Saturday 21 January 2012
VERTIBRATES
Vertebrates (pronounced / ˈvɜrtɨbrəts/ ) are animals that are members of the subphylum Vertebrata (chordates with backbones and spinal columns). Vertebrates are the largest
group of chordates, with
currently about 58,000 species described.[2] Vertebrates include the jawless fishes , bony fishes , sharks and rays , amphibians, reptiles, mammals, and birds. Extant vertebrates range in size from
the frog species Paedophryne amauensis, at as little as 7.7 mm (0.3 inch), to the blue whale, at up to 33 m (110 ft). Vertebrates make up about
5% of all described animal
species; the rest are invertebrates , which lack backbones. The vertebrates traditionally
include the hagfishes, which do not have proper vertebrae,
though their closest living
relatives, the lampreys , do have vertebrae. [3] Hagfishes do, however, possess a cranium. For this reason, the vertebrate subphylum is
sometimes referred to as
"Craniata" when discussing morphology. Molecular
analysis since 1992 has
suggested that the hagfishes
are most closely related to lampreys [4], and so also are vertebrates in a monophyletic sense. Others consider them a
sister group of vertebrates in
the common taxon of Craniata.[5] Etymology The word vertebrate derives
from the Latin word
vertebratus ( Pliny), meaning joint of the spine. [6] It is closely related to the word vertebra , which refers to any of the bones or segments of the spinal column.[7] Anatomy and
morphology All vertebrates are built along
the basic chordate body plan : a stiff rod running through
the length of the animal
(vertebral column or notochord),[8] with a hollow tube of nervous tissue (the spinal cord) above it and the gastrointestinal tract below. In all vertebrates, the mouth
is found at, or right below,
the anterior end of the animal,
while the anus opens to the exterior before the end of the
body. The remaining part of
the body continuing aft of the
anus forms a tail with vertebrae and spinal cord, but no gut.[9] Vertebral column The defining characteristic of a
vertebrate is the vertebral column, in which the notochord (a stiff rod of uniform composition) found
in all chordates has been replaced by a segmented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[10] and the Latimeria. Jawed vertebrates are typified by paired appendages (fins or
legs, which may be
secondarily lost), but this is
not part of the definition of
vertebrates as a whole. Fossilized skeleton of Diplodocus carnegii, showing an extreme example of the backbone that characterizes the vertebrates. Exhibited at the Museum für Naturkunde (Museum of Natural Science), Berlin. Gills Gill arches bearing gills in a pike All basal vertebrates breathe with gills. The gills are carried right behind the head,
bordering the posterior
margins of a series of
openings from the esophagus to the exterior. Each gill is
supported by a cartilagenous or bony gill arch.[11] The bony fish have three pairs of arches, cartilaginous fish have five to seven pairs, while the
primitive jawless fish have seven. The vertebrate
ancestor no doubt had more
arches, as some of their chordate relatives have more than 50 pairs of gills.[9] In amphibians and some primitive bony fishes, the larvae bear external gills, branching off from the gill arches.[12] These are reduced in adulthood, their function
taken over by the gills proper
in fishes and by lungs in most amphibians. Some amphibans
retain the external larval gills
in adulthood, the complex
internal gill system as seen in
fish apparently being
irrevocably lost very early in the evolution of tetrapods. [13] While the higher vertebrates do not have gills, the gill
arches form during fetal developement , and lay the basis of essential structures
such as jaws , the thyroid gland, the larynx , the columella (corresponding to
the stapes in mammals) and in mammals the malleus and incus.[9] Central nervous system The vertebrates are the only chordate group to exhibit a proper brain. A slight swelling of the anterior end of the nerve cord is found in the lancelet, though it lacks the eyes and other complex sense
organs comparable to those of
vertebrates. Other chordates
do not show any trends towards cephalisation. [9] The central nervous system is based on a hollow nerve tube
running along the length of
the animal, form which the peripheral nervous system branches out to enervate 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 (forebrain), mesencephalon (midbrain) and rhombencephalon (hindbrain), further differentiated in the various vertebrate groups. [14] Two laterally placed eyes form around outgrows from
the midbrain, except in hagfish, though this may be a secondary loss. [15][16] The forebrain is well developed
and subdivided in most tetrapods, while the midbrain dominate in many fish and some salamanders. Vesicles of the forebrain are usually
paired, giving rise to
hemispheres like the cerebral hemispheres in mammals.[14] 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 with well developed brains,
like insects, spiders and squids have a ventral rather than
dorsal system of ganglions, with a split brain stem running on each side of the mouth/gut. [9] Evolutionary history First vertebrates The early vertebrate Haikouichthys Vertebrates originated about
525 million years ago during
the Cambrian explosion , which saw the rise in
organism diversity. The
earliest known vertebrate is
believed to be the Myllokunmingia .[1] Another early vertebrate is 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.[17] All of these early vertebrates lacked jaws in the common sense and
relied on filter feeding close to the seabed.[18] From fishes to amphibians Acanthostega , a fish-like early labyrinthodont . The first jawed vertebrates appeared in the Ordovician and became common in the Devonian , often known as the "Age of Fishes". [19] The two groups of bony fishes , the actinopterygii and sarcopterygii , evolved and became common.[20] 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
much of the late Silurian. The Devonian also saw the rise of
the first labyrinthodonts , which was a transitional
between fishes and amphibians. Mesozoic vertebrates Silesaurus, an archosaur The reptiles appeared from labyrinthodonts in the
subsequent Carboniferous period. The anapsid and synapsid reptiles were common during the late Paleozoic, while the diapsids became dominant during the Mesozoic. In the sea, the bony fishes became dominant. The dinosaurs gave rise to the birds in the Jurassic.[21] The demise of the dinosaurs at the
end of the Cretaceous promoted expansion of the mammals, which had evolved from the therapsids, a group of synapsid reptiles, during
the late Triassic Period. After the Mesozoic Frogs like Palaeobatrachus were among the animals to
successfully diversify in the post-Mesozoic world The Cenozoic world has seen great diversification of bony
fishes, frogs, birds and
mammals. Over half of all living
vertebrate species (about
32,000 species) are fishes (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 4,600 metres (15,000 feet)
to flatfishes (order
Pleuronectiformes) in the Challenger Deep, the deepest
ocean trench at about 11,000
metres (36,000 feet). 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
(frogs, with more than 5,800
species; salamanders, with about 580 species; and
caecilians, with about 175
species); mammals (with over
5,400 species); and reptiles and
birds (with more than 18,000
species). Tetrapods dominate the megafauna of most
terrestrial environments
(including fossorial and
arboreal realms) and also
include many partially or fully
aquatic groups (e.g., sea snakes, penguins, cetaceans). Classification There are several ways of
classifying animals. Evolutionary systematics relies on anatomy, physiology and evolutionary history, which is determined through
similarities in anatomy and, if
possible, the genetics of organisms. Phylogenetic classification is based solely on phylogeny .[22] Evolutionary systematics gives an
overview; phylogenetic
systematics gives detail. The
two systems are thus
complementary rather than opposed.[23] Traditional classification Traditional spindle diagram of the evolution of the vertebrates at class level Conventional classification has
living vertebrates grouped
into seven classes based on
traditional interpretations of
gross anatomical and physiological traits. This classification is the one most
commonly encountered in
school textbooks, overviews,
non-specialist, and popular
works. The extant vertebrates are: [9] 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)
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