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Amniotes (from Greek ἀμνίον ''amnion'', "membrane surrounding the fetus", earlier "bowl in which the blood of sacrificed animals was caught", from ἀμνός ''amnos'', "lamb") are a
clade A clade (), also known as a monophyletic group or natural group, is a group of organisms that are monophyly, monophyletic – that is, composed of a common ancestor and all its lineage (evolution), lineal descendants - on a phylogenetic tree. R ...

clade
of
tetrapod Tetrapods (; ) are four-limbed animals constituting the superclass Tetrapoda (). It includes extant Extant is the opposite of the word extinct Extinction is the termination of a kind of organism or of a group of kinds (taxon), usually a sp ...
vertebrates Vertebrates () comprise all species of animal Animals (also called Metazoa) are multicellular A multicellular organism is an organism In biology, an organism () is any organic, life, living system that functions as an indiv ...

vertebrates
comprising
reptile 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 ...

reptile
s (class Reptilia),
bird Birds are a group of warm-blooded vertebrate Vertebrates () comprise all species of animal Animals (also called Metazoa) are multicellular eukaryotic organisms that form the Kingdom (biology), biological kingdom Animalia. With ...

bird
s (class Aves) and
mammal Mammals (from Latin Latin (, or , ) is a classical language A classical language is a language A language is a structured system of communication Communication (from Latin ''communicare'', meaning "to share" or "to be i ...
s (class Mammalia). Amniotes lay their
eggs Egg An egg is the organic vessel containing the in which an develops until it can survive on its own, at which point the animal hatches. An egg results from of an . Most s, (excluding s), and lay eggs, although some, such as s, do ...
on land or retain the fertilized egg within the mother, and are distinguished from the
anamniotes The anamniotes are an informal group comprising the fishes and the amphibians, the so-called "lower vertebrates", which lay their eggs in water. They are distinguished from the amniotes, the "higher vertebrates" ( reptiles, bird Birds ...
(
fishes Fish are aquatic Aquatic means relating to water Water (chemical formula H2O) is an inorganic, transparent, tasteless, odorless, and nearly colorless chemical substance, which is the main constituent of Earth's hydrosphere and the ...

fishes
and
amphibians Amphibians are ectotherm File:Junonia lemonias DSF by Kadavoor.JPG, ''Junonia lemonias'' is basking under the sun. An ectotherm (from the Ancient Greek, Greek ἐκτός (''ektós'') "outside" and θερμός (''thermós'') "hot") is an o ...

amphibians
), which typically lay their eggs in water. Older sources, particularly prior to the 20th century, may refer to amniotes as "higher vertebrates" and anamniotes as "lower vertebrates", based on the discredited idea of the evolutionary great chain of being. Amniotes are
tetrapod Tetrapods (; ) are four-limbed animals constituting the superclass Tetrapoda (). It includes extant Extant is the opposite of the word extinct Extinction is the termination of a kind of organism or of a group of kinds (taxon), usually a sp ...
s (descendants of four-limbed and backboned animals) that are characterised by having an
egg An egg is the organic vessel containing the in which an develops until it can survive on its own, at which point the animal hatches. An egg results from of an . Most s, (excluding s), and lay eggs, although some, such as s, do not. eg ...
equipped with an
amnion :''For the alien race in 's , see .'' The amnion is a membrane that closely covers the when first formed. It fills with the which causes the amnion to expand and become the which serves to provide a protective environment for the developing em ...

amnion
, an adaptation to lay eggs on land rather than in water as the anamniotes (including frogs) typically do. Amniotes include
synapsid Synapsids are a group of animals that includes mammal Mammals (from Latin Latin (, or , ) is a classical language belonging to the Italic branch of the Indo-European languages. Latin was originally spoken in the area around Rome ...

synapsid
s (
mammal Mammals (from Latin Latin (, or , ) is a classical language A classical language is a language A language is a structured system of communication Communication (from Latin ''communicare'', meaning "to share" or "to be i ...
s along with their extinct kin) and
sauropsids Sauropsida ("lizard faces") is a clade A clade (; from grc, , ''klados'', "branch"), also known as a monophyletic group or natural group, is a group of organisms that are monophyly, monophyletic—that is, composed of a common ancestor and al ...
(
reptile 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 ...

reptile
s and
bird Birds are a group of warm-blooded vertebrate Vertebrates () comprise all species of animal Animals (also called Metazoa) are multicellular eukaryotic organisms that form the Kingdom (biology), biological kingdom Animalia. With ...

bird
s), as well as their ancestors, back to amphibians. Amniote embryos, whether laid as eggs or carried by the female, are protected and aided by several extensive membranes. In
eutheria Eutheria (; from Greek , 'good, right' and , 'beast'; ) is the clade consisting of all Theria, therian mammals that are more closely related to Placentalia, placentals than to Marsupial, marsupials. Eutherians are distinguished from noneuther ...

eutheria
n mammals (such as humans), these membranes include the
amniotic sac The amniotic sac, commonly called the bag of waters, sometimes the membranes, is the sac in which the fetus A fetus or foetus (; plural fetuses, feti, foetuses, or foeti) is the unborn offspring that develops from an animal embryo An em ...

amniotic sac
that surrounds the
fetus A fetus or foetus (; plural fetuses, feti, foetuses, or foeti) is the unborn offspring that develops from an animal embryo An embryo is the early stage of development of a multicellular organism A multicellular organism is an organism tha ...
. These embryonic membranes and the lack of a
larval stage A larva (plural larvae ) is a distinct juvenile form many animal Animals (also called Metazoa) are multicellular A multicellular organism is an organism In biology, an organism () is any organic, life, living system that f ...
distinguish amniotes from tetrapod
amphibian Amphibians are ectothermic, tetrapod vertebrates of the Class (biology), class Amphibia. All living amphibians belong to the group Lissamphibia. They inhabit a wide variety of habitats, with most species living within terrestrial animal, ter ...
s. The first amniotes, referred to as "basal amniotes", resembled small lizards and evolved from the amphibian
reptiliomorphs Reptiliomorpha 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 defined by Michel Laurin (2001) and Vallin and Laurin (2004) as the ...

reptiliomorphs
about 312 million years ago, in the
Carboniferous The Carboniferous ( ) is a geologic period The geologic time scale (GTS) is a system of chronological dating that classifies Geology, geological strata (stratigraphy) in time. It is used by geologists, paleontology, paleontologists, and other ...
geologic period The geologic time scale (GTS) is a system of chronological dating that classifies Geology, geological strata (stratigraphy) in time. It is used by geologists, paleontology, paleontologists, and other earth sciences, Earth scientists to describe t ...
. Their eggs could survive out of the water, allowing amniotes to branch out into drier environments. The eggs could also "breathe" and cope with wastes, allowing the eggs and the amniotes themselves to evolve into larger forms. The amniotic egg represents a critical divergence within the vertebrates, one enabling amniotes to reproduce on dry land—free of the need to return to water for reproduction as required of the amphibians. From this point the amniotes spread around the globe, eventually to become the dominant land vertebrates. Very early in their evolutionary history, basal amniotes diverged into two main lines, the
synapsid Synapsids are a group of animals that includes mammal Mammals (from Latin Latin (, or , ) is a classical language belonging to the Italic branch of the Indo-European languages. Latin was originally spoken in the area around Rome ...

synapsid
s and the
sauropsid Sauropsida ("lizard faces") is a clade of Amniote, amniotes, broadly equivalent to the Class (biology), class Reptile, Reptilia. Sauropsida is the Sister group, sister taxon to Synapsid, Synapsida, the clade of amniotes which includes Mammal, mamma ...
s, both of which persist into the modern era. The oldest known fossil synapsid is '' Protoclepsydrops'' from about 312 million years ago, while the oldest known sauropsid is probably ''
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 a nocturn ...

Paleothyris
'', in the order Captorhinida, from the
Middle Pennsylvanian The Pennsylvanian ( , also known as Upper Carboniferous or Late Carboniferous) is, in the ICS geologic timescale The geologic time scale (GTS) is a system of chronological dating that classifies Geology, geological strata (stratigraphy) in ti ...
epoch (c. 306–312 million years ago).


Description

Zoologists characterize amniotes in part by
embryonic development An embryo is the early stage of development of a multicellular organism A multicellular organism is an organism In biology, an organism () is any organic, life, living system that functions as an individual entity. All organisms ar ...

embryonic development
that includes the formation of several extensive membranes, the
amnion :''For the alien race in 's , see .'' The amnion is a membrane that closely covers the when first formed. It fills with the which causes the amnion to expand and become the which serves to provide a protective environment for the developing em ...

amnion
,
chorion The chorion is the outermost fetal membrane The fetal membranes are biological membrane, membranes associated with the developing fetus. The two chorioamniotic membranes are the amnion and the chorion, which make up the amniotic sac that surroun ...

chorion
, and
allantois The allantois (plural ''allantoides'' or ''allantoises'') is a hollow sac-like structure filled with clear fluid that forms part of a developing amniote's conceptus (which consists of all embryonic and extra-embryonic tissues). It helps the embryo ...
. Amniotes develop directly into a (typically) terrestrial form with limbs and a thick stratified
epithelium Epithelium is one of the four basic types of animal Animals (also called Metazoa) are multicellular A multicellular organism is an organism In biology, an organism () is any organic, life, living system that functions as ...
(rather than first entering a feeding larval
tadpole A tadpole is the larval stage A larva (plural larvae ) is a distinct juvenile form many animal Animals (also called Metazoa) are multicellular eukaryotic organisms that form the Kingdom (biology), biological kingdom Animalia. With ...
stage followed by
metamorphosis Metamorphosis is a biological process Biological processes are those processes that are vital for an organism In biology, an organism () is any organic, life, living system that functions as an individual entity. All organisms are ...
, as
amphibian Amphibians are ectothermic, tetrapod vertebrates of the Class (biology), class Amphibia. All living amphibians belong to the group Lissamphibia. They inhabit a wide variety of habitats, with most species living within terrestrial animal, ter ...
s do). In amniotes, the transition from a two-layered periderm to a
cornified Keratin () is one of a family of fibrous structural proteins known as scleroproteins. '' α-Keratin'' is a type of keratin found in vertebrates. It is the key structural material making up scales, hair Hair is a protein filament that g ...
epithelium is triggered by
thyroid hormone File:Thyroid_system.svg, upright=1.5, The thyroid The thyroid, or thyroid gland, is an endocrine gland in vertebrates. In humans it is in the neck and consists of two connected lobe (anatomy), lobes. The lower two thirds of the lobes are connec ...
during embryonic development, rather than by metamorphosis. The unique embryonic features of amniotes may reflect specializations for eggs to survive drier environments; or the increase in size and yolk content of eggs may have permitted, and coevolved with, direct development of the embryo to a large size.


Adaptions for terrestrial living

Features of amniotes evolved for survival on land include a sturdy but porous leathery or hard
eggshell An eggshell is the outer covering of a hard-shelled egg An egg is the organic vessel containing the in which an develops until it can survive on its own, at which point the animal hatches. An egg results from of an . Most s, (excludi ...
and an
allantois The allantois (plural ''allantoides'' or ''allantoises'') is a hollow sac-like structure filled with clear fluid that forms part of a developing amniote's conceptus (which consists of all embryonic and extra-embryonic tissues). It helps the embryo ...
that facilitates
respiration Respiration may refer to: Biology * Cellular respiration, the process in which nutrients are converted into useful energy in a cell ** Anaerobic respiration, cellular respiration without oxygen ** Maintenance respiration, the amount of cellular ...
while providing a reservoir for disposal of wastes. Their
kidney The kidneys are two reddish-brown bean-shaped organs An organ is a group of tissues with similar functions. Plant life and animal life rely on many organs that co-exist in organ systems. A given organ's tissues can be broadly categorized ...

kidney
s and
large intestine The large intestine, also known as the large bowel, is the last part of the gastrointestinal tract The gastrointestinal tract (GI tract, digestive tract, alimentary canal) is the tract or passageway of the digestive system The human dig ...

large intestine
s are also well-suited to water retention. Most mammals do not lay eggs, but corresponding structures develop inside the
placenta The placenta is a temporary fetal organ Organ may refer to: Biology * Organ (anatomy) An organ is a group of Tissue (biology), tissues with similar functions. Plant life and animal life rely on many organs that co-exist in organ systems. ...

placenta
. The ancestors of true amniotes, such as '' Casineria kiddi'', which lived about 340 million years ago, evolved from
amphibian Amphibians are ectothermic, tetrapod vertebrates of the Class (biology), class Amphibia. All living amphibians belong to the group Lissamphibia. They inhabit a wide variety of habitats, with most species living within terrestrial animal, ter ...
reptiliomorphs Reptiliomorpha 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 defined by Michel Laurin (2001) and Vallin and Laurin (2004) as the ...

reptiliomorphs
and resembled small
lizards Lizards (suborder In biological classification, the order ( la, wikt:ordo#Latin, ordo) is # a taxonomic rank used in the classification of organisms and recognized by the nomenclature codes. The well-known ranks in descending order are: lif ...

lizards
. At the late Devonian mass extinction (360 million years ago), all known tetrapods were essentially aquatic and fish-like. Because the reptiliomorphs were already established 20 million years later when all their fishlike relatives were extinct, it appears they separated from the other tetrapods somewhere during Romer's gap, when the adult tetrapods became fully terrestrial (some forms would later become secondarily aquatic). The modest-sized ancestors of the amniotes laid their eggs in moist places, such as depressions under fallen logs or other suitable places in the Carboniferous swamps and forests; and dry conditions probably do not account for the emergence of the soft shell. Indeed, many modern-day amniotes require moisture to keep their eggs from desiccating. Although some modern amphibians lay eggs on land, all amphibians lack advanced traits like an amnion. The amniotic egg formed through a series of evolutionary steps. After
internal fertilization Internal fertilization is the union of an egg An egg is the organic vessel containing the in which an develops until it can survive on its own, at which point the animal hatches. An egg results from of an . Most s, (excluding s), and ...
and the habit of laying eggs in terrestrial environments became a reproduction strategy amongst the amniote ancestors, the next major breakthrough appears to have involved a gradual replacement of the gelatinous coating covering the amphibian egg with a fibrous shell membrane. This allowed the egg to increase both its size and in the rate of gas exchange, permitting a larger, metabolically more active embryo to reach full development before hatching. Further developments, like
extraembryonic membraneAn extraembryonic membrane is one of the membranes which assist in the development of the embryo. Such membranes occur in a range of animals from humans to insects. They originate from the embryo, but are not considered part of it. They typically ...
s (amnion, chorion, and allantois) and a calcified shell, were not essential and probably evolved later. It has been suggested that shelled terrestrial eggs without extraembryonic membranes could still not have been more than about 1 cm (0.4 inch) in diameter because of diffusion problems, like the inability to get rid of carbon dioxide if the egg was larger. The combination of small eggs and the absence of a larval stage, where posthatching growth occurs in anamniotic tetrapods before turning into juveniles, would limit the size of the adults. This is supported by the fact that extant squamate species that lay eggs less than 1 cm in diameter have adults whose snout-vent length is less than 10 cm. The only way for the eggs to increase in size would be to develop new internal structures specialized for respiration and for waste products. As this happened, it would also affect how much the juveniles could grow before they reached adulthood.


The egg membranes

Fish and amphibian eggs have only one inner membrane, the embryonic membrane. Evolution of the amniote egg required increased exchange of gases and wastes between the embryo and the atmosphere. Structures to permit these traits allowed further adaption that increased the feasible size of amniote eggs and enabled breeding in progressively drier habitats. The increased size of eggs permitted increase in size of offspring and consequently of adults. Further growth for the latter, however, was limited by their position in the terrestrial food-chain, which was restricted to level three and below, with only
invertebrate Invertebrates are animals that neither possess nor develop a vertebral column (commonly known as a ''backbone'' or ''spine''), derived from the notochord. This includes all animals apart from the chordata, chordate subphylum vertebrate, Vertebra ...
s occupying level two. Amniotes would eventually experience
adaptive radiation In evolutionary biology Evolutionary biology is the subfield of biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular ...
s when some species evolved the ability to digest plants and new
ecological niche In ecology Ecology (from el, οἶκος, "house" and el, -λογία, label=none, "study of") is the study of the relationships between living organisms, including humans, and their physical environment. Ecology considers organisms ...

ecological niche
s opened up, permitting larger body-size for herbivores, omnivores and predators.


Amniote traits

While the early amniotes resembled their amphibian ancestors in many respects, a key difference was the lack of an
otic notch Otic notches are invaginations in the posterior margin of the skull roof The skull roof, or the roofing bones of the skull The skull is a bone A bone is a Stiffness, rigid tissue (anatomy), tissue that constitutes part of the vertebrate s ...
at the back margin of the
skull roof The skull roof, or the roofing bones of the skull The skull is a bone A bone is a Stiffness, rigid tissue (anatomy), tissue that constitutes part of the vertebrate skeleton in animals. Bones protect the various organs of the body, produce ...
. In their ancestors, this notch held a spiracle, an unnecessary structure in an animal without an aquatic larval stage. There are three main lines of amniotes, which may be distinguished by the structure of the
skull The skull is a bone A bone is a rigid tissue Tissue may refer to: Biology * Tissue (biology), an ensemble of similar cells that together carry out a specific function * ''Triphosa haesitata'', a species of geometer moth found in North A ...

skull
and in particular the number of temporal fenestrae (openings) behind each eye. In
anapsid An anapsid is an amniote Amniotes (from Greek ἀμνίον ''amnion'', "membrane surrounding the fetus", earlier "bowl in which the blood of sacrificed animals was caught", from ἀμνός ''amnos'', "lamb") are a clade A clade (; fro ...

anapsid
s, the ancestral condition, there are none, in
synapsid Synapsids are a group of animals that includes mammal Mammals (from Latin Latin (, or , ) is a classical language belonging to the Italic branch of the Indo-European languages. Latin was originally spoken in the area around Rome ...

synapsid
s (mammals and their extinct relatives) there is one, and most
diapsid Diapsids ("two arches") are a group of amniote tetrapods that developed two holes (temporal fenestra) in each side of their skulls about 300 million years ago during the late Carboniferous period. The diapsids are extremely diverse, and include all ...

diapsid
s (including
bird Birds are a group of warm-blooded vertebrate Vertebrates () comprise all species of animal Animals (also called Metazoa) are multicellular eukaryotic organisms that form the Kingdom (biology), biological kingdom Animalia. With ...

bird
s,
crocodilia Crocodilia (or Crocodylia, both ) is an order Order or ORDER or Orders may refer to: * Orderliness Orderliness is associated with other qualities such as cleanliness Cleanliness is both the abstract state of being clean and free from germs, ...

crocodilia
ns,
squamates Squamata (, Latin ''squamatus'' (“scaly, having scales”)) is the largest Order (biology), order of reptiles, comprising lizards, snakes and amphisbaenians (worm lizards), which are collectively known as squamates or scaled reptiles. With ove ...

squamates
, and
tuatara Tuatara (''Sphenodon punctatus'') are reptiles Endemism, endemic to New Zealand. Although resembling most lizards, they are part of a distinct lineage, the order Rhynchocephalia. Their name derives from the Māori language, and means "peaks on ...

tuatara
s), have two. Turtles were traditionally classified as anapsids because they lack fenestrae, but molecular testing firmly places them in the diapsid line of descent - they therefore secondarily lost their fenestrae. Post-cranial remains of amniotes can be identified from their
Labyrinthodont Labyrinthodontia (Greek language, Greek, 'maze-toothed') is an Extinction, extinct amphibian subclass, which constituted some of the dominant animals of late Paleozoic and early Mesozoic eras (about 390 to 150 million years ago). The group evolve ...

Labyrinthodont
ancestors by their having at least two pairs of
rib In vertebrate anatomy, ribs ( la, costae) are the long curved bones which form the rib cage, part of the axial skeleton. In most tetrapods, ribs surround the chest, enabling the lungs to expand and thus facilitate breathing by expanding the ches ...
s, a
sternum The sternum or breastbone is a long flat bone located in the central part of the chest. It connects to the ribs via cartilage and forms the front of the rib cage, thus helping to protect the heart, human lung, lungs, and major blood vessels from in ...

sternum
in the
pectoral girdle The shoulder girdle or pectoral girdle is the set of bones in the appendicular skeleton The appendicular skeleton is the portion of the skeleton The skeleton refers to the frames of support of animal bodies. There are several different skele ...
(some amniotes have lost it) and an
astragalus ''Astragalus'' is a large genus of over 3,000 species of herb In general use, herbs are a widely distributed and widespread group of plants, excluding vegetables Vegetables are parts of plants that are consumed by humans or other animals ...
bone in the
ankle The ankle, or the talocrural region, is the region where the foot The foot (plural: feet) is an anatomical Anatomy (Greek ''anatomē'', 'dissection') is the branch of biology Biology is the natural science that studies life and liv ...

ankle
.


Definition and classification

Amniota was first formally described by the embryologist
Ernst Haeckel Ernst Heinrich Philipp August Haeckel (; 16 February 1834 – 9 August 1919) was a German zoologist Zoology ()The pronunciation of zoology as is usually regarded as nonstandard, though it is not uncommon. is the branch of biology that stud ...

Ernst Haeckel
in 1866 on the presence of the
amnion :''For the alien race in 's , see .'' The amnion is a membrane that closely covers the when first formed. It fills with the which causes the amnion to expand and become the which serves to provide a protective environment for the developing em ...

amnion
, hence the name. A problem with this definition is that the trait (
apomorphy 279px, trait states. In phylogenetics, apomorphy and synapomorphy refer to derived characters of a clade A clade (; from grc, , ''klados'', "branch"), also known as a monophyletic group or natural group, is a group of organisms that are mon ...

apomorphy
) in question does not
fossil A fossil (from Classical Latin Classical Latin is the form of Latin language Latin (, or , ) is a classical language A classical language is a language A language is a structured system of communication used by humans, inc ...

fossil
ize, and the status of fossil forms has to be inferred from other traits.


Traditional classification

Classifications of the amniotes have traditionally recognised three
classes 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 differently f ...
based on major traits and
physiology Physiology (; ) is the scientific Science () is a systematic enterprise that builds and organizes knowledge Knowledge is a familiarity or awareness, of someone or something, such as facts A fact is an occurrence in the real world. ...
: Romer A S and Parsons T S (1985) ''The Vertebrate Body.'' (6th ed.) Saunders, Philadelphia.'' 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 — . * Class
Reptilia Reptiles are tetrapod Tetrapods (; from Greek 'four' and 'foot') are four-limbed animals constituting the superclass Tetrapoda . It includes extant and extinct amphibians, reptiles (including dinosaurs and therefore birds), and synapsids ...

Reptilia
(reptiles) ** Subclass
Anapsid An anapsid is an amniote Amniotes (from Greek ἀμνίον ''amnion'', "membrane surrounding the fetus", earlier "bowl in which the blood of sacrificed animals was caught", from ἀμνός ''amnos'', "lamb") are a clade A clade (; fro ...

Anapsid
a (" proto-reptiles", possibly including
turtle Turtles are an order Order, ORDER or Orders may refer to: * Orderliness Orderliness is a quality that is characterized by a person’s interest in keeping their surroundings and themselves well organized, and is associated with other qu ...

turtle
s) ** Subclass
Diapsid Diapsids ("two arches") are a group of amniote tetrapods that developed two holes (temporal fenestra) in each side of their skulls about 300 million years ago during the late Carboniferous period. The diapsids are extremely diverse, and include all ...

Diapsid
a (majority of reptiles, progenitors of
bird Birds are a group of warm-blooded vertebrate Vertebrates () comprise all species of animal Animals (also called Metazoa) are multicellular eukaryotic organisms that form the Kingdom (biology), biological kingdom Animalia. With ...

bird
s) ** Subclass Euryapsida (
plesiosaurs The Plesiosauria (; Greek Greek may refer to: Greece Anything of, from, or related to Greece Greece ( el, Ελλάδα, , ), officially the Hellenic Republic, is a country located in Southeast Europe. Its population is approximately 10.7 ...
,
placodonts Placodontia ("Tablet (pharmacy), Tablet tooth, teeth") is an Extinction, extinct order (biology), order of marine reptiles that lived during the Triassic period, becoming extinct at the end of the period. It is believed that they were part of Saur ...

placodonts
, and
ichthyosaurs Ichthyosaurs (Ancient Greek for "fish lizard" – ἰχθύς or ''ichthys'' meaning "fish" and or ''sauros'' meaning "lizard") are large extinct marine reptiles. Ichthyosaurs belong to the order known as Ichthyosauria or Ichthyopterygia ('fish fli ...

ichthyosaurs
) ** Subclass
Synapsid Synapsids are a group of animals that includes mammal Mammals (from Latin Latin (, or , ) is a classical language belonging to the Italic branch of the Indo-European languages. Latin was originally spoken in the area around Rome ...

Synapsid
a (mammal-like reptiles, progenitors of
mammal Mammals (from Latin Latin (, or , ) is a classical language A classical language is a language A language is a structured system of communication Communication (from Latin ''communicare'', meaning "to share" or "to be i ...
s) * Class
Aves Birds are a group of warm-blooded vertebrate Vertebrates () comprise all species of animal Animals (also called Metazoa) are multicellular A multicellular organism is an organism In biology, an organism () is any ...

Aves
(birds) ** Subclass Archaeornithes (reptile-like birds, progenitors of all other birds) ** Subclass
Enantiornithes The Enantiornithes, also known as enantiornithines or enantiornitheans in literature, are a group of extinct Avialae, avialans ("birds" in the broad sense), the most abundant and diverse group known from the Mesozoic era. Almost all retained teeth ...
(early birds with an alternative shoulder joint) ** Subclass
Hesperornithes Hesperornithes is an extinct and highly specialized group of aquatic avialans closely related to the ancestors of modern bird Birds are a group of warm-blooded vertebrates constituting the class (biology), class Aves , characterised by f ...
(toothed aquatic flightless birds) ** Subclass Ichthyornithes (toothed, but otherwise modern birds) ** Subclass
Neornithes Birds are a group of warm-blooded vertebrate Vertebrates () comprise all species of animal Animals (also called Metazoa) are multicellular eukaryotic organisms that form the Kingdom (biology), biological kingdom Animalia. With ...

Neornithes
(all living birds) * Class
Mammal Mammals (from Latin Latin (, or , ) is a classical language A classical language is a language A language is a structured system of communication Communication (from Latin ''communicare'', meaning "to share" or "to be i ...
ia (mammals) ** Subclass Prototheria ( Monotremata, egg-laying mammals) ** Subclass
Theria Theria (; Greek#REDIRECT Greek Greek may refer to: Greece Anything of, from, or related to Greece Greece ( el, Ελλάδα, , ), officially the Hellenic Republic, is a country located in Southeast Europe. Its population is approximatel ...
(
metatheria Metatheria is a mammalian clade A clade (), also known as a monophyletic group or natural group, is a group of organism In biology, an organism () is any organic, life, living system that functions as an individual entity. All or ...
,
marsupial Marsupials are any members of the mammal Mammals (from Latin Latin (, or , ) is a classical language A classical language is a language A language is a structured system of communication Communication (from Latin ' ...
s and
eutheria Eutheria (; from Greek , 'good, right' and , 'beast'; ) is the clade consisting of all Theria, therian mammals that are more closely related to Placentalia, placentals than to Marsupial, marsupials. Eutherians are distinguished from noneuther ...

eutheria
,
placental mammals Placentalia is one of the three extant subdivisions of the class of animals Mammalia; the other two are Monotremata and Marsupial Marsupials are any members of the mammal Mammals (from Latin language, Latin , 'breast') are a group ...
) This rather orderly scheme is the one most commonly found in popular and basic scientific works. It has come under critique from
cladistics Cladistics (; ) is an approach to biological classification In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular ...

cladistics
, as the class Reptilia is
paraphyletic In taxonomy Taxonomy (general) is the practice and science of classification of things or concepts, including the principles that underlie such classification. The term may also refer to a specific classification scheme. Originally used only ...
—it has given rise to two other classes not included in Reptilia.


Classification into monophyletic taxa

A different approach is adopted by writers who reject paraphyletic groupings. One such classification, by Michael Benton, is presented in simplified form below. *Series Amniota **Class
Synapsid Synapsids are a group of animals that includes mammal Mammals (from Latin Latin (, or , ) is a classical language belonging to the Italic branch of the Indo-European languages. Latin was originally spoken in the area around Rome ...

Synapsid
a ***A series of unassigned families, corresponding to
Pelycosauria The pelycosaurs (pronounced PEL-ih-ko-saurz) were previously considered an order, but are now only an informal grouping composed of basal or primitive Late Paleozoic The Paleozoic (or Palaeozoic) Era ( ; from the Greek ''palaiós'' (), "old ...
***Order ****Class
Mammal Mammals (from Latin Latin (, or , ) is a classical language A classical language is a language A language is a structured system of communication Communication (from Latin ''communicare'', meaning "to share" or "to be i ...
ia – mammals **Class
Reptilia Reptiles are tetrapod Tetrapods (; from Greek 'four' and 'foot') are four-limbed animals constituting the superclass Tetrapoda . It includes extant and extinct amphibians, reptiles (including dinosaurs and therefore birds), and synapsids ...

Reptilia
*** Subclass
Parareptilia Parareptilia ("at the side of reptiles") is a subclass or clade A clade (), also known as a monophyletic group or natural group, is a group of organisms that are monophyly, monophyletic – that is, composed of a common ancestor and all its ...
**** Family
Mesosaur Mesosaurs ("middle lizards") were a group of small aquatic reptiles that lived during the early Permian The Permian ( ) is a geologic period and stratigraphic system which spans 47 million years from the end of the Carboniferous period milli ...
idae **** Family
Millerettidae Millerettidae is an extinct Extinction is the termination of a kind of organism In biology, an organism (from Ancient Greek, Greek: ὀργανισμός, ''organismos'') is any individual contiguous system that embodies the Life#Biol ...
**** Family
Bolosauridae Bolosauridae is an extinct family (biology), family of ankyramorph parareptiles known from the latest Carboniferous (Gzhelian) or earliest Permian (Asselian) to the early Guadalupian Epoch (geology), epoch (latest Roadian stage) of North America, ...
**** Family
Procolophonidae Procolophonidae is an extinct family In human society, family (from la, familia) is a group of people related either by consanguinity (by recognized birth) or affinity (by marriage or other relationship). The purpose of families is to ma ...
**** Order
Pareiasauromorpha Pareiasauromorpha is a group of parareptilian amniotes Amniotes (from Greek ἀμνίον ''amnion'', "membrane surrounding the fetus", earlier "bowl in which the blood of sacrificed animals was caught", from ἀμνός ''amnos'', "lamb") ...
***** Family
Nycteroleteridae Nycteroleteridae is a family In human society, family (from la, familia) is a group of people related either by consanguinity (by recognized birth) or affinity (by marriage or other relationship). The purpose of families is to maintain t ...
***** Family
Pareiasaur Pareiasaurs (meaning "cheek lizards") were a clade of parareptiles comprising the family In human society, family (from la, familia) is a group of people related either by consanguinity (by recognized birth) or affinity (by marriage or ...
idae *** Subclass
Eureptilia Eureptilia ("true reptiles") is one of the two major clade A clade (), also known as a monophyletic group or natural group, is a group of organisms that are monophyly, monophyletic – that is, composed of a common ancestor and all its linea ...
**** Family
Captorhinidae Captorhinidae (also known as cotylosaurs) is one of the earliest and most basal reptile families, all members of which are extinct. Description '' Captorhinids are a clade of small to very large lizard-like reptiles that date from the late Carb ...
**** Infraclass
Diapsida Diapsids ("two arches") are a group of amniote Amniotes (from Greek ἀμνίον ''amnion'', "membrane surrounding the fetus", earlier "bowl in which the blood of sacrificed animals was caught", from ἀμνός ''amnos'', "lamb") are a cla ...
***** Family ***** Family
Weigeltisauridae Weigeltisauridae is a family (biology), family of gliding neodiapsid reptiles that lived during the Permian period. Fossils of weigeltisaurids have been found in Madagascar, Germany, Great Britain, and Russia. Possible weigeltisaurid fossils have ...
***** Order Younginiformes ***** Infraclass Neodiapsida ****** Order Testudinata ******* Suborder Testudines – turtles ****** Infraclass Lepidosauromorpha ******* Unnamed infrasubclass ******** Infraclass Ichthyosauria ******** Order Thalattosauria ******** Superorder Lepidosauriformes ********* Order Sphenodontida – tuatara ********* Order Squamata – lizards & snakes ******* Infrasubclass Sauropterygia ******** Order Placodontia ******** Order Eosauropterygia ********* Suborder Pachypleurosauria ********* Suborder Nothosauria ******** Order Plesiosauria ****** Infraclass Archosauromorpha ******* Family Trilophosauridae ******* Order Rhynchosauria ******* Order Protorosauria ******* Division Archosauriformes ******** Subdivision Archosauria ********* Infradivision Crurotarsi ********** Order Phytosauria† ********** Family Ornithosuchidae ********** Family Aetosaur, Stagonolepididae ********** Family Rauisuchidae ********** Superfamily Poposauroidea ********** Superorder Crocodylomorpha *********** Order Crocodilia, Crocodylia – crocodilians ********* Infradivision Avemetatarsalia ********** Infrasubdivision Ornithodira *********** Order Pterosauria *********** Family Lagerpetidae *********** Family Silesauridae *********** Superorder Dinosauria – dinosaurs ************Order Ornithischia ************Order Saurischia *************Suborder Theropoda - theropods **************Class Aves – birds


Phylogenetic classification

With the advent of cladistics, other researchers have attempted to establish new classes, based on phylogeny, but disregarding the physiological and anatomical unity of the groups. Unlike Benton, for example, Jacques Gauthier and colleagues forwarded a definition of Amniota in 1988 as "the most recent common ancestor of extant taxon, extant mammals and reptiles, and all its descendants".Gauthier, J., Kluge, A.G. and Rowe, T. (1988). "The early evolution of the Amniota." Pp. 103–155 in Benton, M.J. (ed.), ''The phylogeny and classification of the tetrapods, Volume 1: amphibians, reptiles, birds.'' Oxford: Clarendon Press. As Gauthier makes use of a crown group definition, Amniota has a slightly different content than the biological amniotes as defined by an apomorphy.


Cladogram

The cladistics#Cladograms, cladogram presented here illustrates the phylogeny (family tree) of amniotes, and follows a simplified version of the relationships found by Laurin & Reisz (1995). The cladogram covers the group as defined under Gauthier's definition. The inclusion of Testudines within Parareptilia is unsupported by more recent morphological phylogenetics, phylogenetic studies, which placed turtles firmly within
diapsid Diapsids ("two arches") are a group of amniote tetrapods that developed two holes (temporal fenestra) in each side of their skulls about 300 million years ago during the late Carboniferous period. The diapsids are extremely diverse, and include all ...

diapsid
s. All Molecular phylogenetics, molecular studies have also strongly upheld the placement of turtles within diapsids. Within diapsids, some place turtles within Archosauria, or, more commonly, as a sister group to extant archosaurs, though an analysis conducted by Lyson ''et al.'' (2012) recovered turtles as the sister group of lepidosaurs instead.


References

{{Taxonbar, from=Q181537 Amniotes, Extant Pennsylvanian first appearances Taxa named by Ernst Haeckel