Reptiles, as most commonly defined are the animals in the
class
Class or The Class may refer to:
Common uses not otherwise categorized
* Class (biology), a taxonomic rank
* Class (knowledge representation), a collection of individuals or objects
* Class (philosophy), an analytical concept used differentl ...
Reptilia ( ), a
paraphyletic grouping comprising all
sauropsids
Sauropsida ("lizard faces") is a clade of amniotes, broadly equivalent to the class Reptilia. Sauropsida is the sister taxon to Synapsida, the other clade of amniotes which includes mammals as its only modern representatives. Although early sy ...
except
birds.
Living reptiles comprise
turtle
Turtles are an order of reptiles known as Testudines, characterized by a special shell developed mainly from their ribs. Modern turtles are divided into two major groups, the Pleurodira (side necked turtles) and Cryptodira (hidden necked t ...
s,
crocodilia
Crocodilia (or Crocodylia, both ) is an order of mostly large, predatory, semiaquatic reptiles, known as crocodilians. They first appeared 95 million years ago in the Late Cretaceous period ( Cenomanian stage) and are the closest livi ...
ns,
squamates
Squamata (, Latin ''squamatus'', 'scaly, having scales') is the largest order of reptiles, comprising lizards, snakes, and amphisbaenians (worm lizards), which are collectively known as squamates or scaled reptiles. With over 10,900 species, it ...
(
lizards and
snake
Snakes are elongated, limbless, carnivorous reptiles of the suborder Serpentes . Like all other squamates, snakes are ectothermic, amniote vertebrates covered in overlapping scales. Many species of snakes have skulls with several more j ...
s) and
rhynchocephalians (
tuatara
Tuatara (''Sphenodon punctatus'') are reptiles endemic to New Zealand. Despite their close resemblance to lizards, they are part of a distinct lineage, the order Rhynchocephalia. The name ''tuatara'' is derived from the Māori language and m ...
). As of March 2022, the
Reptile Database
The Reptile Database is a scientific database that collects taxonomic information on all living reptile species (i.e. no fossil species such as dinosaurs). The database focuses on species (as opposed to higher ranks such as families) and has entrie ...
includes about 11,700 species. In the traditional
Linnaean classification system, birds are considered a separate class to reptiles. However, crocodilians are more closely related to birds than they are to other living reptiles, and so modern
cladistic
Cladistics (; ) is an approach to biological classification in which organisms are categorized in groups ("clades") based on hypotheses of most recent common ancestry. The evidence for hypothesized relationships is typically shared derived char ...
classification systems include birds within Reptilia, redefining the term as a
clade. Other cladistic definitions abandon the term reptile altogether in favor of the clade
Sauropsida, which refers to all amniotes more closely related to modern reptiles than to mammals. The study of the traditional reptile
orders, historically combined with that of modern
amphibians, is called
herpetology.
The earliest known proto-reptiles originated around 312 million years ago during the
Carboniferous period, having evolved from advanced
reptiliomorph
Reptiliomorpha (meaning reptile-shaped; in PhyloCode known as ''Pan-Amniota'') is a clade containing the amniotes and those tetrapods that share a more recent common ancestor with amniotes than with living amphibians (lissamphibians). It was defi ...
tetrapods which became increasingly adapted to life on dry land. The earliest known
eureptile
Eureptilia ("true reptiles") is one of the two major subgroups of the clade Sauropsida, the other one being Parareptilia. Eureptilia includes Diapsida (the clade containing all modern reptiles and birds), as well as a number of primitive Permo-C ...
("true reptile") was ''
Hylonomus
''Hylonomus'' (; ''hylo-'' "forest" + ''nomos'' "dweller") is an extinct genus of reptile that lived 312 million years ago during the Late Carboniferous period.
It is the earliest unquestionable reptile (''Westlothiana'' is older, but in fact it ...
,'' a small and superficially lizard-like animal. Genetic and fossil data argues that the two largest lineages of reptiles,
Archosauromorpha (crocodilians, birds, and kin) and
Lepidosauromorpha
Lepidosauromorpha (in PhyloCode known as ''Pan-Lepidosauria'') is a group of reptiles comprising all diapsids closer to lizards than to archosaurs (which include crocodiles and birds). The only living sub-group is the Lepidosauria, which cont ...
(lizards, and kin), diverged near the end of the
Permian
The Permian ( ) is a geologic period and System (stratigraphy), stratigraphic system which spans 47 million years from the end of the Carboniferous Period million years ago (Mya), to the beginning of the Triassic Period 251.9 Mya. It is the last ...
period.
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
The Cretaceous–Paleogene (K–Pg) extinction event (also known as the Cretaceous–Tertiary extinction) was a sudden mass extinction of three-quarters of the plant and animal species on Earth, approximately 66 million years ago. With the ...
wiped out the
pterosaur
Pterosaurs (; from Greek ''pteron'' and ''sauros'', meaning "wing lizard") is an extinct clade of flying reptiles in the order, Pterosauria. They existed during most of the Mesozoic: from the Late Triassic to the end of the Cretaceous (228 ...
s,
plesiosaurs
The Plesiosauria (; Greek: πλησίος, ''plesios'', meaning "near to" and ''sauros'', meaning "lizard") or plesiosaurs are an order or clade of extinct Mesozoic marine reptiles, belonging to the Sauropterygia.
Plesiosaurs first appeared i ...
, and all non-avian
dinosaurs alongside many species of
crocodyliforms
Crocodyliformes is a clade of crurotarsan archosaurs, the group often traditionally referred to as "crocodilians". They are the first members of Crocodylomorpha to possess many of the features that define later relatives. They are the only pseudo ...
, and
squamates
Squamata (, Latin ''squamatus'', 'scaly, having scales') is the largest order of reptiles, comprising lizards, snakes, and amphisbaenians (worm lizards), which are collectively known as squamates or scaled reptiles. With over 10,900 species, it ...
(e.g.,
mosasaur
Mosasaurs (from Latin ''Mosa'' meaning the 'Meuse', and Greek ' meaning 'lizard') comprise a group of extinct, large marine reptiles from the Late Cretaceous. Their first fossil remains were discovered in a limestone quarry at Maastricht on ...
s). Modern non-bird reptiles inhabit all the continents except
Antarctica
Antarctica () is Earth's southernmost and least-populated continent. Situated almost entirely south of the Antarctic Circle and surrounded by the Southern Ocean, it contains the geographic South Pole. Antarctica is the fifth-largest cont ...
.
Reptiles are tetrapod
vertebrates
Vertebrates () comprise all animal taxa within the subphylum Vertebrata () (chordates with backbones), including all mammals, birds, reptiles, amphibians, and fish. Vertebrates represent the overwhelming majority of the phylum Chordata, with c ...
, 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
Among animals, viviparity is development of the embryo inside the body of the parent. This is opposed to oviparity which is a reproductive mode in which females lay developing eggs that complete their development and hatch externally from the ...
, as were some extinct aquatic clades
– the fetus develops within the mother, using a
(non-mammalian) placenta rather than contained in 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
fetus
A fetus or foetus (; plural fetuses, feti, foetuses, or foeti) is the unborn offspring that develops from an animal embryo. Following embryonic development the fetal stage of development takes place. In human prenatal development, fetal dev ...
es 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
''Sphaerodactylus ariasae'', commonly called the Jaragua sphaero or the Jaragua dwarf gecko, is the smallest species of lizard in the family Sphaerodactylidae.
Description
''Sphaerodactylus ariasae'' is the world's smallest known reptile. The s ...
'', which can grow up to to the
saltwater crocodile
The saltwater crocodile (''Crocodylus porosus'') is a crocodilian native to saltwater habitats and brackish wetlands from India's east coast across Southeast Asia and the Sundaic region to northern Australia and Micronesia. It has been list ...
, ''Crocodylus porosus'', which can reach over in length and weigh over .
Classification
Research history
![Reptile003d](https://upload.wikimedia.org/wikipedia/commons/c/c4/Reptile003d.jpg)
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 ( la, Vincentius Bellovacensis or ''Vincentius Burgundus''; c. 1264) was a Dominican friar at the Cistercian monastery of Royaumont Abbey, France. He is known mostly for his ''Speculum Maius'' (''Great mirror''), a major work ...
in his ''Mirror of Nature''.
In the 18th century, the reptiles were, from the outset of classification, grouped with the
amphibians.
Linnaeus
Carl Linnaeus (; 23 May 1707 – 10 January 1778), also known after his ennoblement in 1761 as Carl von Linné Blunt (2004), p. 171. (), was a Swedish botanist, zoologist, taxonomist, and physician who formalised binomial nomenclature, the ...
, working from species-poor
Sweden, where the
common adder
''Vipera berus'', the common European adderMallow D, Ludwig D, Nilson G. (2003). ''True Vipers: Natural History and Toxinology of Old World Vipers''. Malabar, Florida: Krieger Publishing Company. . or common European viper,Stidworthy J. (1974). ...
and
grass snake
The grass snake (''Natrix natrix''), sometimes called the ringed snake or water snake, is a Eurasian non-venomous colubrid snake. It is often found near water and feeds almost exclusively on amphibians.
Subspecies
Many subspecies are recognized ...
are often found hunting in water, included all reptiles and amphibians in
class
Class or The Class may refer to:
Common uses not otherwise categorized
* Class (biology), a taxonomic rank
* Class (knowledge representation), a collection of individuals or objects
* Class (philosophy), an analytical concept used differentl ...
"III –
Amphibia
Amphibians are four-limbed and ectothermic vertebrates of the class Amphibia. All living amphibians belong to the group Lissamphibia. They inhabit a wide variety of habitats, with most species living within terrestrial, fossorial, arbor ...
" 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
Josephus Nicolaus Laurenti (4 December 1735, Vienna – 17 February 1805, Vienna) was an Austrian naturalist and zoologist of Italian origin.
Laurenti is considered the auctor of the class Reptilia ( reptiles) through his authorship of ' (1 ...
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 single heading
herpetology.
![MosasaurDiscovery](https://upload.wikimedia.org/wikipedia/commons/6/64/MosasaurDiscovery.jpg)
It was not until the beginning of the 19th century that it became clear that reptiles and amphibians are, in fact, quite different animals, and
Pierre André Latreille
Pierre André Latreille (; 29 November 1762 – 6 February 1833) was a French zoologist, specialising in arthropods. Having trained as a Roman Catholic priest before the French Revolution, Latreille was imprisoned, and only regained his freedom ...
erected the class ''Batracia'' (1825) for the latter, dividing the
tetrapod
Tetrapods (; ) are four-limbed vertebrate animals constituting the superclass Tetrapoda (). It includes extant and extinct amphibians, sauropsids ( reptiles, including dinosaurs and therefore birds) and synapsids ( pelycosaurs, extinct t ...
s into the four familiar classes of reptiles, amphibians, birds, and mammals. The British anatomist
Thomas Henry Huxley
Thomas Henry Huxley (4 May 1825 – 29 June 1895) was an English biologist and anthropologist specialising in comparative anatomy. He has become known as "Darwin's Bulldog" for his advocacy of Charles Darwin's theory of evolution.
The stori ...
made Latreille's definition popular and, together with
Richard Owen, expanded Reptilia to include the various fossil "
antediluvian
The antediluvian (alternatively pre-diluvian or pre-flood) period is the time period chronicled in the Bible between the fall of man and the Genesis flood narrative in biblical cosmology. The term was coined by Thomas Browne. The narrative takes ...
monsters", including
dinosaur
Dinosaurs are a diverse group of reptiles of the clade Dinosauria. They first appeared during the Triassic period, between 243 and 233.23 million years ago (mya), although the exact origin and timing of the evolution of dinosaurs is t ...
s and the mammal-like (
synapsid
Synapsids + (, 'arch') > () "having a fused arch"; synonymous with ''theropsids'' (Greek, "beast-face") are one of the two major groups of animals that evolved from basal amniotes, the other being the sauropsids, the group that includes reptil ...
) ''
Dicynodon
''Dicynodon'' ("two dog-teeth") is a genus of dicynodont therapsid that flourished during the Upper Permian period. Like all dicynodonts, it was herbivorous animal. This reptile was toothless, except for prominent tusks, hence the name. It proba ...
'' he helped describe. This was not the only possible classification scheme: In the Hunterian lectures delivered at the
Royal College of Surgeons
The Royal College of Surgeons is an ancient college (a form of corporation) established in England to regulate the activity of surgeons. Derivative organisations survive in many present and former members of the Commonwealth. These organisations a ...
in 1863, Huxley grouped the vertebrates into
mammals, sauroids, and ichthyoids (the latter containing the fishes and amphibians). He subsequently proposed the names of
Sauropsida and
Ichthyopsida
The anamniotes are an informal group of craniates comprising all fishes and amphibians, which lay their eggs in aquatic environments. They are distinguished from the amniotes (reptiles, birds and mammals), which can reproduce on dry land either ...
for the latter two groups. In 1866,
Haeckel
Ernst Heinrich Philipp August Haeckel (; 16 February 1834 – 9 August 1919) was a German zoologist, naturalist, eugenicist, philosopher, physician, professor, marine biologist and artist. He discovered, described and named thousands of new sp ...
demonstrated that vertebrates could be divided based on their reproductive strategies, and that reptiles, birds, and mammals were united by the
amniotic egg
Amniotes are a clade of tetrapod vertebrates that comprises sauropsids (including all reptiles and birds, and extinct parareptiles and non-avian dinosaurs) and synapsids (including pelycosaurs and therapsids such as mammals). They are distingu ...
.
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 Sauropsida included
Procolophonia
The Procolophonia are a suborder of herbivorous reptiles that lived from the Middle Permian till the end of the Triassic period. They were originally included as a suborder of the Cotylosauria (later renamed Captorhinida Carroll 1988) but are ...
,
Eosuchia
Eosuchians are an extinct order of diapsid reptiles. Depending on which taxa are included the order may have ranged from the late Carboniferous to the Eocene but the consensus is that eosuchians are confined to the Permian and Triassic.
Eosuchi ...
,
Millerosauria
Millerosauria is an order of Parareptiles that contains the families †Millerettidae and †Eunotosauridae. It is the sister group to the order Procolophonomorpha. It was named in 1957 by Watson. It was once considered a suborder of the disused ...
,
Chelonia
The green sea turtle (''Chelonia mydas''), also known as the green turtle, black (sea) turtle or Pacific green turtle, is a species of large sea turtle of the family Cheloniidae. It is the only species in the genus ''Chelonia''. Its range exten ...
(turtles),
Squamata
Squamata (, Latin ''squamatus'', 'scaly, having scales') is the largest order of reptiles, comprising lizards, snakes, and amphisbaenians (worm lizards), which are collectively known as squamates or scaled reptiles. With over 10,900 species ...
(lizards and snakes),
Rhynchocephalia,
Crocodilia
Crocodilia (or Crocodylia, both ) is an order of mostly large, predatory, semiaquatic reptiles, known as crocodilians. They first appeared 95 million years ago in the Late Cretaceous period ( Cenomanian stage) and are the closest livi ...
, "
thecodonts" (
paraphyletic basal Archosauria), non-
avian
Avian may refer to:
*Birds or Aves, winged animals
*Avian (given name) (russian: Авиа́н, link=no), a male forename
Aviation
*Avro Avian, a series of light aircraft made by Avro in the 1920s and 1930s
*Avian Limited, a hang glider manufacture ...
dinosaur
Dinosaurs are a diverse group of reptiles of the clade Dinosauria. They first appeared during the Triassic period, between 243 and 233.23 million years ago (mya), although the exact origin and timing of the evolution of dinosaurs is t ...
s,
pterosaur
Pterosaurs (; from Greek ''pteron'' and ''sauros'', meaning "wing lizard") is an extinct clade of flying reptiles in the order, Pterosauria. They existed during most of the Mesozoic: from the Late Triassic to the end of the Cretaceous (228 ...
s,
ichthyosaurs, and
sauropterygia
Sauropterygia ("lizard flippers") is an extinct taxon of diverse, aquatic reptiles that developed from terrestrial ancestors soon after the end-Permian extinction and flourished during the Triassic before all except for the Plesiosauria became ...
ns.
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
The occipital condyles are undersurface protuberances of the occipital bone in vertebrates, which function in articulation with the superior facets of the atlas vertebra.
The condyles are oval or reniform (kidney-shaped) in shape, and their anteri ...
, a jaw joint formed by the
quadrate and
articular
The articular bone is part of the lower jaw of most vertebrates, including most jawed fish, amphibians, birds and various kinds of reptiles, as well as ancestral mammals.
Anatomy
In most vertebrates, the articular bone is connected to two oth ...
bones, and certain characteristics of the
vertebrae
The spinal column, a defining synapomorphy shared by nearly all vertebrates, Hagfish are believed to have secondarily lost their spinal column is a moderately flexible series of vertebrae (singular vertebra), each constituting a characteristi ...
. The animals singled out by these formulations, the
amniotes other than the mammals and the birds, are still those considered reptiles today.
![Captorhinus aguti p](https://upload.wikimedia.org/wikipedia/commons/f/f3/Captorhinus_aguti_p.jpg)
The synapsid/sauropsid division supplemented another approach, one that split the reptiles into four subclasses based on the number and position of
temporal fenestrae
The skull is a bone protective cavity for the brain. The skull is composed of four types of bone i.e., cranial bones, facial bones, ear ossicles and hyoid bone. However two parts are more prominent: the cranium and the mandible. In humans, th ...
, openings in the sides of the skull behind the eyes. This classification was initiated by
Henry Fairfield Osborn
Henry Fairfield Osborn, Sr. (August 8, 1857 – November 6, 1935) was an American paleontologist, geologist and eugenics advocate. He was the president of the American Museum of Natural History for 25 years and a cofounder of the American Euge ...
and elaborated and made popular by
Romer
A Reference Card or "Romer" is a device for increasing the accuracy when reading a grid reference from a map. Made from transparent plastic, paper or other materials, they are also found on most baseplate compasses. Essentially, it is a speciall ...
's classic ''
Vertebrate Paleontology
Vertebrate paleontology is the subfield of paleontology that seeks to discover, through the study of fossilized remains, the behavior, reproduction and appearance of extinct animals with vertebrae or a notochord. It also tries to connect, by us ...
''.
[, 3rd ed., 1966.] Those four subclasses were:
*
Anapsid
An anapsid is an amniote whose skull lacks one or more skull openings (fenestra, or fossae) near the temples. Traditionally, the Anapsida are the most primitive subclass of amniotes, the ancestral stock from which Synapsida and Diapsida evolve ...
a – no fenestrae –
cotylosaur
Captorhinidae (also known as cotylosaurs) is an extinct family of tetrapods, traditionally considered primitive reptiles, known from the late Carboniferous to the Late Permian. They had a cosmopolitan distribution across Pangea.
Description
C ...
s and
Chelonia
The green sea turtle (''Chelonia mydas''), also known as the green turtle, black (sea) turtle or Pacific green turtle, is a species of large sea turtle of the family Cheloniidae. It is the only species in the genus ''Chelonia''. Its range exten ...
(
turtle
Turtles are an order of reptiles known as Testudines, characterized by a special shell developed mainly from their ribs. Modern turtles are divided into two major groups, the Pleurodira (side necked turtles) and Cryptodira (hidden necked t ...
s and relatives)
[This taxonomy does not reflect modern molecular evidence, which places turtles within Diapsida.]
*
Synapsid
Synapsids + (, 'arch') > () "having a fused arch"; synonymous with ''theropsids'' (Greek, "beast-face") are one of the two major groups of animals that evolved from basal amniotes, the other being the sauropsids, the group that includes reptil ...
a – one low fenestra –
pelycosaur
Pelycosaur ( ) is an older term for basal or primitive Late Paleozoic synapsids, excluding the therapsids and their descendants. Previously, the term ''mammal-like reptile'' had been used, and pelycosaur was considered an order, but this is ...
s and
therapsid
Therapsida is a major group of eupelycosaurian synapsids that includes mammals, their ancestors and relatives. Many of the traits today seen as unique to mammals had their origin within early therapsids, including limbs that were oriented more ...
s (the '
mammal-like reptiles
Pelycosaur ( ) is an older term for basal or primitive Late Paleozoic synapsids, excluding the therapsids and their descendants. Previously, the term ''mammal-like reptile'' had been used, and pelycosaur was considered an order, but this is ...
')
*
Euryapsida
__NOTOC__
Euryapsida is a polyphyletic (unnatural, as the various members are not closely related) group of sauropsids that are distinguished by a single temporal fenestra, an opening behind the orbit, under which the post-orbital and squamosal b ...
– one high fenestra (above the postorbital and squamosal) –
protorosaurs (small, early lizard-like reptiles) and the marine
sauropterygia
Sauropterygia ("lizard flippers") is an extinct taxon of diverse, aquatic reptiles that developed from terrestrial ancestors soon after the end-Permian extinction and flourished during the Triassic before all except for the Plesiosauria became ...
ns and
ichthyosaurs
Ichthyosaurs (Ancient Greek for "fish lizard" – and ) are large extinct marine reptiles. Ichthyosaurs belong to the order known as Ichthyosauria or Ichthyopterygia ('fish flippers' – a designation introduced by Sir Richard Owen in 1842, alt ...
, the latter called
Parapsida in Osborn's work.
*
Diapsid
Diapsids ("two arches") are a clade of sauropsids, distinguished from more primitive eureptiles by the presence of two holes, known as temporal fenestrae, in each side of their skulls. The group first appeared about three hundred million years a ...
a – two fenestrae – most reptiles, including
lizards,
snake
Snakes are elongated, limbless, carnivorous reptiles of the suborder Serpentes . Like all other squamates, snakes are ectothermic, amniote vertebrates covered in overlapping scales. Many species of snakes have skulls with several more j ...
s,
crocodilians,
dinosaur
Dinosaurs are a diverse group of reptiles of the clade Dinosauria. They first appeared during the Triassic period, between 243 and 233.23 million years ago (mya), although the exact origin and timing of the evolution of dinosaurs is t ...
s and
pterosaur
Pterosaurs (; from Greek ''pteron'' and ''sauros'', meaning "wing lizard") is an extinct clade of flying reptiles in the order, Pterosauria. They existed during most of the Mesozoic: from the Late Triassic to the end of the Cretaceous (228 ...
s
![Varanodon1DB](https://upload.wikimedia.org/wikipedia/commons/4/40/Varanodon1DB.jpg)
The composition of Euryapsida was uncertain.
Ichthyosaurs
Ichthyosaurs (Ancient Greek for "fish lizard" – and ) are large extinct marine reptiles. Ichthyosaurs belong to the order known as Ichthyosauria or Ichthyopterygia ('fish flippers' – a designation introduced by Sir Richard Owen in 1842, alt ...
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 ''problematica'' is a term used for a taxonomic group where its broader relationships are unknown or undefined. Alternatively, such groups are frequently referred to as "enigmatic taxa". In the system of open nomenclature, uncertain ...
'' 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
By the early 21st century, vertebrate paleontologists were beginning to adopt
phylogenetic
In biology, phylogenetics (; from Greek φυλή/ φῦλον [] "tribe, clan, race", and wikt:γενετικός, γενετικός [] "origin, source, birth") is the study of the evolutionary history and relationships among or within groups o ...
taxonomy, in which all groups are defined in such a way as to be
monophyletic; that is, groups which 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 extant reptiles.
Colin Tudge
Colin Hiram Tudge (born 22 April 1943) is a British biologist, science writer and broadcaster.
Tudge was born and brought up in south London and attended Dulwich College, from where he won a scholarship to Peterhouse, Cambridge, studying zool ...
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
Amniotes are a clade of tetrapod vertebrates that comprises sauropsids (including all reptiles and birds, and extinct parareptiles and non-avian dinosaurs) and synapsids (including pelycosaurs and therapsids such as mammals). They are dis ...
: the section that is left after the Mammalia and Aves have been hived off. It cannot be defined by synapomorphies, as is the proper way. 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, non-mammalian amniotes'.
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.
![Bearded Dragon Skeleton](https://upload.wikimedia.org/wikipedia/commons/7/7a/Bearded_Dragon_Skeleton.jpg)
When Sauropsida was used, it often had the same content or even the same definition as Reptilia. In 1988,
Jacques Gauthier proposed a
cladistic
Cladistics (; ) is an approach to biological classification in which organisms are categorized in groups ("clades") based on hypotheses of most recent common ancestry. The evidence for hypothesized relationships is typically shared derived char ...
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. While Gauthier's definition was close to the modern consensus, nonetheless, it became considered inadequate because the actual relationship of turtles to other reptiles was not yet well understood at this time.
[ Major revisions since have included the reassignment of synapsids as non-reptiles, and classification of turtles as diapsids.]
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
The ''International Code of Phylogenetic Nomenclature'', known as the ''PhyloCode'' for short, is a formal set of rules governing phylogenetic nomenclature. Its current version is specifically designed to regulate the naming of clades, leaving the ...
, 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
The sand lizard (''Lacerta agilis'') is a Lacertidae, lacertid lizard distributed across most of Europe from France and across the continent to Lake Baikal in Russia. It does not occur in European Turkey. Its distribution is often patchy. In the ...
'' and ''Crocodylus niloticus
''Crocodylus'' is a genus of true crocodiles in the family Crocodylidae.
Taxonomy
The generic name, ''Crocodylus'', was proposed by Josephus Nicolaus Laurenti in 1768. ''Crocodylus'' contains 13–14 extant (living) species and 5 extinct spe ...
'' than to ''Homo sapiens
Humans (''Homo sapiens'') are the most abundant and widespread species of primate, characterized by bipedalism and exceptional cognitive skills due to a large and complex brain. This has enabled the development of advanced tools, culture, ...
''. 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 crocodiles.]
Taxonomy
General classification of extinct and living reptiles, focusing on major groups.
* Reptilia/ Sauropsida
**Parareptilia
Parareptilia ("at the side of reptiles") is a subclass or clade of basal sauropsids (reptiles), typically considered the sister taxon to Eureptilia (the group that likely contains all living reptiles and birds). Parareptiles first arose near th ...
** Eureptilia
Eureptilia ("true reptiles") is one of the two major subgroups of the clade Sauropsida, the other one being Parareptilia. Eureptilia includes Diapsida (the clade containing all modern reptiles and birds), as well as a number of primitive Permo ...
***Captorhinidae
Captorhinidae (also known as cotylosaurs) is an extinct family of tetrapods, traditionally considered primitive reptiles, known from the late Carboniferous to the Late Permian. They had a cosmopolitan distribution across Pangea.
Description
...
***Diapsid
Diapsids ("two arches") are a clade of sauropsids, distinguished from more primitive eureptiles by the presence of two holes, known as temporal fenestrae, in each side of their skulls. The group first appeared about three hundred million years a ...
a
****Araeoscelidia
Araeoscelidia or Araeoscelida is a clade of extinct diapsid reptiles superficially resembling lizards, extending from the Late Carboniferous to the Early Permian.
The group contains the genera ''Araeoscelis'', ''Petrolacosaurus'', the possi ...
****Neodiapsida
Neodiapsida is a clade, or major branch, of the reptilian family tree, typically defined as including all diapsids apart from some early primitive types known as the araeoscelidians. Modern reptiles and birds belong to the neodiapsid subclade S ...
*****Drepanosauromorpha
Drepanosaurs (members of the clade Drepanosauromorpha) are a group of extinct reptiles that lived between the Carnian and Rhaetian stages of the late Triassic Period, approximately between 230 and 210 million years ago. The various species of ...
(placement uncertain)
*****Younginiformes
Younginiformes is a replacement name for the taxon Eosuchia, proposed by Alfred Romer in 1947.
The Eosuchia having become a wastebasket taxon for many probably distantly-related primitive diapsid reptiles ranging from the late Carboniferous to ...
( paraphyletic)
*****Ichthyosauromorpha
The Ichthyosauromorpha are an extinct clade of marine reptiles consisting of the Ichthyosauriformes and the Hupehsuchia, living during the Mesozoic.
The node clade Ichthyosauromorpha was first defined by Ryosuke Motani ''et al.'' in 2014 as t ...
(placement uncertain)
*****Thalattosauria
Thalattosauria (Greek for "sea lizards") is an extinct order of prehistoric marine reptiles that lived in the middle to late Triassic period. Thalattosaurs were diverse in size and shape, and are divided into two superfamilies: Askeptosauroidea ...
(placement uncertain)
*****Sauria
Sauria is the clade containing the most recent common ancestor of archosaurs (such as crocodilians, dinosaurs, etc.) and lepidosaurs ( lizards and kin), and all its descendants. Since most molecular phylogenies recover turtles as more closely ...
******Lepidosauromorpha
Lepidosauromorpha (in PhyloCode known as ''Pan-Lepidosauria'') is a group of reptiles comprising all diapsids closer to lizards than to archosaurs (which include crocodiles and birds). The only living sub-group is the Lepidosauria, which cont ...
******* Lepidosauriformes
******** Rhynchocephalia (tuatara)
********Squamata
Squamata (, Latin ''squamatus'', 'scaly, having scales') is the largest order of reptiles, comprising lizards, snakes, and amphisbaenians (worm lizards), which are collectively known as squamates or scaled reptiles. With over 10,900 species ...
(lizards & snakes)
****** Choristodera (placement uncertain)
******Sauropterygia
Sauropterygia ("lizard flippers") is an extinct taxon of diverse, aquatic reptiles that developed from terrestrial ancestors soon after the end-Permian extinction and flourished during the Triassic before all except for the Plesiosauria became ...
(placement uncertain)
******Pantestudines
Pantestudines or Pan-Testudines is the group of all reptiles more closely related to turtles than to any other living animal. It includes both modern turtles (crown group turtles, also known as Testudines) and all of their extinct relatives (also ...
(turtles and kin, placement uncertain)
****** Archosauromorpha
*******Protorosauria
Protorosauria is an extinct polyphyletic group of archosauromorph reptiles from the latest Middle Permian ( Capitanian stage) to the end of the Late Triassic (Rhaetian stage) of Asia, Europe and North America. It was named by the English anatom ...
(paraphyletic)
*******Rhynchosauria
Rhynchosaurs are a group of extinct herbivorous Triassic archosauromorph reptiles, belonging to the order Rhynchosauria. Members of the group are distinguished by their triangular skulls and elongated, beak like premaxillary bones. Rhynchosaurs ...
*******Allokotosauria
Allokotosauria is a clade of early archosauromorph reptiles from the Middle to Late Triassic known from Asia, Africa, North America and Europe. Allokotosauria was first described and named when a new monophyletic grouping of specialized he ...
*******Archosauriformes
Archosauriformes (Greek for 'ruling lizards', and Latin for 'form') is a clade of diapsid reptiles that developed from archosauromorph ancestors some time in the Latest Permian (roughly 252 million years ago). It was defined by Jacques Gauthier ...
********Phytosaur
Phytosaurs (Φυτόσαυροι in greek) are an extinct group of large, mostly semiaquatic Late Triassic archosauriform reptiles. Phytosaurs belong to the order Phytosauria. Phytosauria and Phytosauridae are often considered to be equivalent g ...
ia
******** Archosauria
*********Pseudosuchia
Pseudosuchia is one of two major divisions of Archosauria, including living crocodilians and all archosaurs more closely related to crocodilians than to birds. Pseudosuchians are also informally known as "crocodilian-line archosaurs". Prior to ...
**********Crocodilia
Crocodilia (or Crocodylia, both ) is an order of mostly large, predatory, semiaquatic reptiles, known as crocodilians. They first appeared 95 million years ago in the Late Cretaceous period ( Cenomanian stage) and are the closest livi ...
(crocodilians)
*********Avemetatarsalia
Avemetatarsalia (meaning "bird metatarsals") is a clade of diapsid reptiles containing all archosaurs more closely related to birds than to crocodilians. The two most successful groups of avemetatarsalians were the dinosaurs and pterosaurs. ...
/Ornithodira
Avemetatarsalia (meaning "bird metatarsals") is a clade of diapsid reptiles containing all archosaurs more closely related to birds than to crocodilians. The two most successful groups of avemetatarsalians were the dinosaurs and pterosaurs. Dinos ...
**********Pterosaur
Pterosaurs (; from Greek ''pteron'' and ''sauros'', meaning "wing lizard") is an extinct clade of flying reptiles in the order, Pterosauria. They existed during most of the Mesozoic: from the Late Triassic to the end of the Cretaceous (228 ...
ia
**********Dinosaur
Dinosaurs are a diverse group of reptiles of the clade Dinosauria. They first appeared during the Triassic period, between 243 and 233.23 million years ago (mya), although the exact origin and timing of the evolution of dinosaurs is t ...
ia
*********** Ornithischia
***********Saurischia
Saurischia ( , meaning "reptile-hipped" from the Greek ' () meaning 'lizard' and ' () meaning 'hip joint') is one of the two basic divisions of dinosaurs (the other being Ornithischia), classified by their hip structure. Saurischia and Ornithis ...
(including birds ( Aves))
Phylogeny
The cladogram
A cladogram (from Greek ''clados'' "branch" and ''gramma'' "character") is a diagram used in cladistics to show relations among organisms. A cladogram is not, however, an evolutionary tree because it does not show how ancestors are related to ...
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 Archosauromorpha, though a few have recovered turtles as Lepidosauromorpha instead. The cladogram below used a combination of genetic (molecular) and fossil (morphological) data to obtain its results.[
]
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. As of 2013, three turtle genomes have been sequenced. The results place turtles as a sister clade
In phylogenetics, a sister group or sister taxon, also called an adelphotaxon, comprises the closest relative(s) of another given unit in an evolutionary tree.
Definition
The expression is most easily illustrated by a cladogram:
Taxon A and t ...
to the archosaurs, the group that includes crocodiles, dinosaurs, and birds. However, in their comparative analysis of the timing of organogenesis
Organogenesis is the phase of embryonic development that starts at the end of gastrulation and continues until birth. During organogenesis, the three germ layers formed from gastrulation (the ectoderm, endoderm, and mesoderm) form the internal org ...
, Werneburg and Sánchez-Villagra (2009) found support for the hypothesis that turtles belong to a separate clade within Sauropsida, outside the sauria
Sauria is the clade containing the most recent common ancestor of archosaurs (such as crocodilians, dinosaurs, etc.) and lepidosaurs ( lizards and kin), and all its descendants. Since most molecular phylogenies recover turtles as more closely ...
n clade altogether.
Evolutionary history
Origin of the reptiles
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
''Casineria'' is an extinct genus of tetrapod which lived about 340-334 million years ago in the Mississippian epoch of the Carboniferous period. Its generic name, ''Casineria'', is a latinization of Cheese Bay, the site near Edinburgh, Scotla ...
'' (though it may have been a temnospondyl
Temnospondyli (from Greek τέμνειν, ''temnein'' 'to cut' and σπόνδυλος, ''spondylos'' 'vertebra') is a diverse order of small to giant tetrapods—often considered primitive amphibians—that flourished worldwide during the Carbo ...
). A series of footprints from the fossil strata of Nova Scotia
Nova Scotia ( ; ; ) is one of the thirteen provinces and territories of Canada. It is one of the three Maritime provinces and one of the four Atlantic provinces. Nova Scotia is Latin for "New Scotland".
Most of the population are native Eng ...
dated to show typical reptilian toes and imprints of scales. These tracks are attributed to ''Hylonomus
''Hylonomus'' (; ''hylo-'' "forest" + ''nomos'' "dweller") is an extinct genus of reptile that lived 312 million years ago during the Late Carboniferous period.
It is the earliest unquestionable reptile (''Westlothiana'' is older, but in fact it ...
'', the oldest unquestionable reptile known.
It was a small, lizard-like animal, about long, with numerous sharp teeth indicating an insectivorous diet. Other examples include ''Westlothiana
''Westlothiana'' ("animal from West Lothian") is a genus of reptile-like tetrapod that lived about 338 million years ago during the latest part of the Visean age of the Carboniferous. Members of the genus bore a superficial resemblance to modern ...
'' (for the moment considered a reptiliomorph
Reptiliomorpha (meaning reptile-shaped; in PhyloCode known as ''Pan-Amniota'') is a clade containing the amniotes and those tetrapods that share a more recent common ancestor with amniotes than with living amphibians (lissamphibians). It was defi ...
rather than a true amniote) and ''Paleothyris
''Paleothyris'' was a small, agile, anapsid romeriidan reptile which lived in the Middle Pennsylvanian epoch in Nova Scotia (approximately 312 to 304 million years ago). ''Paleothyris'' had sharp teeth and large eyes, meaning that it was like ...
'', both of similar build and presumably similar habit.
However, microsaur
Microsauria ("small lizards") is an extinct, possibly polyphyletic order of tetrapods from the late Carboniferous and early Permian periods. It is the most diverse and species-rich group of lepospondyls. Recently, Microsauria has been consider ...
s have been at times considered true reptiles, so an earlier origin is possible.
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 Carboniferous Rainforest 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
''Mesosaurus'' (meaning "middle lizard") is an extinct genus of reptile from the Cisuralian, Early Permian of southern Africa and South America. Along with it, the genera ''Brazilosaurus'' and ''Stereosternum'', it is a member of the family (bio ...
'', a genus from the Early Permian that had returned to water, feeding on fish.
A 2021 examination of reptile diversity in the Carboniferous and Permian suggests a much higher degree of diversity than previously thought, comparable or even exceeding that of synapsids. Thus, the "First Age of Reptiles" was proposed.
Anapsids, synapsids, diapsids, and sauropsids
It was traditionally assumed that the first reptiles retained an anapsid
An anapsid is an amniote whose skull lacks one or more skull openings (fenestra, or fossae) near the temples. Traditionally, the Anapsida are the most primitive subclass of amniotes, the ancestral stock from which Synapsida and Diapsida evolve ...
skull inherited from their ancestors.[Coven, R (2000): History of Life. ]Blackwell Science
Wiley-Blackwell is an international scientific, technical, medical, and scholarly publishing business of John Wiley & Sons. It was formed by the merger of John Wiley & Sons Global Scientific, Technical, and Medical business with Blackwell Publish ...
, Oxford, UK
p 154
from Google Books This type of skull has a skull roof with only holes for the nostrils, eyes and a pineal eye
A parietal eye, also known as a third eye or pineal eye, is a part of the epithalamus present in some vertebrates. The eye is located at the top of the head, is photoreceptive and is associated with the pineal gland, regulating circadian rhythm ...
.[ Romer, A.S. & T.S. Parsons. 1977. ''The Vertebrate Body.'' 5th ed. Saunders, Philadelphia. (6th ed. 1985)] The discoveries of synapsid
Synapsids + (, 'arch') > () "having a fused arch"; synonymous with ''theropsids'' (Greek, "beast-face") are one of the two major groups of animals that evolved from basal amniotes, the other being the sauropsids, the group that includes reptil ...
-like openings (see below) in the skull roof of the skulls of several members of Parareptilia
Parareptilia ("at the side of reptiles") is a subclass or clade of basal sauropsids (reptiles), typically considered the sister taxon to Eureptilia (the group that likely contains all living reptiles and birds). Parareptiles first arose near th ...
(the clade containing most of the amniotes traditionally referred to as "anapsids"), including lanthanosuchoids, millerettids, bolosaurids, some nycteroleterids, some procolophonoids and at least some mesosaur
Mesosaurs ("middle lizards") were a group of small aquatic reptiles that lived during the early Permian period, roughly 299 to 270 million years ago. Mesosaurs were the first known aquatic reptiles, having apparently returned to an aquatic life ...
s 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 lineage called 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
Mammals () are a group of vertebrate animals constituting the class Mammalia (), characterized by the presence of mammary glands which in females produce milk for feeding (nursing) their young, a neocortex (a region of the brain), fur o ...
. Soon after, another group evolved a similar trait, this time with a double opening behind each eye, earning them the name Diapsida
Diapsids ("two arches") are a clade of sauropsids, distinguished from more primitive eureptiles by the presence of two holes, known as temporal fenestrae, in each side of their skulls. The group first appeared about three hundred million years ag ...
("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 bite.]
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
In biology, phylogenetics (; from Greek φυλή/ φῦλον [] "tribe, clan, race", and wikt:γενετικός, γενετικός [] "origin, source, birth") is the study of the evolutionary history and relationships among or within groups o ...
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 archosaurs.
Permian reptiles
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
Pelycosaur ( ) is an older term for basal or primitive Late Paleozoic synapsids, excluding the therapsids and their descendants. Previously, the term ''mammal-like reptile'' had been used, and pelycosaur was considered an order, but this is now ...
, such as ''Edaphosaurus
''Edaphosaurus'' (, meaning "pavement lizard" for dense clusters of teeth) is a genus of extinct edaphosaurid synapsids that lived in what is now North America and Europe around 303.4 to 272.5 million years ago, during the Late Carboniferous to ...
'' 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
Therapsida is a major group of eupelycosaurian synapsids that includes mammals, their ancestors and relatives. Many of the traits today seen as unique to mammals had their origin within early therapsids, including limbs that were oriented mo ...
.[ Colbert, E.H. & Morales, M. (2001): '' Colbert's Evolution of the Vertebrates: A History of the Backboned Animals Through Time''. 4th edition. John Wiley & Sons, Inc, New York. .]
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 survivors).
Early in the period, the modern reptiles, or crown-group reptiles, evolved and split into two main lineages: the Archosauromorpha (forebears of turtle
Turtles are an order of reptiles known as Testudines, characterized by a special shell developed mainly from their ribs. Modern turtles are divided into two major groups, the Pleurodira (side necked turtles) and Cryptodira (hidden necked t ...
s, crocodiles, and dinosaur
Dinosaurs are a diverse group of reptiles of the clade Dinosauria. They first appeared during the Triassic period, between 243 and 233.23 million years ago (mya), although the exact origin and timing of the evolution of dinosaurs is t ...
s) and the Lepidosauromorpha
Lepidosauromorpha (in PhyloCode known as ''Pan-Lepidosauria'') is a group of reptiles comprising all diapsids closer to lizards than to archosaurs (which include crocodiles and birds). The only living sub-group is the Lepidosauria, which cont ...
(predecessors of modern lizards and tuatara
Tuatara (''Sphenodon punctatus'') are reptiles endemic to New Zealand. Despite their close resemblance to lizards, they are part of a distinct lineage, the order Rhynchocephalia. The name ''tuatara'' is derived from the Māori language and m ...
s). Both groups remained lizard-like and relatively small and inconspicuous during the Permian.
Mesozoic reptiles
The close of the Permian saw the greatest mass extinction known (see the Permian–Triassic extinction event
The Permian–Triassic (P–T, P–Tr) extinction event, also known as the Latest Permian extinction event, the End-Permian Extinction and colloquially as the Great Dying, formed the boundary between the Permian and Triassic geologic periods, as ...
), 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
Archosauromorpha (Greek for "ruling lizard forms") is a clade of diapsid reptiles containing all reptiles more closely related to archosaurs (such as crocodilians and dinosaurs, including birds) rather than lepidosaurs (such as tuataras, liz ...
. 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
The Triassic ( ) is a geologic period and system (stratigraphy), system which spans 50.6 million years from the end of the Permian Period 251.902 million years ago (Year#Abbreviations yr and ya, Mya), to the beginning of the Jurassic Period 201.36 ...
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 dinosaur
Dinosaurs are a diverse group of reptiles of the clade Dinosauria. They first appeared during the Triassic period, between 243 and 233.23 million years ago (mya), although the exact origin and timing of the evolution of dinosaurs is t ...
s and pterosaur
Pterosaurs (; from Greek ''pteron'' and ''sauros'', meaning "wing lizard") is an extinct clade of flying reptiles in the order, Pterosauria. They existed during most of the Mesozoic: from the Late Triassic to the end of the Cretaceous (228 ...
s, as well as the ancestors of crocodiles. Since reptiles, first rauisuchia
"Rauisuchia" is a paraphyletic group of mostly large and carnivorous Triassic archosaurs. Rauisuchians are a category of archosaurs within a larger group called Pseudosuchia, which encompasses all archosaurs more closely related to crocodilians ...
ns and then dinosaurs, dominated the Mesozoic era, the interval is popularly known as the "Age of Reptiles". The dinosaurs also developed smaller forms, including the feather-bearing smaller theropods
Theropoda (; ), whose members are known as theropods, is a dinosaur clade that is characterized by hollow bones and three toes and claws on each limb. Theropods are generally classed as a group of saurischian dinosaurs. They were ancestrally ca ...
. In the Cretaceous
The Cretaceous ( ) is a geological period that lasted from about 145 to 66 million years ago (Mya). It is the third and final period of the Mesozoic Era, as well as the longest. At around 79 million years, it is the longest geological period of ...
period, these gave rise to the first true birds.
The sister group to Archosauromorpha is Lepidosauromorpha
Lepidosauromorpha (in PhyloCode known as ''Pan-Lepidosauria'') is a group of reptiles comprising all diapsids closer to lizards than to archosaurs (which include crocodiles and birds). The only living sub-group is the Lepidosauria, which cont ...
, containing lizards and tuatara
Tuatara (''Sphenodon punctatus'') are reptiles endemic to New Zealand. Despite their close resemblance to lizards, they are part of a distinct lineage, the order Rhynchocephalia. The name ''tuatara'' is derived from the Māori language and m ...
s, as well as their fossil relatives. Lepidosauromorpha contained at least one major group of the Mesozoic sea reptiles: the mosasaurs
Mosasaurs (from Latin ''Mosa'' meaning the 'Meuse', and Greek ' meaning 'lizard') comprise a group of extinct, large marine reptiles from the Late Cretaceous. Their first fossil remains were discovered in a limestone quarry at Maastricht on th ...
, which lived during the Cretaceous
The Cretaceous ( ) is a geological period that lasted from about 145 to 66 million years ago (Mya). It is the third and final period of the Mesozoic Era, as well as the longest. At around 79 million years, it is the longest geological period of ...
period. The phylogenetic placement of other main groups of fossil sea reptiles – the ichthyopterygia
Ichthyopterygia ("fish flippers") was a designation introduced by Sir Richard Owen in 1840 to designate the Jurassic ichthyosaurs that were known at the time, but the term is now used more often for both true Ichthyosauria and their more primitiv ...
ns (including ichthyosaurs) and the sauropterygia
Sauropterygia ("lizard flippers") is an extinct taxon of diverse, aquatic reptiles that developed from terrestrial ancestors soon after the end-Permian extinction and flourished during the Triassic before all except for the Plesiosauria became ...
ns, which evolved in the early Triassic – is more controversial. Different authors linked these groups either to lepidosauromorphs[Gauthier J.A. (1994): ''The diversification of the amniotes''. In: D.R. Prothero and R.M. Schoch (ed.) Major Features of Vertebrate Evolution: 129–159. Knoxville, Tennessee: The Paleontological Society.] or to archosauromorphs, and ichthyopterygians were also argued to be diapsids that did not belong to the least inclusive clade containing lepidosauromorphs and archosauromorphs.
Cenozoic reptiles
The close of the Cretaceous
The Cretaceous ( ) is a geological period that lasted from about 145 to 66 million years ago (Mya). It is the third and final period of the Mesozoic Era, as well as the longest. At around 79 million years, it is the longest geological period of ...
period saw the demise of the Mesozoic era reptilian megafauna (see the Cretaceous–Paleogene extinction event
The Cretaceous–Paleogene (K–Pg) extinction event (also known as the Cretaceous–Tertiary extinction) was a sudden mass extinction of three-quarters of the plant and animal species on Earth, approximately 66 million years ago. With the ...
, also known as K-T extinction event). Of the large marine reptiles, only sea turtle
Sea turtles (superfamily Chelonioidea), sometimes called marine turtles, are reptiles of the order Testudines and of the suborder Cryptodira. The seven existing species of sea turtles are the flatback, green, hawksbill, leatherback, loggerhe ...
s were left; and of the non-marine large reptiles, only the semi-aquatic crocodiles
Crocodiles (family Crocodylidae) or true crocodiles are large semiaquatic reptiles that live throughout the tropics in Africa, Asia, the Americas and Australia. The term crocodile is sometimes used even more loosely to include all extant memb ...
and broadly similar choristoderes survived the extinction, with last members of the latter, the lizard-like ''Lazarussuchus
''Lazarussuchus'' (meaning " Lazarus's crocodile") is an extinct genus of amphibious reptile, known from the Cenozoic of Europe. It is the youngest known member of Choristodera, an extinct order of aquatic reptiles that first appeared in the Midd ...
'', becoming extinct in the Miocene
The Miocene ( ) is the first epoch (geology), geological epoch of the Neogene Period and extends from about (Ma). The Miocene was named by Scottish geologist Charles Lyell; the name comes from the Greek words (', "less") and (', "new") and mea ...
. 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 Cenozoic. 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 large and giant tortoise
Tortoises () are reptiles of the family Testudinidae of the order Testudines (Latin: ''tortoise''). Like other turtles, tortoises have a shell to protect from predation and other threats. The shell in tortoises is generally hard, and like oth ...
s.
After the extinction of most archosaur and marine reptile lines by the end of the Cretaceous, reptile diversification continued throughout the Cenozoic. Squamates
Squamata (, Latin ''squamatus'', 'scaly, having scales') is the largest order of reptiles, comprising lizards, snakes, and amphisbaenians (worm lizards), which are collectively known as squamates or scaled reptiles. With over 10,900 species, it ...
took a massive hit during the K–Pg 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
Domestication is a sustained multi-generational relationship in which humans assume a significant degree of control over the reproduction and care of another group of organisms to secure a more predictable supply of resources from that group. A ...
species).
Morphology and physiology
Circulation
All Lepidosauria, lepidosaurs and turtle
Turtles are an order of reptiles known as Testudines, characterized by a special shell developed mainly from their ribs. Modern turtles are divided into two major groups, the Pleurodira (side necked turtles) and Cryptodira (hidden necked t ...
s have a three-chambered heart consisting of two atrium (heart), atria, one variably partitioned ventricle (heart), 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 fitness (biology), fitness advantage.
For example, Iguana hearts, like the majority of the squamates
Squamata (, Latin ''squamatus'', 'scaly, having scales') is the largest order of reptiles, comprising lizards, snakes, and amphisbaenians (worm lizards), which are collectively known as squamates or scaled reptiles. With over 10,900 species, it ...
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 Atrium (heart), left atrium, the Atrium (heart), right atrium, the Heart valve, atrioventricular valve, the cavum venosum, cavum arteriosum, the cavum pulmonale, the muscular ridge, the ventricular ridge, pulmonary 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 cardiac cycle, ventricular diastole and completely divides it during systole (medicine), ventricular systole. Because of this ridge, some of these squamata, squamates are capable of producing ventricular pressure differentials that are equivalent to those seen in mammalian and avian hearts.
Crocodilia
Crocodilia (or Crocodylia, both ) is an order of mostly large, predatory, semiaquatic reptiles, known as crocodilians. They first appeared 95 million years ago in the Late Cretaceous period ( Cenomanian stage) and are the closest livi ...
ns have an anatomically four-chambered heart, similar to birds, but also have two systemic aortas and are therefore capable of bypassing their pulmonary circulation.
Metabolism
Modern non-avian reptiles exhibit some form of Ectotherm, 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 dinosaur
Dinosaurs are a diverse group of reptiles of the clade Dinosauria. They first appeared during the Triassic period, between 243 and 233.23 million years ago (mya), although the exact origin and timing of the evolution of dinosaurs is t ...
s 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 Thermoregulation, 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.
Respiratory system
All reptiles breathe using lungs. Aquatic turtle
Turtles are an order of reptiles known as Testudines, characterized by a special shell developed mainly from their ribs. Modern turtles are divided into two major groups, the Pleurodira (side necked turtles) and Cryptodira (hidden necked t ...
s 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. Lung ventilation is accomplished differently in each main reptile group. In squamata, 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. Tupinambis, Tegu lizards are known to possess a proto-thoracic diaphragm, 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.
Crocodilia
Crocodilia (or Crocodylia, both ) is an order of mostly large, predatory, semiaquatic reptiles, known as crocodilians. They first appeared 95 million years ago in the Late Cretaceous period ( Cenomanian stage) and are the closest livi ...
ns 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 "Liver, hepatic piston". The Bronchus, airways 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 iguanas.
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 struggling prey). Skinks (family Skink, 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
How turtle, 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 Breathing, breathe. The varied results indicate that turtles and tortoises have found a variety of solutions to this problem.
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 (''Indian flapshell turtle, 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 thoracic diaphragm, 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 their shells.[
]
Sound production
Compared with frogs, birds, and mammals, reptiles are less vocal. Sound production is usually limited to wikt:hiss, hissing, which is produced merely by forcing air though a partly closed glottis and is not considered to be a true vocalization. The ability to vocalize exists in crocodilians, some lizards and turtles; and typically involves vibrating fold-like structures in the larynx or glottis. Some geckos and turtles possess true vocal cords, which have elastin-rich connective tissue.
Hearing in snakes
Hearing in humans relies on 3 parts of the ear; the outer ear that directs sound waves into the ear canal, the middle ear that transmits incoming sound waves to the inner ear, and the inner ear that helps in hearing and keeping your balance. Unlike humans and other animals, snakes do not possess an outer ear, a middle ear, and a Tympanum (anatomy), tympanum but have an inner ear structure with Cochlea, cochleas directly connected to their jawbone. They are able to feel the vibrations generated from the sound waves in their jaw as they move on the ground. This is done by the use of Mechanoreceptor, mechanoreceptors, sensory nerves that run along the body of snakes directing the vibrations along the spinal nerves to the brain. Snakes have a sensitive auditory perception and can tell which direction sound being made is coming from so that they can sense the presence of prey or predator but it is still unclear how sensitive snakes are to sound waves traveling through the air.
Skin
Reptilian skin is covered in a horny Epidermis (zoology), 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 dermis, dermal layer that produces leather in mammals.
Exposed parts of reptiles are protected by reptile scales, scales or scutes, sometimes with a bony base (osteoderms), forming armour (zoology), armor. In lepidosaurians, such as lizards and snakes, the whole skin is covered in overlapping epidermis (zoology), 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. The scales found in turtles and crocodiles are of dermis, dermal, rather than epidermal, origin and are properly termed scutes. In turtles, the body is hidden inside a hard shell composed of fused scutes.
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. Proteolysis, Proteolytic enzymes and Lymph, 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
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 (anatomy), colon to aid in the reabsorption of water. Some are also able to take up water stored in the Urinary bladder, bladder. Excess salts are also excreted by nasal and lingual salt glands in some reptiles.
In all reptiles the urinogenital ducts and the anus both empty into an organ called a cloaca. In some reptiles, a midventral wall in the cloaca may open into a urinary bladder, but not all. It is present in all turtles and tortoises as well as most lizards, but is lacking in the monitor lizard, the legless lizards. It is absent in the snakes, alligators, and crocodiles.
Many turtles, tortoises, and lizards have proportionally very large bladders. Charles Darwin noted that the Galapagos tortoise had a bladder which could store up to 20% of its body weight. Such adaptations are the result of environments such as remote islands and deserts where water is very scarce. Other desert-dwelling reptiles have large bladders that can store a long-term reservoir of water for up to several months and aid in osmoregulation.
Turtles have two or more accessory urinary bladders, located lateral to the neck of the urinary bladder and dorsal to the pubis, occupying a significant portion of their body cavity. Their bladder is also usually bilobed with a left and right section. The right section is located under the liver, which prevents large stones from remaining in that side while the left section is more likely to have Bladder stone (animal), calculi.
Digestion
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
Mammals () are a group of vertebrate animals constituting the class Mammalia (), characterized by the presence of mammary glands which in females produce milk for feeding (nursing) their young, a neocortex (a region of the brain), fur o ...
, 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 ]dinosaur
Dinosaurs are a diverse group of reptiles of the clade Dinosauria. They first appeared during the Triassic period, between 243 and 233.23 million years ago (mya), although the exact origin and timing of the evolution of dinosaurs is t ...
s. Today, turtle
Turtles are an order of reptiles known as Testudines, characterized by a special shell developed mainly from their ribs. Modern turtles are divided into two major groups, the Pleurodira (side necked turtles) and Cryptodira (hidden necked t ...
s are the only predominantly herbivorous reptile group, but several lines of Agamidae, agamas and Iguanidae, 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 Sailing ballast, 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.
]
Nerves
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
Snakes are elongated, limbless, carnivorous reptiles of the suborder Serpentes . Like all other squamates, snakes are ectothermic, amniote vertebrates covered in overlapping scales. Many species of snakes have skulls with several more j ...
'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.
Intelligence
Reptiles are generally considered less intelligent than mammals and birds. The Brain-to-body mass ratio, 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 monitor lizard, monitors, are known to exhibit complex behavior, including cooperation and cognitive abilities allowing them to optimize their foraging and Territory (animal), territoriality over time. 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 mazes. Another study found that giant tortoises are capable of learning through operant conditioning, visual discrimination and retained learned behaviors with long-term memory. Sea turtles have been regarded as having simple brains, but their flippers are used for a variety of foraging tasks (holding, bracing, corralling) in common with marine mammals.
Vision
Most reptiles are Diurnality, 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.
Reptiles usually have excellent vision, allowing them to detect shapes and motions at long distances. They often have only a few Rod cells and have poor vision in low-light conditions. At the same time they have cells called "double Cone cell, cones" which give them sharp color vision and enable them to see ultraviolet wavelengths. In some species, such as blind snakes, vision is reduced.
Many Lepidosauria, lepidosaurs have a photosensory organ on the top of their heads called the parietal eye, which are also called third eye, pineal eye
A parietal eye, also known as a third eye or pineal eye, is a part of the epithalamus present in some vertebrates. The eye is located at the top of the head, is photoreceptive and is associated with the pineal gland, regulating circadian rhythm ...
or pineal gland. This "eye" does not work the same way as a normal eye does as it has only a rudimentary retina and lens and thus, cannot form images. It is, however, sensitive to changes in light and dark and can detect movement.[
Some snakes have extra sets of visual organs (in the loosest sense of the word) in the form of Loreal pit, pits sensitive to infrared radiation (heat). Such heat-sensitive pits are particularly well developed in the pit vipers, but are also found in boidae, boas and pythonidae, pythons. These pits allow the snakes to sense the body heat of birds and mammals, enabling pit vipers to hunt rodents in the dark.
Most reptiles, as well as birds, possess a nictitating membrane, a translucent third eyelid which is drawn over the eye from the inner corner. In crocodilians , it protects its eyeball surface while allowing a degree of vision underwater. However, many squamates, geckos and snakes in particular, lack eyelids, which are replaced by a transparent scale. This is called the brille, spectacle, or eyecap. The brille is usually not visible, except for when the snake molts, and it protects the eyes from dust and dirt.
]
Reproduction
Reptiles generally sexual reproduction, 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 hemipenis, 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 development. 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, use 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 the Early Permian
The Permian ( ) is a geologic period and System (stratigraphy), stratigraphic system which spans 47 million years from the end of the Carboniferous Period million years ago (Mya), to the beginning of the Triassic Period 251.9 Mya. It is the last ...
mesosaur
Mesosaurs ("middle lizards") were a group of small aquatic reptiles that lived during the early Permian period, roughly 299 to 270 million years ago. Mesosaurs were the first known aquatic reptiles, having apparently returned to an aquatic life ...
s, 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 mosasaur
Mosasaurs (from Latin ''Mosa'' meaning the 'Meuse', and Greek ' meaning 'lizard') comprise a group of extinct, large marine reptiles from the Late Cretaceous. Their first fossil remains were discovered in a limestone quarry at Maastricht on ...
s, ichthyosaurs, and Sauropterygia
Sauropterygia ("lizard flippers") is an extinct taxon of diverse, aquatic reptiles that developed from terrestrial ancestors soon after the end-Permian extinction and flourished during the Triassic before all except for the Plesiosauria became ...
, a group that include pachypleurosaurs and Plesiosauria.
Asexual reproduction has been identified in squamata, 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 teiidae, teiids (especially ''Aspidocelis'') and lacertidae, lacertids (''Lacerta (genus), Lacerta''). In captivity, Komodo dragons (Varanidae) have reproduced by parthenogenesis.
Parthenogenetic species are suspected to occur among chameleons, agamidae, agamids, night lizard, xantusiids, and typhlopidae, 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.
Defense mechanisms
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,Conflict avoidance, 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
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 Crypsis, 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 off approaching danger.
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 Batesian mimicry.
Alternative defense in snakes
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 species, like cobras or hognose snakes, 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 snake venom, 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
The grass snake (''Natrix natrix''), sometimes called the ringed snake or water snake, is a Eurasian non-venomous colubrid snake. It is often found near water and feeds almost exclusively on amphibians.
Subspecies
Many subspecies are recognized ...
, thanatosis, 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 tongue. If this does not 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.
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 tooth, canine-like teeth. These are used primarily for seizing prey, but are also used in fighting and display.
Shedding and regenerating tails
Geckos, skinks, and some 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 thrash, 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. The tail grows back in most species, but some, like crested geckos, lose their tails for the rest of their lives. 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 Tiliqua rugosa, 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 front part.
Reptiles that are capable of shedding their tails can partially regeneration (biology), 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
In cultures and religions
Dinosaurs have been widely depicted in culture since the English palaeontologist Richard Owen coined the name ''dinosaur
Dinosaurs are a diverse group of reptiles of the clade Dinosauria. They first appeared during the Triassic period, between 243 and 233.23 million years ago (mya), although the exact origin and timing of the evolution of dinosaurs is t ...
'' in 1842. As soon as 1854, the Crystal Palace Dinosaurs were on display to the public in south London.[Torrens, Hugh. "Politics and Paleontology". ''The Complete Dinosaur,'' 175–190.] One dinosaur appeared in literature even earlier, as 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 documentary film, 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 Serpent (symbolism), symbolic role in different cultures. In History of Egypt, Egyptian history, the Nile cobra adorned the crown of the pharaoh. It was snake worship, worshipped as one of the gods and was also used for sinister purposes: murder of an adversary and ritual suicide (Cleopatra VII of Egypt, Cleopatra). In Greek mythology snakes are associated with deadly antagonists, as a chthonic symbol, roughly translated as ''earthbound''. The nine-headed Lernaean Hydra that Hercules defeated and the three Gorgon sisters are children of Gaia (mythology), Gaia, the earth. Medusa was one of the three Gorgon sisters who Perseus defeated. 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 Titan (mythology), 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, snake worship, snakes are worshipped as gods, with many women pouring milk on snake pits. The cobra is seen on the neck of Shiva, while 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 from 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 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.[Plotkin, Pamela, T., 2007, ''Biology and Conservation of Ridley Sea Turtles'', Johns Hopkins University, .] A tortoise's longevity is suggested by its long lifespan and its shell, which was thought to protect it from any foe.[Ball, Catherine, 2004, ''Animal Motifs in Asian Art'', Courier Dover Publications, .] In the cosmological myths of several cultures a ''World Turtle'' carries the world upon its back or supports the heavens.[Stookey, Lorena Laura, 2004, ''Thematic Guide to World Mythology'', Greenwood Press, .]
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.
Lizards such as the Gila monster produce toxins with medical applications. Gila toxin reduces plasma glucose; the substance is now synthesised for use in the anti-diabetes drug exenatide (Byetta). Another toxin from Gila monster saliva has been studied for use as an anti-Alzheimer's drug.
Geckos have also been used as medicine, especially in China. Turtles have been used in Chinese traditional medicine for thousands of years, with every part of the turtle believed to have medical benefits. There is a lack of scientific evidence that would correlate claimed medical benefits to turtle consumption. Growing demand for turtle meat has placed pressure on vulnerable wild populations of turtles.
Commercial farming
Crocodiles are protected in many parts of the world, and are crocodile farm, 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 purposes.
Snakes are also farmed, primarily in East Asia, East and Southeast Asia, and their production has become more intensive in the last decade. Snake farming has been troubling for conservation in the past as it can lead to overexploitation of wild snakes and their natural prey to supply the farms. However, farming snakes can limit the hunting of wild snakes, while reducing the slaughter of higher-order vertebrates like cows. The energy efficiency of snakes is higher than expected for carnivores, due to their ectothermy and low metabolism. Waste protein from the poultry and pig industries is used as feed in snake farms. Snake farms produce meat, Snakeskin, snake skin, and antivenom.
Turtle farming is another known but controversial practice. Turtles have been farmed for a variety of reasons, ranging from food to traditional medicine, the pet trade, and scientific conservation. Demand for turtle meat and medicinal products is one of the main threats to turtle conservation in Asia. Though commercial breeding would seem to insulate wild populations, it can stoke the demand for them and increase wild captures. Even the potentially appealing concept of raising turtles at a farm to release into the wild is questioned by some veterinarians who have had some experience with farm operations. They caution that this may introduce into the wild populations infectious diseases that occur on the farm, but have not (yet) been occurring in the wild.
Reptiles in captivity
In the Western world, some snakes (especially relatively docile species such as the Python regius, ball python and corn snake) are sometimes kept as pets. Numerous species of lizard are kept as pets, including Pogona, bearded dragons, iguanas, anoles, and geckos (such as the popular leopard gecko and the crested gecko).
Turtles and tortoises are increasingly popular pets, but keeping them can be challenging due to their particular requirements, such as temperature control, the need for UV light sources, and a varied diet. The long lifespans of turtles and especially tortoises mean they can potentially outlive their owners. Good hygiene and significant maintenance is necessary when keeping reptiles, due to the risks of ''Salmonella'' and other pathogens. Regular hand-washing after handling is an important measure to prevent infection.
A herpetarium is a zoological exhibition space for reptiles and amphibians.
See also
* List of reptiles
* Lists of reptiles by region
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Further reading
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Notes
References
External links
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Reptile Phylogeny
Reptile images
Sri Lanka Wild Life Information Database
''Biology of the Reptilia''
is an online copy of the full text of a 22 volume 13,000 page summary of the state of research of reptiles.
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Reptiles,
Extant Pennsylvanian first appearances
Paraphyletic groups
Articles containing video clips