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Vertebrates () comprise all species of
animal Animals (also called Metazoa) are multicellular A multicellular organism is an organism In biology, an organism () is any organic, life, living system that functions as an individual entity. All organisms are composed of cells ...

animal
s within the
subphylum In zoological nomenclature The International Code of Zoological Nomenclature (ICZN) is a widely accepted Convention (norm), convention in zoology that rules the formal scientific name, scientific naming of organisms treated as animals. It is als ...
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
Chordata
, with currently about 69,963
species In biology, a species is the basic unit of biological classification, classification and a taxonomic rank of an organism, as well as a unit of biodiversity. A species is often defined as the largest group of organisms in which any two individu ...

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 Eels are ray-finned fish belonging to the order Anguilliformes (), which consists of eight suborders, 19 families, 111 genera Genus (plural ...

hagfish
and
lampreys Lampreys (sometimes inaccurately called lamprey eels) are an ancient extant lineage of jawless fish Agnatha (, Ancient Greek Ancient Greek includes the forms of the Greek language used in ancient Greece and the classical antiquity ...

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 (; ) 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 ...

cartilaginous fish
(
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 withi ...

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 taxonomic Taxonomy (general) is the practice and science of classification of things or concepts, including the principles that underlie such classification. The term may al ...

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 (; ) are four-limbed animals constituting the superclass Tetrapoda (). It includes extant Extant is the opposite of the word extinct Extinction is the termination of a kind of organism or of a group of kinds (taxon), usually a sp ...

tetrapods
(limbed vertebrates)
Extant Extant is the opposite of the word extinct Extinction is the termination of a kind of organism or of a group of kinds (taxon), usually a species. The moment of extinction is generally considered to be the death of the endling, last individual o ...
vertebrates range in size from the
frog A frog is any member of a diverse and largely carnivorous A carnivore , meaning "meat Meat is animal flesh that is eaten as food. Humans have hunted and killed animals for meat since prehistoric times. The advent of civilization all ...

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 Eels are ray-finned fish belonging to the order Anguilliformes (), which consists of eight suborders, 19 families, 111 genera Genus (plural ...

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 c ...
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 Eels are ray-finned fish belonging to the order Anguilliformes (), which consists of eight suborders, 19 families, 111 genera Genus (plural ...

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 In cladistics for a group of organisms, monophyly is the condition of being a clade—that is, a group of taxa composed only of a common ancestor (or more precisely an ancestral population) and all of its lineal descendants. Monophyletic grou ...

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 ...

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 c ...
'', 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 organisms and their parts. Anatomy is a branch of natural science which deals with the structural organization of li ...
), 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 contain ...

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 gastrointestinal tract The gastrointestinal tract, (GI tract, GIT, d ...
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 A classical language is a language A language is a structured system of communication Communication (from Latin ''communicare'', meaning "to share" or "to be in ...

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 organisms and their parts. Anatomy is a branch of natural science which deals with the structural organization of li ...
(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 s ...

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 that many aquatic Aquatic means relating to water Water (chemical formula H2O) is an inorganic, transparent, tasteless, odorless, and nearly colorless chemical substance, which is the main constituent ...

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 respiratory organ that many aquatic Aquatic means relating to water Water (chemical formula H2O) is an inorganic, transparent, tasteless, odorless, and nearly colorless chemical substance, which is the main constituent ...
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 taxonomic Taxonomy (general) is the practice and science of classification of things or concepts, including the principles that underlie such classification. The term may al ...
have three pairs of arches,
cartilaginous fish 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 ...
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 Animals (also called Metazoa) are multicellular A multicellular organism is an organism In biology, an organism () is any organic, life, living system that functions as an individual entity. All ...

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, ter ...
s and some primitive bony
fishes Fish are aquatic Aquatic means relating to water Water (chemical formula H2O) is an inorganic, transparent, tasteless, odorless, and nearly colorless chemical substance, which is the main constituent of Earth's hydrosphere and the ...

fishes
, the
larva A larva (plural larvae ) is a distinct juvenile form many animal Animals (also called Metazoa) are multicellular A multicellular organism is an organism In biology, an organism () is any organic, life, living system that f ...
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 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 ...

lung
s in most amphibians. Some amphibians retain the external larval gills in adulthood, the complex internal
gill A gill () is a respiratory organ that many aquatic Aquatic means relating to water Water (chemical formula H2O) is an inorganic, transparent, tasteless, odorless, and nearly colorless chemical substance, which is the main constituent ...
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 extant Extant is the opposite of the word extinct Extinction is the termination of a kind of organism or of a group of kinds (taxon), usually a sp ...
s.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 , typically used for grasping and manipulating food. The term ''jaws'' is also broadly applied to the whole of the structures constituting the vault of the mouth and serving t ...

jaw
s, the
thyroid gland The thyroid, or thyroid gland, is an endocrine gland Endocrine glands are ductless glands of the endocrine system The endocrine system is a messenger system comprising feedback loops of the hormones released by internal glands of an organis ...

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 ...

larynx
, the ''columella'' (corresponding to the
stapes The ''stapes'' or stirrup is a bone in the middle ear of humans and other animals which is involved in the conduction of sound vibrations to the inner ear. This bone is connected to the oval window by its Annular ligament of stapes, annular ligam ...
in
mammal Mammals (from Latin Latin (, or , ) is a classical language A classical language is a language A language is a structured system of communication Communication (from Latin ''communicare'', meaning "to share" or "to be i ...
s) 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 is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecu ...

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 kingd ...

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 ...
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 Animals (also called Metazoa) are multicellular A multicellular organism is an organism In biology, an organism () is any organic, life, living system that functions as an individual entity. All ...
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 Organ may refer to: Biology * Organ (anatomy) An organ is a group of Tissue (biology), tissues with similar functions. Plant life and animal life rely on many organs that co-exist in organ systems. A given organ's tis ...

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 is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, ...
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. They provide living organisms with vision, the ability to receive and process visual detail, as well as enabling several photo response functions that are independent of vision. Eyes detect light L ...

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 Eels are ray-finned fish belonging to the order Anguilliformes (), which consists of eight suborders, 19 families, 111 genera Genus (plural ...

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 extant Extant is the opposite of the word extinct Extinction is the termination of a kind of organism or of a group of kinds (taxon), usually a sp ...
s, while the midbrain dominates in many
fish Fish are aquatic Aquatic means relating to water Water (chemical formula H2O) is an inorganic, transparent, tasteless, odorless, and nearly colorless chemical substance, which is the main constituent of Earth's hydrosphere and the ...

fish
and some
salamander Salamanders are a group of 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 spe ...

salamander
s. Vesicles of the forebrain are usually paired, giving rise to hemispheres like the
cerebral hemisphere The vertebrate cerebrum (brain) is formed by two cerebral hemispheres that are separated by a groove, the longitudinal fissure. The brain can thus be described as being divided into left and right cerebral hemispheres. Each of these hemispheres ...
s in
mammal Mammals (from Latin Latin (, or , ) is a classical language A classical language is a language A language is a structured system of communication Communication (from Latin ''communicare'', meaning "to share" or "to be i ...
s. 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 Latin (, or , ) is a classical language A classical language is a language A language is a structured system of communication Communication (from Latin ''communicare'', meaning "to share" or "to be in ...

insect
s,
spider Spiders (order Order or ORDER or Orders may refer to: * Orderliness Orderliness is associated with other qualities such as cleanliness Cleanliness is both the abstract state of being clean and free from germs, dirt, trash, or waste, and the ...

spider
s and
squid Squid are cephalopod A cephalopod is any member of the mollusca Mollusca is the second-largest phylum In biology, a phylum (; plural The plural (sometimes list of glossing abbreviations, abbreviated ), in many languages, is ...

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 The peripheral nervous system (PNS) is one of two components that make up the nervous system In biology Biology is the natural science that studies li ...

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 Organ may refer to: Biology * Organ (anatomy) An organ is a group of Tissue (biology), tissues with similar functions. Plant life and anima ...
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 animal, a member of the subphylum Tunicata (). It is part of the Chordata, a phylum which includes all animals with dorsal nerve cords and notochords (including vertebrates). The subphylum was at one time cal ...
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 animal, a member of the subphylum Tunicata (). It is part of the Chordata, a phylum which includes all animals with dorsal nerve cords and notochords (including vertebrates). The subphylum was at one time cal ...
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 , typically used for grasping and manipulating food. The term ''jaws'' is also broadly applied to the whole of the structures constituting the vault of the mouth and serving t ...

jaw
s in the common sense and relied on filter feeding close to the seabed. A vertebrate group of uncertain phylogeny, small eel-like conodonts, 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 (~450 mya) and became common in the Devonian, often known as the "Age of Fishes". The two groups of bony fishes, the actinopterygii and 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, a group of armoured fish that dominated the entirety of that period since the late Silurian. The Devonian also saw the rise of the first Labyrinthodontia, 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, ter ...
s.


Mesozoic vertebrates

Amniotes branched from labyrinthodonts in the subsequent Carboniferous period. The Parareptilia and synapsid amniotes were common during the late Paleozoic, while diapsids became dominant during the Mesozoic. In the sea, the bony fishes became dominant. Birds, a derived form of dinosaurs, dinosaur, evolved in the Jurassic. The demise of the non-avian dinosaurs at the end of the Cretaceous allowed for the expansion of
mammal Mammals (from Latin Latin (, or , ) is a classical language A classical language is a language A language is a structured system of communication Communication (from Latin ''communicare'', meaning "to share" or "to be i ...
s, which had evolved from the therapsids, a group of synapsid amniotes, during the late Triassic Period.


After the Mesozoic

The 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 snakes, penguins, cetaceans).


Classification

There are several ways of classifying animals. Evolutionary taxonomy, Evolutionary systematics relies on anatomy, physiology and evolutionary history, which is determined through similarities in anatomy and, if possible, the genetics of organisms. Cladistics, Phylogenetic classification is based solely on 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 anatomy, anatomical and Physiology, physiological traits. This classification is the one most commonly encountered in school textbooks, overviews, non-specialist, and popular works. The extant taxon, extant vertebrates are: * Subphylum Vertebrata ** Class Agnatha (jawless fishes) ** Class Chondrichthyes (cartilaginous fishes) ** Class Osteichthyes (bony fishes) ** Class Amphibia (amphibians) ** Class Reptilia (reptiles) ** Class Aves (birds) ** Class Mammalia (mammals) In addition to these, there are two classes of extinct armoured fishes, the Placodermi and the Acanthodii, both of which are considered paraphyletic. Other ways of classifying the vertebrates have been devised, particularly with emphasis on the phylogeny of labyrinthodontia, early amphibians and reptiles. An example based on Janvier (1981, 1997), Shu ''et al.'' (2003), and Benton (2004) is given here († = extinct): * Subphylum Vertebrata **''Palaeospondylus'' ** Infraphylum Agnatha 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 (extinct armoured fishes) *** Class Chondrichthyes (cartilaginous fishes) *** Class Acanthodii (extinct spiny "sharks") *** Superclass 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, 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, 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 synapsids 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, 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 clade, whereas all other clades have living extant taxa, descendants. Note that Acanthodii, the "spiny sharks", were shown to be either a paraphyletic or a polyphyletic group, with some taxa being more closely related with
cartilaginous fish 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 ...
, others more closely related with
bony fish Osteichthyes (), popularly referred to as the bony fish, is a diverse taxonomic Taxonomy (general) is the practice and science of classification of things or concepts, including the principles that underlie such classification. The term may al ...
, and again others being more basal (phylogeny), basal on the tree of life. Similarly, the Placodermi and Ostracodermi are not anymore considered
monophyletic In cladistics for a group of organisms, monophyly is the condition of being a clade—that is, a group of taxa composed only of a common ancestor (or more precisely an ancestral population) and all of its lineal descendants. Monophyletic grou ...

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, ter ...
s,
mammal Mammals (from Latin Latin (, or , ) is a classical language A classical language is a language A language is a structured system of communication Communication (from Latin ''communicare'', meaning "to share" or "to be i ...
s, reptiles, birds) each make up about 50% of today's vertebrate biodiversity, diversity, while all other groups are either extinct or rare. The next cladogram shows the extant clades of tetrapods (the four-limbed vertebrates), and a selection of extinct (†) groups: Note that Reptiliomorpha, reptile-like amphibians, mammal-like reptiles, and non-avian dinosaurs are all 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 Agnatha (, Ancient Greek Ancient Greek includes the forms of the Greek language used in ancient Greece and the classical antiquity ...

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 extant Extant is the opposite of the word extinct Extinction is the termination of a kind of organism or of a group of kinds (taxon), usually a sp ...
s and
fish Fish are aquatic Aquatic means relating to water Water (chemical formula H2O) is an inorganic, transparent, tasteless, odorless, and nearly colorless chemical substance, which is the main constituent of Earth's hydrosphere and the ...

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