22] Later morphologicalphylogenetic studies with this in mind placed turtles firmly within Diapsida.[53] Allmolecular studies have strongly upheld the placement of turtles within diapsids, most commonly as a sister group to extantarchosaurs.[27][28][29][30]
With the close of theCarboniferous, the amniotes became the dominant tetrapod fauna. While primitive, terrestrialreptiliomorphs still existed, the synapsid amniotes evolved the first truly terrestrialmegafauna (giant animals) in the form ofpelycosaurs, such asEdaphosaurus and the carnivorousDimetrodon. In the mid-Permian period, the climate became drier, resulting in a change of fauna: The pelycosaurs were replaced by thetherapsids.[54]
The close of the Permian saw the greatest mass extinction known (see thePermian–Triassic extinction event), an event prolonged by the combination of two or more distinct extinction pulses.[55] Most of the earlier parareptile and synapsid megafauna disappeared, being replaced by the true reptiles, particularlyarchosauromorphs. These were characterized by elongated hind legs and an erect pose, the early forms looking somewhat like long-legged crocodiles. Thearchosaurs became the dominant group during theTriassic period, though it took 30 million years before their diversity was as great as the animals that lived in the Permian.[55] Archosaurs developed into the well-knowndinosaurs andpterosaurs, as well as the ancestors ofcrocodilians. Since reptiles, firstrauisuchians and then dinosaurs, dominated the Mesozoic era, the interval is popularly known as the "Age of Reptiles". The dinosaurs also developed smaller forms, including the feather-bearing smallertheropods. In theCretaceous period, these gave rise to the first truebirds.[56]
Thesister group to Archosauromorpha isLepidosauromorpha, containinglizards andtuataras, as well as their fossil relatives. Lepidosauromorpha contained at least one major group of the Mesozoic sea reptiles: themosasaurs, which lived during theCretaceous period. The phylogenetic placement of other main groups of fossil sea reptiles – theichthyopterygians (includingichthyosaurs) and thesauropterygians, which evolved in the early Triassic – is more controversial. Different authors linked these groups either to lepidosauromorphs[4] or to archosauromorphs,[57][58][59] and ichthyopterygians were also argued to be diapsids that did not belong to the least inclusive clade containing lepidosauromorphs and archosauromorphs.[60]
Varanus priscus was a giant carnivorousgoanna lizard, perhaps as long as 7 metres and weighing up to 1,940 kilograms[61]Skeleton ofChampsosaurus, achoristodere, the latest surviving order of extinct reptiles. The last known choristoderes are known from theMiocene, around 11.3 million years ago
The close of theCretaceous period saw the demise of the Mesozoic era reptilian megafauna (see theCretaceous–Paleogene extinction event, also known as K-T extinction event). Of the largemarine reptiles, onlysea turtles were left; and of the non-marine large reptiles, only the semi-aquaticcrocodilians and broadly similarchoristoderes survived the extinction, with last members of the latter, the lizard-likeLazarussuchus, becoming extinct in theMiocene.[62] Of the great host of dinosaurs dominating the Mesozoic, only the small beakedbirds survived. This dramatic extinction pattern at the end of the Mesozoic led into the Cenozoic. Mammals and birds filled the empty niches left behind by the reptilian megafauna and, while reptile diversification slowed, bird and mammal diversification took an exponential turn.[46] However, reptiles were still important components of the megafauna, particularly in the form of large and gianttortoises.[63][64]
After the extinction of most archosaur and marine reptile lines by the end of the Cretaceous, reptile diversification continued throughout the Cenozoic.Squamates took a massive hit during the K–Pg event, only recovering ten million years after it,[65] but they underwent a great radiation event once they recovered, and today squamates make up the majority of living reptiles (> 95%).[66][67] Approximately 10,000 extant species of traditional reptiles are known, with birds adding about 10,000 more, almost twice the number of mammals, represented by about 5,700 living species (excludingdomesticated species).[68]
Alllepidosaurs andturtles have a three-chamberedheart consisting of twoatria, one variably partitionedventricle, and two aortas that lead to thesystemic circulation. The degree of mixing ofoxygenated 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 afitness advantage.[70]
Juvenileiguanaheart bisected through the ventricle, bisecting the left and right atrium
For example, iguana hearts, like the majority of the squamate hearts, are composed of three chambers with two aorta and one ventricle, cardiac involuntary muscles.[71] The main structures of the heart are the sinus venosus, the pacemaker, the left atrium, the right atrium, the atrioventricular valve, the cavum venosum, cavum arteriosum, the cavum pulmonale, the muscular ridge, the ventricular ridge, pulmonary veins, and paired aortic arches.[72]
Some squamate 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 duringventricular diastole and completely divides it duringventricular systole. Because of this ridge, some of thesesquamates are capable of producing ventricular pressure differentials that are equivalent to those seen in mammalian and avian hearts.[73]
Crocodilians have an anatomically four-chambered heart, similar tobirds, but also have two systemic aortas and are therefore capable of bypassing theirpulmonary circulation.[74] In turtles, the ventricle is not perfectly divided, so a mix of aerated and nonaerated blood can occur.[75]
Sustained energy output (joules) of a typical reptile versus a similar size mammal as a function of core body temperature. The mammal has a much higher peak output, but can only function over a very narrow range of body temperature.
Modern non-avian reptiles exhibit some form ofcold-bloodedness (i.e. some mix ofpoikilothermy,ectothermy, andbradymetabolism) so that they have limited physiological means of keeping the body temperature constant and often rely on external sources of heat. Due to a less stable core temperature thanbirds andmammals, reptilian biochemistry requiresenzymes capable of maintaining efficiency over a greater range of temperatures than in the case forwarm-blooded animals. The optimum body temperature range varies with species, but is typically below that of warm-blooded animals; for many lizards, it falls in the 24–35 °C (75–95 °F) range,[76] while extreme heat-adapted species, like the Americandesert iguanaDipsosaurus dorsalis, can have optimal physiological temperatures in the mammalian range, between 35 and 40 °C (95 and 104 °F).[77] While the optimum temperature is often encountered when the animal is active, the low basal metabolism makes body temperature drop rapidly when the animal is inactive.
As in all animals, reptilian muscle action produces heat. In large reptiles, likeleatherback turtles, the low surface-to-volume ratio allows this metabolically produced heat to keep the animals warmer than their environment even though they do not have awarm-blooded metabolism.[78] This form of homeothermy is calledgigantothermy; it has been suggested as having been common in largedinosaurs and other extinct large-bodied reptiles.[79][80]
The benefit of a low resting metabolism is that it requires far less fuel to sustain bodily functions. By using temperature variations in their surroundings, or by remaining cold when they do not need to move, reptiles can save considerable amounts of energy compared to endothermic animals of the same size.[81] A crocodile needs from a tenth to a fifth of the food necessary for alion of the same weight and can live half a year without eating.[82] Lower food requirements and adaptive metabolisms allow reptiles to dominate the animal life in regions where netcalorie availability is too low to sustain large-bodied mammals and birds.
It is generally assumed that reptiles are unable to produce the sustained high energy output necessary for long distance chases or flying.[83] Higher energetic capacity might have been responsible for the evolution ofwarm-bloodedness in birds and mammals.[84] However, investigation of correlations between active capacity andthermophysiology show a weak relationship.[85] Most extant reptiles are carnivores with a sit-and-wait feeding strategy; whether reptiles are cold blooded due to their ecology is not clear. Energetic studies on some reptiles have shown active capacities equal to or greater than similar sized warm-blooded animals.[86]
X-rayfluoroscopy videos of a female American alligator showing contraction of the lungs while breathing
All reptiles breathe usinglungs. Aquaticturtles have developed more permeable skin, and some species have modified theircloaca to increase the area forgas exchange.[87] Even with these adaptations, breathing is never fully accomplished without lungs. Lung ventilation is accomplished differently in each main reptile group. Insquamates, the lungs are ventilated almost exclusively by the axial musculature. This is also the same musculature that is used during locomotion. Because of thisconstraint, 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, employbuccal 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.[88]
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 "hepaticpiston". Theairways form a number of double tubular chambers within each lung. On inhalation and exhalation air moves through the airways in the same direction, thus creating a unidirectional airflow through the lungs. A similar system is found in birds,[89] monitor lizards[90] and iguanas.[91]
Most reptiles lack asecondary 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 (familyScincidae) 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.[92]
Howturtles 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 those turtlesbreathe. The varied results indicate that turtles have found a variety of solutions to this problem.
The difficulty is that mostturtle 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 adiaphragm), 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).[93]
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.[94] This is because they use 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.[94]
Compared with frogs, birds, and mammals, reptiles are less vocal. Sound production is usually limited tohissing, which is produced merely by forcing air though a partly closedglottis and is not considered to be a true vocalization. The ability to vocalize exists in crocodilians, some lizards and turtles; and typically involves vibrating fold-like structures in thelarynx or glottis. Somegeckos and turtles possess truevocal cords, which haveelastin-rich connective tissue.[95][96]
Hearing in humans relies on 3 parts of the ear; the outer ear that directs sound waves into the ear canal, the middle ear that transmits incoming sound waves to the inner ear, and the inner ear that helps in hearing and keeping their balance. Unlike humans and other mammals, snakes do not possess an outer ear, a middle ear, and atympanum but have an inner ear structure withcochleas directly connected to their jawbone.[97] They are able to feel the vibrations generated from the sound waves in their jaw as they move on the ground. This is done by the use ofmechanoreceptors, sensory nerves that run along the body of snakes directing the vibrations along the spinal nerves to the brain. Snakes have a sensitive auditory perception and can tell which direction sound being made is coming from so that they can sense the presence of prey or predator but it is still unclear how sensitive snakes are to sound waves traveling through the air.[98]
Reptilian skin is covered in a hornyepidermis, 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 thickdermal layer that producesleather in mammals.[99]Exposed parts of reptiles are protected byscales orscutes, sometimes with a bony base (osteoderms), formingarmor. Inlepidosaurs, such as lizards and snakes, the whole skin is covered in overlappingepidermal scales. Such scales were once thought to be typical of the class Reptilia as a whole, but are now known to occur only in lepidosaurs.[citation needed] The scales found in turtles and crocodiles are ofdermal, rather than epidermal, origin and are properly termed scutes.[citation needed] In turtles, the body is hidden inside a hard shell composed of fused scutes.
Lacking a thick dermis, reptilian leather is not as strong as mammalian leather. It is used in leather-wares for decorative purposes for shoes, belts and handbags, particularly crocodile skin.
Reptiles shed their skin through a process calledecdysis which occurs continuously throughout their lifetime. In particular, younger reptiles tend to shed once every five to six weeks while adults shed three to four times a year.[100] Younger reptiles shed more because of their rapid growth rate. Once full size, the frequency of shedding drastically decreases. The process of ecdysis involves forming a new layer of skin under the old one.Proteolytic enzymes andlymphatic fluid is secreted between the old and new layers of skin. Consequently, this lifts the old skin from the new one allowing shedding to occur.[101] Snakes will shed from the head to the tail while lizards shed in a "patchy pattern".[101]Dysecdysis, a common skin disease in snakes and lizards, will occur when ecdysis, or shedding, fails.[102] There are numerous reasons why shedding fails and can be related to inadequate humidity and temperature, nutritional deficiencies, dehydration and traumatic injuries.[101] 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 disrupt the process of ecdysis.[102]
Excretion is performed mainly by two smallkidneys. In diapsids,uric acid is the mainnitrogenous waste product; turtles, likemammals, excrete mainlyurea. Unlike thekidneys 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 aloop of Henle, which is present in thenephrons of birds and mammals. Because of this, many reptiles use thecolon to aid in thereabsorption of water. Some are also able to take up water stored in thebladder. Excess salts are also excreted by nasal and lingualsalt glands in some reptiles.
In all reptiles, the urinogenital ducts and therectum both empty into an organ called acloaca. In some reptiles, a midventral wall in the cloaca may open into a urinary bladder, but not all. It is present in all turtles and tortoises as well as most lizards, but is lacking in themonitor lizard, thelegless lizards. It is absent in the snakes, alligators, and crocodiles.[103]
Many turtles and lizards have proportionally very large bladders.Charles Darwin noted that theGalapagos tortoise had a bladder which could store up to 20% of its body weight.[104] Such adaptations are the result of environments such as remote islands and deserts where water is very scarce.[105]: 143 Other desert-dwelling reptiles have large bladders that can store a long-term reservoir of water for up to several months and aid inosmoregulation.[106]
Turtles have two or more accessory urinary bladders, located lateral to the neck of the urinary bladder and dorsal to the pubis, occupying a significant portion of their body cavity.[107] Their bladder is also usually bilobed with a left and right section. The right section is located under the liver, which prevents large stones from remaining in that side while the left section is more likely to havecalculi.[108]
Most reptiles are insectivorous or carnivorous and have simple and comparatively short digestive tracts due to meat being fairly simple to break down and digest.Digestion is slower than inmammals, reflecting their lower restingmetabolism and their inability to divide andmasticate their food.[109] Theirpoikilotherm metabolism has very low energy requirements, allowing large reptiles like crocodiles and large constrictors to live from a single large meal for months, digesting it slowly.[82]
While modern reptiles are predominantly carnivorous, during the early history of reptiles several groups produced some herbivorousmegafauna: in thePaleozoic, thepareiasaurs; and in theMesozoic several lines ofdinosaurs.[46] Today,turtles are the only predominantly herbivorous reptile group, but several lines ofagamas andiguanas have evolved to live wholly or partly on plants.[110]
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 calledgastroliths) to aid in digestion: The rocks are washed around in the stomach, helping to grind up plant matter.[110] Fossil gastroliths have been found associated with bothornithopods andsauropods, though whether they actually functioned as a gastric mill in the latter is disputed.[111][112]Salt water crocodiles also use gastroliths asballast, stabilizing them in the water or helping them to dive.[113] A dual function as both stabilizing ballast and digestion aid has been suggested for gastroliths found inplesiosaurs.[114]
The reptilian nervous system contains the same basic part of theamphibian brain, but the reptilecerebrum andcerebellum are slightly larger. Most typical sense organs are well developed with certain exceptions, most notably thesnake's lack of external ears (middle and inner ears are present). There are twelve pairs ofcranial nerves.[115] Due to their short cochlea, reptiles useelectrical tuning to expand their range of audible frequencies.
Most reptiles arediurnal animals. The vision is typically adapted to daylight conditions, with color vision and more advanced visualdepth perception than in amphibians and most mammals.
Reptiles usually have excellent vision, allowing them to detect shapes and motions at long distances. They often have poor vision in low-light conditions. Birds, crocodiles and turtles have three types ofphotoreceptor:rods, singlecones and double cones, which gives them sharp color vision and enables them to seeultraviolet wavelengths.[116] The lepidosaurs appear to have lost theduplex retina and only have a single class of receptor that is cone-like or rod-like depending on whether the species is diurnal or nocturnal.[117] In many burrowing species, such asblind snakes, vision is reduced.
Manylepidosaurs have a photosensory organ on the top of their heads called theparietal eye, which are also calledthird eye,pineal eye orpineal gland. This "eye" does not work the same way as a normal eye does as it has only a rudimentary retina and lens and thus, cannot form images. It is, however, sensitive to changes in light and dark and can detect movement.[116]
Some snakes have extra sets of visual organs (in the loosest sense of the word) in the form ofpits sensitive toinfrared radiation (heat). Such heat-sensitive pits are particularly well developed in thepit vipers, but are also found inboas andpythons. These pits allow the snakes to sense the body heat of birds and mammals, enabling pit vipers to hunt rodents in the dark.[b]
Most reptiles, as well as birds, possess anictitating membrane, a translucent third eyelid which is drawn over the eye from the inner corner. In crocodilians, it protects its eyeball surface while allowing a degree of vision underwater.[119] However, many squamates, geckos and snakes in particular, lack eyelids, which are replaced by a transparent scale. This is called thebrille, spectacle, or eyecap. The brille is usually not visible, except for when the snake molts, and it protects the eyes from dust and dirt.[120]
Crocodilian egg diagram (1) eggshell, (2) yolk sac, (3) yolk (nutrients), (4) vessels, (5) amnion, (6) chorion, (7) air space, (8) allantois, (9) albumin (egg white), (10) amniotic sac, (11) crocodile embryo, (12) amniotic fluidCommon house geckos mating, ventral view withhemipenis inserted in thecloacaMost reptiles reproduce sexually, for example thisTrachylepis maculilabrisskinkReptiles haveamniotic eggs with hard or leathery shells, requiringinternal fertilization when mating.
Reptiles generallyreproduce sexually,[121] though some are capable ofasexual reproduction. All reproductive activity occurs through thecloaca, the single exit/entrance at the base of the tail where waste is also eliminated. Most reptiles havecopulatory organs, which are usually retracted or inverted and stored inside the body. In turtles and crocodilians, the male has a single medianpenis, while squamates, including snakes and lizards, possess a pair ofhemipenes, only one of which is typically used in each session. Tuatara, however, lack copulatory organs, and so the male and female simply press their cloacas together as the male discharges sperm.[122]
Most reptiles lay amniotic eggs covered with leathery or calcareous shells. Anamnion (5),chorion (6), andallantois (8) are present duringembryonic life. The eggshell (1) protects the crocodile embryo (11) and keeps it from drying out, but it is flexible to allow gas exchange. The chorion (6) aids in gas exchange between the inside and outside of the egg. It allows carbon dioxide to exit the egg and oxygen gas to enter the egg. The albumin (9) further protects the embryo and serves as a reservoir for water and protein. The allantois (8) is a sac that collects the metabolic waste produced by the embryo. The amniotic sac (10) contains amniotic fluid (12) which protects and cushions the embryo. The amnion (5) aids in osmoregulation and serves as a saltwater reservoir. The yolk sac (2) surrounding the yolk (3) contains protein and fat rich nutrients that are absorbed by the embryo via vessels (4) that allow the embryo to grow and metabolize. The air space (7) provides the embryo with oxygen while it is hatching. This ensures that the embryo will not suffocate while it is hatching. There are nolarval stages of development.Viviparity andovoviviparity have evolved in squamates and many extinct clades of reptiles. Among squamates, 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 nourishment to supplement the yolk, and yet others lack any yolk and provide all nutrients via a structure similar to the mammalianplacenta. The earliest documented case of viviparity in reptiles is the EarlyPermianmesosaurs,[123] although some individuals or taxa in that clade may also have been oviparous because a putative isolated egg has also been found. Several groups of Mesozoic marine reptiles also exhibited viviparity, such asmosasaurs,ichthyosaurs, andSauropterygia, a group that includespachypleurosaurs andPlesiosauria.[7]
Asexual reproduction has been identified insquamates 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, calledparthenogenesis, occurs in several species ofgecko, and is particularly widespread in theteiids (especiallyAspidocelis) andlacertids (Lacerta). In captivity,Komodo dragons (Varanidae) have reproduced byparthenogenesis.
Some reptiles exhibittemperature-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 tuatara.[124] To date, there has been no confirmation of whether TDSD occurs in snakes.[125]
Gianttortoises are among the longest-lived vertebrate animals (over 100 years by some estimates) and have been used as a model for studyinglongevity.[126] DNA analysis of thegenomes ofLonesome George, the iconic last member ofChelonoidis abingdonii, and theAldabra giant tortoiseAldabrachelys gigantea led to the detection of lineage-specific variants affectingDNA repair genes that might contribute to our understanding of increased lifespan.[126]
Reptiles are generally considered less intelligent than mammals and birds.[32] Thesize of their brain relative to their body is much less than that of mammals, theencephalization quotient being about one tenth of that of mammals,[127] though larger reptiles can show more complex brain development. Larger lizards, like themonitors, are known to exhibit complex behavior, including cooperation[128] and cognitive abilities allowing them to optimize theirforaging andterritoriality over time.[129] Crocodiles have relatively larger brains and show a fairly complex social structure. TheKomodo dragon is even known to engage in play,[130] as are turtles, which are also considered to be social creatures,[131] and sometimes switch between monogamy and promiscuity in their sexual behavior.[citation needed] One study found thatwood turtles were better thanwhite rats at learning to navigate mazes.[132] Another study found that giant tortoises are capable of learning throughoperant conditioning, visual discrimination and retained learned behaviors with long-term memory.[133] Sea turtles have been regarded as having simple brains, but their flippers are used for a variety of foraging tasks (holding, bracing, corralling) in common with marine mammals.[134]
Many small reptiles, such as snakes and lizards, that 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.[136] 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 tend to avoid confrontation throughcamouflage. Two major groups of reptile predators are birds and other reptiles, both of which have well-developed color vision. Thus the skins of many reptiles havecryptic coloration of plain or mottled gray, green, and brown to allow them to blend into the background of their natural environment.[137] Aided by the reptiles' capacity for remaining motionless for long periods, the camouflage of many snakes is so effective that people or domestic animals are most typically bitten because they accidentally step on them.[138]
When camouflage fails to protect them,blue-tongued skinks will try to ward off attackers by displaying their blue tongues, and thefrill-necked lizard will display its brightly colored frill. These same displays are used in territorial disputes and during courtship.[139] If danger arises so suddenly that flight is useless, 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 to ward off approaching danger.
In contrast to the normal drab coloration of most reptiles, the lizards of the genusHeloderma (theGila monster and thebeaded lizard) and many of thecoral snakes have high-contrast warning coloration, warning potential predators they are venomous.[140] A number of non-venomous North American snake species have colorful markings similar to those of the coral snake, an oft cited example ofBatesian mimicry.[141][142]
Camouflage does not always fool a predator. When caught out, snake species adopt different defensive tactics and use a complicated set of behaviors when attacked. Some species, like cobras or hognose snakes, first elevate their head and spread out the skin of their neck in an effort to look large and threatening. Failure of this strategy may lead to other measures practiced particularly by cobras, vipers, and closely related species, which usevenom to attack. The venom is modified saliva, delivered through fangs from avenom gland.[143][144] Some non-venomous snakes, such as Americanhognose snakes or Europeangrass snake,play dead when in danger; some, including the grass snake, exude a foul-smelling liquid to deter attackers.[145][146]
When acrocodilian is concerned about its safety, it will gape to expose the teeth and tongue. If this does not work, the crocodilian gets a little more agitated and typically begins to make hissing sounds. After this, the crocodilian will start to change 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 to bite immediately. Some will use their heads assledgehammers and literally smash an opponent, some will rush or swim toward the threat from a distance, even chasing the opponent onto land or galloping after it.[147] The main weapon in all crocodiles is the bite, which can generate very high bite force. Many species also possesscanine-like teeth. These are used primarily for seizing prey, but are also used in fighting and display.[148]
Geckos,skinks, and some other lizards that are captured by the tail will shed part of the tail structure through a process calledautotomy and thus be able to flee. The detached tail will continue to thrash, creating a deceptive sense of continued struggle and distracting the predator's attention from the fleeing prey animal. The detached tails ofleopard geckos can wiggle for up to 20 minutes. The tail grows back in most species, but some, like crested geckos, lose their tails for the rest of their lives.[149] In many species the tails are of a separate and dramatically more intense color than the rest of the body so as to encourage potential predators to strike for the tail first. In theshingleback skink and some species of geckos, the tail is short and broad and resembles the head, so that the predators may attack it rather than the more vulnerable front part.[150]
Reptiles that are capable of shedding their tails can partiallyregenerate them over a period of weeks. The new section will however contain cartilage rather than bone, and will never grow to the same length as the original tail. It is often also distinctly discolored compared to the rest of the body and may lack some of the external sculpting features seen in the original tail.[151]
Dinosaurs have been widely depicted in culture since the English palaeontologistRichard Owen coined the namedinosaur in 1842. As soon as 1854, theCrystal Palace Dinosaurs were on display to the public in south London.[152][153] One dinosaur appeared in literature even earlier, asCharles Dickens placed aMegalosaurus in the first chapter of his novelBleak House in 1852.[c]The dinosaurs featured in books, films, television programs, artwork, and other media have been used for both education and entertainment. The depictions range from the realistic, as in the televisiondocumentaries of the 1990s and first decade of the 21st century, to the fantastic, as in themonster movies of the 1950s and 1960s.[153][155][156]
The snake or serpent has played a powerfulsymbolic role in different cultures. InEgyptian history, the Nile cobra adorned the crown of thepharaoh. It wasworshipped as one of the gods and was also used for sinister purposes: murder of an adversary and ritual suicide (Cleopatra). InGreek mythology, snakes are associated with deadly antagonists, as achthonic symbol, roughly translated asearthbound. The nine-headedLernaean Hydra thatHercules defeated and the threeGorgon sisters are children ofGaia, the earth.Medusa was one of the three Gorgon sisters whoPerseus defeated. Medusa is described as a hideous mortal, with snakes instead of hair and the power to turn men to stone with her gaze. After killing her, Perseus gave her head toAthena who fixed it to her shield called theAegis. TheTitans are depicted in art with their legs replaced by bodies of snakes for the same reason: They are children of Gaia, so they are bound to the earth.[157] In Hinduism,snakes are worshipped as gods, with many women pouring milk on snake pits. The cobra is seen on the neck ofShiva, whileVishnu is depicted often as sleeping on a seven-headed snake or within the coils of a serpent. There are temples in India solely for cobras sometimes calledNagraj (King of Snakes), and it is believed that snakes are symbols of fertility. In the annual Hindu festival ofNag Panchami, snakes are venerated and prayed to.[158] In religious terms, the snake andjaguar are arguably the most important animals in ancientMesoamerica. "In states of ecstasy, lords dance a serpent dance; great descending snakes adorn and support buildings fromChichen Itza toTenochtitlan, and theNahuatl wordcoatl meaning serpent or twin, forms part of primary deities such asMixcoatl,Quetzalcoatl, andCoatlicue."[159] In Christianity and Judaism, a serpent appears in Genesis to temptAdam and Eve with theforbidden fruit from theTree of Knowledge of Good and Evil.[160]
The turtle has a prominent position as a symbol of steadfastness and tranquility in religion, mythology, and folklore from around the world.[161] A tortoise's longevity is suggested by its long lifespan and its shell, which was thought to protect it from any foe.[162] In thecosmological myths of several cultures aWorld Turtle carries the world upon its back or supports the heavens.[163]
Deaths fromsnakebites are uncommon in many parts of the world, but are still counted in tens of thousands per year in India.[164] Snakebite can be treated withantivenom made from the venom of the snake. To produce antivenom, a mixture of the venoms of different species of snake is injected into the body of a horse in ever-increasing dosages until the horse is immunized. Blood is then extracted; the serum is separated, purified and freeze-dried.[165] Thecytotoxic effect of snake venom is being researched as a potential treatment for cancers.[166]
Gila monsters produce compounds that reduce plasma glucose; one of these substances is now used in the anti-diabetes drugexenatide (Byetta), aglucagon-like peptide-1 (GLP-1) receptor agonist likesemiglutide (Ozempic).[167][168] Another toxin from Gila monster saliva has been studied for use as an anti-Alzheimer's drug.[169]
Geckos have also been used asfolk medicine, especially in China, without any evidence that they have any active compounds.[170] Turtles have been used in Chinese traditional medicine for thousands of years, with every part of the turtle believed to have medical benefits (again, without scientific evidence). Growing demand forturtle meat has placed pressure on vulnerable wild populations of turtles.[171]
Crocodiles are protected in many parts of the world, and arefarmed commercially. Their hides are tanned and used to make leather goods such as shoes andhandbags;crocodile meat is also considered a delicacy.[172] The most commonly farmed species are the saltwater and Nile crocodiles. Farming has resulted in an increase in the saltwater crocodile population inAustralia, as eggs are usually harvested from the wild, so landowners have an incentive to conserve their habitat.Crocodile leather is made into wallets, briefcases, purses, handbags, belts, hats, and shoes.Crocodile oil has been used for various purposes.[173]
Snakes are also farmed, primarily inEast andSoutheast Asia, and their production has become more intensive in the last decade.Snake farming has been troubling for conservation in the past as it can lead tooverexploitation of wild snakes and their natural prey to supply the farms. However, farming snakes can limit the hunting of wild snakes, while reducing the slaughter of higher-order vertebrates like cows. The energy efficiency of snakes is higher than expected for carnivores, due to their ectothermy and low metabolism. Waste protein from the poultry and pig industries is used as feed in snake farms.[174] Snake farms produce meat,snake skin, and antivenom.
Turtle farming is another known but controversial practice. Turtles have been farmed for a variety of reasons, ranging from food to traditional medicine, the pet trade, and scientific conservation. Demand for turtle meat and medicinal products is one of the main threats to turtle conservation in Asia. Though commercial breeding would seem to insulate wild populations, it can stoke the demand for them and increase wild captures.[175][171] Even the potentially appealing concept of raising turtles at a farm to release into the wild is questioned by some veterinarians who have had some experience with farm operations. They caution that this may introduce into the wild populations infectious diseases that occur on the farm, but have not (yet) been occurring in the wild.[176][177]
Turtles and tortoises are increasingly popular pets, but keeping them can be challenging due to their particular requirements, such as temperature control, the need for UV light sources, and a varied diet. The long lifespans of turtles and especially tortoises mean they can potentially outlive their owners. Good hygiene and significant maintenance is necessary when keeping reptiles, due to the risks ofSalmonella and other pathogens.[181] Regular hand-washing after handling is an important measure to prevent infection.
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^"The copperhead is a pit viper and, like others pit vipers, it has heat-sensitive pit organs on each side of its head between the eye and the nostril. These pits detect objects that are warmer than the environment and enable copperheads to locate nocturnal, mammalian prey."[118]
^"Michaelmas term lately over, and the Lord Chancellor sitting in Lincoln's Inn Hall. Implacable November weather. As much mud in the streets, as if the waters had but newly retired from the face of the earth, and it would not be wonderful to meet aMegalosaurus, forty feet long or so, waddling like an elephantine lizard up Holborne Hill."[154]
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