Reptile

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Reptiles Fossil range: 320–0 Ma PreЄ Є O S D C P T J K Pg N Carboniferous – Recent
From left to right: Spectacled Caiman (Caiman crocodilus), Green Sea Turtle (Chelonia mydas,), Eastern Diamondback Rattlesnake (Crotalus adamanteus) and Tuatara (Sphenodon punctatus)
Scientific classification
Kingdom: Animalia Phylum: Chordata Subphylum: Vertebrata (unranked) Amniota Class: Reptilia Laurenti, 1768
Included groups
Crocodilia Sphenodontia Squamata Testudines For extinct orders, see text
Excluded groups
Aves Mammalia

Reptiles, or members of the class Reptilia, are air-breathing, cold-blooded amniotes that have skin covered in scales or scutes as opposed to hair or feathers. They are tetrapods (having or having descended from vertebrates with four limbs) and lay amniote eggs, whose embryos are surrounded by the amnion membrane. Modern reptiles inhabit every continent with the exception of Antarctica, and four living orders are currently recognized:

• Crocodilia (crocodiles, gavials, caimans, and alligators): 23 species
• Sphenodontia (tuatara from New Zealand): 2 species
• Squamata (lizards, snakes, and amphisbaenids ("worm-lizards"): approximately 7,900 species
• Testudines (turtles, tortoises, and terrapins): approximately 300 species

The majority of reptile species are oviparous (egg-laying) although certain species of squamates are capable of giving live birth. This is achieved, either through ovoviviparity (egg retention), or viviparity (offspring born without use 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 that may reach 6 m in length and weigh over 1,000 kg. The science dealing with reptiles is called herpetology.

Contents 1 Classification 1.1 History of classification 1.2 Taxonomy 1.3 Phylogeny 2 Evolutionary history 2.1 Rise of the reptiles 2.2 Anapsids, synapsids and sauropsids 2.3 Permian reptiles 2.4 Mesozoic, the "Age of Reptiles" 2.5 Demise of the dinosaurs 3 Systems 3.1 Circulatory 3.2 Respiratory 3.2.1 Reptilian lungs 3.2.2 Turtles and tortoises 3.2.3 Palate 3.3 Skin 3.4 Excretory 3.5 Digestive systems 3.6 Nervous system 3.7 Vision 3.8 Reproductive 4 References 5 Further reading 6 See also 7 External links

Classification

History of classification

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.^[1] The terms "reptile" and "amphibian" were largely interchangeable, "reptiles" being preferred by the French.^[2] Josephus Nicolaus Laurenti were the first to formally use the term "Reptilia" for an expanded, though basically similar selection of reptiles and amphibians to that of Linnaeus.^[3] Not until the turn of the century did it become clear that reptiles and amphibians are in fact quite different animals, and Pierre André Latreille erected the class Batracia for the latter, dividing the tetrapods into the four familiar classes of reptiles, amphibians, birds and mammals.^[4]

The British anatomist Thomas Henry Huxley made Latreille's definition popular, and together with Richard Owen expanded Reptilia to include the various fossil “Antediluvian monster”, including the mammal-like 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.^[5]

Around the end of the 19th century, the class reptilia had come to included all the amniotes except birds and mammals. Thus reptiles were defined as the set of animals that includes crocodiles, alligators, tuatara, lizards, snakes, amphisbaenians, and turtles, grouped together as the class Reptilia (Latin repere, "to creep"). This is still the usual definition of the term. However, in recent years, many taxonomists^[who?] 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 developed 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 Reptila are 'non-avian, non-mammalian amniotes'.^[6]

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 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.^[7]

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 among 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 sauropyterygians.^[8]

This classification supplemented, but was never as popular as, the classification of the reptiles (according to Romer's classic Vertebrate Paleontology^[9]) into four subclasses according to the positioning of temporal fenestrae, openings in the sides of the skull behind the eyes. Those divisions were:

• Anapsida – no fenestrae
• Synapsida – one low fenestra (no longer considered true reptiles)
• Euryapsida – one high fenestra (now included within Diapsida)
• Diapsida – two fenestrae

All of the above but Synapsida fall under Sauropsida.

Taxonomy

Classification to order level, after Benton, 2004.^[10]

• Series Amniota
  • Class Synapsida
    • Order Pelycosauria*
    • Order Therapsida
      • Class Mammalia
  • Class Sauropsida
    • Subclass Anapsida
      • Order Testudines (turtles)
    • Subclass Diapsida
      • Order Araeoscelidia
      • Order Younginiformes
      • Infraclass Ichthyosauria
      • Infraclass Lepidosauromorpha
        • Superorder Sauropterygia
          • Order Placodontia
          • Order Nothosauroidea
          • Order Plesiosauria
        • Superorder Lepidosauria
          • Order Sphenodontia (tuatara)
          • Order Squamata (lizards & snakes)
      • Infraclass Archosauromorpha
        • Order Prolacertiformes
        • Division Archosauria
          • Subdivision Crurotarsi
            • Superorder Crocodylomorpha
              • Order Crocodilia
              • Order Phytosauria
              • Order Rauisuchia
              • Order Rynchosauria
          • Subdivision Avemetatarsalia
            • Infradivision Ornithodira
              • Order Pterosauria
              • Superorder Dinosauria
                • Order Saurischia
                  • Class Aves
                • Order Ornithischia

Phylogeny

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.^[11]

Amniota

Synapsida Reptilia unnamed Anapsida Mesosauridae unnamed Millerettidae unnamed Lanthanosuchidae unnamed Nyctiphruretia unnamed Pareiasauria Procolophonoidea ?Testudines (turtles, tortoises, and terrapins) Romeriida Captorhinidae unnamed Protorothyrididae* Diapsida Araeoscelidia unnamed Younginiformes Sauria ?Ichthyosauria ?Sauropterygia Lepidosauromorpha (lizards, snakes, tuatara, and their extinct relatives) Archosauromorpha (crocodiles, birds, and their extinct relatives)

Evolutionary history

Rise of the reptiles

The origin of the reptiles lays about 320-310 million years back, in the steaming swamps of the late Carboniferous, when the first reptiles evolved from advanced reptilomorph labyrinthodonts.^[12] The oldest traces of reptiles is a series of footprints from the fossil strata of Nova Scotia, dated to 315 million years old.^[13] The tracks are attributed to Hylonomus, the oldest known reptile in the biological sense of the word.^[14] It was a small, lizard-like animal, about 20 to 30 cm (8-12 inche) long, with numerous sharp teeth indicating an insectivorous diet.^[15] Other examples include Westlothiana (for the moment considered to be more related to amphibians than amniotes)^{[citation needed]} and Paleothyris, both of similar build and presumably habit. One of the best known early reptiles is Mesosaurus, a genus of early reptiles from the early Permian that had returned to water, living off 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.

Anapsids, synapsids and sauropsids

The first reptiles are categorized as Anapsids, having a solid skull with holes only for nose, eyes, spinal cord, etc.^[16] Turtles are believed by some to be surviving Anapsids, as they also 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).^[17] Both sides have strong evidence, and the conflict has yet to be resolved.^{[18][19] [20]}

Very early after the first reptiles appeared, two branches split off.^[21] One lead to the Synapsida (the "mammal-like reptiles" or "stem mammals"), having two openings in the skull roof behind the eyes high , the other group, 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 bout groups was to lighten the skull and give room for the jaw muscles to move, allowing for a more powerful bite.^[22] The diapsids and later anapsids are classed as the "true reptiles", the Sauropsida.^[7].

Permian reptiles

With the close of the Carboniferous, reptiles became the dominant tetrapod fauna. While the terrestrial reptilomorph labyrinthodonts still existed, the mammal-like reptiles evolved the first terrestrial megafauna in the form of pelycosaurs like Edaphosaurus and the carnivorous Dimetrodon. In the mid-Permian the climate turned dryer, resulting in a faunal turnover. The primitive pelycosaurs where replaced by the more advanced therapsids.^[23]

The anapsid reptiles, with their massive skulls without 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).^[24]

Early in the period, the diapside reptiles split into two lineages, the lepidosaurs (forefathers of modern snakes, lizards, and tuataras). The group remained lizard-like and relatively small and inconspicuous during the whole periode.

Mesozoic, the "Age of Reptiles"

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, making room for the archosauromorph diapsids. The archosaurs was 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, 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 popularly known as the "Age of Reptiles". The dinosaurs also deveoped smaller forms, including the feather-bearing smaller theropds. In the mid Jurassic, these gave rise to the first birds.^[25]

The lepidosauromorph diapsids may have been ancestral to the sea reptiles.^[26] Developing into the ichthyosaurs and sauropterygians, they came to dominate the Mesozoic seas.

The Therpasids came under increasing pressure from the archosaurs the early Mesozoic and developed into increasingly smaller and more nocturnal forms, the first mammals being the only survivors of the line by late Jurassic.

Demise of the dinosaurs

The close of the Cretacious saw the demise of the Mesozoic 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”, opened up for the “Age of Mammals”. Despite this, reptiles are still a major fauna component, particularly in tropical climates. There are about 8200 extant species of reptiles (whereof almost half are snakes), compared to 5400 species of mammals (of which ⅔ are rodents and bats). The most numerous modern group with reptilian roots are the birds, with over 9000 species.

Systems

Circulatory

Most reptiles have a three-chamber heart consisting of two atria, one variably-partitioned ventricle, and two aorta that go the systemic circulation. The degree of mixing of oxygenated and deoxygenated blood in the three-chamber heart is variable 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.^[27]

There are some interesting exceptions to the general physiology. For instance, crocodilians have an anatomically four-chambered heart, but also have two systemic aorta and are therefore capable only of bypassing their pulmonary circulation.^[28] Also, some snake and lizard species (e.g., monitor lizards and pythons) have three-chamber hearts that become functional four-chamber 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.^[29]

Respiratory

Reptilian lungs

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."

Turtles and tortoises

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 & tortoises have found a variety of solutions to this problem. The problem 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 red-eared sliders, which also breathe during locomotion, but they had 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).

Palate

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 from getting kicked in 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.

Skin

Reptilian skin is covered in a horny epidermis, making it watertight and enable reptiles to live on dry land, in contrast to the amphibians. Compared to mammals, reptilian skin is rather thin, and lack the thick dermal layer that produces leather in mammals.^[30] Exposed parts of reptiles are protected by scales or scutes, sometimes with a bony base, forming armour. In turtles, the body is hidden inside a hard shell composed on fused scutes. In the lepidosaurians like lizards and snakes, the whole skin is covered in epidermal scales. Such scales where once thought to be typical of the class Reptilia as a whole, but are actually found only in lepidosaurians. The scales found in turtles and crocodiles are of dermal origin rather than epidermal, and are properly termed scutes.

Excretory

Excretion is performed mainly by two small kidneys. In diapsids, uric acid is the main nitrogenous waste product; turtles, like mammals, mainly excrete 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 present in the nephrons of birds and mammals, called a Loop of Henle. 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.

Digestive systems

Most reptiles are carnivorous and have rather simple and not overly long guts, meat being fairly simple to break down and digest. Digestion is slower than in mammals, reflecting about the fact that they can not divide and masticate their food like mammals do, and their lower metabolism. Being cold blooded their energy requirement is about a 5th to a 10th of that of a mammal of the same size. Large reptiles like crocodiles and the large constrictors can basically live from a single large meal for months, digesting it slowly.

While modern reptiles are predominately carnivorous, this has not always been so. During the early history of reptiles, several groups produced big-bodied 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 developed to live wholly or partly from plants.

Herbivorous reptiles face the same problems of mastication as herbivorous mammals, but lacking the complex mammal teeth, quite a few species swallow rocks and pebbles to aid in digestion, so called gastrolithes. The rocks are washed around in the stomach helping to grind up plant matter. Fossil gastrolithes has also been found associated with sauropods. Sea turtles, crocodiles and marine iguanas also use the gastrolithes as ballast, helping them to dive.

Nervous system

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.[2]

Reptiles are not generally considered particularly intelligent when compared to mammals and birds.^[31] Their brains fall well below those of mammals in size relative to the body, the encephalisation quotient being about one tenth of that of mammals.^[32] The crocodiles have brains in the higher size range and show a fairly complex social structure. Larger lizards like the monitors are known to exhibit complex behaviour, including cooporation.^[33] The Komodo dragon is known to engage in play.^[34]

Vision

Most reptiles are diurnal animals. The vision is typically adapted to daylight condition, with colour vision and advanced visual depth perception compared to amphibians and most mammals. In some species vision is reduced, such as blindsnakes.^[35] 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 also found in boas and pythons. These allows the snakes to sense the body heat from birds and mammals, making pitvipers able to hunt rodents in the dark.

Reproductive

Most reptiles reproduce sexually, though some are capable of asexual reproduction. All reproductive activity occurs with the cloaca, the single exit/entrance at the base of the tail where waste is also eliminated. Tuataras lack copulatory organs, so the male and female simply press their cloacas together as the male excretes sperm.^[36] Most reptiles, however, 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.

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 only evolved in Squamates, and a substantial fraction of the species utilize this mode of reprduction, including all boas and most vipers. 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 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 are able to produce a unisexual diploid clone of the mother. This 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.

Parthenogenetic species are also suspected to occur among chameleons, agamids, xantusiids, and typhlopids.

References

Further reading

See also

• List of reptiles
• Lists of reptiles by region
• Amniote
• Anapsida
• Synapsida
• Diapsida

External links

Wikispecies has information related to: Reptilia

The Wikibook Dichotomous Key has a page on the topic of Reptilia

• The Reptile Database
• Reptile Phylogeny
• World Reptile Amphibian Information Center
• Reptile images

v • d • e Extant Chordata classes by subphylum Kingdom Animalia · Subkingdom Eumetazoa · (unranked) Bilateria · Superphylum Deuterostomia Urochordata (Tunicates) Ascidiacea (Ascidians) · Thaliacea · Appendicularia (Larvaceans) · Sorberacea Cephalochordata (Lancelets) Leptocardii Craniata Myxini (Hagfish) · Hyperoartia (Lampreys) · Chondrichthyes (Cartilaginous fish) · Actinopterygii (Ray-finned fish) · Sarcopterygii (Lobe-finned fish) · Amphibia (Amphibians) · Sauropsida (Reptiles) · Aves (Birds) · Mammalia (Mammals)

v • d • e Extant sauropsida orders by subclass Kingdom: Animalia · Phylum: Chordata · Subphylum: Vertebrata Anapsida Chelonia Testudines Diapsida Lepidosauromorpha Sphenodontia · Squamata Archosauromorpha Crocodilia