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Vertebrates () comprise all species of
animal Animals (also called Metazoa) are multicellular eukaryotic organisms that form the Kingdom (biology), biological kingdom Animalia. With few exceptions, animals Heterotroph, consume organic material, Cellular respiration#Aerobic respiration, ...
s within the
subphylum In zoological nomenclature, a subphylum is a taxonomic rank 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 ...
Vertebrata () (
chordates A chordate () is an animal of the phylum Chordata (). All chordates possess 5 Apomorphy and synapomorphy , synapomorphies, or primary characteristics, at some point during their larval or adulthood stages that distinguish them from all other ta ...
with
backbones
backbones
). Vertebrates represent the overwhelming majority of the
phylum In biology, a phylum (; plural The plural (sometimes list of glossing abbreviations, abbreviated ), in many languages, is one of the values of the grammatical number, grammatical category of number. The plural of a noun typically denotes a q ...
Chordata, with currently about 69,963
species In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, molecular interactions, Physiology, physiological mechanis ...

species
described. Vertebrates comprise such groups as the following: *
jawless fish Agnatha (, Ancient Greek Ancient Greek includes the forms of the Greek language used in ancient Greece and the classical antiquity, ancient world from around 1500 BC to 300 BC. It is often roughly divided into the following periods: Mycen ...
, which include
hagfish Hagfish, of the class Myxini (also known as Hyperotreti) and order Myxiniformes , are eel-shaped, slime-producing marine fish (occasionally called slime eels). They are the only known living animals that have a skull but no vertebral column, al ...

hagfish
and
lampreys Lampreys (sometimes inaccurately called lamprey eels) are an ancient extant lineage of jawless fish of the order Petromyzontiformes , placed in the superclass Cyclostomata Cyclostomata is a group of agnathans that comprises the living ...

lampreys
*
jawed vertebrates Gnathostomata are the jawed vertebrates. The term derives from Greek language, Greek: (') "jaw" + (') "mouth". Gnathostome diversity comprises roughly 60,000 species, which accounts for 99% of all living vertebrates. In addition to opposing ja ...

jawed vertebrates
, which include: **
cartilaginous fish Chondrichthyes (; from Greek χονδρ- 'cartilage', ἰχθύς 'fish') is a class that contains the cartilaginous fishes that have skeletons primarily composed of cartilage. They can be contrasted with the Osteichthyes or ''bony fishes'' ...
(
shark Sharks are a group of elasmobranch fish characterized by a Chondrichthyes#Skeleton, cartilaginous skeleton, five to seven gill slits on the sides of the head, and pectoral fins that are not fused to the head. Modern sharks are classified within ...

shark
s, rays, and
ratfish Chimaeras are Chondrichthyes, cartilaginous fish in the order (biology), order Chimaeriformes , known informally as ghost sharks, rat fish, spookfish, or rabbit fish; the last three names are not to be confused with rattails, Barreleye, Opisthop ...
) ** bony vertebrates, which include: *** ray-fins (the majority of living
bony fish Osteichthyes (), popularly referred to as the bony fish, is a diverse Taxonomy (biology), taxonomic group of fish that have skeletons primarily composed of bone tissue. They can be contrasted with the Chondrichthyes, which have skeletons primaril ...

bony fish
) *** lobe-fins, which include: **** coelacanths and
lungfish Lungfish are freshwater rhipidistian vertebrates belonging to the Order (taxonomic rank), order Dipnoi. Lungfish are best known for retaining ancestral characteristics within the Osteichthyes, including the ability to breathe air, and ancestral ...
****
tetrapods Tetrapods (; from Greek 'four' and 'foot') are four-limbed animals constituting the superclass Tetrapoda . It includes extant and extinct Extinction is the termination of a kind of organism In biology, an organism (from Ancient ...

tetrapods
(limbed vertebrates)
Extant Extant 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 * Extant taxon, a taxon which is not extinct, s ...
vertebrates range in size from the
frog A frog is any member of a diverse and largely Carnivore, carnivorous group of short-bodied, tailless amphibians composing the order (biology), order Anura (literally ''without tail'' in Ancient Greek). The oldest fossil "proto-frog" ''Triadobat ...

frog
species ''
Paedophryne amauensis ''Paedophryne amauensis'' is a species of microhylid frog from Papua New Guinea Papua New Guinea (PNG; , ; tpi, Papua Niugini; ho, Papua Niu Gini; tcs, Op Deudai), officially the Independent State of Papua New Guinea ( tpi, Independen ...
'', at as little as , to the
blue whale The blue whale (''Balaenoptera musculus'') is a marine mammal belonging to the baleen whale suborder Mysticeti. Reaching a maximum confirmed length of and a weighing up to , it is the largest animal known to have existed. The blue whale's lo ...
, at up to . Vertebrates make up less than five percent of all described
animal species Animals (also called Metazoa) are multicellular eukaryotic Eukaryotes () are organism In biology, an organism (from Ancient Greek, Greek: ὀργανισμός, ''organismos'') is any individual contiguous system that embodie ...
; the rest are
invertebrates 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, Vertebrat ...
, which lack
vertebral column The vertebral column, also known as the backbone or spine, is part of the axial skeleton. The vertebral column is the defining characteristic of a vertebrate in which the notochord (a flexible rod of uniform composition) found in all chordata, ...

vertebral column
s. The vertebrates traditionally include the
hagfish Hagfish, of the class Myxini (also known as Hyperotreti) and order Myxiniformes , are eel-shaped, slime-producing marine fish (occasionally called slime eels). They are the only known living animals that have a skull but no vertebral column, al ...

hagfish
, which do not have proper
vertebra In the vertebrate spinal column The vertebral column, also known as the backbone or spine, is part of the axial skeleton. The vertebral column is the defining characteristic of a vertebrate in which the notochord (a flexible rod of uniform co ...
e due to their loss in evolution, though their closest living relatives, the
lamprey Lampreys (sometimes inaccurately called lamprey eels) are an ancient extant lineage of Agnatha, jawless fish of the order (biology), order Petromyzontiformes , placed in the superclass Cyclostomata. The adult lamprey may be characterized by ...

lamprey
s, do. Hagfish do, however, possess a
cranium 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 red blood cell, red and white blood cells, store mi ...

cranium
. For this reason, the vertebrate subphylum is sometimes referred to as "
Craniata A craniate is a member of the Craniata (sometimes called the Craniota), a proposed clade A clade (; from grc, , ''klados'', "branch"), also known as a monophyletic group or natural group, is a group of organisms that are monophyly, monophyle ...
" when discussing morphology. Molecular analysis since 1992 has suggested that
hagfish Hagfish, of the class Myxini (also known as Hyperotreti) and order Myxiniformes , are eel-shaped, slime-producing marine fish (occasionally called slime eels). They are the only known living animals that have a skull but no vertebral column, al ...

hagfish
are most closely related to
lamprey Lampreys (sometimes inaccurately called lamprey eels) are an ancient extant lineage of Agnatha, jawless fish of the order (biology), order Petromyzontiformes , placed in the superclass Cyclostomata. The adult lamprey may be characterized by ...

lamprey
s, and so also are vertebrates in a
monophyletic 300px, A cladogram of the primates, showing a ''monophyletic'' taxon: ''the simians'' (in yellow); a ''paraphyletic'' taxon: ''the prosimians'' (in cyan, including the red patch); and a ''polyphyletic'' group: ''the night-active primates, i.e., ...

monophyletic
sense. Others consider them a sister group of vertebrates in the common taxon of craniata. The populations of vertebrates have dropped in the past 50 years.


Etymology

The word ''vertebrate'' derives from the
Latin Latin (, or , ) is a classical language belonging to the Italic languages, Italic branch of the Indo-European languages. Latin was originally spoken in the area around Rome, known as Latium. Through the power of the Roman Republic, it became the ...

Latin
word ''vertebratus'' (
Pliny Pliny may refer to: People from antiquity * Pliny the Elder (AD 23–79), ancient Roman nobleman, scientist, historian, and author of ''Naturalis Historia'' (''Pliny's Natural History'') * Pliny the Younger (died 113), ancient Roman statesman, ...
), meaning ''joint of the spine.'' ''Vertebrate'' is derived from the word ''
vertebra In the vertebrate spinal column The vertebral column, also known as the backbone or spine, is part of the axial skeleton. The vertebral column is the defining characteristic of a vertebrate in which the notochord (a flexible rod of uniform co ...
'', which refers to any of the bones or segments of the
spinal column The vertebral column, also known as the backbone or spine, is part of the axial skeleton. The vertebral column is the defining characteristic of a vertebrate in which the notochord (a flexible rod of uniform composition) found in all chordata, c ...
.


Anatomy and morphology

All vertebrates are built along the basic chordate
body plan A body plan, ''Bauplan'' (German plural ''Baupläne''), or ground plan is a set of morphological features common to many members of a phylum In biology, a phylum (; plural The plural (sometimes list of glossing abbreviations, abbreviated ) ...
: a stiff rod running through the length of the animal (vertebral column and/or
notochord In anatomy Anatomy (Greek ''anatomē'', 'dissection') is the branch of biology concerned with the study of the structure of organism In biology, an organism (from Ancient Greek, Greek: ὀργανισμός, ''organismos'') is any in ...
), with a hollow tube of nervous tissue (the
spinal cord The spinal cord is a long, thin, tubular structure made up of nervous tissue, which extends from the medulla oblongata in the brainstem to the lumbar region of the vertebral column. It encloses the central canal of the spinal cord, which co ...

spinal cord
) above it and the
gastrointestinal tract The gastrointestinal tract (GI tract, digestive tract, alimentary canal) is the tract or passageway of the digestive system The human digestive system consists of the human gastrointestinal tract, gastrointestinal tract plus the accessory or ...
below. In all vertebrates, the mouth is found at, or right below, the anterior end of the animal, while the
anus The anus (from Latin Latin (, or , ) is a classical language belonging to the Italic languages, Italic branch of the Indo-European languages. Latin was originally spoken in the area around Rome, known as Latium. Through the power of the Rom ...

anus
opens to the exterior before the end of the body. The remaining part of the body continuing after the anus forms a
tail The tail is the section at the rear end of certain kinds of animal Animals (also called Metazoa) are multicellular eukaryotic organisms that form the Kingdom (biology), biological kingdom Animalia. With few exceptions, animals Heterotrop ...
with vertebrae and spinal cord, but no gut.Romer, A.S. (1949): ''The Vertebrate Body.'' W.B. Saunders, Philadelphia. (2nd ed. 1955; 3rd ed. 1962; 4th ed. 1970)


Vertebral column

The defining characteristic of a vertebrate is the
vertebral column The vertebral column, also known as the backbone or spine, is part of the axial skeleton. The vertebral column is the defining characteristic of a vertebrate in which the notochord (a flexible rod of uniform composition) found in all chordata, ...

vertebral column
, in which the
notochord In anatomy Anatomy (Greek ''anatomē'', 'dissection') is the branch of biology concerned with the study of the structure of organism In biology, an organism (from Ancient Greek, Greek: ὀργανισμός, ''organismos'') is any in ...
(a stiff rod of uniform composition) found in all
chordates A chordate () is an animal of the phylum Chordata (). All chordates possess 5 Apomorphy and synapomorphy , synapomorphies, or primary characteristics, at some point during their larval or adulthood stages that distinguish them from all other ta ...

chordates
has been replaced by a segmented series of stiffer elements (vertebrae) separated by mobile joints (intervertebral discs, derived embryonically and evolutionarily from the notochord). However, a few vertebrates have secondarily lost this anatomy, retaining the notochord into adulthood, such as the
sturgeon Sturgeon is the common name Common may refer to: Places * Common, a townland in County Tyrone, Northern Ireland * Boston Common Boston Common (also known as the Common) is a central public park in downtown Boston, Massachusetts. It is so ...

sturgeon
and
coelacanth The coelacanths ( ) constitute a now-rare order of fish Fish are Aquatic animal, aquatic, craniate, gill-bearing animals that lack Limb (anatomy), limbs with Digit (anatomy), digits. They form a sister group to the tunicates, together for ...

coelacanth
.
Jawed vertebrates Gnathostomata are the jawed vertebrates. The term derives from Greek language, Greek: (') "jaw" + (') "mouth". Gnathostome diversity comprises roughly 60,000 species, which accounts for 99% of all living vertebrates. In addition to opposing ja ...

Jawed vertebrates
are typified by paired appendages (fins or legs, which may be secondarily lost), but this trait is not required in order for an animal to be a vertebrate.


Gills

All
basal Basal or basilar is a term meaning ''base'', ''bottom'', or ''minimum''. Science * Basal (anatomy), an anatomical term of location for features associated with the base of an organism or structure * Basal (medicine), a minimal level that is neces ...
vertebrates breathe with
gills A gill () is a respiratory organ found in many aquatic organisms that extracts dissolved oxygen Oxygen is the chemical element with the chemical symbol, symbol O and atomic number 8. It is a member of the chalcogen Group (periodic ...

gills
. The gills are carried right behind the head, bordering the posterior margins of a series of openings from the
pharynx The pharynx (plural: pharynges) is the part of the throat behind the human mouth, mouth and nasal cavity, and above the esophagus and trachea – the tubes going down to the stomach and the lungs. It is found in vertebrates and invertebrates, thou ...

pharynx
to the exterior. Each
gill A gill () is a respiration organ, respiratory organ found in many aquatic ecosystem, aquatic organisms that extracts dissolved oxygen from water and excretes carbon dioxide. The gills of some species, such as hermit crabs, have adapted to allow r ...
is supported by a cartilagenous or bony
gill arch Branchial arches, or gill arches, are a series of bony "loops" present in fish, which support the gills. As gills are the primitive condition of vertebrates, all vertebrate embryos develop pharyngeal arch The pharyngeal arches, also known as ...
. The
bony fish Osteichthyes (), popularly referred to as the bony fish, is a diverse Taxonomy (biology), taxonomic group of fish that have skeletons primarily composed of bone tissue. They can be contrasted with the Chondrichthyes, which have skeletons primaril ...
have three pairs of arches,
cartilaginous fish Chondrichthyes (; from Greek χονδρ- 'cartilage', ἰχθύς 'fish') is a class that contains the cartilaginous fishes that have skeletons primarily composed of cartilage. They can be contrasted with the Osteichthyes or ''bony fishes'' ...
have five to seven pairs, while the primitive
jawless fish Agnatha (, Ancient Greek Ancient Greek includes the forms of the Greek language used in ancient Greece and the classical antiquity, ancient world from around 1500 BC to 300 BC. It is often roughly divided into the following periods: Mycen ...
have seven. The vertebrate ancestor no doubt had more arches than this, as some of their
chordate A chordate () is an animal of the phylum Chordata (). All chordates possess 5 Apomorphy and synapomorphy , synapomorphies, or primary characteristics, at some point during their larval or adulthood stages that distinguish them from all other ta ...

chordate
relatives have more than 50 pairs of gills. In
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, terr ...
s and some primitive bony
fishes Fish are aquatic, craniate, gill A gill () is a respiration organ, respiratory organ found in many aquatic ecosystem, aquatic organisms that extracts dissolved oxygen from water and excretes carbon dioxide. The gills of some species, such ...

fishes
, the
larva A larva (plural larvae ) is a distinct juvenile form many animals undergo before metamorphosis into adults. Animals with indirect developmental biology, development such as insects, amphibians, or cnidarians typically have a larval phase of the ...
e bear
external gillsExternal gills are the gill A gill () is a respiration organ, respiratory organ found in many aquatic ecosystem, aquatic organisms that extracts dissolved oxygen from water and excretes carbon dioxide. The gills of some species, such as hermit cra ...
, branching off from the gill arches. These are reduced in adulthood, their function taken over by the gills proper in fishes and by
lung The lungs are the primary organs of the respiratory system in human Humans (''Homo sapiens'') are the most populous and widespread species of primates, characterized by bipedality, opposable thumbs, hairlessness, and intelligence allowi ...

lung
s in most amphibians. Some amphibians retain the external larval gills in adulthood, the complex internal
gill A gill () is a respiration organ, respiratory organ found in many aquatic ecosystem, aquatic organisms that extracts dissolved oxygen from water and excretes carbon dioxide. The gills of some species, such as hermit crabs, have adapted to allow r ...
system as seen in fish apparently being irrevocably lost very early in the evolution of
tetrapod Tetrapods (; ) are four-limbed animals constituting the superclass Tetrapoda (). It includes Neontology#Extant taxa vs. extinct taxa, extant and extinct amphibians, reptiles (including dinosaurs and therefore birds), and synapsids (including mam ...
s.Clack, J. A. (2002): Gaining ground: the origin and evolution of tetrapods. ''
Indiana University Press Indiana University Press, also known as IU Press, is an academic publisher Academic publishing is the subfield of publishing Publishing is the activity of making information, literature, music, software and other content available to the pu ...
'', Bloomington, Indiana. 369 pp
While the more derived vertebrates lack gills, the gill arches form during
fetal development Prenatal development () includes the development of the embryo and of the foetus during a viviparous animal's gestationGestation is the period of development during the carrying of an embryo An embryo is the early stage of development of a ...

fetal development
, and form the basis of essential structures such as
jaw The jaw is any opposable articulated structure at the entrance of the mouth In animal anatomy Anatomy (Greek ''anatomē'', 'dissection') is the branch of biology concerned with the study of the structure of organism In biology, an ...

jaw
s, the
thyroid gland The thyroid, or thyroid gland, is an endocrine gland in the neck consisting of two connected lobes. The lower two thirds of the lobes are connected by a thin band of tissue called the thyroid isthmus. The thyroid is located at the front of th ...

thyroid gland
, the
larynx The larynx (), commonly called the voice box, is an 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 i ...

larynx
, the ''columella'' (corresponding to the
stapes The ''stapes'' or stirrup is a bone A bone is a Stiffness, rigid tissue (anatomy), tissue that constitutes part of the skeleton in most vertebrate animals. Bones protect the various organs of the body, produce red blood cell, red and white bl ...
in
mammal Mammals (from Latin language, Latin , 'breast') are a group of vertebrate animals constituting the class (biology), class Mammalia (), and characterized by the presence of mammary glands which in Female#Mammalian female, females produce milk ...
s) and, in mammals, the malleus and incus.


Central nervous system

The
central nervous system The central nervous system (CNS) is the part of the nervous system In Biology, biology, the nervous system is a Complex system, highly complex part of an animal that coordinates its Behavior, actions and Sense, sensory information by transmi ...

central nervous system
of vertebrates is based on a hollow nerve cord running along the length of the animal. Of particular importance and unique to vertebrates is the presence of
neural crest Neural crest cells are a temporary group of cells unique to vertebrate Vertebrates () comprise all species of animal Animals (also called Metazoa) are multicellular eukaryotic organisms that form the Kingdom (biology), biological kingdo ...

neural crest
cells. These are progenitors of
stem cell In multicellular organisms Multicellular organisms are organism In biology, an organism (from Ancient Greek, Greek: ὀργανισμός, ''organismos'') is any individual contiguous system that embodies the Life#Biology, properties o ...
s, and critical to coordinating the functions of cellular components.Teng, L.; Labosky, P. A. (2006)
"Neural crest stem cells"
In: Jean-Pierre Saint-Jeannet, ''Neural Crest Induction and Differentiation'', pp. 206-212, Springer Science & Business Media. .
Neural crest cells migrate through the body from the nerve cord during development, and initiate the formation of and structures such as the jaws and skull. The vertebrates are the only
chordate A chordate () is an animal of the phylum Chordata (). All chordates possess 5 Apomorphy and synapomorphy , synapomorphies, or primary characteristics, at some point during their larval or adulthood stages that distinguish them from all other ta ...
group with neural
cephalisation Cephalization is an evolution Evolution is change in the Heredity, heritable Phenotypic trait, characteristics of biological populations over successive generations. These characteristics are the Gene expression, expressions of genes that are ...
, the concentration of
brain A brain is an organ (biology), organ that serves as the center of the nervous system in all vertebrate and most invertebrate animals. It is located in the head, usually close to the sensory organs for senses such as Visual perception, vision. It ...

brain
functions in the head. A slight swelling of the anterior end of the nerve cord is found in the
lancelet The lancelets ( or ), also known as amphioxi (singular: amphioxus ), consist of some 30 to 35 species of "fish-like" benthic filter feeding chordates in the order Amphioxiformes. They are the modern representatives of the subphylum Cephalochor ...
, a chordate, though it lacks the eyes and other complex sense
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 as parenchyma Parenchyma () is the bulk of functional ...
comparable to those of vertebrates. Other chordates do not show any trends towards cephalisation. A
peripheral nervous system The peripheral nervous system (PNS) is one of two components that make up the nervous system In Biology, biology, the nervous system is a Complex system, highly complex part of an animal that coordinates its Behavior, actions and Sense, sen ...
branches out from the nerve cord to innervate the various systems. The front end of the nerve tube is expanded by a thickening of the walls and expansion of the central canal of spinal cord into three primary brain vesicles: The
prosencephalon In the anatomy Anatomy (Greek ''anatomē'', 'dissection') is the branch of biology concerned with the study of the structure of organism In biology, an organism (from Ancient Greek, Greek: ὀργανισμός, ''organismos'') is an ...
(forebrain),
mesencephalon The midbrain or mesencephalon is the forward-most portion of the brainstem and is associated with vision, hearing, motor control, sleep and wakefulness, arousal (alertness), and temperature regulation. The name comes from the Greek ''mesos'', "mi ...
(midbrain) and
rhombencephalon The hindbrain or rhombencephalon is a developmental categorization of portions of the central nervous system in vertebrates. It includes the medulla, pons The pons (Latin for "bridge") is part of the brainstem that in human Humans ('' ...

rhombencephalon
(hindbrain), further differentiated in the various vertebrate groups.Hildebrand, M.; Gonslow, G. (2001): Analysis of Vertebrate Structure. 5th edition. ''John Wiley & Sons, Inc''.
New York New York most commonly refers to: * New York City, the most populous city in the United States, located in the state of New York * New York (state), a state in the Northeastern United States New York may also refer to: Film and television * New ...

New York
Two laterally placed
eye Eyes are organs of the visual system The visual system comprises the sensory organ (the eye) and parts of the central nervous system (the retina containing photoreceptor cells, the optic nerve, the optic tract and the visual cortex) which ...

eye
s form around outgrowths from the midbrain, except in
hagfish Hagfish, of the class Myxini (also known as Hyperotreti) and order Myxiniformes , are eel-shaped, slime-producing marine fish (occasionally called slime eels). They are the only known living animals that have a skull but no vertebral column, al ...

hagfish
, though this may be a secondary loss. The forebrain is well-developed and subdivided in most
tetrapod Tetrapods (; ) are four-limbed animals constituting the superclass Tetrapoda (). It includes Neontology#Extant taxa vs. extinct taxa, extant and extinct amphibians, reptiles (including dinosaurs and therefore birds), and synapsids (including mam ...
s, while the midbrain dominates in many
fish Fish are Aquatic animal, aquatic, craniate, gill-bearing animals that lack Limb (anatomy), limbs with Digit (anatomy), digits. Included in this definition are the living hagfish, lampreys, and Chondrichthyes, cartilaginous and bony fish as we ...

fish
and some
salamander Salamanders are a group of amphibians typically characterized by a lizard Lizards are a widespread group of Squamata, squamate reptiles, with over 6,000 species, ranging across all continents except Antarctica, as well as most oceanic isl ...

salamander
s. Vesicles of the forebrain are usually paired, giving rise to hemispheres like the
cerebral hemisphere The vertebrate Vertebrates () comprise all species of animal Animals (also called Metazoa) are multicellular eukaryotic organisms that form the Kingdom (biology), biological kingdom Animalia. With few exceptions, animals Heterotroph, ...
s in
mammal Mammals (from Latin language, Latin , 'breast') are a group of vertebrate animals constituting the class (biology), class Mammalia (), and characterized by the presence of mammary glands which in Female#Mammalian female, females produce milk ...
s. The resulting anatomy of the central nervous system, with a single hollow nerve cord topped by a series of (often paired) vesicles, is unique to vertebrates. All
invertebrates 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, Vertebrat ...
with well-developed brains, such as
insect Insects (from Latin ') are pancrustacean Hexapoda, hexapod invertebrates of the class (biology), class Insecta. They are the largest group within the arthropod phylum. Insects have a chitinous exoskeleton, a three-part body (head, Thorax (inse ...

insect
s,
spider Spiders (order (biology), order Araneae) are air-breathing arthropods that have eight legs, chelicerae with fangs generally able to inject venom, and spinnerets that extrude spider silk, silk. They are the largest order of arachnids and rank se ...

spider
s and
squid Squid are cephalopod A cephalopod is any member of the molluscan Taxonomic rank, class Cephalopoda (Greek language, Greek plural , ; "head-feet") such as a squid, octopus, cuttlefish, or nautilus. These exclusively marine animals are cha ...

squid
s, have a ventral rather than dorsal system of
ganglion A ganglion is a group of neuron cell bodies in the peripheral nervous system. In the somatic nervous system this includes dorsal root ganglia and trigeminal ganglia among a few others. In the autonomic nervous system there are both sympath ...

ganglion
s, with a split
brain stem The brainstem (or brain stem) is the posterior stalk-like part of the brain A brain is an organ (biology), organ that serves as the center of the nervous system in all vertebrate and most invertebrate animals. It is located in the head, usuall ...
running on each side of the mouth or gut.


Molecular Signatures

In addition to the morphological characteristics used to define vertebrates (i.e. the presence of a notochord, the development of a vertebral column from the notochord, a dorsal nerve cord, pharyngeal gills, a post-anal tail, etc.), molecular markers known as
conserved signature indelsConserved signature inserts and deletions (CSIs) in protein sequences provide an important category of molecular markers for understanding phylogenetic relationships. CSIs, brought about by rare genetic changes, provide useful phylogenetic markers t ...
(CSIs) in protein sequences have been identified and provide distinguishing criteria for the subphylum Vertebrata. Specifically, 5 CSIs in the following proteins: protein synthesis elongation factor-2 (EF-2), eukaryotic translational initiation factor 3 (Euk IF-3),
adenosine kinase Adenosine kinase (AdK; EC 2.7.1.20) is an enzyme that catalyzes the transfer of gamma-phosphate from Adenosine triphosphate (Adenosine triphosphate, ATP) to adenosine (Ado) leading to formation of Adenosine monophosphate (Adenosine monophosphate, A ...
(AdK) and a protein related to ubiquitin carboxyl-terminal hydrolase are exclusively shared by all vertebrates and reliably distinguish them from all other
metazoan Animals (also called Metazoa) are multicellular eukaryotic organisms that form the Kingdom (biology), biological kingdom Animalia. With few exceptions, animals Heterotroph, consume organic material, Cellular respiration#Aerobic respiration, ...
. The CSIs in these protein sequences are predicted to play important functionally important in vertebrates. A specific relationship between Vertebrates and
Tunicate A tunicate is a marine 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 subphylum ...
s is also strongly supported by two CSIs found in the proteins predicted exosome complex RRP44 and
serine palmitoyltransferase In enzymology, a serine C-palmitoyltransferase () is an enzyme that catalysis, catalyzes the chemical reaction: :palmitoyl-CoA + L-serine \rightleftharpoons CoA + 3-dehydro-D-sphinganine + CO2 Thus, the two substrate (biochemistry), substrates ...
, that are exclusively shared by species from these two subphyla but not
Cephalochordate A cephalochordate (from Greek language, Greek: ', "head" and ', "chord") is an animal in the chordate subphylum, Cephalochordata. They are commonly called amphioxus or Lancelet, lancelets. They are chordates with all 5 Synapomorphy, synapomorphi ...
s, indicating Vertebrates are more closely related to
Tunicate A tunicate is a marine 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 subphylum ...
s than
Cephalochordate A cephalochordate (from Greek language, Greek: ', "head" and ', "chord") is an animal in the chordate subphylum, Cephalochordata. They are commonly called amphioxus or Lancelet, lancelets. They are chordates with all 5 Synapomorphy, synapomorphi ...
s.


Evolutionary history


First vertebrates

Vertebrates originated about 525 million years ago during the Cambrian explosion, which saw a rise in organism diversity. The earliest known vertebrate is believed to be ''
Myllokunmingia ''Myllokunmingia'' is a genus of basal chordate A chordate () is an animal Animals (also called Metazoa) are multicellular eukaryotic organisms that form the Kingdom (biology), biological kingdom Animalia. With few exceptions, anima ...

Myllokunmingia
''. One of many early vertebrates are '' Haikouichthys ercaicunensis''. Unlike the other fauna that dominated the Cambrian, these groups had the basic vertebrate body plan: a notochord, rudimentary vertebrae, and a well-defined head and tail. All of these early vertebrates lacked
jaw The jaw is any opposable articulated structure at the entrance of the mouth In animal anatomy Anatomy (Greek ''anatomē'', 'dissection') is the branch of biology concerned with the study of the structure of organism In biology, an ...

jaw
s in the common sense and relied on filter feeding close to the seabed. A vertebrate group of uncertain phylogeny, small eel-like
conodont Conodonts (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 million as ...
s, are known from microfossils of their paired tooth segments from the late Cambrian to the end of the Triassic.


From fish to amphibians

The first
jawed vertebrates Gnathostomata are the jawed vertebrates. The term derives from Greek language, Greek: (') "jaw" + (') "mouth". Gnathostome diversity comprises roughly 60,000 species, which accounts for 99% of all living vertebrates. In addition to opposing ja ...

jawed vertebrates
may have appeared in the late
Ordovician The Ordovician ( ) is a geologic period A geological period is one of the several subdivisions of geologic time enabling cross-referencing of rocks and geologic events from place to place. These periods form elements of a hierarchy of divisions i ...

Ordovician
(~450 mya) and became common in the
Devonian The Devonian ( ) is a geologic period and system of the Paleozoic The Paleozoic (or Palaeozoic) Era ( ; from the Greek ''palaiós'' (), "old" and ''zōḗ'' (), "life", meaning "ancient life") is the earliest of three geologic eras of the P ...
, often known as the "Age of Fishes". The two groups of
bony fishes Osteichthyes (), popularly referred to as the bony fish, is a diverse taxonomic group of fish Fish are Aquatic animal, aquatic, craniate, gill-bearing animals that lack Limb (anatomy), limbs with Digit (anatomy), digits. They form a siste ...
, the
actinopterygii Actinopterygii (New Latin New Latin (also called Neo-Latin or modern Latin) is the revival of Latin Latin (, or , ) is a classical language belonging to the Italic languages, Italic branch of the Indo-European languages. Latin was orig ...
and
sarcopterygii Sarcopterygii (; ) — sometimes considered synonymous with Crossopterygii () — is a taxon (traditionally a class (biology), class or subclass) of the Osteichthyes, bony fishes whose members are known as lobe-finned fishes. The group Tetrapoda, ...

sarcopterygii
, evolved and became common. The Devonian also saw the demise of virtually all jawless fishes save for lampreys and hagfish, as well as the
Placodermi Placodermi (from the Greek πλάξ = plate and δέρμα = skin, literally " plate-skinned") is a class Class or The Class may refer to: Common uses not otherwise categorized * Class (biology), a taxonomic rank * Class (knowledge representa ...
, a group of armoured fish that dominated the entirety of that period since the late
Silurian The Silurian ( ) is a geologic period and system spanning 24.6 million years from the end of the Ordovician Period, at million years ago ( Mya), to the beginning of the Devonian The Devonian ( ) is a period (geology), geologic period and sys ...
. The Devonian also saw the rise of the first
labyrinthodonts Labyrinthodontia (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 mil ...

labyrinthodonts
, which was a transitional form between fishes and
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, terr ...
s.


Mesozoic vertebrates

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 (; from grc, , ''klados'', "bran ...
s branched from labyrinthodonts in the subsequent
Carboniferous The Carboniferous ( ) is a geologic period A geological period is one of the several subdivisions of geologic time enabling cross-referencing of rocks and geologic events from place to place. These periods form elements of a hierarchy of divisi ...
period. The
Parareptilia Parareptilia ("at the side of reptiles") is a subclass or 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 o ...
and
synapsid Synapsids are a group of animals that includes mammals and every animal more closely related to mammals than to the other members of the amniote clade, such as reptiles and birds. Unlike other amniotes, they have a Skull#Fenestrae, temporal fenes ...

synapsid
amniotes were common during the late
Paleozoic The Paleozoic (or Palaeozoic) Era ( ; from the 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 popula ...
, while
diapsid 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 clad ...

diapsid
s became dominant during the
Mesozoic The Mesozoic Era ( ), also called the Age of Reptiles and the Age of Conifers, is the second-to-last era of Earth's geological history, lasting from about and comprising the Triassic The Triassic ( ) is a geologic period and system A system ...
. In the sea, the
bony fishes Osteichthyes (), popularly referred to as the bony fish, is a diverse taxonomic group of fish Fish are Aquatic animal, aquatic, craniate, gill-bearing animals that lack Limb (anatomy), limbs with Digit (anatomy), digits. They form a siste ...
became dominant.
Bird Birds are a group of warm-blooded vertebrates constituting 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 indiv ...

Bird
s, a derived form of
dinosaur Dinosaurs are a diverse group of reptile Reptiles, as most commonly defined, are the animals in the Class (biology), class Reptilia , a paraphyletic grouping comprising all amniotes except synapsids (mammals and their extinct relatives) an ...

dinosaur
, evolved in the
Jurassic The Jurassic ( ) is a Geological period, geologic period and System (stratigraphy), stratigraphic system that spanned from the end of the Triassic Period million years ago (Mya) to the beginning of the Cretaceous Period, approximately Mya. The Ju ...
. The demise of the non-avian dinosaurs at the end of the
Cretaceous The Cretaceous ( ) is a geological period A geological period is one of the several subdivisions of geologic time enabling cross-referencing of rocks and geologic events from place to place. These periods form elements of a hierarchy of divisions ...

Cretaceous
allowed for the expansion of
mammal Mammals (from Latin language, Latin , 'breast') are a group of vertebrate animals constituting the class (biology), class Mammalia (), and characterized by the presence of mammary glands which in Female#Mammalian female, females produce milk ...
s, which had evolved from the
therapsid Therapsida is a major group of eupelycosauria Eupelycosauria is a large 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—t ...

therapsid
s, a group of synapsid amniotes, during the late
Triassic The Triassic ( ) is a geologic period A geological period is one of the several subdivisions of geologic time enabling cross-referencing of rocks and geologic events from place to place. These periods form elements of a hierarchy of divisions int ...

Triassic
Period.


After the Mesozoic

The
Cenozoic The Cenozoic ( ; ) is Earth's current geological era An era is a span of time defined for the purposes of chronology or historiography, as in the regnal eras in the history of a given monarchy, a calendar era used for a given calendar, or the ge ...

Cenozoic
world has seen great diversification of bony fishes, amphibians, reptiles, birds and mammals. Over half of all living vertebrate species (about 32,000 species) are fish (non-tetrapod craniates), a diverse set of lineages that inhabit all the world's aquatic ecosystems, from snow minnows (Cypriniformes) in Himalayan lakes at elevations over to flatfishes (order Pleuronectiformes) in the Challenger Deep, the deepest ocean trench at about . Fishes of myriad varieties are the main predators in most of the world's water bodies, both freshwater and marine. The rest of the vertebrate species are tetrapods, a single lineage that includes amphibians (with roughly 7,000 species); mammals (with approximately 5,500 species); and reptiles and birds (with about 20,000 species divided evenly between the two classes). Tetrapods comprise the dominant megafauna of most terrestrial environments and also include many partially or fully aquatic groups (e.g., sea
snake Snakes are elongated, limbless, carnivorous reptiles of the suborder Serpentes . Like all other squamates, snakes are ectothermic, amniote Amniotes (from Greek ἀμνίον ''amnion'', "membrane surrounding the fetus", earlier "bowl ...

snake
s,
penguins Penguins ( order Sphenisciformes , family Spheniscidae ) are a group of aquatic flightless birds. They live almost exclusively in the Southern Hemisphere, with only one species, the Galápagos penguin, found north of the Equator Th ...

penguins
, cetaceans).


Classification

There are several ways of classifying animals.
Evolutionary systematics Evolutionary taxonomy, evolutionary systematics or Darwinian classification is a branch of biological classification In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical ...
relies on
anatomy Anatomy (Greek ''anatomē'', 'dissection') is the branch of biology concerned with the study of the structure of organism In biology, an organism (from Ancient Greek, Greek: ὀργανισμός, ''organismos'') is any individual conti ...

anatomy
,
physiology Physiology (; ) is the scientific study of functions and mechanisms in a living system. As a sub-discipline of biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, B ...
and
evolution Evolution is change in the heritable Heredity, also called inheritance or biological inheritance, is the passing on of Phenotypic trait, traits from parents to their offspring; either through asexual reproduction or sexual reproduction, ...

evolution
ary history, which is determined through similarities in anatomy and, if possible, the
genetics Genetics is a branch of biology concerned with the study of genes, genetic variation, and heredity in organisms.Hartl D, Jones E (2005) Though heredity had been observed for millennia, Gregor Mendel, Moravia, Moravian scientist and Augustinian ...

genetics
of organisms.
Phylogenetic classification Phylogenetic nomenclature is a method of nomenclature Nomenclature (, ) is a system of names or terms, or the rules for forming these terms in a particular field of arts or sciences. The principles of naming vary from the relatively informal naming ...

Phylogenetic classification
is based solely on
phylogeny , based on completely sequenced genomes. A phylogenetic tree (also phylogeny or evolutionary tree Felsenstein J. (2004). ''Inferring Phylogenies'' Sinauer Associates: Sunderland, MA.) is a branching diagram or a tree showing the evolution ...

phylogeny
. Evolutionary systematics gives an overview; phylogenetic systematics gives detail. The two systems are thus complementary rather than opposed.


Traditional classification

Conventional classification has living vertebrates grouped into seven classes based on traditional interpretations of gross
anatomical Anatomy (Greek ''anatomē'', 'dissection') is the branch of biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, mo ...

anatomical
and
physiological Physiology (; ) is the scientific study of functions and mechanisms in a living system. As a sub-discipline of biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, B ...
traits. This classification is the one most commonly encountered in school textbooks, overviews, non-specialist, and popular works. The
extant Extant 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 * Extant taxon, a taxon which is not extinct, s ...
vertebrates are: * Subphylum Vertebrata ** Class
Agnatha Agnatha (, Ancient Greek 'without jaws') is a superclass (biology), superclass of jawless fish in the phylum Chordata, subphylum Vertebrata, consisting of both present (Cyclostomata, cyclostomes) and extinct (conodonts and ostracoderms) species. ...
(jawless fishes) ** Class
Chondrichthyes Chondrichthyes (; ) is a class (biology), class that contains the cartilaginous fishes that have skeletons primarily composed of cartilage. They can be contrasted with the Osteichthyes or ''bony fishes'', which have skeletons primarily composed o ...

Chondrichthyes
(cartilaginous fishes) ** Class
Osteichthyes Osteichthyes (), popularly referred to as the bony fish, is a diverse Taxonomy (biology), taxonomic group of fish that have skeletons primarily composed of bone tissue. They can be contrasted with the Chondrichthyes, which have skeletons primaril ...

Osteichthyes
(bony fishes) ** Class
Amphibia Amphibians are ectothermic, tetrapod vertebrates of the class Amphibia. All living amphibians belong to the group Lissamphibia. They inhabit a wide variety of habitat In ecology Ecology (from el, οἶκος, "house" and ...

Amphibia
(amphibians) ** 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) ** Class
Aves 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 ...

Aves
(birds) ** Class
Mammalia Mammals (from Latin language, Latin , 'breast') are a group of vertebrate animals constituting the class (biology), class Mammalia (), and characterized by the presence of mammary glands which in Female#Mammalian female, females produce milk ...

Mammalia
(mammals) In addition to these, there are two classes of extinct armoured fishes, the
Placodermi Placodermi (from the Greek πλάξ = plate and δέρμα = skin, literally " plate-skinned") is a class Class or The Class may refer to: Common uses not otherwise categorized * Class (biology), a taxonomic rank * Class (knowledge representa ...
and the
Acanthodii Acanthodii or acanthodians is an extinct class of gnathostomes (jawed fishes), typically considered a paraphyletic In taxonomy, a group is paraphyletic if it consists of the group's last common ancestor and all descendants of that ances ...

Acanthodii
, both of which are considered
paraphyletic In taxonomy, a group is paraphyletic if it consists of the group's last common ancestor and all descendants of that ancestor excluding a few—typically only one or two—Monophyly, monophyletic subgroups. The group is said to be paraphyleti ...

paraphyletic
. Other ways of classifying the vertebrates have been devised, particularly with emphasis on the
phylogeny , based on completely sequenced genomes. A phylogenetic tree (also phylogeny or evolutionary tree Felsenstein J. (2004). ''Inferring Phylogenies'' Sinauer Associates: Sunderland, MA.) is a branching diagram or a tree showing the evolution ...

phylogeny
of and reptiles. An example based on Janvier (1981, 1997), Shu ''et al.'' (2003), and Benton (2004) is given here († =
extinct Extinction is the termination of a kind of organism or of a group of kinds (taxon), usually a species. The moment of extinction is generally considered to be the death of the endling, last individual of the species, although the Functional extin ...
): * Subphylum Vertebrata **'''' ** Infraphylum
Agnatha Agnatha (, Ancient Greek 'without jaws') is a superclass (biology), superclass of jawless fish in the phylum Chordata, subphylum Vertebrata, consisting of both present (Cyclostomata, cyclostomes) and extinct (conodonts and ostracoderms) species. ...
or Cephalaspidomorphi (
lamprey Lampreys (sometimes inaccurately called lamprey eels) are an ancient extant lineage of Agnatha, jawless fish of the order (biology), order Petromyzontiformes , placed in the superclass Cyclostomata. The adult lamprey may be characterized by ...

lamprey
s and other jawless fishes) ***Superclass Anaspidomorphi (anaspids and relatives) ** Infraphylum Gnathostomata (vertebrates with jaws) *** Class
Placodermi Placodermi (from the Greek πλάξ = plate and δέρμα = skin, literally " plate-skinned") is a class Class or The Class may refer to: Common uses not otherwise categorized * Class (biology), a taxonomic rank * Class (knowledge representa ...
(extinct armoured fishes) *** Class
Chondrichthyes Chondrichthyes (; ) is a class (biology), class that contains the cartilaginous fishes that have skeletons primarily composed of cartilage. They can be contrasted with the Osteichthyes or ''bony fishes'', which have skeletons primarily composed o ...

Chondrichthyes
(cartilaginous fishes) *** Class
Acanthodii Acanthodii or acanthodians is an extinct class of gnathostomes (jawed fishes), typically considered a paraphyletic In taxonomy, a group is paraphyletic if it consists of the group's last common ancestor and all descendants of that ances ...

Acanthodii
(extinct spiny "sharks") *** Superclass
Osteichthyes Osteichthyes (), popularly referred to as the bony fish, is a diverse Taxonomy (biology), taxonomic group of fish that have skeletons primarily composed of bone tissue. They can be contrasted with the Chondrichthyes, which have skeletons primaril ...

Osteichthyes
(bony vertebrates) **** Class Actinopterygii (ray-finned bony fishes) **** Class Sarcopterygii (lobe-finned fishes, including the tetrapods) *** Superclass Tetrapoda (four-limbed vertebrates) **** Class
Amphibia Amphibians are ectothermic, tetrapod vertebrates of the class Amphibia. All living amphibians belong to the group Lissamphibia. They inhabit a wide variety of habitat In ecology Ecology (from el, οἶκος, "house" and ...

Amphibia
(amphibians, some ancestral to the amniotes)—now a paraphyletic group **** Class Synapsida (mammals and the extinct mammal-like reptiles) **** Class Sauropsida (reptiles and birds) While this traditional classification is orderly, most of the groups are
paraphyletic In taxonomy, a group is paraphyletic if it consists of the group's last common ancestor and all descendants of that ancestor excluding a few—typically only one or two—Monophyly, monophyletic subgroups. The group is said to be paraphyleti ...

paraphyletic
, i.e. do not contain all descendants of the class's common ancestor. For instance, descendants of the first reptiles include modern reptiles as well as mammals and birds; the agnathans have given rise to the
jawed vertebrates Gnathostomata are the jawed vertebrates. The term derives from Greek language, Greek: (') "jaw" + (') "mouth". Gnathostome diversity comprises roughly 60,000 species, which accounts for 99% of all living vertebrates. In addition to opposing ja ...

jawed vertebrates
; the Osteichthyes, bony fishes have given rise to the tetrapoda, land vertebrates; the traditional "Labyrinthodont, amphibians" have given rise to the Reptilia, reptiles (traditionally including the
synapsid Synapsids are a group of animals that includes mammals and every animal more closely related to mammals than to the other members of the amniote clade, such as reptiles and birds. Unlike other amniotes, they have a Skull#Fenestrae, temporal fenes ...

synapsid
s or mammal-like "reptiles"), which in turn have given rise to the mammals and birds. Most scientists working with vertebrates use a classification based purely on
phylogeny , based on completely sequenced genomes. A phylogenetic tree (also phylogeny or evolutionary tree Felsenstein J. (2004). ''Inferring Phylogenies'' Sinauer Associates: Sunderland, MA.) is a branching diagram or a tree showing the evolution ...

phylogeny
, organized by their known evolutionary history and sometimes disregarding the conventional interpretations of their anatomy and physiology.


Phylogenetic relationships

In Phylogenetics, phylogenetic taxonomy, the relationships between animals are not typically divided into ranks but illustrated as a nested "family tree" known as a phylogenetic tree. The cladogram below is based on studies compiled by Philippe Janvier and others for the ''Tree of Life Web Project'' and Delsuc et al.,Janvier, P. 1997. Vertebrata. Animals with backbones. Version 1 January 1997 (under construction). http://tolweb.org/Vertebrata/14829/1997.01.01 in The Tree of Life Web Project, http://tolweb.org/ and complemented (based on and ). A dagger (symbol), dagger (†) denotes an
extinct Extinction is the termination of a kind of organism or of a group of kinds (taxon), usually a species. The moment of extinction is generally considered to be the death of the endling, last individual of the species, although the Functional extin ...
clade, whereas all other clades have living extant taxa, descendants. fish ''Diplacanthus acus'' from the
Devonian The Devonian ( ) is a geologic period and system of the Paleozoic The Paleozoic (or Palaeozoic) Era ( ; from the Greek ''palaiós'' (), "old" and ''zōḗ'' (), "life", meaning "ancient life") is the earliest of three geologic eras of the P ...
period (geology), period File:Cheirolepis canadensis.jpg, The early ray-finned fish, ray-fin ''Cheirolepis, Cheirolepis canadensis'' from the
Devonian The Devonian ( ) is a geologic period and system of the Paleozoic The Paleozoic (or Palaeozoic) Era ( ; from the Greek ''palaiós'' (), "old" and ''zōḗ'' (), "life", meaning "ancient life") is the earliest of three geologic eras of the P ...
period (geology), period Note that
Acanthodii Acanthodii or acanthodians is an extinct class of gnathostomes (jawed fishes), typically considered a paraphyletic In taxonomy, a group is paraphyletic if it consists of the group's last common ancestor and all descendants of that ances ...

Acanthodii
, the "spiny sharks", were shown to be either a
paraphyletic In taxonomy, a group is paraphyletic if it consists of the group's last common ancestor and all descendants of that ancestor excluding a few—typically only one or two—Monophyly, monophyletic subgroups. The group is said to be paraphyleti ...

paraphyletic
or a polyphyletic group, with some taxa being more closely related with
cartilaginous fish Chondrichthyes (; from Greek χονδρ- 'cartilage', ἰχθύς 'fish') is a class that contains the cartilaginous fishes that have skeletons primarily composed of cartilage. They can be contrasted with the Osteichthyes or ''bony fishes'' ...
, others more closely related with
bony fish Osteichthyes (), popularly referred to as the bony fish, is a diverse Taxonomy (biology), taxonomic group of fish that have skeletons primarily composed of bone tissue. They can be contrasted with the Chondrichthyes, which have skeletons primaril ...
, and again others being more basal (phylogeny), basal on the tree of life. Similarly, the
Placodermi Placodermi (from the Greek πλάξ = plate and δέρμα = skin, literally " plate-skinned") is a class Class or The Class may refer to: Common uses not otherwise categorized * Class (biology), a taxonomic rank * Class (knowledge representa ...
and Ostracodermi are not anymore considered
monophyletic 300px, A cladogram of the primates, showing a ''monophyletic'' taxon: ''the simians'' (in yellow); a ''paraphyletic'' taxon: ''the prosimians'' (in cyan, including the red patch); and a ''polyphyletic'' group: ''the night-active primates, i.e., ...

monophyletic
groups. Also note that Teleostei (Neopterygii) and Tetrapoda (
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, terr ...
s,
mammal Mammals (from Latin language, Latin , 'breast') are a group of vertebrate animals constituting the class (biology), class Mammalia (), and characterized by the presence of mammary glands which in Female#Mammalian female, females produce milk ...
s, reptiles, birds) each make up about 50% of today's vertebrate biodiversity, diversity, while all other groups are either
extinct Extinction is the termination of a kind of organism or of a group of kinds (taxon), usually a species. The moment of extinction is generally considered to be the death of the endling, last individual of the species, although the Functional extin ...
or rare. The next cladogram shows the extant clades of tetrapods (the four-limbed vertebrates), and a selection of
extinct Extinction is the termination of a kind of organism or of a group of kinds (taxon), usually a species. The moment of extinction is generally considered to be the death of the endling, last individual of the species, although the Functional extin ...
(†) groups: Note that Reptiliomorpha, reptile-like amphibians, mammal-like reptiles, and non-avian dinosaurs are all
paraphyletic In taxonomy, a group is paraphyletic if it consists of the group's last common ancestor and all descendants of that ancestor excluding a few—typically only one or two—Monophyly, monophyletic subgroups. The group is said to be paraphyleti ...

paraphyletic
. The placement of hagfish on the vertebrate tree of life has been controversial. Their lack of proper vertebrae (among with other characteristics found in lampreys and jawed vertebrates) led phylogenetic analyses based on morphology (biology), morphology to place them outside Vertebrata. Molecular data, however, indicates they are vertebrates closely related to
lampreys Lampreys (sometimes inaccurately called lamprey eels) are an ancient extant lineage of jawless fish of the order Petromyzontiformes , placed in the superclass Cyclostomata Cyclostomata is a group of agnathans that comprises the living ...

lampreys
. A study by Miyashita ''et al''. (2019), 'reconciliated' the two types of analysis as it supports the Cyclostomata hypothesis using only morphological data.


Number of extant species

The number of described vertebrate species are split between
tetrapod Tetrapods (; ) are four-limbed animals constituting the superclass Tetrapoda (). It includes Neontology#Extant taxa vs. extinct taxa, extant and extinct amphibians, reptiles (including dinosaurs and therefore birds), and synapsids (including mam ...
s and
fish Fish are Aquatic animal, aquatic, craniate, gill-bearing animals that lack Limb (anatomy), limbs with Digit (anatomy), digits. Included in this definition are the living hagfish, lampreys, and Chondrichthyes, cartilaginous and bony fish as we ...

fish
. The following table lists the number of described Extant taxa, extant species for each vertebrate Class (biology), class as estimated in the IUCN Red List of Threatened Species, 2014.3.The World Conservation Union. 2014. ''IUCN Red List of Threatened Species'', 2014.3. Summary Statistics for Globally Threatened Species
Table 1: Numbers of threatened species by major groups of organisms (1996–2014)
The IUCN estimates that 1,305,075 Invertebrate#Number of extant species, extant invertebrate species have been described, which means that less than 5% of the Animal#Number of extant species, described animal species in the world are vertebrates.


Vertebrate species databases

The following databases maintain (more or less) up-to-date lists of vertebrate species: * Fish: FishBase, Fishbase * Amphibians
Amphibiaweb
* Reptiles: Reptile Database * Birds
Avibase
* Mammals
Mammal species of the World


Reproductive systems

Nearly all vertebrates undergo sexual reproduction. They produce Ploidy#Haploid and monoploid, haploid gametes by meiosis. The smaller, motile gametes are Spermatozoon, spermatozoa and the larger, non-motile gametes are Egg cell, ova. These fuse by the process of fertilisation to form diploid zygotes, which develop into new individuals.


Inbreeding

During sexual reproduction, mating with a close relative (inbreeding) often leads to inbreeding depression. Inbreeding depression is considered to be largely due to expression of deleterious Dominance (genetics), recessive mutations. The effects of inbreeding have been studied in many vertebrate species. In several species of fish, inbreeding was found to decrease reproductive success. Inbreeding was observed to increase juvenile mortality in 11 small animal species. A common breeding practice for pet dogs is mating between close relatives (e.g. between half- and full siblings). This practice generally has a negative effect on measures of reproductive success, including decreased litter size and puppy survival. Inbreeding, Incestuous matings in birds result in severe Fitness (biology), fitness costs due to inbreeding depression (e.g. reduction in hatchability of eggs and reduced progeny survival).


Inbreeding avoidance

As a result of the negative fitness consequences of inbreeding, vertebrate species have evolved mechanisms to avoid inbreeding. Numerous inbreeding avoidance mechanisms operating prior to mating have been described. Toads and many other amphibians display Philopatry, breeding site fidelity. Individuals that return to natal ponds to breed will likely encounter siblings as potential mates. Although Inbreeding, incest is possible, ''Bufo americanus'' siblings rarely mate. These toads likely recognize and actively avoid close kin as mates. Advertisement vocalizations by males appear to serve as cues by which females recognize their kin. Inbreeding avoidance mechanisms can also operate subsequent to copulation (zoology), copulation. In guppies, a post-copulatory mechanism of inbreeding avoidance occurs based on competition between sperm of rival males for achieving fertilization. In competitions between sperm from an unrelated male and from a full sibling male, a significant bias in paternity towards the unrelated male was observed. When female sand lizards mate with two or more males, sperm competition within the female's reproductive tract may occur. Active selection of sperm by females appears to occur in a manner that enhances female fitness. On the basis of this selective process, the sperm of males that are more distantly related to the female are preferentially used for fertilization, rather than the sperm of close relatives. This preference may enhance the fitness of progeny by reducing inbreeding depression.


Outcrossing

Mating with unrelated or distantly related members of the same species is generally thought to provide the advantage of masking deleterious recessive mutations in progeny (see heterosis). Vertebrates have evolved numerous diverse mechanisms for avoiding close inbreeding and promoting outcrossing (see inbreeding avoidance). Outcrossing as a way of avoiding inbreeding depression has been especially well studied in birds. For instance, inbreeding depression occurs in the great tit (''Parus major'') when the offspring are produced as a result of a mating between close relatives. In natural populations of the great tit, inbreeding is avoided by dispersal of individuals from their birthplace, which reduces the chance of mating with a close relative. Purple-crowned fairywren females paired with related males may undertake Extra-pair copulation, extra-pair matings that can reduce the negative effects of inbreeding, despite ecological and demographic constraints. Southern pied babblers (''Turdoides bicolor'') appear to avoid inbreeding in two ways: through dispersal and by avoiding familiar group members as mates. Although both males and females disperse locally, they move outside the range where genetically related individuals are likely to be encountered. Within their group, individuals only acquire breeding positions when the opposite-sex breeder is unrelated. Cooperative breeding in birds typically occurs when offspring, usually males, delay dispersal from their natal group in order to remain with the family to help rear younger kin. Female offspring rarely stay at home, dispersing over distances that allow them to breed independently or to join unrelated groups.


Parthenogenesis

Parthenogenesis is a natural form of reproduction in which growth and development of embryos occur without fertilization. Reproduction in Squamata, squamate reptiles is ordinarily sexual, with males having a ZZ pair of sex determining chromosomes, and females a ZW pair. However, various species, including the Epicrates maurus, Colombian Rainbow boa (''Epicrates maurus''), ''Agkistrodon contortrix'' (copperhead snake) and ''Agkistrodon piscivorus'' (cotton mouth snake) can also reproduce by facultative parthenogenesis—that is, they are capable of switching from a sexual mode of reproduction to an Asexual reproduction, asexual mode—resulting in production of WW female progeny. The WW females are likely produced by Thelytoky#Automixis with terminal fusion, terminal automixis. Mole salamanders are an ancient (2.4–3.8 million year-old) unisexual vertebrate lineage. In the polyploid unisexual mole salamander females, a premeiotic endomitotic event doubles the number of chromosomes. As a result, the mature eggs produced subsequent to the two meiotic divisions have the same ploidy as the somatic cells of the female salamander. Synapsis and recombination during meiotic prophase I in these unisexual females is thought to ordinarily occur between identical sister chromosomes and occasionally between homologous chromosomes. Thus little, if any, genetic variation is produced. Recombination between Ploidy#Homoeologous, homeologous chromosomes occurs only rarely, if at all. Since production of genetic variation is weak, at best, it is unlikely to provide a benefit sufficient to account for the long-term maintenance of meiosis in these organisms.


Self-fertilization

Two killifish species, the mangrove killifish (''Kryptolebias marmoratus'') and ''Kryptolebias hermaphroditus'', are the only known vertebrates to self-fertilize. They produces both eggs and sperm by meiosis and routinely reproduces by Reproduction#Autogamy, self-fertilisation. This capacity has apparently persisted for at least several hundred thousand years. Each individual hermaphrodite normally fertilizes itself through uniting inside the fish's body of an egg and a sperm that it has produced by an internal organ. In nature, this mode of reproduction can yield highly homozygous lines composed of individuals so genetically uniform as to be, in effect, identical to one another. Although inbreeding, especially in the extreme form of self-fertilization, is ordinarily regarded as detrimental because it leads to expression of deleterious recessive alleles, self-fertilization does provide the benefit of ''fertilization assurance'' (Fertilisation#Self-Pollination, reproductive assurance) at each generation.


Population trends

The Living Planet Index, following 16,704 populations of 4,005 species of vertebrates, shows a decline of 60% between 1970 and 2014. Since 1970, freshwater species declined 83%, and tropical populations in South and Central America declined 89%. The authors note that, "An average trend in population change is not an average of total numbers of animals lost." According to World Wide Fund for Nature, WWF, this could lead to a sixth Extinction event, major extinction event. The five main causes of biodiversity loss are land-use change, Overexploitation, overexploitation of natural resources, climate change, pollution and invasive species.


See also

* * Exoskeleton * *


References


Bibliography

* *


External links


Tree of Life


*[http://entomology.ifas.ufl.edu/fasulo/vector/chapter_07.htm Vertebrate Pests] chapter in United States Environmental Protection Agency and University of Florida/Institute of Food and Agricultural Sciences National Public Health Pesticide Applicator Training Manual
The Vertebrates


Marc W. Kirschner, ''iBioSeminars'', 2008. {{Authority control Vertebrates, * Terreneuvian first appearances Extant Cambrian first appearances