amniotes
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Amniotes are
tetrapod A tetrapod (; from Ancient Greek :wiktionary:τετρα-#Ancient Greek, τετρα- ''(tetra-)'' 'four' and :wiktionary:πούς#Ancient Greek, πούς ''(poús)'' 'foot') is any four-Limb (anatomy), limbed vertebrate animal of the clade Tetr ...
vertebrate Vertebrates () are animals with a vertebral column (backbone or spine), and a cranium, or skull. The vertebral column surrounds and protects the spinal cord, while the cranium protects the brain. The vertebrates make up the subphylum Vertebra ...
animal Animals are multicellular, eukaryotic organisms in the Biology, biological Kingdom (biology), kingdom Animalia (). With few exceptions, animals heterotroph, consume organic material, Cellular respiration#Aerobic respiration, breathe oxygen, ...
s belonging to the
clade In biology, a clade (), also known as a Monophyly, monophyletic group or natural group, is a group of organisms that is composed of a common ancestor and all of its descendants. Clades are the fundamental unit of cladistics, a modern approach t ...
Amniota, a large group that comprises the vast majority of living terrestrial and semiaquatic vertebrates. Amniotes evolved from amphibious stem tetrapod ancestors during the
Carboniferous The Carboniferous ( ) is a Geologic time scale, geologic period and System (stratigraphy), system of the Paleozoic era (geology), era that spans 60 million years, from the end of the Devonian Period Ma (million years ago) to the beginning of the ...
period. Amniota is defined as the smallest crown clade containing humans, the Greek tortoise, and the
Nile crocodile The Nile crocodile (''Crocodylus niloticus'') is a large crocodilian native to freshwater habitats in Africa, where it is present in 26 countries. It is widely distributed in sub-Saharan Africa, occurring mostly in the eastern, southern, and ce ...
. Amniotes are distinguished from the other living tetrapod clade — the non-amniote lissamphibians (
frog A frog is any member of a diverse and largely semiaquatic group of short-bodied, tailless amphibian vertebrates composing the order (biology), order Anura (coming from the Ancient Greek , literally 'without tail'). Frog species with rough ski ...
s/ toads,
salamander Salamanders are a group of amphibians typically characterized by their lizard-like appearance, with slender bodies, blunt snouts, short limbs projecting at right angles to the body, and the presence of a tail in both larvae and adults. All t ...
s/ newts and
caecilian Caecilians (; ) are a group of limbless, vermiform (worm-shaped) or serpentine (snake-shaped) amphibians with small or sometimes nonexistent eyes. They mostly live hidden in soil or in streambeds, and this cryptic lifestyle renders caecilians ...
s) — by: the development of three extraembryonic membranes ( amnion for
embryo An embryo ( ) is the initial stage of development for a multicellular organism. In organisms that reproduce sexually, embryonic development is the part of the life cycle that begins just after fertilization of the female egg cell by the male sp ...
nic protection, chorion for gas exchange, and allantois for metabolic waste disposal or storage); thicker and keratinized
skin Skin is the layer of usually soft, flexible outer tissue covering the body of a vertebrate animal, with three main functions: protection, regulation, and sensation. Other animal coverings, such as the arthropod exoskeleton, have different ...
; costal respiration (breathing by expanding/constricting the rib cage); the presence of adrenocortical and chromaffin tissues as a discrete pair of glands near their
kidney In humans, the kidneys are two reddish-brown bean-shaped blood-filtering organ (anatomy), organs that are a multilobar, multipapillary form of mammalian kidneys, usually without signs of external lobulation. They are located on the left and rig ...
s; more complex kidneys; the presence of an astragalus for better extremity range of motion; the diminished role of skin breathing; and the complete loss of
metamorphosis Metamorphosis is a biological process by which an animal physically develops including birth transformation or hatching, involving a conspicuous and relatively abrupt change in the animal's body structure through cell growth and different ...
,
gill A gill () is a respiration organ, respiratory organ that many aquatic ecosystem, aquatic organisms use to extract dissolved oxygen from water and to excrete carbon dioxide. The gills of some species, such as hermit crabs, have adapted to allow r ...
s, and lateral lines. The presence of an amniotic buffer, of a water-impermeable
skin Skin is the layer of usually soft, flexible outer tissue covering the body of a vertebrate animal, with three main functions: protection, regulation, and sensation. Other animal coverings, such as the arthropod exoskeleton, have different ...
, and of a robust, air-breathing,
respiratory system The respiratory system (also respiratory apparatus, ventilatory system) is a biological system consisting of specific organs and structures used for gas exchange in animals and plants. The anatomy and physiology that make this happen varies grea ...
, allow amniotes to live on land as true
terrestrial animal Terrestrial animals are animals that live predominantly or entirely on land (e.g. cats, chickens, ants, most spiders), as compared with aquatic animals, which live predominantly or entirely in the water (e.g. fish, lobsters, octopuses), ...
s. Amniotes have the ability to procreate without water bodies. Because the amnion and the fluid it secretes shields the embryo from environmental fluctuations, amniotes can reproduce on
dry land Land, also known as dry land, ground, or earth, is the solid terrestrial surface of Earth not submerged by the ocean or another body of water. It makes up 29.2% of Earth's surface and includes all continents and islands. Earth's land surface ...
by either laying shelled eggs (reptiles, birds and monotremes) or nurturing fertilized eggs within the mother (
marsupial Marsupials are a diverse group of mammals belonging to the infraclass Marsupialia. They are natively found in Australasia, Wallacea, and the Americas. One of marsupials' unique features is their reproductive strategy: the young are born in a r ...
and placental mammals). This distinguishes amniotes from anamniotes (
fish A fish (: fish or fishes) is an aquatic animal, aquatic, Anamniotes, anamniotic, gill-bearing vertebrate animal with swimming fish fin, fins and craniate, a hard skull, but lacking limb (anatomy), limbs with digit (anatomy), digits. Fish can ...
and amphibians) that have to spawn in aquatic environments. Most amniotes still require regular access to drinking water for rehydration, like the semiaquatic amphibians do. They have better
homeostasis In biology, homeostasis (British English, British also homoeostasis; ) is the state of steady internal physics, physical and chemistry, chemical conditions maintained by organism, living systems. This is the condition of optimal functioning fo ...
in drier environments, and more efficient non-aquatic gas exchange to power
terrestrial locomotion Terrestrial locomotion has evolution, evolved as animals adapted from ecoregion#Marine, aquatic to ecoregion#Terrestrial, terrestrial environments. Animal locomotion, Locomotion on land raises different problems than that in water, with reduced f ...
, which is facilitated by their astragalus. Basal amniotes resembled small lizards and evolved from semiaquatic reptiliomorphs, with fossil evidence suggesting they appeared no later than the earliest
Carboniferous The Carboniferous ( ) is a Geologic time scale, geologic period and System (stratigraphy), system of the Paleozoic era (geology), era that spans 60 million years, from the end of the Devonian Period Ma (million years ago) to the beginning of the ...
or late
Devonian The Devonian ( ) is a period (geology), geologic period and system (stratigraphy), system of the Paleozoic era (geology), era during the Phanerozoic eon (geology), eon, spanning 60.3 million years from the end of the preceding Silurian per ...
period. After the Carboniferous rainforest collapse, amniotes spread around Earth's land and became the dominant land vertebrates. Until 2025, it was assumed that amniotes originated during the mid-late Carboniferous, as the earliest body fossils of the group dated to this time. However, the discovery of clawed footprints made by a crown group-amniote (potentially a sauropsid) from the earliest Carboniferous-aged Snowy Plains Formation of Australia (358.9 to 354 million years ago) suggests that they likely originated even earlier, probably during the Devonian. After their origins, they almost immediately diverged into two groups, namely the sauropsids (including all
reptile Reptiles, as commonly defined, are a group of tetrapods with an ectothermic metabolism and Amniotic egg, amniotic development. Living traditional reptiles comprise four Order (biology), orders: Testudines, Crocodilia, Squamata, and Rhynchocepha ...
s and
bird Birds are a group of warm-blooded vertebrates constituting the class (biology), class Aves (), characterised by feathers, toothless beaked jaws, the Oviparity, laying of Eggshell, hard-shelled eggs, a high Metabolism, metabolic rate, a fou ...
s) and synapsids (including
mammal A mammal () is a vertebrate animal of the Class (biology), class Mammalia (). Mammals are characterised by the presence of milk-producing mammary glands for feeding their young, a broad neocortex region of the brain, fur or hair, and three ...
s and extinct ancestors like " pelycosaurs" and therapsids). Excluding the early fossil footprints, the earliest known crown group amniotes known from body fossils are the sauropsid '' Hylonomus'' and the synapsid '' Asaphestera'', both of which are from
Nova Scotia Nova Scotia is a Provinces and territories of Canada, province of Canada, located on its east coast. It is one of the three Maritime Canada, Maritime provinces and Population of Canada by province and territory, most populous province in Atlan ...
during the
Bashkirian The Bashkirian is in the International Commission on Stratigraphy geologic timescale the lowest stage (stratigraphy), stage or oldest age (geology), age of the Pennsylvanian (geology), Pennsylvanian. The Bashkirian age lasted from to Mega annu ...
age of the Late Carboniferous around . This basal divergence within Amniota has also been dated by molecular studies at 310–329 Ma, or 312–330 Ma, and by a fossilized birth–death process study at 322–340 Ma. However, the Snowy Plains footprints suggest a minimum divergence of 358.9–354 Ma.


Etymology

The term ''amniote'' comes from the amnion, which derives from Greek ἀμνίον (''amnion''), which denoted the membrane that surrounds a fetus. The term originally described a bowl in which the blood of sacrificed animals was caught, and derived from ἀμνός (''amnos''), meaning "lamb".


Description

Zoologists characterize amniotes in part by
embryonic development In developmental biology, animal embryonic development, also known as animal embryogenesis, is the developmental stage of an animal embryo. Embryonic development starts with the fertilization of an egg cell (ovum) by a sperm, sperm cell (spermat ...
that includes the formation of several extensive membranes, the amnion, chorion, and allantois. Amniotes develop directly into a (typically) terrestrial form with limbs and a thick stratified
epithelium Epithelium or epithelial tissue is a thin, continuous, protective layer of cells with little extracellular matrix. An example is the epidermis, the outermost layer of the skin. Epithelial ( mesothelial) tissues line the outer surfaces of man ...
(rather than first entering a feeding larval tadpole stage followed by
metamorphosis Metamorphosis is a biological process by which an animal physically develops including birth transformation or hatching, involving a conspicuous and relatively abrupt change in the animal's body structure through cell growth and different ...
, as
amphibian Amphibians are ectothermic, anamniote, anamniotic, tetrapod, four-limbed vertebrate animals that constitute the class (biology), class Amphibia. In its broadest sense, it is a paraphyletic group encompassing all Tetrapod, tetrapods, but excl ...
s do). In amniotes, the transition from a two-layered periderm to a cornified epithelium is triggered by thyroid hormone 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.


Adaptation for terrestrial living

Features of amniotes evolved for survival on land include a sturdy but porous leathery or hard eggshell and an allantois that facilitates respiration while providing a reservoir for disposal of wastes. Their
kidney In humans, the kidneys are two reddish-brown bean-shaped blood-filtering organ (anatomy), organs that are a multilobar, multipapillary form of mammalian kidneys, usually without signs of external lobulation. They are located on the left and rig ...
s (metanephros) and
large intestine The large intestine, also known as the large bowel, is the last part of the gastrointestinal tract and of the Digestion, digestive system in tetrapods. Water is absorbed here and the remaining waste material is stored in the rectum as feces befor ...
s are also well-suited to water retention. Most mammals do not lay eggs, but corresponding structures develop inside the placenta. The ancestors of true amniotes, such as '' Casineria kiddi'', which lived about 340 million years ago, evolved from
amphibian Amphibians are ectothermic, anamniote, anamniotic, tetrapod, four-limbed vertebrate animals that constitute the class (biology), class Amphibia. In its broadest sense, it is a paraphyletic group encompassing all Tetrapod, tetrapods, but excl ...
reptiliomorphs and resembled small 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). This was confirmed by the discovery of fossil footprints dated to the Gap in 2025.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 Desiccation is the state of extreme dryness, or the process of extreme drying. A desiccant is a hygroscopic (attracts and holds water) substance that induces or sustains such a state in its local vicinity in a moderately sealed container. The ...
. 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 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 membranes (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. A similar pattern can be seen in modern amphibians. Frogs that have evolved terrestrial reproduction and direct development have both smaller adults and fewer and larger eggs compared to their relatives that still reproduce in water. An alternative hypothesis is that amniotes evolved as a result of extended embryo retention (EER), where the extraembryonic membranes originated in the oviducts of the fertilized female to control the interaction between the embryos and the female. The eggs in groups like turtles, crocodilians and birds, which are laid at a much earlier developmental stage, would be a secondary evolved trait.


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 develop nor retain a vertebral column (commonly known as a ''spine'' or ''backbone''), which evolved from the notochord. It is a paraphyletic grouping including all animals excluding the chordata, chordate s ...
s occupying level two. Amniotes would eventually experience adaptive radiations when some species evolved the ability to digest plants and new
ecological niche In ecology, a niche is the match of a species to a specific environmental condition. Three variants of ecological niche are described by It describes how an organism or population responds to the distribution of Resource (biology), resources an ...
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 at the back margin of the skull roof. 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 and in particular the number of holes behind each eye. In anapsids, the ancestral condition, there are none; in synapsids (mammals and their extinct relatives) there is one; and in diapsids (including birds,
crocodilia Crocodilia () is an order of semiaquatic, predatory reptiles that are known as crocodilians. They first appeared during the Late Cretaceous and are the closest living relatives of birds. Crocodilians are a type of crocodylomorph pseudosuchia ...
ns, squamates, and
tuatara The tuatara (''Sphenodon punctatus'') is a species of reptile endemic to New Zealand. Despite its close resemblance to lizards, it is actually the only extant member of a distinct lineage, the previously highly diverse order Rhynchocephal ...
s), there are two. Turtles have secondarily lost their fenestrae, and were traditionally classified as anapsids because of this. Molecular testing firmly places them in the diapsid line of descent. Post-cranial remains of amniotes can be identified from their Labyrinthodont ancestors by their having at least two pairs of sacral ribs, a
sternum The sternum (: sternums or sterna) 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, lungs, and major bl ...
in the pectoral girdle (some amniotes have lost it) and an astragalus bone in the 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, natural history, naturalist, eugenics, eugenicist, Philosophy, philosopher, physician, professor, marine biology, marine biologist and artist ...
in 1866 on the presence of the amnion, hence the name. A problem with this definition is that the trait (
apomorphy In phylogenetics, an apomorphy (or derived trait) is a novel Phenotypic trait, character or character state that has evolution, evolved from its ancestral form (or Plesiomorphy and symplesiomorphy, plesiomorphy). A synapomorphy is an apomorphy sh ...
) in question does not
fossil A fossil (from Classical Latin , ) is any preserved remains, impression, or trace of any once-living thing from a past geological age. Examples include bones, shells, exoskeletons, stone imprints of animals or microbes, objects preserve ...
ize, and the status of fossil forms has to be inferred from other traits.


Traditional classification

Older classifications of the amniotes traditionally recognised three classes based on major traits and
physiology Physiology (; ) is the science, scientific study of function (biology), functions and mechanism (biology), mechanisms in a life, living system. As a branches of science, subdiscipline of biology, physiology focuses on how organisms, organ syst ...
: 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) ** Subclass Anapsida (" proto-reptiles", possibly including turtles) ** Subclass Diapsida (majority of reptiles, progenitors of birds) ** Subclass Euryapsida (
plesiosaurs The Plesiosauria or plesiosaurs are an Order (biology), order or clade of extinct Mesozoic marine reptiles, belonging to the Sauropterygia. Plesiosaurs first appeared in the latest Triassic Period (geology), Period, possibly in the Rhaetian st ...
, placodonts, and
ichthyosaurs Ichthyosauria is an taxonomy (biology), order of large extinction, extinct marine reptiles sometimes referred to as "ichthyosaurs", although the term is also used for wider clades in which the order resides. Ichthyosaurians thrived during much of ...
) ** Subclass Synapsida (stem or proto-mammals, progenitors of
mammal A mammal () is a vertebrate animal of the Class (biology), class Mammalia (). Mammals are characterised by the presence of milk-producing mammary glands for feeding their young, a broad neocortex region of the brain, fur or hair, and three ...
s) * Class Aves (birds) ** Subclass
Archaeornithes The Archaeornithes, classically Archæornithes, is an extinct group of the first primitive, reptile-like birds. It is an evolutionary grade of transitional fossils, the primitive birds halfway between non-avian dinosaur ancestors and the derived m ...
(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 teet ...
(early birds with an alternative shoulder joint) ** Subclass
Hesperornithes Hesperornithes is an extinct and highly specialized group of aquatic Avialae, avialans closely related to the ancestors of modern birds. They inhabited both marine and freshwater habitats in the Northern Hemisphere, and include genus, genera such ...
(toothed aquatic flightless birds) ** Subclass Ichthyornithes (toothed, but otherwise modern birds) ** Subclass Neornithes (all living birds) * Class
Mammal A mammal () is a vertebrate animal of the Class (biology), class Mammalia (). Mammals are characterised by the presence of milk-producing mammary glands for feeding their young, a broad neocortex region of the brain, fur or hair, and three ...
ia (mammals) ** Subclass Prototheria ( Monotremata, egg-laying mammals) ** Subclass
Theria Theria ( or ; ) is a scientific classification, subclass of mammals amongst the Theriiformes. Theria includes the eutherians (including the Placentalia, placental mammals) and the metatherians (including the marsupials) but excludes the egg-lay ...
( metatheria (such as
marsupial Marsupials are a diverse group of mammals belonging to the infraclass Marsupialia. They are natively found in Australasia, Wallacea, and the Americas. One of marsupials' unique features is their reproductive strategy: the young are born in a r ...
s) and
eutheria Eutheria (from Greek , 'good, right' and , 'beast'; ), also called Pan-Placentalia, is the clade consisting of Placentalia, placental mammals and all therian mammals that are more closely related to placentals than to marsupials. Eutherians ...
(such as placental mammals)) This rather orderly scheme is the one most commonly found in popular and basic scientific works. It has come under critique from
cladistics Cladistics ( ; from Ancient Greek 'branch') is an approach to Taxonomy (biology), biological classification in which organisms are categorized in groups ("clades") based on hypotheses of most recent common ancestry. The evidence for hypothesiz ...
, as the class Reptilia is
paraphyletic Paraphyly is a taxonomic term describing a grouping that consists of the grouping's last common ancestor and some but not all of its descendant lineages. The grouping is said to be paraphyletic ''with respect to'' the excluded subgroups. In co ...
—it has given rise to two other classes not included in Reptilia. Most species described as microsaurs, formerly grouped in the extinct and prehistoric amphibian group lepospondyls, has been placed in the newer clade Recumbirostra, and shares many anatomical features with amniotes which indicates they were amniotes themselves.


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) ''Clade'' Synapsida *** A series of unassigned families, corresponding to Pelycosauria *** (Order) ''Clade'' Therapsida **** Class
Mammal A mammal () is a vertebrate animal of the Class (biology), class Mammalia (). Mammals are characterised by the presence of milk-producing mammary glands for feeding their young, a broad neocortex region of the brain, fur or hair, and three ...
ia – mammals ** (Class) ''Clade''
Sauropsida Sauropsida (Greek language, Greek for "lizard faces") is a clade of amniotes, broadly equivalent to the Class (biology), class Reptile, Reptilia, though typically used in a broader sense to also include extinct stem-group relatives of modern repti ...
*** Subclass Parareptilia **** Family
Mesosaur Mesosaurs ("middle lizards") were a group of small aquatic reptiles that lived during the early Permian period ( Cisuralian), roughly 299 to 270 million years ago. Mesosaurs were the first known aquatic reptiles, having apparently returned to a ...
idae **** Family Millerettidae **** Family Bolosauridae **** Family Procolophonidae **** Order Pareiasauromorpha ***** Family Nycteroleteridae ***** Family Pareiasauridae *** (Subclass) ''Clade'' Eureptilia **** Family
Captorhinidae Captorhinidae is an extinct family of tetrapods, traditionally considered primitive Reptile, reptiles, known from the late Carboniferous to the Late Permian. They had a cosmopolitan distribution across Pangea. Description Captorhinids are a cl ...
**** (Infraclass) ''Clade'' Diapsida ***** Family Araeoscelididae ***** Family Weigeltisauridae ***** Order Younginiformes ***** (Infraclass) ''Clade'' Neodiapsida ****** Order
Testudinata Testudinata is the group of all tetrapods with a true turtle shell. It includes both modern turtles (Testudines) and many of their extinct, shelled relatives (stem-turtles), though excluding ''Odontochelys'' and ''Eorhynchochelys,'' which are pla ...
******* Suborder
Testudines Turtles are reptiles of the order (biology), order Testudines, characterized by a special turtle shell, shell developed mainly from their ribs. Modern turtles are divided into two major groups, the Pleurodira (side necked turtles) and Crypt ...
– turtles ****** Infraclass Lepidosauromorpha ******* Unnamed infrasubclass ******** Infraclass Ichthyosauria ******** Order Thalattosauria ******** Superorder Lepidosauriformes ********* Order Sphenodontida – tuatara ********* Order
Squamata Squamata (, Latin ''squamatus'', 'scaly, having scales') is the largest Order (biology), order of reptiles; most members of which are commonly known as Lizard, lizards, with the group also including Snake, snakes. With over 11,991 species, it i ...
– lizards and snakes ******* Infrasubclass Sauropterygia ******** Order
Placodontia Placodonts ("Tablet (pharmacy), tablet tooth, teeth") are an Extinction, extinct order (biology), order of marine reptiles that lived during the Triassic period, becoming extinct at the end of the period. They were part of Sauropterygia, the gro ...
******** Order Eosauropterygia ********* Suborder Pachypleurosauria ********* Suborder Nothosauria ******** Order Plesiosauria ****** (Infraclass) ''Clade'' Archosauromorpha ******* Family Trilophosauridae ******* Order Rhynchosauria ******* Order Protorosauria ******* Division Archosauriformes ******** Subdivision Archosauria ********* Infradivision Crurotarsi ********** Order Phytosauria† ********** Family Ornithosuchidae ********** Family Stagonolepididae ********** Family Rauisuchidae ********** Superfamily Poposauroidea ********** Superorder Crocodylomorpha *********** Order Crocodylia – crocodilians ********* Infradivision
Avemetatarsalia Avemetatarsalia (meaning "bird metatarsals") is a clade of diapsid Reptile, reptiles containing all archosaurs more closely related to birds than to crocodilians. The two most successful groups of avemetatarsalians were the dinosaurs and pterosau ...
********** Infrasubdivision Ornithodira *********** Order Pterosauria *********** Family Lagerpetidae *********** Family Silesauridae *********** (Superorder) ''Clade'' Dinosauria – dinosaurs ************ Order Ornithischia ************ (Order) ''Clade'' Saurischia ************* (Suborder) ''Clade'' Theropoda – theropods ************** Class Aves – birds


Phylogenetic classification

With the advent of cladistics, other researchers have attempted to establish new classes, based on
phylogeny A phylogenetic tree or phylogeny is a graphical representation which shows the evolutionary history between a set of species or Taxon, taxa during a specific time.Felsenstein J. (2004). ''Inferring Phylogenies'' Sinauer Associates: Sunderland, M ...
, 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 Extant or Least-concern species, least concern is the opposite of the word extinct. It may refer to: * Extant hereditary titles * Extant literature, surviving literature, such as ''Beowulf'', the oldest extant manuscript written in English * Exta ...
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. Though traditionally considered reptiliomorphs, some recent research has recovered diadectomorphs as the sister group to Synapsida within Amniota, based on inner ear anatomy.


Cladogram

The
cladogram A cladogram (from Greek language, Greek ''clados'' "branch" and ''gramma'' "character") is a diagram used in cladistics to show relations among organisms. A cladogram is not, however, an Phylogenetic tree, evolutionary tree because it does not s ...
presented here illustrates the
phylogeny A phylogenetic tree or phylogeny is a graphical representation which shows the evolutionary history between a set of species or Taxon, taxa during a specific time.Felsenstein J. (2004). ''Inferring Phylogenies'' Sinauer Associates: Sunderland, M ...
(family tree) of amniotes, and follows a simplified version of the relationships found by Laurin & Reisz (1995), with the exception of turtles, which more recent morphological and molecular
phylogenetic In biology, phylogenetics () is the study of the evolutionary history of life using observable characteristics of organisms (or genes), which is known as phylogenetic inference. It infers the relationship among organisms based on empirical dat ...
studies placed firmly within diapsids. The cladogram covers the group as defined under Gauthier's definition. Following studies in 2022 and 2023, with Drepanosauromorpha placed sister to Weigeltisauridae ('' Coelurosauravus'') in Avicephala based on Senter (2004):


References

{{Authority control Extant Pennsylvanian first appearances Taxa named by Ernst Haeckel Zoological nomenclature