Encyclopedia
Reptiles are
tetrapods and
amniotes, animals whose
embryos are surrounded by an amniotic membrane. Today they are represented by four surviving
orders:
Reptiles are found on every
continent except for
Antarctica, although their main distribution comprises the
tropics and subtropics. Though all cellular
metabolism produces some
heat, modern species of reptiles do not generate enough to maintain a constant body temperature and are thus referred to as "
cold-blooded" . Instead they rely on gathering and losing heat from the environment to regulate their internal temperature, e.g, by moving between
sun and shade, or by preferential
circulation — moving warmed
blood into the body core, while pushing cool blood to the periphery. In their natural habitats, most species are adept at this, and can maintain core body temperatures within a fairly narrow range, comparable to that of mammals and
birds, the two surviving groups of "
warm-blooded" animals. While this lack of adequate internal heating imposes costs relative to temperature regulation through behavior, it also provides a large benefit by allowing reptiles to survive on much less
food than comparably-sized mammals and birds, who burn much of their food for warmth. While warm-blooded animals move faster in general, an attacking lizard, snake or crocodile moves very quickly. A herpetologist is a zoologist who studies reptiles and amphibians.
Except for some members of the Testudines, all reptiles are covered by scales.
Most reptile species are oviparous . Many species of squamates, however, are capable of giving live birth. This is achieved, either through ovoviviparity , or viviparity . Many of the viviparous species feed their
fetuses through various forms of
placenta analogous to those of mammals . They often provide considerable initial care for their hatchlings.
Classification of reptiles
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 . This is still the usual definition of the term.
However, in recent years, many taxonomists have begun to insist that taxa should be
monophyletic, that is, groups should include all descendants of a particular form. The reptiles as defined above would be
paraphyletic, since they exclude both birds and mammals, although these also developed from the original reptile. Colin Tudge writes:
- Mammals are a clade, and therefore the cladists are happy to acknowledge the traditional taxon Mammalia; and birds, too, are a clade, universally ascribed to the formal taxon Aves. Mammalia and Aves are, in fact, subclades within the grand clade of the Amniota. But the traditional class reptilia is not a clade. It is just a section of the clade Amniota: the section that is left after the Mammalia and Aves have been hived off. It cannot be defined by 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, we could say that the traditional Reptila are 'non-avian, non-mammalian amniotes'.
Some cladists thus redefine Reptilia as a monophyletic group, including both the classic reptiles as well as the birds and perhaps the mammals . 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 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, e.g. class Reptilia*. Colin Tudge 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 . If we believe, as some 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.
Recent college-level references, such as Benton , 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.
Reptile Groups
- Class Sauropsida
- Family Captorhinidae
- Family Protorothyrididae - Hylonomus was an early reptile [i]. ...
- Subclass Anapsida
- Family Mesosauridae
- Order Procolophonia - incl. Pareiasaurs
- ?Order Testudines - Turtles
- Subclass Diapsida
- Superorder Ichthyopterygia - Ichthyosaurs
- Infraclass Lepidosauromorpha
- Infraclass Archosauromorpha
Evolution of the reptiles
Hylonomus was an early reptile [i]. ...
is the oldest-known reptile, and was about 8 to 12 inches long.
Westlothiana has been suggested as the oldest reptile, but is for the moment considered to be more related to amphibians than
amniotes.
Petrolacosaurus was a small diapsid [i] reptile, one of the earliest known. ...
and
Mesosaurus are other examples.
The first true "reptiles" are categorized as Anapsids, having a solid skull with holes only for nose, eyes, spinal cord, etc. Turtles are believed by some to be surviving Anapsids, as they also share this skull structure; but this point has become contentious lately, with some arguing that turtles reverted to this primitive state in order to improve their armor. Both sides have strong evidence, and the conflict has yet to be resolved.
Shortly after the first reptiles, two branches split off, one leading to the Anapsids, which did not develop holes in their skulls. The other group,
Diapsida, possessed a pair of holes in their skulls behind the eyes, along with a second pair located higher on the skull. The Diapsida split yet again into two lineages, the lepidosaurs and the archosaurs .
The earliest, solid-skulled
amniotes also gave rise to a separate line, the
Synapsida.
Synapsids developed a pair of holes in their skulls behind the eyes , which were used to both lighten the skull and increase the space for jaw muscles. The synapsids eventually evolved into mammals, and are often referred to as mammal-like reptiles, though they are not true members of the class Sauropsida.
Systems
Circulatory
Most reptiles have closed circulation via a three-chamber
heart consisting of 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 . Also, it has been discovered that some snake and lizard species 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 .
Respiratory
All reptiles breathe using lungs. Aquatic
turtles have developed more permeable skin, and even gills in their anal region, for some species . Even with these adaptations, breathing is never fully accomplished without lungs. Lung ventilation is accomplished differently in each main reptile group. In
squamates the lungs are ventilated almost exclusively by the axial musculature. This is also the same musculature that is used during locomotion. Because of this constraint, most squamates are forced to hold their breath during intense runs. Some, however, have found a way around it. Varanids, and a few other lizard species, employ buccal pumping as a complement to their normal "axial breathing." This allows the animals to completely fill their lungs during intense locomotion, and thus remain aerobically active for a long time. Tegu lizards are known to possess a proto-diaphragm, which separates the pulmonary cavity from the visceral cavity. While not actually capable of movement, it does allow for greater lung inflation, by taking the weight of the viscera off the lungs .
Crocodilians actually have a muscular diaphragm that is analogous to the mammalian diaphragm. The difference is that the muscles for the crocodilian diaphragm pull the pubis 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."
How
Turtles & Tortoises breathe has been the subject of much study. To date, only a few species have been studied thoroughly enough to get an idea of how turtles do it. The results indicate that turtles & tortoises have found a variety of solutions to this problem. The problem is that most turtle shells are rigid and do not allow for the type of expansion and contraction that other amniotes use to ventilate their lungs. Some turtles such as the Indian flapshell 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 , with the bottom of the lungs attached to the rest of the viscera. By using a series of special muscles , 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 . 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 . They are probably using their abdominal muscles to breathe during locomotion. The last species to have been studied is red-eared sliders, which also breathe during locomotion, but they had smaller breaths during locomotion than during small pauses between locomotor bouts, indicating that there may be mechanical interference between the limb movements and the breathing apparatus. Box turtles have also been observed to breathe while completely sealed up inside their shells .
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 . Skinks also have evolved a bony secondary palate, to varying degrees. Snakes took a different approach and extended their trachea instead. Their tracheal extension sticks out like a fleshy straw, and allows these animals to swallow large prey without suffering from asphyxiation.
Also, crocodiles are known to cry while eating. Many myths and folklore have grown around this astonishing fact, such as that the crocodile feels guilty for eating, but in truth, the crocodile cries to release fluid from its body, to make room for oxygen. This is also due to the fact that the crocodile's nasal cavity is exceptionally small.
Excretion
Excretion is performed mainly by two small
kidneys. In diapsids
uric acid is the main
nitrogenous waste product; turtles, like mammals, mainly excrete
urea. Unlike the kidneys of mammals and birds, reptile kidneys are unable to produce liquid urine more concentrated than their body fluid. This is because they lack a specialised structure present in the
nephrons of birds and mammals, called a Loop of Henle. Because of this, many reptiles use the colon and
cloaca to aid in the reabsorption of water. Some are also able to take up water stored in the bladder. Excess salts are also excreted by nasal and lingual salt-glands in some reptiles.
Nervous
Advanced nervous system compared to
amphibians. They have twelve pairs of
cranial nerves.
Sexual
Most reptiles reproduce sexually. All male reptiles except turtles and tortoises have a twin tube like sexual organ called the hemipenes. Turtles and tortoises have a single penis. All testudines lay eggs, none are live bearing as some lizard and snakes are. All reproductive activity occurs with the cloaca, the single exit/entrance at the base of the tail where waste and reproduction happens.
Asexual reproduction has been identified in
squamates in six families of lizards and one snake. In some species of squamates, a population of females are able to produce a unisexual diploid clone of the mother. This asexual reproduction called
parthenogenesis occurs in several species of
gecko, and is particularly widespread in the
teiids and
lacertids . Parthenogentic species are also suspected to occur among
chameleons,
agamids,
xantusiids, and
typhlopids.
Amniotic eggs are covered with leathery or calcareous shells. An
amnion, chorion and allantois are present during
embryonic life. There are no
larval stages of development.
See also
- List of reptiles
- List of regional reptiles lists
External links
References
- Benton, M. J. , Vertebrate Paleontology, 3rd ed. Blackwell Science Ltd.
- Laurin, Michel and Gauthier, Jacques A.: , Version 22 June 2000; part of
