See text for extinct groups.
Global reptile distribution (excluding birds)
Reptiles are tetrapod animals in the class Reptilia, comprising
today's turtles, crocodilians, snakes, amphisbaenians, lizards,
tuatara, and their extinct relatives. The study of these traditional
reptile orders, historically combined with that of modern amphibians,
is called herpetology.
Because some reptiles are more closely related to birds than they are
to other reptiles (e.g., crocodiles are more closely related to birds
than they are to lizards), the traditional groups of "reptiles" listed
above do not together constitute a monophyletic grouping or clade
(consisting of all descendants of a common ancestor). For this reason,
many modern scientists prefer to consider the birds part of Reptilia
as well, thereby making Reptilia a monophyletic class, including all
The earliest known proto-reptiles originated around 312 million years
ago during the
Carboniferous period, having evolved from advanced
reptiliomorph tetrapods that became increasingly adapted to life on
dry land. Some early examples include the lizard-like
Casineria. In addition to the living reptiles, there are many diverse
groups that are now extinct, in some cases due to mass extinction
events. In particular, the Cretaceous–
Paleogene extinction event
wiped out the pterosaurs, plesiosaurs, ornithischians, and sauropods,
as well as many species of theropods, including troodontids,
dromaeosaurids, tyrannosaurids, and abelisaurids, along with many
Crocodyliformes, and squamates (e.g. mosasaurids).
Modern non-avian reptiles inhabit all the continents except
Antarctica, although some birds are found on the periphery of
Antarctica. Several living subgroups are recognized: Testudines
(turtles and tortoises), 350 species;
Rhynchocephalia (tuatara from
New Zealand), 1 species;
Squamata (lizards, snakes, and worm
lizards), over 10,200 species;
Crocodilia (crocodiles, gavials,
caimans, and alligators), 24 species; and
approximately 10,000 species.
Reptiles are tetrapod vertebrates, creatures that either have four
limbs or, like snakes, are descended from four-limbed ancestors.
Unlike amphibians, reptiles do not have an aquatic larval stage. Most
reptiles are oviparous, although several species of squamates are
viviparous, as were some extinct aquatic clades — the fetus
develops within the mother, contained in a placenta rather than an
eggshell. As amniotes, reptile eggs are surrounded by membranes for
protection and transport, which adapt them to reproduction on dry
land. Many of the viviparous species feed their fetuses through
various forms of placenta analogous to those of mammals, with some
providing initial care for their hatchlings. Extant reptiles range in
size from a tiny gecko, Sphaerodactylus ariasae, which can grow up to
17 mm (0.7 in) to the saltwater crocodile, Crocodylus
porosus, which may reach 6 m (19.7 ft) in length and weigh
over 1,000 kg (2,200 lb).
1.1 Research history
Phylogenetics and modern definition
1.5 The position of turtles
2 Evolutionary history
2.1 Origin of the reptiles
2.2 Rise of the reptiles
2.3 Anapsids, synapsids, diapsids, and sauropsids
2.6 Cenozoic reptiles
3 Morphology and physiology
3.3 Respiratory system
3.3.1 Turtles and tortoises
4 Defense mechanisms
Camouflage and warning
4.2 Alternative defense in snakes
4.3 Defense in crocodilians
4.4 Shedding and regenerating tails
5 Relations with humans
5.1 In cultures and religions
6 See also
7 Further reading
10 External links
See also: List of reptiles
See also: Skull roof
Reptiles, from Nouveau Larousse Illustré, 1897-1904: Notice the
inclusion of amphibians (below the crocodiles).
In the 13th century the category of reptile was recognized in Europe
as consisting of a miscellany of egg-laying creatures, including
"snakes, various fantastic monsters, lizards, assorted amphibians, and
worms", as recorded by
Vincent of Beauvais
Vincent of Beauvais in his Mirror of Nature.
In the 18th century, the reptiles were, from the outset of
classification, grouped with the amphibians. Linnaeus, working from
species-poor Sweden, where the common adder and grass snake are often
found hunting in water, included all reptiles and amphibians in class
"III – Amphibia" in his Systema Naturæ. The terms
"reptile" and "amphibian" were largely interchangeable, "reptile"
(from Latin repere, "to creep") being preferred by the French.
Josephus Nicolaus Laurenti was the first to formally use the term
"Reptilia" for an expanded selection of reptiles and amphibians
basically similar to that of Linnaeus. Today, the two groups are
still commonly treated under the same heading as herptiles.
An "antediluvian monster", a
Mosasaurus discovered in a Maastricht
limestone quarry, 1770 (contemporary engraving)
It was not until the beginning of the 19th century that it became
clear that reptiles and amphibians are, in fact, quite different
Pierre André Latreille
Pierre André Latreille erected the class Batracia (1825)
for the latter, dividing the tetrapods into the four familiar classes
of reptiles, amphibians, birds, and mammals. The British anatomist
Thomas Henry Huxley
Thomas Henry Huxley made Latreille's definition popular and, together
with Richard Owen, expanded Reptilia to include the various fossil
"antediluvian monsters", including dinosaurs and the mammal-like
Dicynodon he helped describe. This was not the only
possible classification scheme: In the Hunterian lectures delivered at
the Royal College of Surgeons in 1863, Huxley grouped the vertebrates
into mammals, sauroids, and ichthyoids (the latter containing the
fishes and amphibians). He subsequently proposed the names of
Ichthyopsida for the latter two groups. In 1866,
Haeckel demonstrated that vertebrates could be divided based on their
reproductive strategies, and that reptiles, birds, and mammals were
united by the amniotic egg.
The terms "Sauropsida" ("lizard faces") and "Theropsida" ("beast
faces") were used again in 1916 by E.S. Goodrich to distinguish
between lizards, birds, and their relatives on the one hand
(Sauropsida) and mammals and their extinct relatives (Theropsida) on
the other. Goodrich supported this division by the nature of the
hearts and blood vessels in each group, and other features, such as
the structure of the forebrain. According to Goodrich, both lineages
evolved from an earlier stem group, Protosauria ("first lizards") in
which he included some animals today considered reptile-like
amphibians, as well as early reptiles.
In 1956, D.M.S. Watson observed that the first two groups diverged
very early in reptilian history, so he divided Goodrich's Protosauria
between them. He also reinterpreted
Sauropsida and Theropsida to
exclude birds and mammals, respectively. Thus his
Procolophonia, Eosuchia, Millerosauria, Chelonia (turtles), Squamata
(lizards and snakes), Rhynchocephalia, Crocodilia, "thecodonts"
(paraphyletic basal Archosauria), non-avian dinosaurs, pterosaurs,
ichthyosaurs, and sauropterygians.
In the late 19th century, a number of definitions of Reptilia were
offered. The traits listed by Lydekker in 1896, for example, include a
single occipital condyle, a jaw joint formed by the quadrate and
articular bones, and certain characteristics of the vertebrae. The
animals singled out by these formulations, the amniotes other than the
mammals and the birds, are still those considered reptiles today.
The first reptiles had an anapsid type of skull roof, as seen in the
Permian genus Captorhinus
The synapsid/sauropsid division supplemented another approach, one
that split the reptiles into four subclasses based on the number and
position of temporal fenestrae, openings in the sides of the skull
behind the eyes. This classification was initiated by Henry Fairfield
Osborn and elaborated and made popular by Romer's classic Vertebrate
Paleontology. Those four subclasses were:
Anapsida – no fenestrae – cotylosaurs and Chelonia
(turtles and relatives)[note 1]
Synapsida – one low fenestra – pelycosaurs and
therapsids (the 'mammal-like reptiles')
Euryapsida – one high fenestra (above the postorbital and
squamosal) – protorosaurs (small, early lizard-like reptiles)
and the marine sauropterygians and ichthyosaurs, the latter called
Parapsida in Osborn's work.
Diapsida – two fenestrae – most reptiles, including
lizards, snakes, crocodilians, dinosaurs and pterosaurs
The composition of
Euryapsida was uncertain.
Ichthyosaurs were, at
times, considered to have arisen independently of the other
euryapsids, and given the older name Parapsida.
Parapsida was later
discarded as a group for the most part (ichthyosaurs being classified
as incertae sedis or with Euryapsida). However, four (or three if
Euryapsida is merged into Diapsida) subclasses remained more or less
universal for non-specialist work throughout the 20th century. It has
largely been abandoned by recent researchers: in particular, the
anapsid condition has been found to occur so variably among unrelated
groups that it is not now considered a useful distinction.
Phylogenetics and modern definition
Phylogenetic classifications group the traditional "mammal-like
reptiles", like this Varanodon, with other synapsids, not with extant
By the early 21st century, vertebrate paleontologists were beginning
to adopt phylogenetic taxonomy, in which all groups are defined in
such a way as to be monophyletic; that is, groups include all
descendants of a particular ancestor. The reptiles as historically
defined are paraphyletic, since they exclude both birds and mammals.
These respectively evolved from dinosaurs and from early therapsids,
which were both traditionally called reptiles.
Birds are more
closely related to crocodilians than the latter are to the rest of
Colin Tudge wrote:
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
Aves have been hived off. It cannot be defined by
synapomorphies, as is the proper way. Instead, it is defined by a
combination of the features it has and the features it lacks: reptiles
are the amniotes that lack fur or feathers. At best, the cladists
suggest, we could say that the traditional Reptilia are 'non-avian,
Despite the early proposals for replacing the paraphyletic Reptilia
with a monophyletic Sauropsida, which includes birds, that term was
never adopted widely or, when it was, was not applied consistently.
Sauropsida was used, it often had the same content or even the
same definition as Reptilia. In 1988,
Jacques Gauthier proposed a
cladistic definition of Reptilia as a monophyletic node-based crown
group containing turtles, lizards and snakes, crocodilians, and birds,
their common ancestor and all its descendants. Because the actual
relationship of turtles to other reptiles was not yet well understood
at this time, Gauthier's definition came to be considered
A variety of other definitions were proposed by other scientists in
the years following Gauthier's paper. The first such new definition,
which attempted to adhere to the standards of the PhyloCode, was
published by Modesto and Anderson in 2004. Modesto and Anderson
reviewed the many previous definitions and proposed a modified
definition, which they intended to retain most traditional content of
the group while keeping it stable and monophyletic. They defined
Reptilia as all amniotes closer to
Lacerta agilis and Crocodylus
niloticus than to Homo sapiens. This stem-based definition is
equivalent to the more common definition of Sauropsida, which Modesto
and Anderson synonymized with Reptilia, since the latter is better
known and more frequently used. Unlike most previous definitions of
Reptilia, however, Modesto and Anderson's definition includes
birds, as they are within the clade that includes both lizards and
List of reptiles
List of reptiles and List of snakes
Classification to order level of the reptiles, after Benton,
Squamata (lizards & snakes)
The cladogram presented here illustrates the "family tree" of
reptiles, and follows a simplified version of the relationships found
by M.S. Lee, in 2013. All genetic studies have supported the
hypothesis that turtles are diapsids; some have placed turtles within
archosauriformes, though a few have recovered
turtles as lepidosauriformes instead. The cladogram below used a
combination of genetic (molecular) and fossil (morphological) data to
obtain its results.
Synapsida (mammals and their extinct relatives)
Rhynchocephalia (tuatara and their extinct relatives)
Squamata (lizards and snakes)
Archosauriformes (crocodiles, birds, dinosaurs and extinct relatives)
The position of turtles
The placement of turtles has historically been highly variable.
Classically, turtles were considered to be related to the primitive
anapsid reptiles. Molecular work has usually placed turtles within
the diapsids. So far three turtle genomes have been sequenced. The
results place turtles as a sister clade to the archosaurs, the group
that includes crocodiles, dinosaurs, and birds.
Main article: Evolution of reptiles
Origin of the reptiles
An early reptile Hylonomus
Mesozoic scene showing typical reptilian megafauna: dinosaurs
including Europasaurus holgeri, iguanodonts and Archaeopteryx
lithographica perched on the foreground tree stump.
The origin of the reptiles lies about 310–320 million years ago, in
the steaming swamps of the late
Carboniferous period, when the first
reptiles evolved from advanced reptiliomorphs.
The oldest known animal that may have been an amniote is Casineria
(though it may have been a temnospondyl). A series of
footprints from the fossil strata of
Nova Scotia dated to
7015994064400000000♠315 Ma show typical reptilian toes and
imprints of scales. These tracks are attributed to Hylonomus, the
oldest unquestionable reptile known. It was a small, lizard-like
animal, about 20 to 30 centimetres (7.9 to 11.8 in) long, with
numerous sharp teeth indicating an insectivorous diet. Other
Westlothiana (for the moment considered a
reptiliomorph rather than a true amniote) and Paleothyris, both of
similar build and presumably similar habit.
Rise of the reptiles
The earliest amniotes, including stem-reptiles (those amniotes closer
to modern reptiles than to mammals), were largely overshadowed by
larger stem-tetrapods, such as Cochleosaurus, and remained a small,
inconspicuous part of the fauna until the
Collapse. This sudden collapse affected several large groups.
Primitive tetrapods were particularly devastated, while stem-reptiles
fared better, being ecologically adapted to the drier conditions that
followed. Primitive tetrapods, like modern amphibians, need to return
to water to lay eggs; in contrast, amniotes, like modern
reptiles – whose eggs possess a shell that allows them to be
laid on land – were better adapted to the new conditions.
Amniotes acquired new niches at a faster rate than before the collapse
and at a much faster rate than primitive tetrapods. They acquired new
feeding strategies including herbivory and carnivory, previously only
having been insectivores and piscivores. From this point forward,
reptiles dominated communities and had a greater diversity than
primitive tetrapods, setting the stage for the
Mesozoic (known as the
Age of Reptiles). One of the best known early stem-reptiles is
Mesosaurus, a genus from the early
Permian that had returned to water,
feeding on fish.
Anapsids, synapsids, diapsids, and sauropsids
A = Anapsid,
B = Synapsid,
C = Diapsid
It was traditionally assumed that the first reptiles retained an
anapsid skull inherited from their ancestors. This type of skull
has a skull roof with only holes for the nostrils, eyes and a pineal
eye. The discoveries of synapsid-like openings (see below) in the
skull roof of the skulls of several members of
Parareptilia (the clade
containing most of the amniotes traditionally referred to as
"anapsids"), including lanthanosuchoids, millerettids, bolosaurids,
some nycteroleterids, some procolophonoids and at least some
mesosaurs made it more ambiguous and it's currently
uncertain whether the ancestral amniote had an anapsid-like or
synapsid-like skull. These animals are traditionally referred to
as "anapsids", and form a paraphyletic basic stock from which other
groups evolved. Very shortly after the first amniotes appeared, a
Synapsida split off; this group was characterized by a
temporal opening in the skull behind each eye to give room for the jaw
muscle to move. These are the "mammal-like amniotes", or stem-mammals,
that later gave rise to the true mammals. Soon after, another
group evolved a similar trait, this time with a double opening behind
each eye, earning them the name
Diapsida ("two arches"). The
function of the holes in these groups was to lighten the skull and
give room for the jaw muscles to move, allowing for a more powerful
Turtles have been traditionally believed to be surviving parareptiles,
on the basis of their anapsid skull structure, which was assumed to be
primitive trait. The rationale for this classification has been
disputed, with some arguing that turtles are diapsids that evolved
anapsid skulls in order to improve their armor. Later morphological
phylogenetic studies with this in mind placed turtles firmly within
Diapsida. All molecular studies have strongly upheld the placement
of turtles within diapsids, most commonly as a sister group to extant
With the close of the Carboniferous, the amniotes became the dominant
tetrapod fauna. While primitive, terrestrial reptiliomorphs still
existed, the synapsid amniotes evolved the first truly terrestrial
megafauna (giant animals) in the form of pelycosaurs, such as
Edaphosaurus and the carnivorous Dimetrodon. In the mid-Permian
period, the climate became drier, resulting in a change of fauna: The
pelycosaurs were replaced by the therapsids.
The parareptiles, whose massive skull roofs had no postorbital holes,
continued and flourished throughout the Permian. The pareiasaurian
parareptiles reached giant proportions in the late Permian, eventually
disappearing at the close of the period (the turtles being possible
Early in the period, the modern reptiles, or crown-group reptiles,
evolved and split into two main lineages: the Archosauromorpha
(forebears of turtles, crocodiles, and dinosaurs) and the
Lepidosauromorpha (predecessors of modern lizards and tuataras). Both
groups remained lizard-like and relatively small and inconspicuous
during the Permian.
The close of the
Permian saw the greatest mass extinction known (see
Triassic extinction event), an event prolonged by the
combination of two or more distinct extinction pulses. Most of the
earlier parareptile and synapsid megafauna disappeared, being replaced
by the true reptiles, particularly archosauromorphs. These were
characterized by elongated hind legs and an erect pose, the early
forms looking somewhat like long-legged crocodiles. The archosaurs
became the dominant group during the
Triassic period, though it took
30 million years before their diversity was as great as the animals
that lived in the Permian. Archosaurs developed into the
well-known dinosaurs and pterosaurs, as well as the ancestors of
crocodiles. Since reptiles, first rauisuchians and then dinosaurs,
Mesozoic era, the interval is popularly known as the
"Age of Reptiles". The dinosaurs also developed smaller forms,
including the feather-bearing smaller theropods. In the Cretaceous
period, these gave rise to the first true birds.
The sister group to
Archosauromorpha is Lepidosauromorpha, containing
lizards and tuataras, as well as their fossil relatives.
Lepidosauromorpha contained at least one major group of the Mesozoic
sea reptiles: the mosasaurs, which lived during the
The phylogenetic placement of other main groups of fossil sea reptiles
– the ichthyopterygians (including ichthyosaurs) and the
sauropterygians, which evolved in the early
Triassic – is more
controversial. Different authors linked these groups either to
lepidosauromorphs or to archosauromorphs, and
ichthyopterygians were also argued to be diapsids that did not belong
to the least inclusive clade containing lepidosauromorphs and
Varanus priscus was a giant carnivorous goanna lizard, perhaps as long
as 7 metres and weighing up to 1,940 kilograms.
The close of the
Cretaceous period saw the demise of the
reptilian megafauna (see the Cretaceous–
Paleogene extinction event).
Of the large marine reptiles, only sea turtles were left; and of the
non-marine large reptiles, only the semi-aquatic crocodiles and
broadly similar choristoderes survived the extinction, with the latter
becoming extinct in the Miocene. Of the great host of dinosaurs
dominating the Mesozoic, only the small beaked birds survived. This
dramatic extinction pattern at the end of the
Mesozoic led into the
Mammals and birds filled the empty niches left behind by the
reptilian megafauna and, while reptile diversification slowed, bird
and mammal diversification took an exponential turn. However,
reptiles were still important components of the megafauna,
particularly in the form of giant tortoises.
After the extinction of most archosaur and marine reptile lines by the
end of the Cretaceous, reptile diversification continued throughout
the Cenozoic. Squamates took a massive hit during the KT-event, only
recovering ten million years after it, but they underwent a great
radiation event once they recovered, and today squamates make up the
majority of living reptiles (> 95%). Approximately 10,000
extant species of traditional reptiles are known, with birds adding
about 10,000 more, almost twice the number of mammals, represented by
about 5,700 living species (excluding domesticated species).
Morphology and physiology
Thermographic image of monitor lizards
All squamates and turtles have a three-chambered heart consisting of
two atria, one variably partitioned ventricle, and two aortas that
lead to the systemic circulation. The degree of mixing of oxygenated
and deoxygenated blood in the three-chambered heart varies depending
on the species and physiological state. Under different conditions,
deoxygenated blood can be shunted back to the body or oxygenated blood
can be shunted back to the lungs. This variation in blood flow has
been hypothesized to allow more effective thermoregulation and longer
diving times for aquatic species, but has not been shown to be a
Iguana heart bisected through the ventricle, bisecting the
left and right atrium.
For example, Iguana hearts, like the majority of
the squamates hearts, are composed of three chambers with
two aorta and one ventricle, cardiac involuntary muscles. The main
structures of the heart are the sinus venosus, the pacemaker,
the left atrium, the right atruim, the atrioventriular
valve, the cavum venosum, cavum arteriosum, the cavum pulmonale, the
muscular ridge, the ventricular ridge, pulmanary veins, and
paired aortic arches.
Some squamate species (e.g., pythons and monitor lizards) have
three-chambered hearts that become functionally four-chambered hearts
during contraction. This is made possible by a muscular ridge that
subdivides the ventricle during 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
Crocodilians have an anatomically four-chambered heart, similar to
birds, but also have two systemic aortas and are therefore capable of
bypassing their pulmonary circulation.
Sustained energy output (joules) of a typical reptile versus a similar
size mammal as a function of core body temperature. The mammal has a
much higher peak output, but can only function over a very narrow
range of body temperature.
Modern non-avian reptiles exhibit some form of cold-bloodedness (i.e.
some mix of poikilothermy, ectothermy, and bradymetabolism) so that
they have limited physiological means of keeping the body temperature
constant and often rely on external sources of heat. Due to a less
stable core temperature than birds and mammals, reptilian biochemistry
requires enzymes capable of maintaining efficiency over a greater
range of temperatures than in the case for warm-blooded animals. The
optimum body temperature range varies with species, but is typically
below that of warm-blooded animals; for many lizards, it falls in the
24°–35 °C (75°–95 °F) range, while extreme
heat-adapted species, like the American desert iguana Dipsosaurus
dorsalis, can have optimal physiological temperatures in the mammalian
range, between 35° and 40 °C (95° and 104 °F). While
the optimum temperature is often encountered when the animal is
active, the low basal metabolism makes body temperature drop rapidly
when the animal is inactive.
As in all animals, reptilian muscle action produces heat. In large
reptiles, like leatherback turtles, the low surface-to-volume ratio
allows this metabolically produced heat to keep the animals warmer
than their environment even though they do not have a warm-blooded
metabolism. This form of homeothermy is called gigantothermy; it
has been suggested as having been common in large dinosaurs and other
extinct large-bodied reptiles.
The benefit of a low resting metabolism is that it requires far less
fuel to sustain bodily functions. By using temperature variations in
their surroundings, or by remaining cold when they do not need to
move, reptiles can save considerable amounts of energy compared to
endothermic animals of the same size. A crocodile needs from a
tenth to a fifth of the food necessary for a lion of the same weight
and can live half a year without eating. Lower food requirements
and adaptive metabolisms allow reptiles to dominate the animal life in
regions where net calorie availability is too low to sustain
large-bodied mammals and birds.
It is generally assumed that reptiles are unable to produce the
sustained high energy output necessary for long distance chases or
flying. Higher energetic capacity might have been responsible for
the evolution of warm-bloodedness in birds and mammals. However,
investigation of correlations between active capacity and
thermophysiology show a weak relationship. Most extant reptiles
are carnivores with a sit-and-wait feeding strategy; whether reptiles
are cold blooded due to their ecology is not clear. Energetic studies
on some reptiles have shown active capacities equal to or greater than
similar sized warm-blooded animals.
All reptiles breathe using lungs. Aquatic turtles have developed more
permeable skin, and some species have modified their cloaca to
increase the area for gas exchange. Even with these adaptations,
breathing is never fully accomplished without lungs.
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 (part of the pelvis, which is
movable in crocodilians) back, which brings the liver down, thus
freeing space for the lungs to expand. This type of diaphragmatic
setup has been referred to as the "hepatic piston". The airways
bronchia form a number of double tubular chambers within each lung. On
inhalation and exhalation air moves through the airways in the same
direction, thus creating a unidirectional airflow through the lungs. A
similar system is found in birds, monitor lizards and
Most reptiles lack a secondary palate, meaning that they must hold
their breath while swallowing. Crocodilians have evolved a bony
secondary palate that allows them to continue breathing while
remaining submerged (and protect their brains against damage by
Skinks (family Scincidae) also have evolved a bony
secondary palate, to varying degrees. Snakes took a different approach
and extended their trachea instead. Their tracheal extension sticks
out like a fleshy straw, and allows these animals to swallow large
prey without suffering from asphyxiation.
Turtles and tortoises
Red-eared slider taking a gulp of air
How turtles and tortoises breathe has been the subject of much study.
To date, only a few species have been studied thoroughly enough to get
an idea of how those turtles breathe. The varied results indicate that
turtles and tortoises have found a variety of solutions to this
The difficulty is that most turtle shells are rigid and do not allow
for the type of expansion and contraction that other amniotes use to
ventilate their lungs. Some turtles, such as the Indian flapshell
(Lissemys punctata), have a sheet of muscle that envelops the lungs.
When it contracts, the turtle can exhale. When at rest, the turtle can
retract the limbs into the body cavity and force air out of the lungs.
When the turtle protracts its limbs, the pressure inside the lungs is
reduced, and the turtle can suck air in.
Turtle lungs are attached to
the inside of the top of the shell (carapace), with the bottom of the
lungs attached (via connective tissue) to the rest of the viscera. By
using a series of special muscles (roughly equivalent to a diaphragm),
turtles are capable of pushing their viscera up and down, resulting in
effective respiration, since many of these muscles have attachment
points in conjunction with their forelimbs (indeed, many of the
muscles expand into the limb pockets during contraction).
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. This is because they use their abdominal muscles to
breathe during locomotion. The last species to have been studied is
the red-eared slider, which also breathes during locomotion, but takes
smaller breaths during locomotion than during small pauses between
locomotor bouts, indicating that there may be mechanical interference
between the limb movements and the breathing apparatus. Box turtles
have also been observed to breathe while completely sealed up inside
Skin of a sand lizard, showing squamate reptiles iconic scales
Reptilian skin is covered in a horny epidermis, making it watertight
and enabling reptiles to live on dry land, in contrast to amphibians.
Compared to mammalian skin, that of reptiles is rather thin and lacks
the thick dermal layer that produces leather in mammals. Exposed
parts of reptiles are protected by scales or scutes, sometimes with a
bony base, forming armor. In lepidosaurians, such as lizards and
snakes, the whole skin is covered in overlapping epidermal scales.
Such scales were once thought to be typical of the class Reptilia as a
whole, but are now known to occur only in lepidosaurians.[citation
needed] The scales found in turtles and crocodiles are of dermal,
rather than epidermal, origin and are properly termed scutes.[citation
needed] In turtles, the body is hidden inside a hard shell composed of
Lacking a thick dermis, reptilian leather is not as strong as
mammalian leather. It is used in leather-wares for decorative purposes
for shoes, belts and handbags, particularly crocodile skin.
Shedding. Reptiles shed their skin through a process called ecdysis
which occurs continuously throughout their lifetime. In particular,
younger reptiles tend to shed once every 5–6 weeks while adults shed
3-4 times a year. Younger reptiles shed more because of their
rapid growth rate. Once full size, the frequency of shedding
drastically decreases. The process of ecdysis involves forming a new
layer of skin under the old one. Proteolytic enzymes and lymphatic
fluid is secreted between the old and new layers of skin.
Consequently, this lifts the old skin from the new one allowing
shedding to occur. Snakes will shed from the head to the tail
while lizards shed in a “patchy pattern”. Dysecdysis, a common
skin disease in snakes and lizards, will occur when ecdysis, or
shedding, fails. There are numerous reasons why shedding fails and
can be related to inadequate humidity and temperature, nutritional
deficiencies, dehydration and traumatic injuries. Nutritional
deficiencies decrease proteolytic enzymes while dehydration reduces
lymphatic fluids to separate the skin layers. Traumatic injuries on
the other hand, form scars that will not allow new scales to form and
disrupt the process of ecdysis.
Excretion is performed mainly by two small kidneys. In diapsids, uric
acid is the main nitrogenous waste product; turtles, like mammals,
excrete mainly urea. Unlike the kidneys of mammals and birds, reptile
kidneys are unable to produce liquid urine more concentrated than
their body fluid. This is because they lack a specialized structure
called a loop of Henle, which is present in the nephrons of birds and
mammals. Because of this, many reptiles use the colon to aid in the
reabsorption of water. Some are also able to take up water stored in
the bladder. Excess salts are also excreted by nasal and lingual salt
glands in some reptiles.
A colubrid snake, Dolichophis jugularis, eating a legless lizard,
Pseudopus apodus. Most reptiles are carnivorous, and many primarily
eat other reptiles.
Gastroliths from a plesiosaur
Most reptiles are insectivorous or carnivorous and have simple and
comparatively short digestive tracts due to meat being fairly simple
to break down and digest.
Digestion is slower than in mammals,
reflecting their lower resting metabolism and their inability to
divide and masticate their food. Their poikilotherm metabolism has
very low energy requirements, allowing large reptiles like crocodiles
and large constrictors to live from a single large meal for months,
digesting it slowly.
While modern reptiles are predominantly carnivorous, during the early
history of reptiles several groups produced some herbivorous
megafauna: in the Paleozoic, the pareiasaurs; and in the Mesozoic
several lines of dinosaurs. Today, turtles are the only
predominantly herbivorous reptile group, but several lines of agamas
and iguanas have evolved to live wholly or partly on plants.
Herbivorous reptiles face the same problems of mastication as
herbivorous mammals but, lacking the complex teeth of mammals, many
species swallow rocks and pebbles (so called gastroliths) to aid in
digestion: The rocks are washed around in the stomach, helping to
grind up plant matter. Fossil gastroliths have been found
associated with both ornithopods and sauropods, though whether they
actually functioned as a gastric mill in the latter is
disputed. Salt water crocodiles also use gastroliths as
ballast, stabilizing them in the water or helping them to dive. A
dual function as both stabilizing ballast and digestion aid has been
suggested for gastroliths found in plesiosaurs.
The reptilian nervous system contains the same basic part of the
amphibian brain, but the reptile cerebrum and cerebellum are slightly
larger. Most typical sense organs are well developed with certain
exceptions, most notably the snake's lack of external ears (middle and
inner ears are present). There are twelve pairs of cranial
nerves. Due to their short cochlea, reptiles use electrical
tuning to expand their range of audible frequencies.
Reptiles are generally considered less intelligent than mammals and
birds. The size of their brain relative to their body is much less
than that of mammals, the encephalization quotient being about one
tenth of that of mammals, though larger reptiles can show more
complex brain development. Larger lizards, like the monitors, are
known to exhibit complex behavior, including cooperation.
Crocodiles have relatively larger brains and show a fairly complex
social structure. The
Komodo dragon is even known to engage in
play, as are turtles, which are also considered to be social
creatures, and sometimes switch between monogamy and
promiscuity in their sexual behavior. One study found
that wood turtles were better than white rats at learning to navigate
Most reptiles are diurnal animals. The vision is typically adapted to
daylight conditions, with color vision and more advanced visual depth
perception than in amphibians and most mammals. In some species, such
as blind snakes, vision is reduced.
Some snakes have extra sets of visual organs (in the loosest sense of
the word) in the form of pits sensitive to infrared radiation (heat).
Such heat-sensitive pits are particularly well developed in the pit
vipers, but are also found in boas and pythons. These pits allow the
snakes to sense the body heat of birds and mammals, enabling pit
vipers to hunt rodents in the dark.
Crocodilian egg diagram
1. eggshell, 2. yolk sac, 3. yolk (nutrients), 4. vessels, 5. amnion,
6. chorion, 7. air space, 8. allantois, 9. albumin (egg white), 10.
amniotic sac, 11. crocodile embryo, 12. amniotic fluid
Most reptiles reproduce sexually, for example this Trachylepis
Reptiles have amniotic eggs with hard or leathery shells, requiring
internal fertilization when mating.
Reptiles generally reproduce sexually, though some are capable of
asexual reproduction. All reproductive activity occurs through the
cloaca, the single exit/entrance at the base of the tail where waste
is also eliminated. Most reptiles have copulatory organs, which are
usually retracted or inverted and stored inside the body. In turtles
and crocodilians, the male has a single median penis, while squamates,
including snakes and lizards, possess a pair of hemipenes, only one of
which is typically used in each session. Tuatara, however, lack
copulatory organs, and so the male and female simply press their
cloacas together as the male discharges sperm.
Most reptiles lay amniotic eggs covered with leathery or calcareous
shells. An amnion, chorion, and allantois are present during embryonic
life. The eggshell (1) protects the crocodile embryo (11) and keeps it
from drying out, but it is flexible to allow gas exchange. The chorion
(6) aids in gas exchange between the inside and outside of the egg. It
allows carbon dioxide to exit the egg and oxygen gas to enter the egg.
The albumin (9) further protects the embryo and serves as a reservoir
for water and protein. The allantois (8) is a sac that collects the
metabolic waste produced by the embryo. The amniotic sac (10) contains
amniotic fluid (12) which protects and cushions the embryo. The amnion
(5) aids in osmoregulation and serves as a saltwater reservoir. The
yolk sac (2) surrounding the yolk (3) contains protein and fat rich
nutrients that are absorbed by the embryo via vessels (4) that allow
the embryo to grow and metabolize. The air space (7) provides the
embryo with oxygen while it is hatching. This ensures that the embryo
will not suffocate while it is hatching. There are no larval stages of
Viviparity and ovoviviparity have evolved in many extinct
clades of reptiles and in squamates. In the latter group, many
species, including all boas and most vipers, utilize this mode of
reproduction. The degree of viviparity varies; some species simply
retain the eggs until just before hatching, others provide maternal
nourishment to supplement the yolk, and yet others lack any yolk and
provide all nutrients via a structure similar to the mammalian
placenta. The earliest documented case of viviparity in reptiles is
Permian mesosaurs, although some individuals or taxa in
that clade may also have been oviparous because a putative isolated
egg has also been found. Several groups of
Mesozoic marine reptiles
also exhibited viviparity, such as mosasaurs, ichthyosaurs, and
Sauropterygia, a group that include pachypleurosaurs and
Asexual reproduction has been identified in squamates in six families
of lizards and one snake. In some species of squamates, a population
of females is able to produce a unisexual diploid clone of the mother.
This form of asexual reproduction, called parthenogenesis, occurs in
several species of gecko, and is particularly widespread in the teiids
(especially Aspidocelis) and lacertids (Lacerta). In captivity, Komodo
dragons (Varanidae) have reproduced by parthenogenesis.
Parthenogenetic species are suspected to occur among chameleons,
agamids, xantusiids, and typhlopids.
Some reptiles exhibit temperature-dependent sex determination (TDSD),
in which the incubation temperature determines whether a particular
egg hatches as male or female. TDSD is most common in turtles and
crocodiles, but also occurs in lizards and tuatara. To date,
there has been no confirmation of whether TDSD occurs in snakes.
Many small reptiles, such as snakes and lizards that live on the
ground or in the water, are vulnerable to being preyed on by all kinds
of carnivorous animals. Thus avoidance is the most common form of
defense in reptiles. At the first sign of danger, most snakes and
lizards crawl away into the undergrowth, and turtles and crocodiles
will plunge into water and sink out of sight.
Camouflage and warning
A camouflaged Phelsuma deubia on a palm frond
Reptiles tend to avoid confrontation through camouflage. Two major
groups of reptile predators are birds and other reptiles, both of
which have well developed color vision. Thus the skins of many
reptiles have cryptic coloration of plain or mottled gray, green, and
brown to allow them to blend into the background of their natural
environment. Aided by the reptiles' capacity for remaining
motionless for long periods, the camouflage of many snakes is so
effective that people or domestic animals are most typically bitten
because they accidentally step on them.
When camouflage fails to protect them, blue-tongued skinks will try to
ward off attackers by displaying their blue tongues, and the
frill-necked lizard will display its brightly colored frill. These
same displays are used in territorial disputes and during
courtship. If danger arises so suddenly that flight is useless,
crocodiles, turtles, some lizards, and some snakes hiss loudly when
confronted by an enemy.
Rattlesnakes rapidly vibrate the tip of the
tail, which is composed of a series of nested, hollow beads to ward of
In contrast to the normal drab coloration of most reptiles, the
lizards of the genus Heloderma (the
Gila monster and the beaded
lizard) and many of the coral snakes have high-contrast warning
coloration, warning potential predators they are venomous. A
number of non-venomous North American snake species have colorful
markings similar to those of the coral snake, an oft cited example of
Alternative defense in snakes
Venom and Evolution of snake venom
Camouflage does not always fool a predator. When caught out, snake
species adopt different defensive tactics and use a complicated set of
behaviors when attacked. Some first elevate their head and spread out
the skin of their neck in an effort to look large and threatening.
Failure of this strategy may lead to other measures practiced
particularly by cobras, vipers, and closely related species, which use
venom to attack. The venom is modified saliva, delivered through fangs
from a venom gland. Some non-venomous snakes, such as
American hognose snakes or European grass snake, play dead when in
danger; some, including the grass snake, exude a foul-smelling liquid
to deter attackers.
Defense in crocodilians
When a crocodilian is concerned about its safety, it will gape to
expose the teeth and yellow tongue. If this doesn't work, the
crocodilian gets a little more agitated and typically begins to make
hissing sounds. After this, the crocodilian will start to change its
posture dramatically to make itself look more intimidating. The body
is inflated to increase apparent size. If absolutely necessary it may
decide to attack an enemy.
White-headed dwarf gecko
White-headed dwarf gecko with shed tail
Some species try to bite immediately. Some will use their heads as
sledgehammers and literally smash an opponent, some will rush or swim
toward the threat from a distance, even chasing the opponent onto land
or galloping after it. The main weapon in all crocodiles is the
bite, which can generate very high bite force. Many species also
possess canine-like teeth. These are used primarily for seizing prey,
but are also used in fighting and display.
Shedding and regenerating tails
Main article: Autotomy
Geckos, skinks, and other lizards that are captured by the tail will
shed part of the tail structure through a process called autotomy and
thus be able to flee. The detached tail will continue to wiggle,
creating a deceptive sense of continued struggle and distracting the
predator's attention from the fleeing prey animal. The detached tails
of leopard geckos can wiggle for up to 20 minutes. In many
species the tails are of a separate and dramatically more intense
color than the rest of the body so as to encourage potential predators
to strike for the tail first. In the shingleback skink and some
species of geckos, the tail is short and broad and resembles the head,
so that the predators may attack it rather than the more vulnerable
Reptiles that are capable of shedding their tails can partially
regenerate them over a period of weeks. The new section will however
contain cartilage rather than bone, and will never grow to the same
length as the original tail. It is often also distinctly discolored
compared to the rest of the body and may lack some of the external
sculpting features seen in the original tail.
Relations with humans
Main article: Reptiles in culture
In cultures and religions
Main article: Reptiles in culture
The 1897 painting of fighting "Laelaps" (now Dryptosaurus) by Charles
Dinosaurs have been widely depicted in culture since the English
Richard Owen coined the name dinosaur in 1842. As soon
as 1854, the
Crystal Palace Dinosaurs
Crystal Palace Dinosaurs were on display to the public in
south London. One dinosaur appeared in literature even
Charles Dickens placed a
Megalosaurus in the first chapter
of his novel
Bleak House in 1852. The dinosaurs featured in
books, films, television programs, artwork, and other media have been
used for both education and entertainment. The depictions range from
the realistic, as in the television documentaries of the 1990s and
first decade of the 21st century, or the fantastic, as in the monster
movies of the 1950s and 1960s.
The snake or serpent has played a powerful symbolic role in different
cultures. In Egyptian history, the Nile cobra adorned the crown of the
pharaoh. It was worshipped as one of the gods and was also used for
sinister purposes: murder of an adversary and ritual suicide
Greek mythology snakes are associated with deadly
antagonists, as a chthonic symbol, roughly translated as earthbound.
Lernaean Hydra that
Hercules defeated and the three
Gorgon sisters are children of Gaia, the earth.
Medusa was one of the
Gorgon sisters who
Medusa is described as a
hideous mortal, with snakes instead of hair and the power to turn men
to stone with her gaze. After killing her,
Perseus gave her head to
Athena who fixed it to her shield called the Aegis. The Titans are
depicted in art with their legs replaced by bodies of snakes for the
same reason: They are children of Gaia, so they are bound to the
earth. In Hinduism, snakes are worshipped as gods, with many
women pouring milk on snake pits. The cobra is seen on the neck of
Vishnu is depicted often as sleeping on a seven-headed
snake or within the coils of a serpent. There are temples in India
solely for cobras sometimes called Nagraj (King of Snakes), and it is
believed that snakes are symbols of fertility. In the annual Hindu
festival of Nag Panchami, snakes are venerated and prayed to. In
religious terms, the snake and jaguar are arguably the most important
animals in ancient Mesoamerica. "In states of ecstasy, lords dance a
serpent dance; great descending snakes adorn and support buildings
Chichen Itza to Tenochtitlan, and the
Nahuatl word coatl meaning
serpent or twin, forms part of primary deities such as Mixcoatl,
Quetzalcoatl, and Coatlicue." In Christianity and Judaism, a
serpent appears in Genesis to tempt
Adam and Eve
Adam and Eve with the forbidden
fruit from the Tree of Knowledge of Good and Evil.
The turtle has a prominent position as a symbol of steadfastness and
tranquility in religion, mythology, and folklore from around the
world. A tortoise's longevity is suggested by its long lifespan
and its shell, which was thought to protect it from any foe. In
the cosmological myths of several cultures a World
Turtle carries the
world upon its back or supports the heavens.
Rod of Asclepius
Rod of Asclepius symbolizes medicine
Deaths from snakebites are uncommon in many parts of the world, but
are still counted in tens of thousands per year in India.
Snakebite can be treated with antivenom made from the venom of the
snake. To produce antivenom, a mixture of the venoms of different
species of snake is injected into the body of a horse in
ever-increasing dosages until the horse is immunized. Blood is then
extracted; the serum is separated, purified and freeze-dried. The
cytotoxic effect of snake venom is being researched as a potential
treatment for cancers.
Geckos have also been used as medicine, especially in China.
Crocodilia § Interactions with humans
Crocodiles are protected in many parts of the world, and are farmed
commercially. Their hides are tanned and used to make leather goods
such as shoes and handbags; crocodile meat is also considered a
delicacy. The most commonly farmed species are the saltwater and
Nile crocodiles. Farming has resulted in an increase in the saltwater
crocodile population in Australia, as eggs are usually harvested from
the wild, so landowners have an incentive to conserve their habitat.
Crocodile leather is made into wallets, briefcases, purses, handbags,
belts, hats, and shoes.
Crocodile oil has been used for various
In the Western world, some snakes (especially docile species such as
the ball python and corn snake) are kept as pets.
Evolution of reptiles
List of reptiles
Lists of reptiles by region
List of threatened reptiles and amphibians of the United States
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