reptilelike, adj.reptiloid /rep"tl oyd'/, adj.
/rep"til, -tuyl/, n.
1. any cold-blooded vertebrate of the class Reptilia, comprising the turtles, snakes, lizards, crocodilians, amphisbaenians, tuatara, and various extinct members including the dinosaurs.
2. (loosely) any of various animals that crawl or creep.
3. a groveling, mean, or despicable person.
4. of or resembling a reptile; creeping or crawling.
5. groveling, mean, or despicable.
[1350-1400; ME reptil < LL reptile, n. use of neut. of reptilis creeping, equiv. to L rept(us) (ptp. of repere to creep) + -ilis -ILE]

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Any of the approximately 6,000 species of the class Reptilia, air-breathing vertebrates that have internal fertilization and a scaly body and are cold-blooded.

Most species have short legs (or none) and have long tails, and most lay eggs. Living reptiles include the scaly reptiles (snakes and lizards; order Squamata), the crocodiles (Crocodilia), the turtles (Chelonia), and the unique tuatara (Rhynchocephalia). Being cold-blooded, reptiles are not found in very cold regions, and in regions with cold winters they usually hibernate. They range in size from geckos that measure about 1 in. (3 cm) long to the python, which grows to 30 ft (9 m); the largest turtle, the marine leatherback, weighs about 1,500 lbs (680 kg). Extinct reptiles include the dinosaurs, the pterosaurs, and the dolphinlike ichthyosaurs.

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     any member of the class Reptilia, the group of air-breathing vertebrates (vertebrate) that have internal fertilization, amniotic development (biological development), and epidermal scales (scale) covering part or all of their body. The major groups of living reptiles—the turtles (turtle) (order Testudines), tuataras (tuatara) (order Sphenodontida), lizards (lizard) and snakes (snake) (order Squamata), and crocodiles (crocodile) (order Crocodylia)—account for over 8,700 species. Birds (bird) (class Aves) share a common ancestor with crocodiles in subclass Archosauria (archosaur) and are technically one lineage of reptiles, but they are treated separately (see bird).

      The extinct reptiles included an even more diverse group of animals that ranged from the marine plesiosaurs (plesiosaur), pliosaurs, and ichthyosaurs (ichthyosaur) to the giant plant-eating and meat-eating dinosaurs (dinosaur) of terrestrial environments. Taxonomically, Reptilia and Synapsida (a group of mammal-like reptiles and their extinct relatives) were sister groups that diverged from a common ancestor during the Middle Pennsylvanian Epoch (approximately 312 to 307 million years ago). For millions of years representatives of these two groups were superficially similar; however, slowly lifestyles diverged, and from the synapsid line came hairy mammals (mammal) that possessed an endothermic (warm-blooded (warm-bloodedness)) physiology and mammary glands (mammary gland) for feeding their young. All birds and some groups of extinct reptiles, such as selected groups of dinosaurs, also evolved an endothermic physiology; however, the majority of modern reptiles possess an ectothermic (cold-blooded (cold-bloodedness)) physiology. Today, only the leatherback sea turtle (Dermochelys coriacea) has a near-endothermic physiology. So far no reptile, living or extinct, has developed specialized skin glands for feeding its young.

General features
      Most reptiles have a continuous external covering of epidermal scales (scale). Reptile scales contain a unique type of keratin called beta keratin; the scales and interscalar skin also contain alpha keratin, which is a trait shared with other vertebrates (vertebrate). Keratin is the main component of reptilian scales. Scales may be very small (as in the microscopic tubercular scales of dwarf geckos (gecko) [Sphaerodactylus]) or relatively large (as in the body scales of many groups of lizards (lizard) and snakes (snake)). The largest scales are the scutes covering the shell of a turtle or the plates of a crocodile.

      Other features also define the class Reptilia. The occipital condyle (a protuberance where the skull attaches to the first vertebra (vertebrate)) is single. The cervical vertebrae in reptiles have midventral keels, and the intercentrum of the second cervical vertebra fuses to the axis in adults. Taxa with well-developed limbs have two or more sacral vertebrae. The lower jaw of reptiles is made up of several bones but lacks an anterior coronoid bone. In the ear a single auditory bone, the stapes, transmits sound vibrations from the eardrum ( tympanum) to the inner ear. Sexual reproduction (sex) is internal, and sperm may be deposited by copulation or through the apposition of cloacae (cloaca). Asexual reproduction by parthenogenesis also occurs in some groups. Development (biological development) may be internal, with embryos (embryo) retained in the female's oviducts, and embryos of some species may be attached to the mother by a placenta; however, development in most species is external, with embryos enclosed in shelled eggs (egg). In all cases each embryo is encased in an amnion, a membranous fluid-filled sac.

      In the agriculture industry as a whole, reptiles do not have a great commercial value compared with fowl and hoofed mammals (mammal); nonetheless, they have a significant economic value for food and ecological services (such as insect control) at the local level, and they are valued nationally and internationally for food, medicinal products, leather goods, and the pet trade.

      Reptiles have their greatest economic impact in some temperate and many tropical areas, although this impact is often overlooked because their contribution is entirely local. A monetary value is often not assigned to any vertebrate that provides pest control. Nonetheless, many lizards control insect pests in homes and gardens; snakes are major predators of rodents (rodent), and the importance of rodent control has been demonstrated repeatedly when populations of rodent-eating snakes are decimated by snake harvesting for the leather trade. The absence of such snakes allows rodent populations to explode. Similarly, turtles, crocodiles, snakes, and lizards are regularly harvested as food for local consumption in many tropical areas. When this harvesting becomes commercial, the demands on local reptile populations commonly exceed the ability of species to replace themselves by normal reproductive means. Harvesting is often concentrated on the larger individuals of most species, and these individuals are often the adult females and males in the population; their removal greatly reduces the breeding stock and leads to a preciptious population decline.

      The overharvesting of crocodiles for the leather industry in the 1950s and 1960s caused the widespread extirpation, or localized extinction, of many crocodilian species. In addition, surviving populations experienced a near-worldwide drop in numbers. Since then, regulations at the national and international levels have greatly reduced the harvests, and proactive conservation and management measures have allowed many crocodilian populations to rebound. Regulated harvesting currently provides an adequate number of skins to the leather trade and also allows crocodiles to resume their role as top predators in many aquatic ecosystems (ecosystem). The late 20th-century return of the American alligator (Alligator mississippiensis) from near extinction in the southeastern United States demonstrates that successful management of reptile populations is possible if it is closely supervised.

      Regulated harvesting of large snakes and lizards is also underway in parts of Indonesia. In addition, several groups of reptiles ( tegu lizards in Argentina, freshwater turtles in China, and green iguanas (iguana) [Iguana iguana] in Central America) are raised as livestock. Often the process of regulated harvesting begins with the removal of a few eggs, juveniles, or adults from wild populations. Stocks of reptiles are raised on farms and ranches. Farms and ranches then sell some individuals to commercial interests, while others are retained as breeding stock.

      Reptiles have contributed significantly to a variety of biomedical and basic biological research programs. Snake venom studies contributed greatly to the care of heart-attack (heart attack) patients in the 1960s and 1970s and are widely studied in the development of pain-management drugs. Field studies of lizards and other reptiles and the manipulation of populations of various lizard species (such as the anoles (anole) [Anolis]) have provided scientists opportunities to test hypotheses on different aspects of evolution. Reptile research remains an important area of evolutionary biology. Similarly, lizards and other reptiles have provided experimental models for examining physiological mechanisms, especially those associated with body heat.

Size range
 Most reptiles are measured from snout to vent (that is, the tip of the nose to the cloaca); however, measurements of total length are common for larger species, and shell length is used to gauge the size of turtles. The body size of living reptiles varies widely. Dwarf geckos (Sphaerodactylus parthenopion) are the smallest reptiles and have a snout-to-vent length of 16–18 mm (0.6–0.7 inch). In contrast, giant turtles, such as the leatherback sea turtles (Dermochelys coriacea), possess shell lengths of nearly 2 metres (about 7 feet). In terms of total length, the largest living reptiles are the reticulated pythons (python) (Python reticulatus) and saltwater crocodiles (Crocodylus porosus), which may grow to more than 7 metres (23 feet) as adults. Some ancient reptile groups had members that were the largest animals ever to live on the land—some sauropod dinosaur fossils (fossil) measure 20–30 metres (66–98 feet) in total length. The largest marine reptiles, the pliosaurs, grew to 15 metres (50 feet) long.

      The reptile groups also show a diversity of morphologies. Some groups, such as most lizards and all crocodiles, possess strongly developed limbs, whereas other groups, such as the worm lizards and snakes, are limbless. Reptilian body flexibility ranges from the highly flexible forms found in snakes to the inflexible armoured bodies of turtles. In addition, the tails of most turtles tend to be short, especially when compared with the long heavy tails of crocodiles.

      Giants in any animal group always attract attention and are often exaggerated. Anacondas (anaconda) (Eunectes), gigantic snakes (snake) from South America, are undoubtedly the largest living snakes. The largest species, the green anaconda (E. murinus), likely only rarely exceeds 9 metres (30 feet) in length; nonetheless, persistent but unsubstantiated reports have been made of anacondas that are 12 metres (40 feet) long. The reticulated python (P. reticulatus) of Southeast Asia and the East Indies has been recorded at 10.1 metres (33.3 feet).

 At about 5.5 metres (18 feet), the king cobra (Ophiophagus hannah) of Asia and the East Indies is the longest venomous snake. The heaviest venomous snake is probably the eastern diamondback rattlesnake (Crotalus adamanteus); its length does not exceed 2.4 metres (7.9 feet), but it may weigh as much as 15.5 kg (34 pounds). The largest of the common nonvenomous snakes is probably the keeled rat snake (Ptyas carinatus), at about 3.7 metres (12 feet).

      Five species of crocodiles (crocodile) may grow larger than 6 metres (20 feet). Nile crocodiles (Crocodylus niloticus) and estuarine (or saltwater) crocodiles (C. porosus) regularly exceed this length. The American crocodile (C. acutus), the Orinoco crocodile (C. intermedius), and the gavial (Gavialis gangeticus) may also grow larger than 6 metres; however, this is less common. The gavial normally attains a length of about 4–5 metres (12–15 feet).

      The giant among living turtles is the marine leatherback sea turtle (D. coriacea), which reaches a total length of about 2.7 metres (9 feet) and a weight of about 680 kg (1,500 pounds). The largest of the land turtles is a Galápagos tortoise (Geochelone nigra), weighing about 255 kg (560 pounds).

      The largest modern lizard, the Komodo dragon (Varanus komodoensis) of the East Indies, is a monitor lizard (monitor) that attains a total length of 3 metres (10 feet). In addition, two or three other species of monitors reach 1.8 metres (5.9 feet). The water monitor (V. salvator) may grow to a greater total length than the Komodo dragon, but it does not exceed the latter in weight. The green iguana (I. iguana), which grows to a length of about 2 metres (7 feet) comes close to that size, but no other lizard does.

 Within each reptile group, with the possible exception of snakes, no living member is as large as its largest extinct representative. At about 2.7 metres (9 feet) in total length, the leatherback sea turtle is smaller than Archelon, a genus of extinct marine turtles from the Late Cretaceous (Cretaceous Period) (100 million to 65.5 million years ago) that was about 3.6 metres (11.8 feet) long. No modern crocodile approaches the estimated 15-metre (49-foot) length of Phobosuchus, and the Komodo dragon does not compare with Tylosaurs, a mosasaur that exceeded 6 metres (20 feet) in length. Some quadrapedal browsing dinosaurs grew to lengths of 30 metres (100 feet) and weights of 91,000 kg (200,000 pounds) or more.

  The smallest reptiles are found among the geckos (family Gekkonidae), skinks (skink) (family Scincidae), and microteiids (family Gymnopthalmidae); some of these lizards grow no longer than 4 cm (about 2 inches). Certain blind snakes (blind snake) (family Typhlopidae) are less than 10 cm (4 inches) in total length when fully grown. Several species of turtles weigh less than 450 grams (1 pound) and reach a maximum shell length of 12.5 cm (5 inches). The smallest crocodiles are the dwarf crocodiles (Osteolaemus tetraspis), which grow to about 2 metres (7 feet), and the dwarf caimans (caiman) (Paleosuchus), which typically grow to 1.7 metres (6 feet) or less.

Distribution and ecology
      Reptiles are mainly animals of Earth's temperate and tropical regions, and the greatest number of reptilian species live between 30° N and 30° S latitude. Nevertheless, at least two species, the European viper (Vipera berus) and the wall lizard (Lacerta) (Lacerta vivipara), have populations that edge over the Arctic Circle (66° 33' 39” N latitude). Other species of snakes, lizards, and turtles also live at high latitudes and altitudes and have evolved lifestyles that allow them to survive and reproduce with little more than three months of activity each year.

      Reptile activity is strongly dependent on the temperature of the surrounding environment. Reptiles are ectothermic—that is, they require an external heat source to elevate their body temperature. They are also considered cold-blooded (cold-bloodedness) animals, although this label can be misleading, as the blood of many desert reptiles is often relatively warm. The body temperatures of many species approximate the surrounding air or the temperature of the substrate, hence a reptile can feel cold to the human touch. Many species, particularly lizards, have preferred body temperatures above 28 °C (82 °F) and only pursue their daily activities when they have elevated their body temperatures to those levels. These species maintain elevated body temperatures at a relatively constant level by shuttling in and out of sunlight.

      Reptiles occur in most habitats (habitat), from the open sea to the middle elevations in mountainous habitats. The yellow-bellied sea snake (Pelamis platurus) spends all its life in marine environments. It feeds and gives birth far from any coastline and is helpless if washed ashore, whereas other sea snakes live in coastal waters of estuaries (estuary) and coral reefs (coral reef). The sea turtles are also predominately coastal animals, although most species have a pelagic, or open-ocean, phase that lasts from the hatchling stage to the young juvenile stage. Many snakes, crocodiles, and a few lizards are aquatic and live in freshwater habitats ranging from large rivers and lakes to small mountain streams. On land, turtles, snakes, and lizards also occur widely in forests, in grasslands, and even in true deserts. In many arid lands lizards and snakes are the major small-animal carnivores.

      Reptiles of the North Temperate Zone include many ecological types. Aquatic groups are represented in both hemispheres by the water snakes (snake), many testudinoid turtles, and the two species of Alligator. Terrestrial groups include tortoises, ground-dwelling snakes, and many genera of lizards (lizard). Arboreal snakes (tree snake) are few, and arboreal lizards are almost nonexistent. There are few specialized burrowing lizards in this region, but burrowing snakes are common.

      The wall lizard (L. vivipara) and the European viper (V. berus) are the most northerly distributed reptiles. A portion of each reptile's geographic range occurs just north of the Arctic Circle, at least in Scandinavia. Other reptiles—the slowworm (Anguis fragilis), the sand lizard (L. agilis), the grass snake (Natrix natrix), and the smooth snake (Coronella austriaca)—also appear at high latitudes and reach to 60° N in Europe. Of these six northern species, all but the grass snake are viviparous (viviparity) (live-bearing). Across Siberia only L. vivipara and V. berus live north of 60° N.

      In North America no reptile is found at 60° N latitude or higher. Two species of garter snakes (garter snake) (Thamnophis) live as far north as 55° N in western Canada; however, it is only south of 40° N that numerous species of reptiles occur. In the eastern United States and eastern Asia, several colubrid snake species, northern skinks (Plestiodon), glass lizards (glass lizard) (Ophisaurus), and softshell turtles (softshell turtle) (Trionychidae) are common.

      Across North America and Eurasia, the northern limit of turtles (turtle) is about 55° N. Even though these regions are characterized by many species of turtles, most families and genera are unique to one region or another. This phenomenon also occurs in other groups of reptiles. Many lizards of temperate Eurasia belong to the families Agamidae and Lacertidae, which do not occur in the Americas, whereas many lizards of North America are in the families Iguanidae and Teiidae, which do not live in Eurasia. Nonetheless, of the two living species of Alligator, one (A. mississippiensis) lives in the southeastern United States, and the other (A. sinensis) lives in China.

 The reptiles of the eastern United States are almost as distinct from those of the western United States and northern Mexico as they are from those of eastern Asia. The eastern region (that is, the eastern United States) has many genera and species of emydid (Emydidae) turtles. In contrast, the western region (that is, the western United States and northern Mexico), which is defined by a diagonal line running southeast to northwest through Texas, then northward along the Continental Divide, has only four or five species of emydids. Few genera and species of iguanid lizards inhabit the eastern region, whereas the western region has many. Although the eastern region has more species of water snakes (water snake), the western region contains more garter snakes. Whereas more species of snakes appear in the eastern United States than in the western areas, the converse is true of lizard species.

Central and South America
      Nearer to the Equator, reptiles become more numerous and diverse. This is true of crocodiles; Mexico is home to three species, but nine countries in South America are home to four or more crocodilian species. Turtles, lizards, and snakes are also particularly diverse in this region.

      Some groups of North American turtles are represented in the New World tropics. For example, the mud turtles (mud turtle) (Kinosternon) and sliders (Trachemys) appear in both regions, but the majority of species are members of genera and even families (such as the side-necked turtles (side-necked turtle) of families Podocnemididae and Chelidae) that are unknown in North America. In addition, Central America has three endemic genera of turtles (Dermatemys, Claudius, and Staurotypus).

      Many of the genera of iguanid lizards occurring in the western United States have species in Mexico; one genus of spiny lizards (Sceloporus) is most diverse in Mexico. South of Mexico the North American iguanids disappear and are replaced by tropical groups such as the black iguanas (Ctenosaura), the helmeted iguanids (Corythophanes), the casque-headed iguanids (Laemanctus), and the basilisks (basilisk) (Basiliscus). Iguanid lizards of the anole genus (Anolis) are represented in northern South America and the West Indies by more than 200 species. Other iguanid genera—the long-legged Polychrus—make their appearance.

      The lizard family Teiidae, though represented in the United States by the racerunners and whiptails of the genus Cnemidophorus, is primarily tropical, and its area of greatest biological diversity begins in Central America with the large, conspicuous, and active ameivas (Ameiva). The Gymnophthalmidae (or microteiids), close relatives of the teiids, are a diverse group of small-bodied lizards that live largely in and under leaf litter in the West Indies, Central America, and South America north of central Argentina.

      Among snakes, fer-de-lances (fer-de-lance) (Bothrops), coral snakes (coral snake) (Micrurus), rear-fanged snakes (such as the cat-eyed snakes (mangrove snake) [Leptodeira]), and certain nonvenomous genera (such as the tropical green snakes (green snake) [Leptophis]) do not occur north of Central America. Farther south these groups become more diverse. Vine snakes (vine snake) (Oxybelis and Imantodes), false coral snakes (Erythrolamprus), coral snakes (Micrurus), slender ground snakes (Drymobius), and the burrowing spindle snakes (Atractus) are some of the most biologically diverse (biodiversity) groups in this region.

      Several groups of reptiles that form important, if not dominant, elements of the fauna of the Eastern Hemisphere are largely or completely absent from the American tropics. Such groups include the lizard families Agamidae, Chamaeleonidae, Lacertidae, and Scincidae and many genera, subfamilies, and families of snakes.

      South of the tropics, in the temperate regions of South America, the diversity of reptiles diminishes rapidly. Crocodiles and turtles do not occur south of northern Argentina; however, the range of one viviparous pit viper reaches to almost 50° S, and the ranges of two iguanid lizards—Magellan's tree iguana (Liolaemus magellanicus) and the Cascabel rattlesnake (Crotalus durissus)—extend to almost to 55° S.

      The temperate zone of Eurasia is noted for its many lizards of the families Agamidae and Lacertidae, and, to lesser degrees, Gekkonidae and Scincidae. Most of the lizards are terrestrial, and extremely specialized burrowers include desert-dwelling skinks (Ophiomorphus and Scincus). Most of the snakes characteristic of this vast area are also terrestrial, and the leaf-nosed snakes (leaf-nosed snake) (Lytorhynchus) and the sand boas (boa) (Eryx) are the distinctive burrowing snakes of the region. Arboreal snakes are represented almost exclusively by the rat snakes (Elaphe).

      A few types of reptiles characteristic of the Asian tropics extend into the temperate zone—such as several rear-fanged snakes (Boiga trigonata and Psammodynastes), some cobras of the genus Naja, several species of softshell turtles (Trionychidae), and some species of skinks and geckos. Except for the Chinese alligator (Alligator sinensis) and the Indian gavial (Gavialis gangeticus), crocodiles are absent from temperate Eurasia.

      In the Asian tropics, the reptilian fauna is extremely rich in species and encompasses several diverse types. Aquatic reptile groups are represented by snakes of various genera (such as Natrix, Enhydris, and Acrochordus), several groups of lizards (Tropidophorus among the skinks and Hydrosaurus among the agamids), many batagurids and soft-shelled turtles, and five species of crocodiles. Asia's numerous terrestrial reptiles include the small kukri snakes (kukri snake) (Oligodon), the big Asian rat snakes (Ptyas), cobras (family Elapidae), monitor lizards (Varanus), many species and genera of skinks, some geckos, and several land turtles (Cuora (box turtle), Indotestudo, and Geochelone). Specialized burrowing snakes (such as those of family Uropeltidae and the colubrid genus Calamaria) and lizards (such as the family Dibamidae and the skink genus Brachymeles) contain many.

      Many distinctive life forms of reptiles in tropical Asia are arboreal. They include pythons and Asian pit vipers (Trimeresurus), vine snakes (Ahaetulla), slug-eating snakes (Pareas), flying snakes (flying snake) (Chrysopelea), and tree racers (Gonyosoma). Some lizards, such as the monitors, climb only with the aid of claws. A few others—such as the deaf agamids, Cophotis—climb with the help of prehensile, or grasping, tails. Other Asian reptiles, such as several species of geckos, climb with the help of clinging pads under the digits. The most striking arboreal reptiles of this area are the flying lizards ( Draco), which possess spreadable rib wings, and the parachuting gecko (Ptychozoon), which has fully webbed digits, a fringed tail, and wide flaps of skin along its sides.

      As a result of close geological relationships and faunal similarities, the general characteristics of reptiles in the Australian faunal region also apply in New Guinea. Australia is the only continent in the world in which venomous snake species outnumber nonvenomous ones. The family Colubridae, which encompasses the majority of the nonvenomous or slightly venomous snakes of the world, is poorly represented in Australia, with fewer than a dozen species. The reptilian fauna also includes several pythons and minute blind snakes (family Typhlopidae); a great variety of geckos, skinks, and agamid lizards; side-necked turtles (family Chelidae); and three species of crocodiles. The Australian region is home to a diverse group of cobras (family Elapidae) but no vipers.

 The reptilian fauna of Africa has two main components. The first, the fauna of North Africa, is akin to that of central and southwestern Asia and southern Europe and thus is mainly a Eurasian fauna. The racers (Coluber), the burrowing sand skink (Scincus), and the batagurid turtle (Mauremys caspica) are elements of this fauna in North Africa.

      North African reptiles, though representing many families, are principally terrestrial and burrowing. Many lacertid and agamid lizards scamper over rocks and sand by day; they are replaced at night by small geckos and are preyed upon by the racers and sand snakes. In addition to cobras, the venomous snakes of North Africa include the common vipers, the saw-scaled viper (Echis carinatus), and the horned vipers ( Cerastes). The last two are true desert animals and also occur in Southwest Asia.

      Some reptilian genera from sub-Saharan Africa also occur in North Africa and in southwestern Asia. Examples include the sand snakes (Psammophis), cobras, and chameleons (chameleon) (family Chamaeleonidae).

      The second and much larger component of the African herpetofauna is the sub-Saharan assemblage that ranges from the Sahara southward to the Cape of Good Hope. In common with tropical Asia, this vast area is home to cobras and many skinks and geckos. Its fauna differs from that of Asia by the absence of pit vipers (subfamily Crotalinae), the absence of batagurid turtles, and the presence of a few agamid lizards. These groups are replaced in tropical Africa by the many true vipers (subfamily Viperinae), the side-necked turtles (family Pelomedusidae), the wall lizards (family Lacertidae), and the spiny-tailed lizards (spiny-tailed lizard) (family Cordylidae). Numerous species of chameleons and tortoises and three species of crocodiles occur in sub-Saharan Africa.

      The large island of Madagascar, off the eastern coast of Africa, has a peculiar fauna with its affinities mainly to African reptile groups. Because of its long isolation, Madagascar possesses distinct genera and subfamilies of chameleons and other reptiles.

Natural history

Life cycle and life history
      The diversity of reptile life histories is amazingly wide and often reveals nearly unimaginable reproductive (reproduction) adaptations (adaptation). Some reptiles are annual species that hatch, mature, reproduce, and die in one year or, at most, two years (as in side-blotched lizards (Uta) [Uta stansburiana]). Others, such as loggerhead sea turtles (sea turtle) (Caretta caretta), are long-lived species that require 25 or more years to mature and have life spans that exceed 50 years. Numerous other species fall between these extremes. Some reptiles lay eggs, whereas others are live-bearers. Some species lay 1 or 2 eggs (egg), whereas others lay 100 or more eggs in each nesting event. Some reptiles nest year round, whereas others may nest once each year or allow two or more years between breeding cycles.

Courtship and fertilization
      The evolution of amniotic development (biological development) and the shelled egg enabled vertebrates (vertebrate) to become fully terrestrial. These two evolutionary advances required the previous development of internal fertilization. In other words, the deposition of sperm by the male into the female's reproductive tract and the sperm's subsequent penetration of the egg cell was necessary before the shelled egg could exist.

      In living reptiles the deposition of the male's sperm inside the body of the female occurs by cloacal apposition or the use of an intromittent, or copulatory, organ. The former method is characteristic of only one group, the tuataras (tuatara) (Sphenodon), which copulate via the close alignment of the male's cloaca (that is, a common chamber and outlet into which the intestinal, urinary, and genital tracts open) with that of the female. The male then discharges semen into the female's cloaca. In all other reptiles, males have either a penis (as in turtles (turtle) [order Testudines] and crocodiles (crocodile) [order Crocodylia]) or hemipenes (as in lizards (lizard) and snakes (snake) [order Squamata]). The penis is a homologue (homology) of the mammalian penis, and its presence in reptiles indicates that this organ arose early in the evolution of the amniotes (Amniota) and prior to the origin of reptiles and synapsids. In contrast, the hemipenes are structurally quite different. They are labeled “hemi” because two occur in each male, although only one is used during a single copulatory event. Whether a penis or hemipenis, this organ is inserted into the female cloaca.

      Once semen is deposited, the sperm must move out of the female's cloaca and into each oviduct. They move up the oviduct to an opening adjacent to an ovary. The mechanism of how the sperm find this pathway remains largely unknown, but for successful fertilization the sperm must be above the oviduct glands that will secrete the shell of the egg. When ovulation occurs, the eggs are shed from the ovary and drop directly into the oviduct, one on each side. In reptiles copulation may stimulate ovulation, occur simultaneously with ovulation, take place within an hour to a week of ovulation (presumably the most frequent situation), or occur months prior to complete the development of the eggs and their ovulation.

      Although spring is the main period of courtship and copulation for most temperate-zone reptiles, males commonly complete spermatogenesis (that is, the production of sperm) in late summer. Occasionally, a male will mate, and his sperm will be stored in the oviducts of the female until the eggs are ovulated in the spring (such as in snapping turtles (snapping turtle) [Chelydra serpentine]). This ability to store sperm seems to be widespread in snakes and turtles, although the phenomenon has not been rigorously tested. One study showed that the diamondback terrapin (Malaclemys terrapin) could produce viable eggs four years after copulation, although the percent of fertile eggs declined sharply after one year and progressively to the fourth and final year of the experiment.

 For a successful copulation to occur, cooperation between the female and male is required. In most reptiles the male courts the female with a series of behaviours to assess her reproductive readiness and receptivity. Many lizards also have a distinct pattern of head bobs and forebody push-ups. Combined with water vibrations and sprays, male crocodiles also use body movements to court females and warn off other males. In anoles (anole) (Anolis) and flying lizards ( Draco), males have well-developed and brightly coloured throat fans, or dewlaps, that open and close. Throat fans are used to attract females and play a large role in territorial disputes with other males. Turtles use visual and olfactory displays and tactile cues in courtship. These signals occur in various combinations and are species-dependent. For example, in some turtle species the female seems to be pestered into submission.

      Courtship in snakes (snake) and many scleroglossan lizards may also involve the use of pheromones (pheromone) that ensure that courtship and copulation occur between members of the same species. Pheromones may also help to attract a member of the opposite sex and thus illicit the female's cooperation in the reproductive process (reproduction). Snakes rely mainly on pheromone and tactile stimulation. The male crawls over the female and regularly taps his chin on her back; this behaviour presumably results in an exchange of pheromones, which simultaneously stimulates the participants.

      The so-called courtship dance of many snakes is often mistakenly interpreted as a dance in which the forepart of the bodies of a male and a female are held high and entwined. It is actually a power struggle between two males competing over the same female. The goal of the courtship dance is to press the body of the opponent to the ground. The swaying wrestling match continues until one male concedes defeat and crawls away. Often by that time the female, who was probably in the midst of being courted by one of the males, has departed, and she must be tracked (through her odour trail) by the victorious male for copulation to occur.

      Other male reptiles also decide dominance and access to females by combat. Monitor lizards (monitor) (Varanus) wrestle, truly grappling with one another; they may stand on their hind limbs and tail to attempt to force one another to the ground. Among turtles, male tortoises (tortoise) commonly ram each other with their heads and bodies. The objective is to drive away the opposing male, and it is best if an opponent can be rolled onto his back. A pair of mated sea turtles (sea turtle) is regularly accompanied by other males that bite at the mounted male in an effort to displace him. The male's objective is to ensure that his sperm fertilizes the female's egg so that the offspring will share his genes. One method, aside from combat, to ensure limited insemination of the female is through the deposition of a mucous copulatory plug. Male garter snakes (garter snake) (Thamnophis) deposit this plug into the female's cloaca at the end of copulation. The plug prevents any other mating and remains for a day or two.

      In a few species of lizards—including certain geckos (gecko) (Gekkonidae), racerunners (racerunner) (Tediidae), rock lizards (Lacertidae), monitor lizards (Varanus), and the brahminy blind snake (Ramphotyphlops braminus)—females may reproduce by parthenogenesis (that is, their eggs require no sperm activation or fertilization). Instead, the eggs are self-activated and spontaneously begin cell division and differentiation once they are ovulated and deposited in a nest. In many cases the entire species is unisexual and contains only females. In Komodo dragons (V. komodoensis) and other bisexual species, some females may reproduce parthenogenetically, whereas other females reproduce sexually (sex). Because no sperm are used, male chromosomes (chromosome) are not available, and recombination does not occur. Consequently, the resulting offspring have the same genetic makeup as the mother. In unisexual species such as R. braminus all individuals have the same genetic composition, and the entire species is likely to have arisen from one female.

Embryonic development and parental care
      Once the eggs are fertilized, development begins, and the egg becomes an embryo as it divides into successively smaller cells. The time delay between fertilization and egg deposition (that is, egg laying) is poorly documented for the majority of reptile species. Whereas copulation and the delivery of sperm into the female's reproductive tract can occur weeks or months before the eggs are ovulated, fertilization and egg deposition typically appear to occur within hours to days of ovulation. Apparently, many egg-laying (or oviparous (oviparity)) reptiles have a mechanism to retard or stop development in the oviduct once the early gastrula stage is attained; however, in most species, development continues as soon as the egg is deposited. During periods of high stress and other relatively unusual conditions (such as in captivity), females have been known to retain shelled eggs in their oviduct for weeks to months. In some situations where protracted egg retention results, eggs have eroded the oviductal wall and have fallen into the body cavity.

 Egg-laying and nest-building behaviours vary widely among reptiles. These behaviours range from the “casual” dropping of the eggs in a relatively suitable site to the preparation of an elaborate nest; however, in a few groups parental care may also occur. Most turtles dig an egg chamber exclusively with their hind limbs, and attention is given to the selection of the nest site, the excavation of the egg chamber, and its closure. Thereafter, the female departs, and the eggs and hatchlings must survive on their own. Most lizards and snakes also depart after the eggs are lain; the egg chamber can be little more than a hollow as the lizard or snake crawls through leaf litter or soil, or it may be more elaborate. For example, the common, or green, iguana (I. iguana) digs a deep burrow with a combination of its fore- and hind limbs; this chamber is often so deep that the female is totally hidden from view. At the end of this burrow, she lays her eggs and fills the entire burrow with loosened soil. Often a group of females will return to the same nesting site within the same nesting colony year after year.

      Snakes can also dig elaborate and deep chambers; the pine snake (Pituophis melanoleucus) lives on sandy soil and uses its head and the forepart of its body to scoop soil from its burrows and egg chambers. Many geckos (gecko) deposit their eggs in cracks or crevices in rock faces, in tree bark, or in plant tissues beneath the bark of trees. The eggs of some geckos are adhesive and may be attached to vertical surfaces; in other geckos several females will share a good nesting site beneath a slab of rock or behind the loose bark on the side of a tree. Such locations may contain dozens of eggs at different stages of development.

      Although a few species of lizards and snakes remain with their clutch—often curling their bodies around their eggs for the entire duration of the incubation period—the most intricate examples of parental care occur in crocodiles (crocodile). Even though there are species-specific variants in behaviour, the female crocodile typically creates a nest mound of soil and vegetation, using her mouth, limbs, body, and tail in its construction. After she digs a hole in the mound and lays her eggs, her attention remains focused on her eggs, and she stays nearby to watch over them. As the eggs begin to hatch, the hatchlings begin to chirp and squeak, bringing their mother to the nest. She uncovers the eggs and may even use her tongue to help some of the hatchlings out of their eggshells. She then carries her young to the water in her mouth and will stay with them for several months until they are large enough to survive on their own.

      Some reptiles may bear their young alive. This mode, called viviparity, is widespread and has evolved independently dozens of times in the squamates (that is, the lizards and snakes). No living crocodiles, turtles, or tuataras are live-bearers; however, in the squamates, live-bearing ranges from retention of unshelled eggs in the oviducts to the development of placentae (placenta) between the mother and her fetuses. The evolutionary steps from egg laying to placental development are demonstrated by extant species. For example, the rough green snake (Opheodrys aestivus) retains eggs for varying periods, and it can deposit eggs containing full-term embryos (embryo) that hatch within days of deposition. In other taxa the eggs are not shelled but remain in the oviducts throughout development. The yolk nourishes each embryo, although gas exchange does occur across the amniotic membranes and the oviducal walls. Placental development ranges from simple wall contact and gas exchange between the mother and a developing embryo to the full interdigitation of maternal and fetal tissue for nutrition and gas exchange (as in garter snakes [Thamnophis]). There are several types of placentae that have evolved in squamates that use various components of the amniotic membranes.

      Clutches of eggs and litters of neonates vary widely in reptiles and are species-dependent. Among egg layers a clutch may range from a single egg to more than 100. Among live-bearing reptiles, a litter may range from 1 to about 50 neonates. Adult body size is just one aspect associated with number of offspring; genetic constitution and nutrition are also major factors.

      The smallest of the living reptiles typically have the fewest offspring, often laying only one or two eggs or producing only one or two neonates. Many geckos and some skinks (skink) have genetically fixed clutch sizes of two eggs, and one egg is usually produced by each ovary during a given reproductive cycle. Conversely, turtles and crocodiles produce some of the largest clutches among living reptiles; sea turtles often produce more than 100 eggs each time, whereas the larger crocodiles average 40–50 eggs per clutch. Some of the larger snakes also produce clutches or litters of 40–50 eggs or embryos, but most squamates, even large-bodied species, produce less than 20 eggs or embryos during each reproductive cycle.

      Nutrition clearly affects the number of offspring produced, with malnourished females laying fewer eggs or giving birth to fewer offspring. A female lizard suffering through a drought year or coping with loss of her tail may resorb maturing egg follicles in the ovary or forego egg development altogether during that year.

      The frequency of reproduction is also governed by energy availability. Female timber rattlesnakes (rattlesnake) (Crotalus horridus) commonly breed every third year because the female eats little during the summer of her pregnancy. She requires the following summer to rebuild her fat (energy) stores for the subsequent year's pregnancy and egg development.

      The duration of egg incubation and pregnancy is temperature-dependent. Because reptiles are ectothermic, the embryos of live-bearing females and the eggs of oviparous females deposited in the soil or other locations are subject to fluctuating temperatures. In general, cool temperatures slow development and warm temperatures speed development, but extreme heat and cold are lethal to developing embryos. On average, temperate-zone reptiles have incubations or pregnancies of 8–12 weeks. Tropical species tend to have similar incubation periods; however, incubations of some species may last nearly one year or longer (as in the Fijian iguana [Brachylophus fasciatus]).

      In addition to hereditary, or genetic, factors, the sex of many reptile species may be manipulated by the environment in which embryonic development takes place. Environment-dependent sex determination (ESD) is the collective term for all factors (such as temperature, moisture, and others) that affect the ratio of males to females produced in a given clutch of eggs or a litter of neonates. Temperature-dependent sex determination (TSD), discovered in the early 1970s, is the most researched of these factors. The sex of the offspring in species with TSD is influenced by the temperature during one critical period of incubation, instead of by hereditary factors. In most turtles females are produced at high temperatures and males at low temperatures. At a narrow range of intermediate temperatures, roughly equal numbers of males and females are produced. The reverse occurs in many crocodiles, and females result from cooler temperatures. Some squamates also display TSD, but the sex of most species appears to be primarily determined by genetics.

      In egg-laying reptiles the hatchling must break through the eggshell. For this purpose turtles, crocodiles, and tuataras bear a horny pointed caruncle on their snout. The hatchling uses the caruncle to slice open the amniotic membranes and then the eggshell. Squamates have an egg tooth, a special premaxillary tooth that extends forward and out of the mouth, to cut through membranes and shell. Generally, the hatchling rests briefly once out of the shell. If the nest is buried under soil or other material, a hatchling must dig upward to emerge on the surface. Sometimes this occurs in concert with other hatchlings in the nest; a coordinated behaviour is necessary for sea turtles and other species whose eggs are buried deep. In a few species of turtles, such as the North American painted turtle (Chrysemys picta), the hatchlings leave the eggshell, but they remain in the nest through the winter and emerge in the spring. Individual painted turtle hatchlings can tolerate short periods of extreme cold that freezes much of the water in their bodies.

      Live-bearing reptiles give birth in the same manner as mammals. If the amniotic membranes do not rupture during birth, the neonate must struggle to break free from the encapsulating membranes.

Growth and longevity
      Reptiles, especially turtles (turtle), are noted for their extreme longevity (life span). Many turtles have long lives, but few species have individuals that live more than a century. Records of longevity are derived from captive animals that led protected and catered life. Many North American turtle species require 12 to 18 years to reach sexual maturity. Once they reach adulthood, mortality rates decline substantially, and many individuals reach and exceed 30 years (as in Blanding's turtle [Emydoidea blandingii] and the eastern box turtle [Terrapene carolina]). Generally, the larger the animal, the greater is its life span, so crocodiles, large snakes (such as boas (boa) and pythons (python)), and large lizards often live more than 20 years.

      Although patterns of growth are poorly documented for the majority of reptiles, most species probably follow a pattern of determinate, or asymptotic, growth as they mature. Most reptiles are characterized by a period of rapid juvenile growth that slows upon reaching full adulthood. Growth then ceases altogether a few years after maturity.

      In contrast, some large-bodied species likely have what is known as indeterminate, or attenuated, growth. Typically, rapid growth occurs in juveniles and slows as the individual approaches maturity and shifts its energy resources to reproduction. During most of the adult years, growth is ether extremely slow or nonexistent; however, when food resources are high, active growth can occur. Thus, the size of an individual of a species characterized by attenuated growth is only limited by its food supply.


Avoidance and noise
 Avoidance (avoidance behaviour), the most common form of defense in the animal kingdom, is also the most common form of defense in reptiles. At the first recognition of danger, most snakes (snake) and lizards (lizard) crawl or scamper away into the undergrowth; turtles (turtle) and crocodiles (crocodile) plunge into water and sink out of sight. Even so, should danger arise so suddenly and so close at hand that flight may be hazardous, other behaviours are adopted.

      Crocodiles, turtles, some lizards, and some snakes hiss loudly when confronted by an enemy. Rattlesnakes (rattlesnake) rapidly vibrate the tip of the tail, which consists of loose, dry, horny rings. Even snakes without rattles, such as the fox snake (Elaphe vulpina) of the United States, often rapidly vibrate the ends of their tails. Often, the tail will come into contact with dry leaves, and the resulting sound will seem deceptively like the rattle of a rattlesnake.

Body form and posturing
 Change in body form is relatively common in snakes. It usually involves spreading the neck, as in the cobras (cobra) (family Elapidae (elapid)), or the whole body, as in the harmless hognose snakes (hognose snake) (Heterodon) and DeKay's snake (Storeria dekayi) of the United States. Some snakes inflate the forward parts of their bodies; inflation is one of the defensive behaviours of the large South American bird snake Pseustes poecilonotus and the African boomslang (Dispholidus typus).

      Snakes may also assume threat postures as they change their body form. A cobra raises the forepart of its body and spreads its hood when threatened. The typical defensive posture of vipers (viper) is the body coiled and the neck held in an S-curve, the head poised to strike.

 Some lizards (lizard) flatten their bodies, puff out their throats, and turn broadside to the enemy. The helmeted iguanids (Corythophanes) of Central America and the chameleons (chameleon) of Africa increase their apparent size in this way when approached by snakes. The Australian bearded lizard (Pogona barbata) spreads its throat downward and outward. The Australian frilled lizard (Chlamydosaurus kingii) suddenly raises a wide membrane, or frill, which extends backward from the throat. Many lizards and snakes open their mouths when threatened but do not strike. A common African lizard, the black-necked agama (Acanthocercus atricollis), faces an enemy with head held high and mouth open to show the brilliant orange interior.

Display of colour
 The display of bright colours (coloration) is often defensive. This behaviour occurs in some red- or yellow-bellied snakes that turn over or curl up their tails, exposing the brightly coloured undersurface. This behaviour is known in harmless snakes, such as the American ring-necked snake (Diadophis), as well as venomous snakes, such as the Southern coral snake (Micrurus frontalis), with red, orange, or yellow undersides. Although not yet fully understood, these colours must have some significance to predators. Many other animals coloured red, orange, or yellow are either distasteful to predators or possess defenses capable of killing or injuring them. Hence, these colours are thought to serve as warning coloration to potential predators.

       camouflage that involves both form and colour is common in reptiles. For example, many arboreal snakes and lizards are green; some of the green-coloured snakes, such as the vine snakes (vine snake) of South America (Oxybelis) and southern Asia (Ahaetulla), are very slender and resemble plants (plant) common in the habitat. Likewise, lizards of semiarid and rocky habitats are frequently pale and have blotched patterns that resemble pebbles and gravel—as in the leopard lizard (Crotaphytus wislizeni) of the southwestern United States.

       mimicry of dangerous species by harmless ones is a passive defense; however, its validity as an actual mechanism of defense is sometimes challenged. Nonetheless, evidence of mimicry appears among different groups of snakes. For example, the venomous American coral snakes (Micrurus) have various ringed patterns of red, yellow, white, and black. These patterns are matched often by non- or mildly venomous snake species occurring in the same area.

Striking and biting
      If a threatening posture does not succeed in driving off an enemy, many reptiles may become more aggressive. Some snakes (such as DeKay's snake [S. dekayi]) strike, but with their mouths closed. Others (such as the hognose snakes [Heterodon]) strike with their mouth open but do not bite, but snakes of many species will strike and bite viciously. Among the nonvenomous snakes of North America, few are as quick to bite as the water snakes (water snake) of genus Nerodia; however, they are nonvenomous.

      Most of the dangerously venomous snakes (vipers (viper), pit vipers, and cobras (cobra)) bite in self-defense. Vipers and pit vipers usually strike from a horizontally coiled posture. From this position, the head can be rapidly shot forward, stab the enemy, and be pulled back in readiness for the next strike. From the typical raised posture, a cobra sweeps its head forward and downward to bite. To strike again, it raises its head and neck once more; such aggressive, defensive movements of cobras are slower than those of pit vipers.

      Many lizards, regardless of family and size, also bite in defense. For example, the tokay gecko (Gekko gecko) of Southeast Asia bites if sufficiently threatened. Although small lizards have a bite that is effective against only the smallest predators, a large monitor lizard (monitor) (Varanus) can inflict a painful wound with its large teeth and strong jaws. Some turtles, particularly the softshell turtles (softshell turtle) of family Trionychidae and snapping turtles (snapping turtle) of family Chelydridae, bite frequently and vigorously.

      The spitting of venom by some Asian and African cobras (Naja) and the ringhals (Hemachatus haemachatus) is a purely defensive act directed against large animals. Instead of a straight canal ending in a long opening near the tip of each fang as in most cobras, the specialized fang of the spitting cobra has a canal that turns sharply forward to a small round opening on the front surface. At the moment of ejection, the mouth is opened slightly, and a fine stream of venom is forced out of the fangs by the contraction of the muscle enveloping the poison gland. A spitting cobra usually raises its head and the forepart of its body in the characteristic cobra defensive posture prior to spitting, but venom can be ejected from any position. The effect on skin is negligible; the eyes, however, may be severely damaged, and blindness can result unless the venom is washed out quickly.

Use of the tail
 A few lizards, representing different families, have thick tails (tail) covered by large, hard, spiny scales (scale). Such a tail swung vigorously from side to side is an effective defense against snakes, especially when the head and body of the lizard are in a burrow or wedged between rocks.

      The tails of some lizard species are useful in defense in another way. When captured, some lizards voluntarily shed, or autotomize (autotomy), their tails, which wriggle violently, temporarily confusing the predator and allowing the lizard to escape. Each vertebra of the tails of tail-shedding lizards has a fracture plane that can voluntarily split by the appropriate twitch of the tail muscles. Simultaneous stimulation of the nerves (nervous system) in the severed portion keeps it twitching for a few seconds after separation. Usually the tail is broken in only one place, but a few lizards, particularly the so-called glass snakes (Ophisaurus), break their tails into several pieces. The stump heals quickly, and a new tail grows; often, however, the regenerated (regeneration) tail is not as long as the original and has simpler scales.

      Snakes, turtles, and crocodiles may have their tails bitten off by predators; however, they cannot break them voluntarily or regenerate them. In confrontations with enemies, some snakes use their tails as diversions by raising them and moving them slowly. Species with this habit commonly have thick, blunt, brightly coloured tails. For example, the small African burrowing python (Calabaria reinhardtii) waves its tail in the air as it moves slowly away from a threat.

      Many snakes, both harmless and venomous, attempt to hide their heads under coils of their bodies. For most species with this habit, the body may be coiled loosely; however, it may also be tightly coiled so that it forms a compact ball with the head in the centre. Balling, as the latter habit is called, is a characteristic response of Calabaria and another African python, Python regius. The African armadillo lizard (Cordylus cataphractus), a species with heavy scales on its head and hard spiny scales covering its body and tail, rolls on its back and grasps its tail in its mouth to present an imposing ring of hard spines to a predator.

Odours (odour)
      Some reptiles use musk-secreting glands (gland) when other defensive measures fail. The water snakes (Nerodia), the garter snakes (garter snake) (Thamnophis), and the alligator lizards (alligator lizard) (Gerrhonotus) emit a foul-smelling substance from their cloacal glands. An assortment of turtles, such as the mud turtle and the musk turtle (Kinosternidae), have glands on the bridge of their shells that excrete a vile-smelling fluid that likely makes them distasteful to many predators.

Feeding habits
      With few exceptions, modern reptiles feed (feeding behaviour) on some form of animal life (such as insects (insect), mollusks (mollusk), birds (bird), frogs (frog), mammals (mammal), fishes (fish), or even other reptiles). Land tortoises are vegetarians, eating leaves, grass, and even cactus in some cases. The green iguana (I. iguana) of Central and South America, the chuckwalla (Sauromalus obesus) of the southwestern United States and northern Mexico, and the spiny-tailed agamids (agama) (Uromastyx) of North Africa and southwestern Asia also are herbivorous. The marine iguana (Amblyrhynchus cristatus) of the Galápagos Islands dives into the sea for seaweed.

      The majority of carnivorous reptiles have nonspecialized diets and feed on a variety of animals. In general, the smaller the reptile, the smaller is its prey. Only the very largest of living snakes—the reticulated python (Python reticulatus), the Indian python (P. molurus), and the green anaconda (Eunectes murinus)—are capable of eating large mammals such as small pigs (pig) and deer. Among crocodiles the largest species—the Nile crocodile (Crocodylus niloticus), the estuarine, or saltwater, crocodile (C. porosus), and the Orinoco crocodile (C. intermedius)—have been known to attack and eat humans. Presumably, the great carnivorous dinosaurs—such as Allosaurus and Tyrannosaurus (tyrannosaur)—devoured even larger prey. These predators were almost certainly capable of killing the largest of their herbivorous contemporaries.

Walking and crawling
 In the typical reptilian posture, limbs project nearly perpendicular from the body and bend downward toward the ground at the elbows and knees. This limb posture produces a sprawled gait that some biologists label as inefficient and awkward. Its continued persistence in thousands of amphibians (amphibian) and reptiles shows its effectiveness and high efficiency for lifestyles designed for energy conservation. At rest the reptilian trunk and tail lie on the substrate; during walking and running the body is held only slightly above the substrate and bends from side to side to increase the length of each step from each sprawled limb. A few terrestrial reptile groups exhibit an evolutionary shift in limb posture from the horizontal to the vertical. This same shift produces the erect posture seen today in birds and mammals. This vertical posture was typical of late dinosaurs, and presumably, like those of birds and mammals, the dinosaur joints had locking mechanisms to reduce the muscle energy required to hold the body erect when standing still.

      The only living reptiles that use a vertical limb posture in walking are the crocodiles (crocodile). The “high walk” of these animals employs the quadrupedal limb-movement sequence with only a slight lateral undulation. Some young crocodiles use a galloping gait, much like that of a bounding rabbit, for high-speed escape; the body flexes up and down rather than from side to side for this type of locomotion. Crocodiles also use a belly-slide gait. This is also an escape behaviour, even though the body rests on the substrate; lateral undulations of the body and tail and the quadrupedal limb sequence propel the crocodile down the river bank into the water.

      A snake moves by pushing backward against rocks, sticks, or any relatively fixed point—such as a lump of earth or a small depression in uneven ground—with the rear (ventrolateral) surface of the body's curves. Each joint of the body passes through the same curves, pressing against the same object and thrusting forward. Heavy-bodied snakes such as pythons and certain rattlesnakes can move forward without undulation. This rectilinear movement depends on the ability of snakes to stretch or contract their bodies along its longitudinal axis. By raising a part of its belly, stretching that part forward, lowering it to the ground, and repeating the process alternately with other parts of the body, a heavy snake can move forward smoothly in a straight line.

      A variety of modern lizards (lizard) are bipedal when running. The collared lizard (Crotaphytus collaris) of the United States and the frilled lizard (Chlamydosaurus kingii) of Australia are capable of bipedal movement, a phenomenon that was widespread among the dinosaurs. These present-day lizards run on their long hind limbs, with the forward parts of their bodies at an angle of about 60° off the horizontal.

      Presumably, bipedalism among the dinosaurs (dinosaur) began, as it did among modern lizards, as a means of obtaining a faster running speed. Because the centre of gravity is in front of the hips, modern bipedal lizards must move forward continuously in order to maintain a semierect posture; they can stand still in that position only for very short periods.

      The sprawled limb posture of bipedal lizards causes each limb to swing outward as it is brought forward and to push the body sideways and forward when each leg thrusts backward against the ground. Bipedal dinosaurs eliminated this side-to-side motion by shifting to a vertical hind-limb posture. This posture supports the body in an upright position, and the limbs swing directly forward and backward. So successful was this mode of locomotion that dinosaurs utilizing it dominated terrestrial life for millions of years.

Clinging and climbing
      Arboreal animals possess groups of anatomical features that help them cling to branches and other substrates. The most common clinging structures in vertebrates (vertebrate) are claws; they seem to be the only arboreal adaptations of some lizards, such as the common iguana (I. iguana). Similar structures appear in many geckos (gecko) (family Gekkonidae), in the anoles (anole) (Anolis; family Iguanidae), and in some skinks (skink) (family Scincidae).

      Other adaptations for climbing include footpads. Pads on the feet consist of wide plates or scales under the fingers and toes. The outer layer of each scale is composed of numerous microscopic hooks formed by the free, bent tips of cells. These minute hooks can catch in the slightest irregularities of a surface, and they enable geckos to run up apparently smooth walls and even upside down on plaster ceilings. Because the hooklike cells are bent downward and to the rear, a gecko must curl its pads upward to disengage them. Thus, when walking or running up a tree or wall, a gecko must curl and uncurl its pad surface with every step.

      The giant Solomon Islands skink (Corucia), true chameleons (chameleon) (Chamaeleonidae), arboreal vipers, boas (boa), and pythons use prehensile tails—that is, tails that are capable of supporting most of the weight of the animal or are used habitually for grasping—for clinging to their aerial supports. Still, true chameleons rely mainly on the tonglike arrangement of the digits in their feet. The toes of each foot are united into two opposed bundles—three on the inside and two on the outside of the forefoot, and two on the inside and three on the outside of the hind foot.

      Slender vine snakes (vine snake) of several genera of the family Colubridae are capable of extending half the body length in a horizontal plane without support; they do so habitually in bridging the gap between tree branches. Most snakes can reach across an open space; however, all except the vine snakes can extend only a short distance, and that portion of the body invariably sags like a cable. In contrast, the vine snakes bridge an open space like an I-beam. This ability is based partly on their reduced body weight and partly on their deepened and strengthened vertebrae.

      In water, of course, limb movements—whether bipedal or quadrupedal—that work well in terrestrial environments are not very effective. Aquatic reptiles, with some exceptions, use the same means of propulsion as do fish—that is, lateral undulations of the rear half of the body and tail. Crocodiles and aquatic lizards, such as some monitors (family Varanidae) and the marine iguana (Amblyrhynchus cristatus), undulate their bodies and tails from side to side while holding the limbs against the body. The ancient mesosaurs (Mesosaurus) (order Mesosauria) and ichthyosaurs (ichthyosaur) (order Ichthyosauria) used the same method. The marine ichthyosaurs, which were the reptilian counterpart of the porpoises (porpoise) (family Phocoenidae) in class Mammalia, may have used their flippers as rudders.

      The limbless snakes are good swimmers and make lateral undulations similar to those of eels. This fishlike swimming mode requires a flexible body and, usually, a tail of moderate length. Sea snakes (sea snake) have laterally flattened tails that increase their locomotor power. Turtles propel themselves by using their feet as paddles as a part of their quadrupedal limb-movement sequence. Freshwater turtles have webbed feet, whereas the forelimbs of marine turtles are essentially flattened flippers that are moved in a sweeping figure-eight pattern through the water.

      Several groups of reptiles have experimented with flight. One group within the Archosauria (archosaur) (the “ruling reptiles” that include dinosaurs and crocodiles) became highly successful at this means of locomotion and evolved into birds.

      Another group of archosaurs, the Pterosauria (pterosaur), developed wings that were supported along the front margin by the arm and an extremely elongated finger. The pterosaur wing was made of skin; since it lacked both internal supports and feathers (feather), it probably lacked the flexibility or durability of a bird wing. Pterosaurs seem to have emphasized soaring and gliding during flight, but they also engaged in flapping flight. It is possible that pterosaurs had clumsy takeoffs like those of the modern albatross (Diomedea). Since most pterosaur remains have been found in marine deposits, it is assumed that many of the species lived along ocean shores, probably roosting on cliffs from which takeoff would have been easier.

      Among modern lizards, flying lizards ( Draco) are expert gliders. The “wing” of these small reptiles is made up of skin supported by five or six elongated ribs between the forelimbs and hind limbs. At rest the ribs and wings are folded against the sides of the body. During flight the wings form broad semicircles on each side between the limbs. These lizards, which live in the forested country of Southeast Asia and the East Indies, are gliders; in spite of their label as flying lizards, they are not flyers. To glide, the lizard launches itself from a tree into the air toward another tree, turning upward sharply to slow its body just before lighting on the new perch. Since the limbs are not modified, this lizard walks and runs like any other arboreal lizard.

Form and function

External covering
 The external covering of reptiles is dry and composed of scales (scale) made of keratin. It bears few glands (gland) or none at all and differs in this respect from the skin of amphibians (amphibian) and mammals (mammal). The epidermis has cycles of growth, and in the outer layer the cells die and harden into a tough and horny surface. Bony plates (osteoderms) develop in the dermis, the layer just below the epidermis, of some reptiles and are best seen on the backs of crocodiles (crocodile).

Internal features
Skeletal system
      The skeletons (skeleton) of reptiles fit the general pattern of vertebrates (vertebrate). They have a bony skull, a long vertebral column that encloses the spinal nerve cord (spinal cord), ribs that form a protective bony basket around the viscera, and a framework of limbs.

      Each group of reptiles developed its own particular variations on this major pattern in accord with the general adaptive trends of the group. Snakes (snake), for example, have lost the limb bones, although a few retain vestiges of the hind limbs. The limbs of several types of marine reptiles have been modified into fins or flippers. In other types, such as the extinct marine-dwelling ichthyosaurs (ichthyosaur) and plesiosaurs (plesiosaur), the bones of the limbs, which no longer needed to support the weight of the body against the pull of gravity, became much shorter. At the same time, the bones of other reptiles that composed the digits multiplied in number, forming a long flipper.

      Groups of reptiles whose modes of life came to depend heavily on passive defense also developed specializations of the skeleton. The bony and horny shells of turtles (turtle) and the rows of bony plates on the backs of crocodiles and the Ankylosaurus (a genus of dinosaurs (dinosaur) that lived between the Early Jurassic Period and the end of the Cretaceous (Cretaceous Period)) are cases in point.

Skull and dentition
 The skulls (skull) of the several subclasses and orders vary in the ways mentioned below. In addition to differences in openings on the side of the skull and in general shape and size, the most significant variations in reptilian skulls are those affecting movements within the skull.

      As a group, reptilian skulls differ from those of early amphibians. Reptiles lack an otic notch (an indentation at the rear of the skull) and several small bones at the rear of the skull roof. The skulls of modern reptiles are also sharply set off from those of mammals in many ways, but the clearest differences occur in the lower jaw and adjacent regions. Reptiles have a number of bones in the lower jaw, only one of which, the dentary, bears teeth (tooth). Behind the dentary a small bone, the articular, forms a joint with the quadrate bone near the rear of the skull. In contrast, the lower jaw of a mammal is made up of a single bone, the dentary; the articular and quadrate have become part of the chain of little bones in the middle ear (ear, human). An almost complete transition between these two very different arrangements is known from fossils of early synapsids (order Therapsida (therapsid)).

      The dentition of most reptiles shows little specialization in a given row of teeth. A dentition that divides groups of teeth into distinctive bladelike incisors, tusklike canines, and flat-crowned molars occurs in mammals but does not occur in reptiles. Instead, the entire tooth row is usually made up of long conical teeth. Venomous snakes have one or several hollow or grooved fangs, but they have the same shape as most snake teeth. The principal differences between species lie in the number, length, and position of the teeth. Crocodiles among the living forms and dinosaurs among the extinct forms have but a single upper and a single lower tooth row. Snakes and many extinct reptilian groups have teeth on the palatal bones (vomer, palatine, pterygoid) and on the bones of the upper jaw (premaxilla, maxilla); however, only one row of teeth is present on the lower jaw.

      Lizards (lizard) have conical or bladelike bicuspid or tricuspid teeth. Some species have conical teeth at the front of the jaws and cuspid teeth toward the rear, but the latter are not comparable to the molars of mammals in either form or function. (They are neither flat-crowned nor used to grind food.) Turtles, except for the earliest extinct species, lack teeth. Instead, they have upper and lower horny plates that serve to bite off chunks of food.

      The teeth of reptiles are also less specialized in function than are mammalian teeth. The larger carnivorous (carnivore) reptiles are equipped only to tear off or bite off large pieces of their prey and swallow them without chewing. Insectivorous lizards, which constitute the majority of all lizards, usually crack the exoskeleton of their insect prey, and then they swallow the prey without grinding it up. Snakes simply swallow their prey whole without any mechanical reduction, although the puncture wounds permit digestive enzymes (enzyme) to enter the prey to aid digestion.

      Many reptiles developed joints (in addition to the hinge for the lower jaw) within the skull that permit the slight movement of one part relative to others. The capacity for such movement within the skull, called kinesis, enables an animal to increase the gape of the mouth and thus is an adaptation for swallowing large objects. Apparently some of the large carnivorous theropod dinosaurs (such as Allosaurus) had a joint between the frontal and parietal bones in the roof of the skull. All reptiles of the super order Lepidosauria (lizards, snakes, and tuataras (tuatara)) have kinetic skulls, but they differ from the dinosaurs in that the joint on the floor of the skull occurs at the juncture of basisphenoid and pterygoid bones in lepidosaurians.

      The skulls of the lepidosaurians became increasingly kinetic as new groups evolved. The Sphenodontia (which include living tuataras (tuatara) [Sphenodon]) and their antecedents, the Rhynchocephalia, had only the basisphenoidal-pterygoidal joint. The lizards lost the lower temporal bar, thereby freeing the quadrate bone and allowing greater movement to the lower jaw, which is hinged to the quadrate. Finally, in the snakes (snake), this trend culminates in the most kinetic skull among the vertebrates. The skulls of snakes possess the ancestral basisphenoidal-pterygoidal joint, a highly mobile quadrate (which gives even greater mobility to the lower jaw), and upper jaws capable of rotating on their longitudinal axes and moving both forward and backward. Many snake species also have a hinge on the roof of the skull between the nasal and frontal bones that allows the snout to be raised slightly. In short, the only part of a snake's skull incapable of movement is the braincase.

 As in all vertebrates, the nervous system of reptiles consists of a brain, a spinal nerve cord, nerves running from the brain or spinal cord, and sense organs. When compared with mammals, reptiles have proportionately smaller brains. The most important difference between the brains of these two vertebrate groups lies in the size of the cerebral hemispheres, the principal associative centres of the brain. These hemispheres make up the bulk of the brain in mammals and, when viewed from above, almost hide the rest of the brain. In reptiles the relative and absolute size of the cerebral hemispheres is much smaller. The brain of snakes and alligators (alligator) forms less than 11,500 of total body weight, whereas in mammals such as squirrels (squirrel) and cats (cat, domestic) the brain accounts for about 1/100 of body weight.

 Modern reptiles do not have the capacity for the rapid sustained activity found in birds (bird) and mammals. It is generally accepted that this lower capacity is related to differences in the circulatory and respiratory systems. Before the origin of lungs (lung), the vertebrate circulatory system (circulation) had a single circuit: in the fishes (fish), blood flows from heart to gills to body and back to the heart. The heart consists of four chambers arranged in a linear sequence.

      With the evolution of lungs in early tetrapods (tetrapod), a new and apparently more efficient circulatory system evolved. Two chambers of the heart, the atrium (or auricle) and ventricle, became increasingly important, and the beginnings of double circulation appeared. An early stage in this evolution can be seen in amphibians today, where one of the main arteries from the heart (the pulmonary artery (pulmonary circulation)) goes directly to the lungs, whereas the others (the systemic arteries) carry blood to the general body. In amphibians the blood is aerated in the lungs and carried back to the atrium of the heart. From the left side of the atrium, which is at least partially divided, the aerated blood is pumped into the ventricle to mix with nonaerated blood; nonaerated blood from the body is returned to the heart via the right half of the atrium. Then the cycle begins again. One aspect of the amphibian system is that the blood leaving the heart for the body is only partially aerated; part of it is made up of deoxygenated blood returned from the body.

      All groups of modern reptiles have a completely divided atrium; it is safe to assume, therefore, that this was true of most, if not all, extinct reptiles. In the four major living groups of reptiles, the ventricle is at least partially divided. When the two atria of a lizard's heart contract, the two streams of blood (aerated blood from the lungs in the left atrium and nonaerated blood from the body in the right atrium) flow into the left chamber of the ventricle. As pressure builds up in that chamber, the nonaerated blood is forced through the gap in the partition into the right chamber of the ventricle. Then, when the ventricle contracts, nonaerated blood is pumped into the pulmonary artery and thence to the lungs, whereas aerated blood is pumped into the systemic arteries (the aortas (aorta)) and so to the body.

      In snakes (snake) all three arterial trunks come out of the chamber of the ventricle that receives the nonaerated blood of the right atrium. During ventricular contraction a muscular ridge forms a partition that guides the nonaerated blood into the pulmonary artery, while the aerated blood received by the other chamber of the ventricle is forced through the opening in the ventricular septum and out through the aortas.

      In crocodiles (crocodile) the ventricular septum is complete, but the two aortas come out of different ventricular chambers. A semilunar valve (valve) at the entrance to the left aorta prevents nonaerated blood in the right ventricle from flowing into the aorta. Instead, part of the aerated blood from the left ventricular chamber pumped into the right aorta flows into the left by way of an opening.

      The ventricle of the turtle is not perfectly divided, and some slight mixing of aerated and nonaerated blood can occur.

      Despite the peculiar and complex circulation, lizards, snakes, and crocodilians have achieved a double system. Tests of the blood in the various chambers and arteries have shown that the oxygen content in both systemic aortas is as high as that of the blood just received by the left atrium from the lungs and is much higher than that of the blood in the pulmonary artery. Except for the turtles, limitation of activity in reptiles cannot be explained on the basis of heart circulation. An explanation may lie in the chemistry of the blood. The blood of reptiles has less hemoglobin and thus carries less oxygen than that of mammals.

      The form of the lungs (lung) and the methods of irrigating them may also influence activity by affecting the efficiency of gas exchange. In snakes the lungs are simple saclike structures having small pockets, or alveoli, in the walls. In the lungs of all crocodiles and many lizards and turtles, the surface area is increased by the development of partitions that, in turn, have alveoli. Because exchange of respiratory gases takes place across surfaces, an increase of the ratio of surface area to volume leads to an increase in respiratory efficiency. In this regard the lungs of snakes are not as effective as the lungs of crocodiles. The elaboration of the internal surface of lungs in reptiles is simple, however, compared with that reached by mammalian lungs, with their enormous number of very fine alveoli.

      Most reptiles breathe (breathing) by changing the volume of the body cavity. By contractions of the muscles moving the ribs, the volume of the body cavity is increased, creating a negative pressure, which is restored to atmospheric level by air rushing into the lungs. By contraction of body muscles, the volume of the body cavity is reduced, forcing air out of the lungs.

      This system applies to all modern reptiles except turtles (turtle), which, because of the fusion of the ribs with a rigid shell, are unable to breathe by this means; they do use the same mechanical principle of changing pressure in the body cavity, however. Contraction of two flank muscles enlarges the body cavity, causing inspiration. Contraction of two other muscles, coincident with relaxation of the first two, forces the viscera upward against the lungs, causing exhalation.

      The rate of respiration, like so many physiological activities of reptiles, is highly variable, depending in part upon the temperature of the environment and in part upon the emotional state of the animal.

Digestive and urogenital systems
      The digestive system of modern reptiles is similar in general plan to that of all higher vertebrates. It includes the mouth and its salivary glands (salivary gland), the esophagus, the stomach, and the intestine and ends in a cloaca. Of the few specializations of the reptilian digestive system, the evolution of one pair of salivary glands into poison glands in the venomous snakes is the most remarkable.

      During development the embryos of higher vertebrates (reptiles, birds, and mammals) consecutively develop three separate sets of kidneys (kidney); these are arranged in longitudinal sequence in the body cavity. The first set, the pronephroi, are vestigial organs left over from the evolutionary past that soon degenerate and disappear without having had any function. The second set, the mesonephroi, are the functional kidneys of adult amphibians, but their only contribution to the lives of reptiles is in providing the duct (the Wolffian duct) that forms a connection between the testes (testis) and the cloaca. The operational kidneys of reptiles, birds, and mammals are the last set, the metanephroi (metanephros), which have separate ducts to the cloaca. The principal functions of the kidney are the removal of nitrogenous wastes resulting from the oxidation (oxidation–reduction reaction) of proteins (protein) and the regulation of water loss. Vertebrates eliminate three kinds of nitrogenous wastes: ammonia, urea, and uric acid. Ammonia and urea are highly soluble in water, but uric acid is not. Ammonia is highly poisonous, urea is slightly poisonous, and uric acid is not poisonous at all.

      Among reptiles the form taken by the nitrogenous wastes is closely related to the habits and habitat of the animal. Aquatic reptiles tend to excrete a large proportion of these wastes as ammonia in aqueous solution. This method uses large amounts of water and is no problem for a freshwater resident, such as an alligator, which eliminates between 40 and 75 percent of its nitrogenous wastes as ammonia. Terrestrial reptiles, such as most snakes and lizards, must conserve body water, and they convert their nitrogenous wastes to insoluble, harmless uric acid, which forms a more or less solid mass in the cloaca. In snakes and lizards, these wastes are eliminated from the cloaca together with wastes from the digestive system.

      Prior to the evolution of the metanephric kidney, the products of the male gonad, the testis, traveled through the same duct with the nitrogenous wastes from the kidney. But with the appearance of the metanephros, the two systems became separated. The female reproductive system never shared a common tube with the kidney. Oviducts (fallopian tube) in all female vertebrates arise as separate tubes with openings usually near, but not connected to, the ovaries (ovary). The oviducts, like the Wolffian ducts of the testes, open to the cloaca. Both ovaries and testes lie in the body cavity near the kidneys.

      With the evolution of the reptilian egg, internal fertilization became necessary. The males of all modern reptiles, with the exception of tuataras, have functional copulatory organs. The structures vary from group to group, but all include erectile tissue as an important element of the operating mechanism, and all are protruded through the male's cloaca into that of the female during copulation. Unlike the penis of turtles and crocodiles, the copulatory organ of lizards and snakes is paired, with each unit being called a hemipenis. The hemipenes of lizards and snakes are elongated tubular structures lying in the tail. The penis of a crocodile or turtle is protruded through the cloacal opening wholly by means of a filling of blood space (sinuses (sinus)) in the penis; protrusion of a lizard's or snake's hemipenis, however, is begun by a pair of propulsor muscles. Completion of the erection is brought about by blood filling the sinuses in the erectile tissue. Only one hemipenis is inserted into a female, but which one is a matter of chance. Unlike the penis of mammals, the copulatory organs of reptiles do not transport sperm through a tube. The ducts from the testes, as already mentioned, empty into the cloaca, and the sperm flow along a groove on the surface of the penis or hemipenis.

Sense organs
 In general construction the eyes (eye, human) of reptiles are like those of other vertebrates. Accommodation for near vision in all living reptiles except snakes is accomplished by pressure being exerted on the lens by the surrounding muscular ring (ciliary body), which thus makes the lens more spherical. In snakes the same end is achieved by the lens being brought forward. The lens moves as a result of the pressure built up on the vitreous humour (eyeball) by contractions of muscles located at the base of the iris. The pupil shape varies remarkably among living reptiles, from the round opening characteristic of all turtles and many diurnal lizards and snakes to the vertical slit of crocodiles and nocturnal snakes and the horizontal slits of a few tree snakes (tree snake). Undoubtedly the most bizarre pupil shape is that of some geckos (gecko), in which the pupil contracts to form a series of pinholes, one above the other. The lower eyelid has the greater range of movement in most reptiles. In crocodiles the upper lid is more mobile. Snakes have no movable eyelids, their eyes being covered by a fixed transparent scale. Tuataras and all crocodiles have a third eyelid, the nictitating membrane, a transparent sheet that moves sideways across the eye from the inner corner, cleansing and moistening the cornea without shutting out the light.

      Visual acuity varies greatly among living reptiles, being poorest in the burrowing lizards and snakes (which often have very small eyes) and greatest in active diurnal species (which usually have large eyes). Judging by the size of the skull opening in which the eye is situated, similar variation existed among the extinct reptiles. Extinct forms, such as the ichthyosaurs, that hunted active prey had large eyes and presumably excellent vision; many herbivorous types, such as the horned dinosaur Triceratops, had relatively small eyes and weak vision. Colour vision has been demonstrated in few living reptiles.

      The power of hearing is variously developed among living reptiles. Crocodiles and most lizards hear reasonably well. Snakes and turtles are sensitive to low-frequency vibrations, thus they “hear” mostly earth-borne, rather than aerial, sound waves (sound). The reptilian auditory apparatus is typically made up of a tympanum, a thin membrane located at the rear of the head; the stapes, a small bone running between the tympanum and the skull in the tympanic cavity (the middle ear); the inner ear; and a eustachian tube connecting the middle ear with the mouth cavity. In reptiles that can hear, the tympanum vibrates in response to sound waves and transmits the vibrations to the stapes. The inner end of the stapes abuts against a small opening (the foramen ovale) to the cavity in the skull containing the inner ear. The inner ear consists of a series of hollow interconnected parts: the semicircular canals; the ovoidal or spheroidal chambers called the utriculus and sacculus; and the lagena, a small outgrowth of the sacculus. The tubes of the inner ear, suspended in a fluid called perilymph, contain another fluid, the endolymph. When the stapes is set in motion by the tympanum, it develops vibrations in the fluid of the inner ear; these vibrations activate cells in the lagena, the seat of the sense of hearing. The semicircular canals are concerned with equilibrium (inner ear).

      Most lizards (lizard) can hear. The majority have their best hearing in the range of 400 to 1,500 hertz and possess a tympanum, a tympanic cavity, and a eustachian tube. The tympanum, usually exposed at the surface of the head or at the end of a short open tube, may be covered by scales or may be absent. In general the last two conditions are characteristic of lizards that lead a more or less completely subterranean life. For subterranean lizards airborne sounds are less important than the low-frequency sounds passing through the ground. The middle ear of these burrowers is usually degenerate as well, often lacking the tympanic cavity and eustachian tube.

      Snakes (snake) have neither tympanum nor eustachian tube, and the stapes is attached to the quadrate bone on which the lower jaw swings. Snakes are obviously more sensitive to vibrations in the ground than to airborne sounds. A loud sound above a snake does not elicit any response, provided that the object making the sound does not move or, if it does, the movements are not seen by the snake. On the other hand, the same snake will raise its head slightly and flick its tongue in and out rapidly if the ground behind it is tapped or scratched. Snakes undoubtedly “hear” these vibrations by means of bone conduction. Sound waves travel more rapidly and strongly in solids than in the air and are probably transmitted first to the inner ear of snakes through the lower jaw, which is normally touching the ground, thence to the quadrate bone, and finally to the stapes. Burrowing lizards presumably hear ground vibrations in the same way.

      All crocodiles (crocodile) have rather keen hearing and have an external ear made up of a short tube closed by a strong valvular flap that ends at the tympanum. The American alligator (Alligator mississippiensis) can hear sounds within a range of 50 to 4,000 hertz. The hearing of crocodiles is involved not only in the detection of prey and enemies but also in their social behaviour (Social behaviour in animals); males roar or bellow to either threaten other males or to attract females.

      Turtles (turtle) have well-developed middle ears and usually large tympana. Measurements of the impulses of the auditory nerve between the inner ear and the auditory centre of the brain show that the inner ear in several species of turtles is sensitive to airborne sounds in the range of 50 to 2,000 hertz.

 Chemically sensitive organs, used by many reptiles to find their prey, are located in the nose and in the roof of the mouth. Part of the lining of the nose is made up of cells subserving the function of smell and corresponding to similar cells in other vertebrates. The second chemoreceptor (chemoreception) is the Jacobson's organ, which originated as an outpocketing of the nasal sac in amphibians; it remained as such in tuataras and crocodiles. The Jacobson's organ is most developed in lizards and snakes, in which its connection with the nasal cavity has been closed and is replaced by an opening into the mouth. The nerve connecting Jacobson's organ to the brain is a branch of the olfactory nerve. In turtles the Jacobson's organ has been lost.

      The use of the Jacobson's organ is most obvious in snakes. If a strong odour or vibration stimulates a snake, its tongue is flicked in and out rapidly. With each retraction, the forked tip touches the roof of the mouth near the opening of the Jacobson's organ, transferring any odour particles adhering to the tongue. In effect, the Jacobson's organ is a short-range chemoreceptor of nonairborne odours, as contrasted to the detection of airborne odours, smelling in the usual sense, by olfactory sensory patches in the nasal tube.

      Some snakes (notably the large vipers (viper)) and scleroglossan lizards (such as skinks (skink), monitors (monitor), and burrowing species of other families) rely upon the olfactory tissue and the Jacobson's organ to locate food, almost to the exclusion of other senses. Other reptiles, such as certain diurnal lizards and crocodiles, appear not to use scent in searching for prey, though they may use their sense of smell for locating a mate.

      The pit vipers (family Viperidae), boas (boa) and pythons (python) (family Boidae), and a few other snakes have special heat-sensitive organs (infrared (infrared radiation) receptors) on their heads as part of their food-detecting apparatus. Just below and behind the nostril of a pit viper is the pit that gives the group its common name. The lip scales of many pythons and boas have depressions (labial pits) that are analogous to the viper's pit. The labial pits of pythons and boas are lined with skin thinner than that covering the rest of the head and are supplied with dense networks of blood capillaries and nerve fibres. The facial pit of the viper is relatively deeper than the boa's labial pits and consists of two chambers separated by a thin membrane bearing a rich supply of fine blood vessels and nerves. In experiments using warm and cold covered electric light bulbs, pit vipers and pitted boas have been shown to detect temperature differences of less than 0.6 °C (1.1 °F).

      Many pit vipers, pythons, and boas are nocturnal and feed largely on mammals and birds. Infrared receptors, located on the face, enable these reptiles to direct their strikes accurately in the dark, once their warm-blooded prey arrives within range. The approach of prey is likely identified by the vibrations they make on the ground; however, the sense of sight and perhaps even the sense of smell are also used. The pit organs simply confirm the identity of the prey and aim the strike.

Thermal relationships
 Reptiles are often described as being cold-blooded (cold-bloodedness) animals; however this is not always true. They have no internal mechanism for the production of heat and maintenance of an elevated body temperature; they are dependent upon heat from their surroundings; that is, they are ectothermic. As ectotherms, many reptiles have body temperatures which fluctuate with that of the environment. This condition is called poikilothermy. Mammals and birds, often described as warm-blooded (warm-bloodedness) animals, produce heat by a cellular process and maintain relatively high body temperatures independent of the environment. In mammals, body temperature is kept relatively constant, and this condition is termed homoiothermy. For example, when the body temperature of a dog or a human being falls below the normal range, shivering begins, and blood vessels in the skin contract. Subsequent muscular activity generates heat, and the contraction of the superficial blood vessels, by reducing the volume of blood flow at the surface, reduces heat loss by radiation. By contrast, when the body temperature of a reptile falls below the optimum, it must move to a part of the environment with a higher ambient temperature. When environmental temperatures fall below a critical minimum, a reptile's metabolic activity decreases; its movements become sluggish, its heartbeat slows, and its rate of breathing drops. In short, it becomes incapable of the normal activities required for growth, reproduction, and survival.

      In higher-temperature environments mammals and birds have some physiological means of cooling their bodies. They can pant or sweat, and superficial blood vessels may expand; however, a reptile must ordinarily move away from a spot in which the temperature is too high, or it will perish very quickly. Some reptiles also pant, but their temperature accommodations are largely behavioral; they might change their orientation with respect to the Sun or wind or raise their body from the ground.

      Each group of reptiles has its own characteristic thermal range. One genus of lizards, for example, may require temperatures of 29–32 °C (84–90 °F) for maximum efficiency, whereas another may require temperatures of 24–27 °C (75–81 °F). As a result of such physiological differences, lizards of the two groups will be active at different times of the day or occupy slightly different habitats.

      In reptiles the body temperatures at which normal activities occur are generally lower than those of most mammals; however, a few sun-loving (heliothermic) lizards, such as the greater earless lizard (Holbrookia texana) of the southwestern United States, have average activity temperatures above 38 °C (100 °F). This temperature is slightly higher than the average human body temperature. Such high temperatures are exceptional, and the majority of lizards have normal activity temperatures in the 27–35 °C (81–95 °F) range.

Evolution and paleontology

Historical development
      The first land vertebrates (vertebrate), the Tetrapoda (tetrapod), appeared during Upper Devonian (Devonian Period) times, about 380 million years ago. Despite having limbs rather than fins, early tetrapods were not completely terrestrial because their eggs (egg) and larvae depended upon a moist aquatic habitat. The first tetrapods apparently soon diverged; one lineage became the amphibians (amphibian) (which retained the requirement for moisture-associated reproduction), whereas a second lineage yielded the Amniota during the Early Pennsylvanian (Pennsylvanian Subperiod) Epoch (318 million to 312 million years ago). Fossils (fossil) of these early amniotes are lacking; however, they must have appeared at this time because, for the Middle Pennsylvanian Epoch (312 million to 307 million years ago), fossils of synapsids (mammal-like reptiles) and early reptiles occur together in the same fossil beds. These earliest known synapsids and reptiles had already developed some traits that would persist in their descendants, modern mammals and reptiles. One example of a feature both groups held in common was the presence of extra-embryonic membranes (essentially, the amniotic sac) in early development, an adaptation that permitted the shift to a fully terrestrial egg.

Fossil distribution
      The earliest known reptiles, Hylonomus and Paleothyris, date from Late Carboniferous (Carboniferous Period) deposits of North America. These reptiles were small lizardlike animals that apparently lived in forested habitats. They are the Eureptilia (true reptiles), and their presence during this suggests that they were distinct from a more primitive group, the anapsids (or Parareptilia). The early reptiles were usually small animals and generally were not as abundant as some of the synapsids, such as the sailback pelycosaurs ( Edaphosaurus, Dimetrodon, and others). Assorted parareptiles occurred throughout the Permian Period (299 million to 251 million years ago), but they largely disappeared from the fossil record by the beginning of what was to become known as the “Age of Reptiles,” the Mesozoic Era (251 million to 65.5 million years ago). Nonetheless, they reappeared during the Late Triassic (Triassic Period) Epoch (228 million to 200 million years ago) as the first turtles (turtle); the most primitive of which was Proganochelys. Turtles regularly appear in fossil records thereafter. Of the eureptiles, the captorhinids were present throughout most of the Permian. These broad-headed lizardlike reptiles appear to have been agile carnivores (carnivore) of moderate size. They disappeared, apparently leaving no descendants, in the Late Permian, or Lopingian, Epoch (260 million to 251 million years ago).

      With the possible exception of turtles (which are often labeled anapsids), modern reptiles and most reptiles of the Mesozoic Era are diapsids. One of the most recognizable groups of diapsids is the lepidosauromorphs. This lineage, which is ancestral to today's tuataras (tuatara) and squamates (lizards and snakes (snake)), appeared first during the Late Permian. Assorted squamates or squamate relatives began appearing in the Jurassic Period (200 million to 146 million years ago); however, representatives of modern lizards and snakes do not occur in the fossil record until the middle of the Cretaceous Period (146 million to 65.5 million years ago).

      One of the main diversifications occurred within the suborder Sauria. Some of the most specialized saurians, the ichthyosaurs (ichthyosaur) and sauropterygians (sauropterygian), appear first in the Early Triassic (251 million to 245 million years ago), and representatives of both groups occurred in the seas until the middle of the Cretaceous. The ichthyosaurs are reptiles with fishlike bodies; they were live-bearers because their body form prevented beaching to lay eggs. The sauropterygians included an assortment of marine creatures; this group included the plesiosaurs (plesiosaur) as well as forms that resembled modern-day turtles and walruses (walrus). The plesiosaurs have no modern-day analogs.

      The archosauromorphs, a group of diapsids that includes the dinosaurs (dinosaur) as well as modern crocodiles (crocodile) and birds (bird), did not appear in the fossil record until the middle of the Triassic Period. The leather-winged pterosaurs (pterosaur), or “winged lizards,” were also archosauromorphs; they persisted throughout the remainder of the Mesozoic Era. Crocodylomorphs and dinosaurs were present in the Early Jurassic Epoch (200 million to 176 million years ago), and their descendants live today in the forms of the crocodiles and birds.


Distinguishing taxonomic features
      Today's reptiles represent only a fraction of the reptile groups and species that have lived; thus, reptilian classification depends upon fossil remains. As such, the higher levels of reptilian classification rely heavily on skeletal characters. Reptiles (class Reptilia) and mammals (mammal) (class Mammalia) are the two surviving branches of the Amniota, which is a group characterized by the presence of amniotic membranes. Obviously, these embryonic structures are not present in the fossil record; however, one can recognize that they existed in the common ancestor of reptiles and synapsids by their presence in modern forms of each group. Cranial, vertebral, and limb-girdle skeletal traits are the major characters used for the higher categories of classification, and soft (fleshy) anatomical traits are used in addition in those groups with living relatives or where the fossil record has preserved such characters.

Annotated classification
      Reptilian classification is highly mutable. Changes in group names and composition occur every few months. These changes derive from the discovery of new fossils (fossil), new data sets, new phylogenetic analytical techniques, and different taxonomic philosophies (taxonomy). Furthermore, many biologists are abandoning the use of group titles (such as class and order) in favour of an indented hierarchical arrangement that reflects the phylogenetic branching pattern (phylogenetic tree). Group titles are used below, but the same title may not depict equivalent phylogenetic branching events; thus, the titles do not reflect equivalent hierarchical positions. The following classification derives mainly from the Tree of Life web project, a collaborative effort by several biologists to classify the diversity of Earth's organisms. Further, this classification contains a listing of the more familiar reptilian groups and only occasionally uses a different taxon name from that proposed in the Tree of Life web project. For example, Parareptilia is called Anapsida in the Tree of Life web project, and Eureptilia is called Romeriida. Groups marked with a dagger (†) are extinct and known only from fossils. For more-detailed taxonomies of individual reptile groups, see dinosaur, lizard, snake, turtle, and crocodile.

Class Reptilia
 Air-breathing, amniotic vertebrate animals, usually with a body covering of keratinous epidermal scales. The occipital condyle (a protuberance where the skull attaches to the first vertebra) is single. Cervical vertebrae have midventral keels; the intercentrum of the second cervical vertebra fuses to the axis in adults; taxa with well-developed limbs have two or more sacral vertebrae. The single auditory bone, the stapes, transmits sound vibrations from the eardrum (tympanum) to the inner ear. The lower jaw consists of several bones but lacks an anterior coronoid bone. Reproduction is internal, with sperm deposition by copulation or cloacal apposition. Development is either internal, with embryos retained in the females' oviducts, with or without a placenta, or external, with embryos in shelled eggs. Whether developing internally or externally, each embryo is encased in amniotic membranes. Excluding birds, there are over 8,700 species of living reptiles.
      Subclass Parareptilia or Anapsida (parareptiles)
 Pennsylvanian to present. Skull typically without temporal openings; prefrontal-palatine contact present.

      †Order Mesosauria (mesosaurs (Mesosaurus))
 Lower Permian. One family, three genera. Aquatic reptiles with slender elongate jaws filled with long pointed teeth. Tail as long as or longer than body and flattened side to side; limbs well developed, hind feet enlarged and paddlelike. Total length to about 1 metre (3 feet).

      †Order Pareisauria (pareisaurs)
 Middle to Upper Permian. Two or 3 families, 10 or more genera. Small to moderately large (2 metres [about 7 feet]), terrestrial reptiles; appearance from lizardlike to sprawl-limbed and cowlike. Dermal sculpturing of large tuberosities and deep pits on skull; limbs well developed; often possess a robust limb and trunk skeleton.

      †Order Procolophonia (procolophonians)
 Upper Permian to Upper Triassic. Three or 4 families, about 30 genera. Small (typically less than 0.5 metres [1.6 feet]) terrestrial lizardlike reptiles. Pineal eye foramen near frontoparietal suture on top of skull.

      Order Testudines (turtles (turtle))
 Upper Triassic to present. Three infraorders. Small (16 cm [6 inches]) to large (3.6 metres [12 feet] in shell length) armoured reptiles, terrestrial to marine. Skull without pineal opening; jaws toothless; armor in form of a shell encasing the body above (carapace) and below (plastron).

      Eureptilia (eureptiles)
 Late Pennsylvanian to present. Skull typically with temporal openings; prefrontal-palatine contact usually absent; supratemporal small. All taxa except for the captorhinids have diapsid skulls characterized by upper and lower temporal fenestrae.

      †Family Captorhinidae (captorhinids (Captorhinus))
 Lower through Upper Permian. One family and about 12 genera. Prefrontal-palatine contact present; dermal sculpturing honeycomblike. Small to moderate-sized terrestrial reptiles.

      † Order Araeoscelidia (araeoscelidians)
 Lower Permian to Upper Triassic. Small lizardlike terrestrial reptiles

      † Infraclass Ichthyosauria (ichthyosaurs (ichthyosaur))
 Lower Triassic to early Upper Cretaceous. Seven or 8 families and more than 20 genera. Highly aquatic reptiles with porpoiselike bodies, a dorsal fin, and a reversed-heterocercal tail (i.e., with the lower lobe longer than the upper). Limbs paddlelike; snout often elongated and beaklike.

      † Superorder Sauropterygia (sauropterygian)
 Lower Triassic to Upper Cretaceous. Three groups of aquatic reptiles (about 10 families and more than 40 genera) with the plesiosaurs largely replacing the nothosaurs temporally. Skull with an upper temporal fenestra (between postorbital, squamosal, and parietal bones) and a broad plate of bone below. Limbs paddlelike in many forms.

      †Order Nothosauroidea (nothosaurs (Nothosaurus))

      †Order Plesiosauria (plesiosaurs (plesiosaur))

      †Order Placodontia (placodonts)
 Lower to Upper Triassic. Aquatic seallike reptiles with long tails and short limbs not modified as paddles; total length typically less than 2 metres (about 7 feet). Head often large with broad flattened jaw and palate teeth, likely for crushing mollusks. Skull with large upper temporal fenestra and a small slitlike suborbital fenestra. Side branch of euryapsids, apparently mollusk eaters. In some the body was armoured and turtlelike in form.

      Subclass Archosauria (archosaurians (archosaur))
 Upper Permian to present. Three major orders. Tiny to giant reptiles with diverse body plans. Teeth in deep sockets (thecodont (thecodontian)); nasal longer than frontal; no pineal foramen on skull roof; hooked metatarsal.

      †Order Pterosauria (pterosaur) (pterodactyls (pterodactyl))
 Upper Triassic to Upper Cretaceous. Two suborders, about 16 families, and more than 30 genera. Highly specialized flying reptiles with hollow bones; fourth digit of the forelimb greatly elongated to support the flying membrane of the wing. Early forms toothed and with long tails; later forms tended to be larger with greatly reduced tails and no teeth.

      †Superorder Dinosauria (dinosaurs (dinosaur))
 Upper Triassic to present. Two major groups of dinosaurs. Skull without prefrontal bones; three or fewer phalanges in fourth digit of forefoot.

      †Order Ornithischia (ornithischians (ornithischian), bird-hipped dinosaurs)
 Upper Triassic to Upper Cretaceous. Five major groups: ornithopods, pachycephalosaurs, stegosaurs, ankylosaurs, and ceratopsina dinosaurs. Teeth triangular with largest tooth in middle of tooth row; predentary bone in lower jaw; and pelvis tetraradiate (i.e., four-branched). Typically with a beaklike structure in the front part of the mouth and grinding teeth in the rear. Both obligate bipedal and quadrupedal forms. Toes often with hooflike structures. Many with heavy armor and horns. Largest about 9 metres (30 ft) long.

      †Order Saurischia (saurischians (saurischian), lizard-hipped dinosaurs)
 Upper Triassic to Upper Cretaceous. Two major groups. Subnarial foramen present; astragulus with wedge-shaped ascending process; pelvis triradiate (i.e., three-branched). Some reduction in digits. Forelimbs usually distinctly shorter than hind limbs. Three to seven sacral vertebrae. Some herbivorous forms were more than 24 metres (78 feet) long.

      Order Crocodylia (crocodiles (crocodile))
 Paleocene to present. Three living families, 8 genera, and 23 species. Aquatic or amphibious reptiles with robust body and tail, short sturdy limbs, a strong flattened and elongate skull with nostrils at tip of snout, and a well-developed secondary palate. Living species 1.5 to about 7 metres (5 to 23 feet) in total length; some extinct species grew to 15 metres (49 feet) long.

      Subclass Lepidosauria (lepidosaurians)
 Upper Jurassic to present. Two orders. No teeth on parasphenoid; teeth attached superficially to upper and lower jaws; parietal eye in parietal; transverse cloacal opening.

      Order Sphenodontida (tuataras (tuatara))
 Lower Triassic to present. Three families, about 20 genera, but only one genus (Sphenodon) surviving, with two living species. Premaxillary downgrowth replaces premaxillary teeth; four to five teeth enlarged at beginning of palatine tooth row.

      Order Squamata (squamates)
 Lizards (lizard), snakes (snake), and amphisbaenians. Upper Jurassic to present. Two suborders. Parietals fused; Jacobson's organ with a fungiform projection and separate from nasal cavity, opening only into mouth cavity; paired functional hemipenes.

Critical appraisal
      Classifications of plants and animals, especially at the levels above the families, were fairly stable for much of the 20th century. Beginning in the late 1980s, however, biologists began to advocate classifications that more accurately reflected phylogeny—that is, the branching evolutionary history of organisms. Because of the numerous branching that occurs within most lineages, the number of formal taxonomic levels available is commonly less than the number of branching events. This situation has caused many systematists (i.e., the biologists who study the relationships of organisms and their classifications) to abandon the formal titles (such as phylum, class, and order) and present their classifications as indented hierarchical lists or tables. Aside from the preceding debate on how to present classifications, several other philosophical debates are ongoing, and it is probable that several academic generations will pass before biological classification stabilizes and the systematists obtain a consensus.

      Because of their long history and great diversity, the Reptilia, or reptiles in the broad phylogenetic sense, are especially difficult to classify in an orderly and consistent manner. The regular discovery of new fossil reptiles (as well as the discovery of more complete specimens of known types), the introduction of new tools (such as X-ray computed tomography scanning and DNA sequencing), and new data analysis techniques all provide fresh insights into the evolutionary history of various groups of reptiles. Often, the newly proposed phylogeny differs from the previous one and entails changes in classification. For example, in the debate involving the relationships of turtles to other reptiles that began in the mid-1990s, one group of systematists proposed that turtles were diapsids (subclass Eureptilia, infraclass Diapsida). For more than a century it was widely accepted that turtles should be classified as anapsids, or parareptiles (subclass Parareptilia). The potential shift of turtles from subclass Parareptilia to Eureptilia would greatly alter the classification of the entire diapsid group and produce an adjustment in the hierarchical arrangement and classification-level names of the various reptile groups.

      The preceding controversy highlights several aspects of science. One aspect is the repeated reexamination (testing) of existing “facts” with new data and new techniques. Facts are not absolutes but hypotheses that have become increasingly accurate by reexamination. This reexamination has improved the knowledge and understanding of reptile evolution and classification, although it has made the latter more complex and less stable.

      Reptile classification may be more complex at present; however, it is also more precise and more accurately conveys the evolutionary relationships within and between groups. The present classification of the Tetrapoda and Reptilia no longer conveys the erroneous impression that amphibians were the intermediate step between fishes and reptiles, that reptiles arose from amphibians, or that birds arose from reptiles. Reptiles derive from an anthracosaurian stock that shares a common tetrapod ancestor with amphibians. Although they are not treated in this article, birds are reptiles. All evidence indicates that birds arose within the Archosauria; however, there is debate whether this origin was among advanced or early dinosaurian archosaurs. Similarly, both fossil evidence and molecular data largely indicate that snakes arose within the scleroglossan lizards. Thus, snakes really are legless (or nearly legless) lizards, and thus they should not be depicted in a classification as a group equal to and at the same level as lizards. .

Herndon G. Dowling George R. Zug

Additional Reading

General works
General surveys of reptiles and their life histories, with many photographs, are found in H.M. Cogger and R.G. Zweifel, Encyclopedia of Reptiles and Amphibians, 2nd ed. (2003); and Tim Halliday and Kraig Adler (eds.), The New Encyclopedia of Reptiles and Amphibians (2002). College-level texts that provide an excellent overview of reptiles and general herpetology include George R. Zug, Laurie J. Vitt, and Jan P. Caldwell, Herpetology: An Introductory Biology of Amphibians and Reptiles, 2nd ed. (2001); and F. Harvey Pough et al., Herpetology, 3rd ed. (2004).Well-organized field guides and taxonomic summaries with excellent maps and photographs include Mark O'Shea and Tim Halliday, Reptiles and Amphibians, ed. by David A. Dickey (2002); Tim Halliday and Kraig Alder (eds.), Firefly Encyclopedia of Reptiles and Amphibians (2002); Roger Conant and Joseph T. Collins, A Field Guide to Reptiles and Amphibians: Eastern and Central North America, 3rd ed. (1998); John Coborn, The Mini-Atlas of Snakes of the World (1994); Roy W. McDiarmid, Jonathan A. Campbell, and T'Shaka A. Touré, Snake Species of the World: A Taxonomic and Geographic Reference, 3 vol. (1999– ); and John B. Iverson, A Revised Checklist with Distribution Maps of the Turtles of the World (1997). In addition, B. Gill and T. Whitaker, New Zealand Frogs and Reptiles (1996), provides a general overview of tuataras.

Advanced works
Carl Gans et al. (eds.), Biology of the Reptilia, 19 vol. (1968–98), is a highly technical series of detailed reviews on various aspects of reptilian anatomy, physiology, behaviour, ecology, and embryology. Ludwig Trutnau and Ralf Sommerlad, Crocodilians: Their Natural History and Captive Husbandry (2006), is an invaluable resource that covers crocodilian evolution and classification, ecology, conservation and management, physiology, and behaviour. Other technical treatments that consider various aspects of the anatomy, natural history, and ecology of living reptiles include Richard A. Seigel, Joseph T. Collins, and Susan S. Novak, Snakes: Ecology and Evolutionary Biology (2001); Jonathan A. Campbell and William W. Lamar, The Venomous Reptiles of the Western Hemisphere, 2 vol. (2004); G.C. Grigg, F. Seebacher, and C. Franklin (eds.), Crocodilian Biology and Evolution (2001); and L.J. Vitt and E.R. Pianka (eds.), Lizard Ecology: Historical and Experimental Perspectives (1994).

Reptile evolution and paleontology
The evolution of reptiles within the context of other vertebrates is carefully considered in Robert L. Carroll, Patterns and Processes of Vertebrate Evolution (1998); and Michael J. Benton Vertebrate Paleontology, 3rd ed. (2005). A succinct overview of the various evolutionary adaptations possessed by lizards is presented in Eric R. Pianka, Lizards: Windows to the Evolution of Diversity (2006). Ronald Orenstein, Turtles, Tortoises, and Terrapins: Survivors in Armor (2001), is a popular and well-illustrated account that addresses turtle conservation and evolution as well as natural history. A highly accessible treatment of the evolution of flight in pterosaurs and other reptiles is found in Phillip J. Currie, The Flying Dinosaurs: The Illustrated Guide to the Evolution of Flight (2002). A semi-technical review of dinosaur evolution, biology, and research is found in James O. Farlow, The Complete Dinosaur, ed. by M.K. Brett-Surman (1999). Anthony J. Martin, Introduction to the Study of Dinosaurs, 2nd ed. (2006), provides a general treatment of dinosaur evolution that highlights the process of paleontological research.George R. Zug

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Universalium. 2010.

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