Eastern Herman's Tortoise
Reptiles are tetrapods (four-legged vertebrates) and amniotes (animals whose embryos are aided and protected by several membranes, whether carried inside the mother or laid outside as part of an egg). Reptiles, which in most cases are covered with scales, have traditionally been defined as including all the amniotes except birds and mammals. Snakes, lizards, turtles, and crocodiles are common examples of reptiles.
Today, reptiles are represented by four surviving orders distributed among more than 7500 species found on every continent except for Antarctica. In the history of life on Earth, reptiles, with their eggs protected by a tough exterior, were the first vertebrate animals to occupy ecological niches far from water. For more than 230 million years, until the Cretaceous-Tertiary extinction event 65 million years ago, reptiles, including the dinosaurs—great and small and wonderfully diverse—were the dominant form of vertebrate land animals.
Modern reptiles in general do not generate enough heat to maintain a constant body temperature and are thus referred to as "cold-blooded" (ectothermic). They rely on gathering heat from and dissipating it to the environment to regulate their internal temperature, and employ such means as moving between areas of sunlight and shade, or preferential circulation (moving warmed blood into the body core, while pushing cool blood to the periphery). While this lack of adequate internal heating imposes costs, it also allows reptiles to survive on much less food than comparably-sized mammals and birds, who burn much of their food for warmth. Although warm-blooded animals move faster in general, an attacking lizard, snake, or crocodile moves very quickly.
While surviving lines of reptiles tend to be cold-blooded, steadily accumulating evidence points to the view that at least some of the dinosaurs dominating Earth's land areas for more than 160 million years (230-65 million years ago) during the Mesozoic era, were warm-blooded reptiles.
Reptiles offer economic, aesthetic, symbolic, and ecological values to humans. They are integral to food chains and in the reduction of agricultural pests, are part of the diet in some cultures, are used in producing leather goods, and are kept as pets. Some reptiles are considered aesthetically pleasing with their vivid colors. In religion, reptiles have served an important symbolic purpose, particularly noteworthy in the appearance of the serpent in Genesis. (See Reptiles and humans.)
According to a report by Uetz in 2000, comprehensive compilations reveal a total of 7,870 species of reptiles, with the majority being lizards (4,470 species) and snakes (2,920), and with 23 described species of living crocodiles, 295 species of turtles, 156 amphisbaenias, and 2 species of tuataras. Uetz reported that 51 percent of known reptile species belong to one of three families: colubrid snakes (1,850 species), skinks (1,200), and geckos (1,000). New reptile species continue to be described at the rate of about 60 species per year (Uetz 2000).
A subsequent tabulation by Uetz in 2005 showed a total of 8,240 extant reptile species, and his March 2006 list revealed 8,364 known species. The International Union for the Conservation of Nature and Natural Resources (IUCN) tabulated 8,163 described species of reptiles in 2004.
Uetz (2000) reported 2,100 living species of reptiles in Asia (including New Guinea), 1,550 in South America, 1,350 in Africa, 1,050 in Central America, and 850 in Australia. There were but 190 species of living reptiles in Europe and 360 in North America.
Most reptiles have closed circulation via a three-chamber heart comprising two atria and one variably-partitioned ventricle. There is usually one pair of aortic arches. In spite of this, due to the fluid dynamics of blood flow through the heart, there is little mixing of oxygenated and deoxygenated blood in the three-chamber heart. Furthermore, the blood flow can be altered to shunt either deoxygenated blood to the body or oxygenated blood to the lungs, which gives the animal greater control over its blood flow, allowing more effective thermoregulation and longer diving times for aquatic species.
There are some interesting exceptions among reptiles. For instance, crocodilians have an incredibly complicated four-chamber heart that is capable of becoming a functionally three-chamber heart during dives (Mazzotti 1989). Also, it has been discovered that some snake and lizard species (for example, monitor lizards and pythons) have three-chamber hearts that become functional four-chamber hearts during contraction. This is made possible by a muscular ridge that subdivides the ventricle during ventricular diastole and completely divides it during ventricular systole. Because of this ridge, some of these squamates are capable of producing ventricular pressure differentials that are equivalent to those seen in mammalian and avian hearts (Wang et al. 2003).
All reptiles breathe using lungs. Aquatic turtles have developed more permeable skin, and for some species even gills in their anal region (Orenstein 2001). Even with these adaptations, breathing is never fully accomplished without lungs.
Lung ventilation is accomplished differently in each main reptile group. In squamates, the lungs are ventilated almost exclusively by the axial musculature, which is also the same musculature used during locomotion. Because of this constraint, most squamates are forced to hold their breath during intense runs. Some, however, have a way around it. Varanids and a few other lizard species employ buccal pumping (a method of respiration using the throat muscles) as a complement to their normal breathing. This allows the animals to completely fill their lungs during intense locomotion, and thus remain aerobically active for a long time. Tegu lizards are known to possess a proto-diaphragm, which separates the pulmonary cavity from the visceral cavity. While not actually capable of movement, it does allow for greater lung inflation, by taking the weight of the viscera off the lungs (Klein et al. 2003). Crocodilians have a muscular diaphragm that is analogous to the mammalian diaphragm. The difference is that the muscles for the crocodilian diaphragm pull the pubis (part of the pelvis, which is movable in crocodilians) back, which brings the liver down, thus freeing space for the lungs to expand. This type of diaphragmatic setup has been referred to as the "hepatic piston."
Turtles and tortoises have found a variety of solutions to breathing, given that most turtle shells are rigid and do not allow for the type of expansion and contraction that other amniotes use to ventilate their lungs. Some turtles such as the Indian flapshell (Lissemys punctata) have a sheet of muscle that envelopes the lungs. When it contracts, the turtle can exhale. When at rest, the turtle can retract the limbs into the body cavity and force air out of the lungs. When the turtle protracts its limbs, the pressure inside the lungs is reduced, and the turtle can suck air in. Turtle lungs are attached to the inside of the top of the shell (carapace), with the bottom of the lungs attached (via connective tissue) to the rest of the viscera. By using a series of special muscles (roughly equivalent to a diaphragm), turtles are capable of pushing their viscera up and down, resulting in effective respiration, since many of these muscles have attachment points in conjunction with their forelimbs (indeed, many of the muscles expand into the limb pockets during contraction).
Breathing during locomotion has been studied in three species, and they show different patterns. Adult female green sea turtles do not breathe as they crutch along their nesting beaches. They hold their breath during terrestrial locomotion and breathe in bouts as they rest. North American box turtles breathe continuously during locomotion, and the ventilation cycle is not coordinated with the limb movements (Landberg et al. 2003). They are probably using their abdominal muscles to breathe during locomotion. The red-eared sliders also breathe during locomotion, but they had smaller breaths during locomotion than during small pauses between locomotion bouts, indicating that there may be mechanical interference between the limb movements and the breathing apparatus. Box turtles have also been observed to breathe while completely sealed up inside their shells (Landberg et al. 2003).
Most reptiles lack a secondary palate, meaning that they must hold their breath while swallowing. Crocodilians have evolved a bony secondary palate that allows them to continue breathing while remaining submerged (and protect their brains from getting kicked in by struggling prey). Skinks (family Scincidae) also have evolved a bony secondary palate to varying degrees. Snakes have a different approach and extend their trachea instead. Their tracheal extension sticks out like a fleshy straw. By thrusting their windpipe into the throat, these animals can swallow large prey without suffering from asphyxiation, despite the fact that swallowing may take several hours.
Land-dwelling reptiles, such as snakes and lizards, excrete nitrogenous wastes in pasty or dry form as crystals of uric acid (Towle 1989). Two small kidneys are used in excretion.
Snakes have hinged upper and lower jaws, which move independently. These jaws stretch when unhinged, allowing snakes to swallow large prey. Saliva begins to digest food before it reaches the stomach, which is basically an enlargement at the end of the esophagus where digestion can slowly proceed (Towle 1989).
Crocodilians have modified salivary glands on their tongue (salt glands), used for excreting excess salt from their body, although they are non-functioning in alligators and caimans. Crocodilians are known to swallow stones, gastroliths ("stomach-stones"), which help to crush up the bones of their prey. The crocodile stomach is divided into two chambers, the first one is described as being powerful and muscular, like a bird gizzard, and this is where the gastroliths are found. The other stomach has the most acidic digestive system of any animal, and it can digest mostly everything from their prey; bones, feathers, and horns.
The tongue of a snake includes highly sensitive smell sensors. Some researchers speculate that the forked nature of the tongue may offer a stereo sense of smell.
Crocodilians see well in daylight and may even have color vision; additionally, their vertical, cat-like pupil gives them excellent night vision. In crocodilians, the upper and lower jaws also are covered with sensory pits, the crocodile version of the lateral line sensory organ found in fish and many amphibians. These pigmented nodules encase bundles of nerve fibers that respond to the slightest disturbance in surface water, detecting vibrations and small pressure changes in water, making it possible for them to detect prey, danger, and intruders even in total darkness. While alligators and caimans have the sensory nodules only on their jaws, crocodiles have similar organs on almost every scale on their body
Most reptiles reproduce sexually. This includes many male snakes that rely on scent to find females and that complete fertilization internally.
Most reptile species are oviparous (egg-laying). Many species of squamates, however, are capable of giving live birth. This is achieved either through ovoviviparity (egg retention) or viviparity (babies born without use of calcified eggs). Many of the viviparous species feed their fetuses through various forms of placenta, just like mammals (Pianka and Vitt 2003). They often provide considerable initial care for their hatchlings.
Amniotic eggs are covered with leathery or calcareous shells and are compartmentalized by four membranes: (1) The amnion encloses the embryo and the amnion fluid in which it floats; (2) the yolk sac encloses the yolk, the embryo's protein-rich food reservoir; (3) the allantois stores the embryo's nitrogenous wastes until hatching, and (4) the chorion is the outer membrane that lines the shell and thereby encloses the egg's other three membrane-bound compartments and the fluid in which they are bathed. (Towle 1989). Eggs are waterproof, but permeable to gases. Sperm are placed inside the female by internal fertilization prior to the formation of the shell.
In some reptiles, the sex of the juvenile is determined by the incubation temperature.
In addition to the common pattern of sexual reproduction among reptiles, a pattern of asexual reproduction has been identified in six families of lizards and one snake family. In some species of squamates (lizards and snakes), a population of females is able to produce a unisexual diploid clone of the mother. This asexual reproduction, called parthenogenesis, occurs in several species of gecko and is particularly widespread in the teiids (especially Aspidocelis) and lacertids (Lacerta). Parthenogenetic species are also suspected to occur among chameleons, agamids, xantusiids, and typhlopids.
Reptiles offer economic, ecological, aesthetic, and symbolic value to humans.
Some species, such as the green turtle, the iguana, and some snakes, are part of the diet, and the giant Galapagos tortoise was so popular as a food among sailors in the nineteenth century that it was nearly exterminated. The skins of crocodilians, snakes, and lizards have been used in leather goods, such as shoes, handbags, gloves, and belts, but international agreements protecting endangered species have prompted a shift of reptile skin sources from hunters of wild species to farmers growing reptiles in captivity. Reptiles also are very popular pets. In the United States, about 3 percent of households have reptiles as pets with many of the reptiles having been imported into the country either legally or illegally as part of the international trade in live exotic animals.
Ecologically, reptiles are a critical element in the food chains of most ecosystems, and sometimes a keystone species whose removal can drastically alter the populations of other organisms. The consumption by reptiles of rodents and insect pests aids in control of these animals, which can be serious agricultural pests.
Aesthetically, many reptiles can be considered beautiful or awe-inspiring, such as the San Francisco garter snake (Thamnophis sirtalis tetrataenia), with its bright orange head, black and red stripes, and turquoise belly, and the chameleons with their color changes. Reptiles appear in designs on apparel and other consumer goods because of their appeal.
Symbolically, reptiles appear in literature and religion in a variety of ways. Perhaps the most famous reference is the Bible reference to the serpent in the Garden of Eden, or Jesus advising his disciples to be "wise as serpents." In South American fame, the Quetezecoatly was the serpent that was the law giver and culture bearer.
Some reptiles also present threats to people, whether because they are venomous, like some snakes, or can attack humans, such as some crocodilians. In addition, salmonella, a bacterial disease, is sometimes picked up from a reptile's skin when touching a reptile kept as a pet.
Hylonomus, the oldest-known reptile, was about 8 to 12 inches (20 to 30 cm) long. Westlothiana, also suggested as the oldest reptile, is for the moment considered to be related more to amphibians than to amniotes. Other examples of fossil animals considered to be ancient reptiles are those of the genera Petrolacosaurus, Araeoscelis, Paleothyris, Ophiacodontidae, Archaeothyris, and Ophiacodon, and also the family of mesosaurs.
The first true "reptiles" or amniotes are categorized as Anapsids (Anapsida), which are vertebrates characterized by solid skulls with the conventional openings for nose, eyes, spinal cord, and so forth, but lacking temporal fenestrae (jaw muscle attachment sites at holes in the sides of the skull behind the eyes near the temples). Turtles are believed by some to be surviving anapsids, indeed the only surviving anapsids, as they also share this skull structure. However, this point has become contentious, with some arguing that turtles reverted to this primitive state in the process of improving their armor. Both sides have marshaled evidence, and the conflict has yet to be resolved.
Shortly after the appearance in the fossil record of the first reptiles, a second branch appeared. The original branch led to the Anapsida, which did not develop the jaw muscle attachment holes in their skulls, and the second led to the Diapsida (diapsids), which developed two pairs of jaw muscle attachment holes in their skulls behind the eye holes. Diapsids ("two arches") are a group of tetrapod animals that appeared in the fossil record about 300 million years ago during the late Carboniferous period. Living diapsids are extremely diverse, and are considered to include all birds, crocodiles, lizards, snakes, and tuataras (and possibly even turtles). While some lost either one hole (lizards), or both holes (snakes), they are still classified as diapsids based on their assumed ancestry.
During the Permian period (299-251 million years ago), the Diapsida line of descent split into two lineages: The lepidosaurs (modern snakes, lizards, and tuataras, as well as, debatably, the extinct sea reptiles of the Mesozoic era) and the archosaurs (living crocodilians and birds as well as the extinct pterosaurs and dinosaurs).
The earliest solid-skulled amniotes in addition to giving rise to the anapsids, are also considered to have given rise about 300 million years ago to a separate line, the Synapsida (synapsids), which have a pair of holes in their skulls behind and above the eyes; this feature has the advantage of lightening the skull and increasing the space for jaw muscles. The synapsids eventually evolved into mammals and the early synapsids have been referred to as mammal-like reptiles by some specialists, while others argue that even the early synapsids were no longer reptiles.
As noted above, from the classical standpoint, reptiles included all the amniotes except birds and mammals. Thus, reptiles were defined as the set of animals that includes crocodiles, alligators, tuatara, lizards, snakes, amphisbaenians, and turtles, grouped together as the class Reptilia (Latin repere, "to creep"). This is still the usual definition of the term.
However, in recent years, many taxonomists have begun to insist that for clear identification of the ancestor-descendant relations of all organisms each defined taxon should be monophyletic, that is, each taxon should include all the descendants from the originating stock. The reptiles as defined are clearly not monophyletic (but rather are paraphyletic), since they exclude both birds and mammals, although these also are considered to be descendant from the original reptile. Colin Tudge (2000) writes:
Mammals are a clade [a monophyletic taxon], and therefore the cladists are happy to acknowledge the traditional taxon Mammalia; and birds, too, are a clade, universally ascribed to the formal taxon Aves. Mammalia and Aves are, in fact, subclades within the grand clade of the Amniota. But the traditional class Reptilia is not a clade. It is just a section of the clade Amniota: The section that is left after the Mammalia and Aves have been hived off. It cannot be defined by synamorphies, as is the proper way. It is instead defined by a combination of the features it has and the features it lacks: Reptiles are the amniotes that lack fur or feathers. At best, the cladists suggest, onee could say that the traditional Reptila are “non-avian, non-mammalian amniotes.”
Some cladists thus redefine Reptilia as a monophyletic group, including the classic reptiles as well as the birds and perhaps the mammals (depending on ideas about their relationships). Others abandon it as a formal taxon altogether, dividing it into several different classes. However, other biologists believe that the common characters of the standard four orders (Crocodilia (crocodiles), Rhynchocephalia (tuataras), Squamata (snakes and lizards), and Testudines (turtles)) are more important than the exact relationships, or feel that redefining the Reptilia to include birds and mammals would be a confusing break with tradition. A number of biologists have adopted a compromise system, marking paraphyletic groups with an asterisk, for example, class Reptilia. Colin Tudge (2000) notes other uses of this compromise system:
By the same token, the traditional class Amphibia becomes Amphibia*, because some ancient amphibian or other gave rise to all the amniotes; and the phylum Crustacea becomes Crustacea*, because it may have given rise to the insects and myriapods (centipedes and millipedes). If we believe, as some (but not all) zoologists do, that myriapods gave rise to insects, then they should be called Myriapoda*…by this convention Reptilia without an asterisk is synonymous with Amniota, and includes birds and mammals, whereas Reptilia* means non-avian, non-mammalian amniotes.
College-level references, such as Benton (2004), offer another compromise by applying traditional ranks to accepted phylogenetic relationships. In this case, reptiles belong to the class Sauropsida, and mammal-like reptiles to the class Synapsida, with birds and mammals separated into their own traditional classes.
The terms Sauropsida ("Lizard Faces") and Theropsida ("Beast Faces") were coined to distinguish between lizards, birds, and their relatives on one hand (Sauropsida) and mammal-like reptiles and mammals (Theropsida) on the other. This classification supplemented, but was never as popular as the classification of the reptiles according to the positioning of temporal fenestrae mentioned above under evolution of the reptiles (Anapsida, Diapsida, Synapsida, and so on).
A diverse group of egg-laying vertebrate animals, the Sauropsida includes all modern and most extinct "reptiles" (excluding synapsids). Living sauropsids include lizards, snakes, turtles, crocodiles, and birds. Extinct sauropsids include dinosaurs (except birds), pterosaurs, plesiosaurs, ichthyosaurs, and many others.
The synapsids were originally defined, at the turn of the twentieth Century, as one of the five main subclasses of reptiles on the basis of their distinctive temporal openings. The synapsids represented the reptilian lineage that led to the mammals, and gradually evolved increasingly mammalian features, hence, "mammal-like reptiles." The traditional classification continued through to the late 1980s.
In the current cladistic based system, the Linnean classification of the class Reptilia in terms of four sub-classes has been replaced. "Sauropsid" (as a monophyletic clade) is retained to refer to all non-synapsid amniotes (or just replaced by "Reptilia" even though the sauropsid group includes birds). The term "Theropsida" is replaced by Synapsida, which now refers to both the old subclass Synapsida and the mammals. In the new edition of his textbook (2004), Michael Benton uses the term "Class Sauropsida" to refer to all non-synapsid reptiles. Because synapsids evolved into mammals, the mammals therefore are included under the clade Synapsida. That is, "synapsids" are now also known as "theropsids."
The following is a very abbreviated classification of the extensive classification system presented by Benton (2000):
The following classification of living reptiles was given by Uetz (2005), which was modified from the overall taxonomy of Zug et al. (2001), and with the Iguania mainly after Frost et al. (2001), and the turtles mainly after Fujita et al. (2004).
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