Reptile

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This is the latest accepted revision, reviewed on 20 March 2023.

Clade of animals including lepidosaurs, testudines, and archosaurs

This article is about the animal class. For other uses, see Reptile (disambiguation).

Reptiles Temporal range: Ma PreꞒ Ꞓ O S D C P T J K Pg N
Reptilians by saurian clade listed in top-to-bottom order: six lepidosaurs and six archelosaurs.
Scientific classification
Kingdom:	Animalia
Phylum:	Chordata
Clade:	Sauropsida
Class:	Reptilia Laurenti, 1768
Extant groups
Lepidosauria (lepidosaurs) Rhynchocephalia (tuatara and relatives) Squamata (lizards, snakes and amphisbaenians) Archelosauria Testudines (turtles) Archosauria (archosaurs) Crocodilia (crocodilians) Aves (birds) See text for extinct groups.

Reptiles, as most commonly defined, are the animals in the

birds.^[1] Living reptiles comprise turtles, crocodilians, squamates (lizards and snakes) and rhynchocephalians (tuatara). As of March 2022, the Reptile Database includes about 11,700 species.^[2] In the traditional Linnaean classification system, birds are considered a separate class to reptiles. However, crocodilians are more closely related to birds than they are to other living reptiles, and so modern cladistic classification systems include birds within Reptilia, redefining the term as a clade. Other cladistic definitions abandon the term reptile altogether in favor of the clade Sauropsida, which refers to all amniotes more closely related to modern reptiles than to mammals. The study of the traditional reptile orders, customarily in combination with the study of modern amphibians, is called herpetology

.

The earliest known proto-reptiles originated around 312 million years ago during the

dinosaurs alongside many species of crocodyliforms, and squamates (e.g., mosasaurs). Modern non-bird reptiles inhabit all the continents except Antarctica

.

Reptiles are tetrapod

oviparous, although several species of squamates are viviparous, as were some extinct aquatic clades^[4]  – the fetus develops within the mother, using a (non-mammalian) placenta rather than contained in an eggshell. 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 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, which can grow up to 17 mm (0.7 in) to the saltwater crocodile

, Crocodylus porosus, which can reach over 6 m (19.7 ft) in length and weigh over 1,000 kg (2,200 lb).

Classification

See also: List of reptiles

Research history

See also: Skull roof

Reptiles, from Nouveau Larousse Illustré, 1897–1904, notice the inclusion of amphibians

(below the crocodiles)

In the 13th century the category of reptile 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 by Beauvais in his Mirror of Nature.^[5] In the 18th century, the reptiles were, from the outset of classification, grouped with the

Systema Naturæ.^[6]

The terms reptile and amphibian were largely interchangeable, reptile (from Latin repere, 'to creep') being preferred by the French.[7] J.N. Laurenti was the first to formally use the term Reptilia for an expanded selection of reptiles and amphibians basically similar to that of Linnaeus.^[8] Today, the two groups are still commonly treated under the single heading herpetology.

"Antediluvian monster", a Mosasaurus discovered in a Maastricht

limestone quarry, 1770 (contemporary engraving)

It was not until the beginning of the 19th century that it became clear that reptiles and amphibians are, in fact, quite different animals, and

amniotic egg

.

The terms Sauropsida ("lizard faces") and Theropsida ("beast faces") were used 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, Protosauria ("first lizards") in which he included some animals today considered reptile-like amphibians, as well as early reptiles.^[11]

In 1956,

Rhynchocephalia, Crocodilia, "thecodonts" (paraphyletic basal Archosauria), non-avian dinosaurs, pterosaurs, ichthyosaurs, and sauropterygians.^[12]

In the late 19th century, a number of definitions of Reptilia were offered. The traits listed by

articular bones, and certain characteristics of the vertebrae.^[13] The animals singled out by these formulations, the amniotes other than the mammals and the birds, are still those considered reptiles today.^[14]

The first reptiles had an anapsid type of skull roof, as seen in the Permian genus Captorhinus

The synapsid/sauropsid division supplemented another approach, one that split the reptiles into four subclasses based on the number and position of

Vertebrate Paleontology.^[15][16]

Those four subclasses were:

• cotylosaurs and chelonia (turtles and relatives)^[a]
• mammal-like reptiles
  ')
• Parapsida
  in Osborn's work.
• crocodilians, dinosaurs and pterosaurs

Phylogenetic classifications group the traditional "mammal-like reptiles", like this Varanodon

, 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 as incertae 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.^[17]

Phylogenetics and modern definition

By the early 21st century, vertebrate paleontologists were beginning to adopt

crocodilians than the latter are to the rest of extant reptiles. Colin Tudge

wrote:

Mammals are a

synapomorphies, 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'.^[14]

Despite the early proposals for replacing the paraphyletic Reptilia with a monophyletic Sauropsida, which includes birds, that term was never adopted widely or, when it was, was not applied consistently.^[19]

Bearded dragon (pogona) skeleton on display at the Museum of Osteology

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 monophyletic node-based crown 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.^[19] Major revisions since have included the reassignment of synapsids as non-reptiles, and classification of turtles as diapsids.^[19]

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 the

Homo 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.^[19]

Taxonomy

See also:

List of snakes

General classification of extinct and living reptiles, focusing on major groups.[20]^[21]

• Reptilia/Sauropsida
  • †Parareptilia
  • Eureptilia
    • †Captorhinidae
    • Diapsida
      • †Araeoscelidia
      • Neodiapsida
        • †
          Drepanosauromorpha
          (placement uncertain)
        • †
          paraphyletic
          )
        • †Ichthyosauromorpha (placement uncertain)
        • †Thalattosauria (placement uncertain)
        • Sauria
          • Lepidosauromorpha
            • Lepidosauriformes
              • Rhynchocephalia (tuatara)
              • Squamata (lizards & snakes)
          • †Choristodera (placement uncertain)
          • †Sauropterygia (placement uncertain)
          • Pantestudines (turtles and kin, placement uncertain)
          • Archosauromorpha
            • †Protorosauria (paraphyletic)
            • †
              Rhynchosauria
            • †Allokotosauria
            • Archosauriformes
              • †Phytosauria
              • Archosauria
                • Pseudosuchia
                  • Crocodilia (crocodilians)
                • Ornithodira
                  • †Pterosauria
                  • Dinosauria
                    • †Ornithischia
                    • Saurischia (including birds (Aves))

Phylogeny

The cladogram presented here illustrates the "family tree" of reptiles, and follows a simplified version of the relationships found by M.S. Lee, in 2013.^[22] All genetic studies have supported the hypothesis that turtles are diapsids; some have placed turtles within Archosauromorpha,^{[22][23][24][25][26][27]} though a few have recovered turtles as Lepidosauromorpha instead.^[28] The cladogram below used a combination of genetic (molecular) and fossil (morphological) data to obtain its results.^[22]

Amniota

Synapsida (mammals and their extinct relatives) Sauropsida / Reptilia †Parareptilia † Millerettidae unnamed †Eunotosaurus †Ankyramorpha † †Procolophonia † † Eureptilia † Romeriida †Paleothyris Diapsida † Neodiapsida † † Crown Reptilia/ Pan-Lepidosauria / † Lepidosauria Lepidosauromorpha Archelosauria/ Pan-Archosauria † s. s. † Prolacertiformes † † Rhynchosauria Pan-Testudines / † Eosauropterygia † †Sinosaurosphargis †Odontochelys Testudinata †Proganochelys Testudines (turtles) Pantestudines s. l. Sauria (total group)

The position of turtles

The placement of turtles has historically been highly variable. Classically, turtles were considered to be related to the primitive anapsid reptiles.

sister clade to the archosaurs, the group that includes crocodiles, non-avian dinosaurs, and birds.^[31] However, in their comparative analysis of the timing of organogenesis, Werneburg and Sánchez-Villagra (2009) found support for the hypothesis that turtles belong to a separate clade within Sauropsida, outside the saurian clade altogether.^[32]

Evolutionary history

Main article: Evolution of reptiles

Origin of the reptiles

An early reptile Hylonomus

Archaeopteryx lithographica

perched on the foreground tree stump

The origin of the reptiles lies about 310–320 million years ago, in the steaming swamps of the late Carboniferous period, when the first reptiles evolved from advanced reptiliomorphs.^[33]

The oldest known animal that may have been an

Ma show typical reptilian toes and imprints of scales.^[37] These tracks are attributed to Hylonomus, the oldest unquestionable reptile known.^[38]

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.[39] Other examples include Westlothiana (for the moment considered a reptiliomorph rather than a true amniote)^[40] and Paleothyris, both of similar build and presumably similar habit.

However,

microsaurs have been at times considered true reptiles, so an earlier origin is possible.^[41]

Rise of the reptiles

The earliest amniotes, including stem-reptiles (those amniotes closer to modern reptiles than to mammals), were largely overshadowed by larger stem-tetrapods, such as

Carboniferous Rainforest Collapse.^[42] 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.^[42] 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).^[43] One of the best known early stem-reptiles is Mesosaurus, a genus from the Early Permian

that had returned to water, feeding on fish.

A 2021 examination of reptile diversity in the Carboniferous and 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.[41]

Anapsids, synapsids, diapsids, and sauropsids

A = Anapsid,
B = Synapsid,
C = Diapsid

It was traditionally assumed that the first reptiles retained an

Diapsida ("two arches").^[44] 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.^[29]

Turtles have been traditionally believed to be surviving parareptiles, on the basis of their anapsid skull structure, which was assumed to be primitive trait.[49] The rationale for this classification has been disputed, with some arguing that turtles are diapsids that evolved anapsid skulls in order to improve their armor.^[33] Later morphological phylogenetic studies with this in mind placed turtles firmly within Diapsida.^[50] All molecular studies have strongly upheld the placement of turtles within diapsids, most commonly as a sister group to extant archosaurs.^{[24][25][26][27]}

Permian reptiles

With the close of the

therapsids.^[51]

The parareptiles, whose massive skull roofs had no postorbital holes, continued and flourished throughout the Permian. The pareiasaurian parareptiles reached giant proportions in the late Permian, eventually disappearing at the close of the period (the turtles being possible survivors).^[51]

Early in the period, the modern reptiles, or crown-group reptiles, evolved and split into two main lineages: the Archosauromorpha (forebears of turtles, crocodiles, and dinosaurs) and the Lepidosauromorpha (predecessors of modern lizards and tuataras). Both groups remained lizard-like and relatively small and inconspicuous during the Permian.

Mesozoic reptiles

The close of the Permian saw the greatest mass extinction known (see the

birds.^[53]

The

mosasaurs, which lived during the Cretaceous period. The phylogenetic placement of other main groups of fossil sea reptiles – the ichthyopterygians (including ichthyosaurs) and the sauropterygians, which evolved in the early Triassic – is more controversial. Different authors linked these groups either to lepidosauromorphs^[1] or to archosauromorphs,^[54][55][56] and ichthyopterygians were also argued to be diapsids that did not belong to the least inclusive clade containing lepidosauromorphs and archosauromorphs.^[57]

Cenozoic reptiles

Varanus priscus was a giant carnivorous goanna lizard, perhaps as long as 7 metres and weighing up to 1,940 kilograms^[58]

choristodere, the latest surviving order of extinct reptiles. The last known choristoderes are known from the Miocene

, around 11.3 million years ago

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,^[62] but they underwent a great radiation event once they recovered, and today squamates make up the majority of living reptiles (> 95%).^[63][64] 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 (excluding domesticated species).^[65]

Morphology and physiology

Circulation

Thermographic image of monitor lizards

Juvenile Iguana heart

bisected through the ventricle, bisecting the left and right atrium

For example, Iguana hearts, like the majority of the squamates hearts, are composed of three chambers with two aorta and one ventricle, cardiac involuntary muscles.^[68] 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.^[69]

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 during

Crocodilians have an anatomically four-chambered heart, similar to birds, but also have two systemic aortas and are therefore capable of bypassing their pulmonary circulation.^[71]

Metabolism

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 of cold-bloodedness (i.e. some mix of poikilothermy, ectothermy, and bradymetabolism) 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 than birds and mammals, reptilian biochemistry requires enzymes capable of maintaining efficiency over a greater range of temperatures than in the case for warm-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,^[72] while extreme heat-adapted species, like the American desert iguana Dipsosaurus dorsalis, can have optimal physiological temperatures in the mammalian range, between 35° and 40 °C (95° and 104 °F).^[73] 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, like

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.[77] A crocodile needs from a tenth to a fifth of the food necessary for a lion of the same weight and can live half a year without eating.^[78] Lower food requirements and adaptive metabolisms allow reptiles to dominate the animal life in regions where net calorie 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.^[79] Higher energetic capacity might have been responsible for the evolution of warm-bloodedness in birds and mammals.^[80] However, investigation of correlations between active capacity and thermophysiology show a weak relationship.^[81] 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.^[82]

Respiratory system

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.^[83] 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.^[84]

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". The airways 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,^[85] monitor lizards^[86] and iguanas.^[87]

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.^[88]

Turtles and tortoises

Red-eared slider

taking a gulp of air

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 those turtles breathe. The varied 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).^[89]

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.^[90] 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.^[90]

Sound production

Compared with frogs, birds, and mammals, reptiles are less vocal. Sound production is usually limited to

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 your balance. Unlike humans and other mammals, snakes do not possess an outer ear, a middle ear, and a tympanum but have an inner ear structure with cochleas directly connected to their jawbone.^[93] 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 of mechanoreceptors, 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.^[94]

Skin