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129-398: Ossea Batsch, 1788 Vertebrates ( / ˈ v ɜːr t ə b r ɪ t s , - ˌ b r eɪ t s / ) are animals with a backbone or spine, consisting of vertebrae and intervertebral discs . The vertebrae are irregular bones , and the intervertebral discs are of fibrocartilage . The vertebral column surrounds and protects the spinal cord . The other feature unique to vertebrates is

258-399: A gastrula with a digestive chamber and two separate germ layers , an external ectoderm and an internal endoderm . In most cases, a third germ layer, the mesoderm , also develops between them. These germ layers then differentiate to form tissues and organs. Repeated instances of mating with a close relative during sexual reproduction generally leads to inbreeding depression within

387-628: A hernia . Spinal stenosis is a narrowing of the spinal canal which can occur in any region of the spine though less commonly in the thoracic region. The stenosis can constrict the spinal canal giving rise to a neurological deficit . Pain at the coccyx (tailbone) is known as coccydynia . Spinal cord injury is damage to the spinal cord that causes changes in its function, either temporary or permanent. Spinal cord injuries can be divided into categories: complete transection, hemisection, central spinal cord lesions, posterior spinal cord lesions, and anterior spinal cord lesions. Scalloping vertebrae

516-405: A vestigial coccyx . A dorsal nerve cord , which folds and fuses into a hollow neural tube during embryonic development and eventually gives rise to the brain and spinal cord , runs more dorsally to the axial endoskeleton (enclosed by protective skeletal extensions known as neural arches ), with a fore-end enlargement that is contained within a distinct skeletonized braincase (hence

645-412: A central cavity, the central canal . Adjacent to each vertebra emerge spinal nerves . The spinal nerves provide sympathetic nervous supply to the body, with nerves emerging forming the sympathetic trunk and the splanchnic nerves . The spinal canal follows the different curves of the column; it is large and triangular in those parts of the column that enjoy the greatest freedom of movement, such as

774-561: A clade Xenambulacraria for the Xenacoelamorpha + Ambulacraria; this is either within Deuterostomia, as sister to Chordata, or the Deuterostomia are recovered as paraphyletic, and Xenambulacraria is sister to the proposed clade Centroneuralia , consisting of Chordata + Protostomia. Eumetazoa , a clade which contains Ctenophora and ParaHoxozoa , has been proposed as a sister group to Porifera . A competing hypothesis

903-449: A curve, convex forward, that begins at the axis (second cervical vertebra) at the apex of the odontoid process or dens and ends at the middle of the second thoracic vertebra; it is the least marked of all the curves. This inward curve is known as a lordotic curve. The thoracic curve, concave forward, begins at the middle of the second and ends at the middle of the twelfth thoracic vertebra. Its most prominent point behind corresponds to

1032-413: A neural arch, while the haemal arch is found underneath the centrum in the caudal (tail) vertebrae of fish , most reptiles , some birds, some dinosaurs and some mammals with long tails. The vertebral processes can either give the structure rigidity, help them articulate with ribs, or serve as muscle attachment points. Common types are transverse process, diapophyses, parapophyses, and zygapophyses (both

1161-408: A pair of secondary enlargements of the hindbrain become the cerebella , which modulate complex motor coordinations . The brain vesicles are usually bilaterally symmetrical , giving rise to the paired cerebral hemispheres in mammals . The resultant anatomy of a central nervous system arising from a single nerve cord dorsal to the gut tube , headed by a series of (typically paired) brain vesicles,

1290-1109: A population due to the increased prevalence of harmful recessive traits. Animals have evolved numerous mechanisms for avoiding close inbreeding . Some animals are capable of asexual reproduction , which often results in a genetic clone of the parent. This may take place through fragmentation ; budding , such as in Hydra and other cnidarians ; or parthenogenesis , where fertile eggs are produced without mating , such as in aphids . Animals are categorised into ecological groups depending on their trophic levels and how they consume organic material . Such groupings include carnivores (further divided into subcategories such as piscivores , insectivores , ovivores , etc.), herbivores (subcategorized into folivores , graminivores , frugivores , granivores , nectarivores , algivores , etc.), omnivores , fungivores , scavengers / detritivores , and parasites . Interactions between animals of each biome form complex food webs within that ecosystem . In carnivorous or omnivorous species, predation

1419-456: A region can vary but overall the number remains the same. The number of those in the cervical region, however, is only rarely changed. The vertebrae of the cervical, thoracic, and lumbar spines are independent bones and generally quite similar. The vertebrae of the sacrum and coccyx are usually fused and unable to move independently. Two special vertebrae are the atlas and axis , on which the head rests. A typical vertebra consists of two parts:

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1548-399: A remnant of the notochord. Reptiles often retain the primitive intercentra, which are present as small crescent-shaped bony elements lying between the bodies of adjacent vertebrae; similar structures are often found in the caudal vertebrae of mammals. In the tail, these are attached to chevron-shaped bones called haemal arches , which attach below the base of the spine, and help to support

1677-438: A result of lifting the head and the lumbar curvature forms as a result of walking. The vertebral column surrounds the spinal cord which travels within the spinal canal , formed from a central hole within each vertebra . The spinal cord is part of the central nervous system that supplies nerves and receives information from the peripheral nervous system within the body. The spinal cord consists of grey and white matter and

1806-435: A single opening, which serves as both mouth and anus. Animals in both phyla have distinct tissues, but these are not organised into discrete organs . They are diploblastic , having only two main germ layers, ectoderm and endoderm. Vertebral column There are around 50,000 species of animals that have a vertebral column. The human spine is one of the most-studied examples, as the general structure of human vertebrae

1935-464: Is a consumer–resource interaction where the predator feeds on another organism, its prey , who often evolves anti-predator adaptations to avoid being fed upon. Selective pressures imposed on one another lead to an evolutionary arms race between predator and prey, resulting in various antagonistic/ competitive coevolutions . Almost all multicellular predators are animals. Some consumers use multiple methods; for example, in parasitoid wasps ,

2064-433: Is a stage in embryonic development that is unique to animals, allowing cells to be differentiated into specialised tissues and organs. All animals are composed of cells, surrounded by a characteristic extracellular matrix composed of collagen and elastic glycoproteins . During development, the animal extracellular matrix forms a relatively flexible framework upon which cells can move about and be reorganised, making

2193-413: Is classed as a spinal disease or dorsopathy and includes the following abnormal curvatures: Individual vertebrae of the human vertebral column can be felt and used as surface anatomy , with reference points are taken from the middle of the vertebral body. This provides anatomical landmarks that can be used to guide procedures such as a lumbar puncture and also as vertical reference points to describe

2322-416: Is curved in several places, a result of human bipedal evolution . These curves increase the vertebral column's strength, flexibility, and ability to absorb shock, stabilising the body in upright position. When the load on the spine is increased, the curvatures increase in depth (become more curved) to accommodate the extra weight. They then spring back when the weight is removed. The upper cervical spine has

2451-463: Is disputed, as they might be water-escape or other structures. Animals are monophyletic , meaning they are derived from a common ancestor. Animals are the sister group to the choanoflagellates , with which they form the Choanozoa . The dates on the phylogenetic tree indicate approximately how many millions of years ago ( mya ) the lineages split. Ros-Rocher and colleagues (2021) trace

2580-605: Is fairly typical ( homologous ) of that found in other mammals , reptiles and birds . The shape of the vertebral body does, however, vary somewhat between different groups of living species. Individual vertebrae are named according to their corresponding body region ( neck , thorax , abdomen , pelvis or tail ). In clinical medicine , features on vertebrae (particularly the spinous process ) can be used as surface landmarks to guide medical procedures such as lumbar punctures and spinal anesthesia . There are also many different spinal diseases in humans that can affect both

2709-450: Is known as zoology , and the study of animal behaviour is known as ethology . The animal kingdom is divided into five infrakingdoms / superphyla , namely Porifera , Ctenophora , Placozoa , Cnidaria and Bilateria . Most living animal species belong to the infrakingdom Bilateria, a highly proliferative clade whose members have a bilaterally symmetric body plan , and the vast majority of bilaterians belong to two large superphyla :

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2838-808: Is often used to refer only to nonhuman animals. The term metazoa is derived from Ancient Greek μετα ( meta ) 'after' (in biology, the prefix meta- stands for 'later') and ζῷᾰ ( zōia ) 'animals', plural of ζῷον zōion 'animal'. Animals have several characteristics that set them apart from other living things. Animals are eukaryotic and multicellular . Unlike plants and algae , which produce their own nutrients , animals are heterotrophic , feeding on organic material and digesting it internally. With very few exceptions, animals respire aerobically . All animals are motile (able to spontaneously move their bodies) during at least part of their life cycle , but some animals, such as sponges , corals , mussels , and barnacles , later become sessile . The blastula

2967-422: Is only produced by sponges and pelagophyte algae. Its likely origin is from sponges based on molecular clock estimates for the origin of 24-ipc production in both groups. Analyses of pelagophyte algae consistently recover a Phanerozoic origin, while analyses of sponges recover a Neoproterozoic origin, consistent with the appearance of 24-ipc in the fossil record. The first body fossils of animals appear in

3096-425: Is replaced by a segmented series of mineralized elements called vertebrae separated by fibrocartilaginous intervertebral discs , which are embryonic and evolutionary remnants of the notochord. Hagfish are the only extant vertebrate whose notochord persists and is not integrated/ replaced by the vertebral column. A few vertebrates have secondarily lost this feature and retain the notochord into adulthood, such as

3225-432: Is shallow, and by the laminae and transverse processes in the thoracic region, where it is deep and broad; these grooves lodge the deep muscles of the back. Lateral to the spinous processes are the articular processes, and still more laterally the transverse processes. In the thoracic region, the transverse processes stand backward, on a plane considerably behind that of the same processes in the cervical and lumbar regions. In

3354-525: Is strongly supported by two CSIs found in the proteins Rrp44 (associated with the exosome complex ) and serine palmitoyltransferase . These are exclusively shared by species from these two subphyla, but not cephalochordates . This indicates hat vertebrates are more closely related to tunicates than cephalochordates. The "Notochordata hypothesis" suggested that the Cephalochordata is the sister taxon to Craniata (Vertebrata). This group, called

3483-547: Is the Benthozoa clade, which would consist of Porifera and ParaHoxozoa as a sister group of Ctenophora . Several animal phyla lack bilateral symmetry. These are the Porifera (sea sponges), Placozoa , Cnidaria (which includes jellyfish , sea anemones , and corals), and Ctenophora (comb jellies). Sponges are physically very distinct from other animals, and were long thought to have diverged first, representing

3612-412: Is the increase in the concavity of the posterior vertebral body. It can be seen on lateral X-ray and sagittal views of CT and MRI scans. Its concavity is due to the increased pressure exerting on the vertebrae due to a mass. Internal spinal mass such as spinal astrocytoma , ependymoma , schwannoma , neurofibroma , and achondroplasia causes vertebrae scalloping. Excessive or abnormal spinal curvature

3741-442: Is the presence of neural crest cells, which are progenitor cells critical to coordinating the functions of cellular components. Neural crest cells migrate through the body from the dorsal nerve cord during development, initiate the formation of neuronal ganglia and various special sense organs. The peripheral nervous system forms when neural crest cells branch out laterally from the dorsal nerve cord and migrate together with

3870-399: Is unique to vertebrates. This is in stark contrast to invertebrates with well-developed central nervous systems such as arthropods and cephalopods , which have an often ladder-like ventral nerve cord made of paired segmental ganglia on the opposite ( ventral ) side of the gut tube, with a split brain stem circumventing the foregut around each side to form a brain on the dorsal side of

3999-521: The Arabian (breed) can have one less vertebrae and pair of ribs. This anomaly disappears in foals that are the product of an Arabian and another breed of horse. Vertebrae are defined by their location in the vertebral column. Cervical vertebrae are those in the neck area. With the exception of the two sloth genera ( Choloepus and Bradypus ) and the manatee genus, ( Trichechus ), all mammals have seven cervical vertebrae. In other vertebrates,

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4128-622: The Cambrian explosion , which began around 539  million years ago (Mya), and most classes during the Ordovician radiation 485.4 Mya. 6,331 groups of genes common to all living animals have been identified; these may have arisen from a single common ancestor that lived about 650 Mya during the Cryogenian period. Historically, Aristotle divided animals into those with blood and those without . Carl Linnaeus created

4257-619: The Ediacaran , represented by forms such as Charnia and Spriggina . It had long been doubted whether these fossils truly represented animals, but the discovery of the animal lipid cholesterol in fossils of Dickinsonia establishes their nature. Animals are thought to have originated under low-oxygen conditions, suggesting that they were capable of living entirely by anaerobic respiration , but as they became specialized for aerobic metabolism they became fully dependent on oxygen in their environments. Many animal phyla first appear in

4386-472: The Izu–Ogasawara Trench at a depth of 8,336 metres (27,349 feet). Many fish varieties are the main predators in most of the world's freshwater and marine water bodies . 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

4515-630: The Jurassic . The Cenozoic world saw 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 the Tibetan stone loach ( Triplophysa stolickai ) in western Tibetan hot springs near Longmu Lake at an elevation of 5,200 metres (17,100 feet) to an unknown species of snailfish (genus Pseudoliparis ) in

4644-573: The Placodermi and the Acanthodii , both considered paraphyletic . Other ways of classifying the vertebrates have been devised, particularly with emphasis on the phylogeny of early amphibians and reptiles. An example based on Janvier (1981, 1997), Shu et al. (2003), and Benton (2004) is given here († = extinct ): While this traditional classification is orderly, most of the groups are paraphyletic , i.e. do not contain all descendants of

4773-469: The Placodermi , a group of armoured fish that dominated the entirety of that period since the late Silurian as well as the eurypterids , dominant animals of the preceding Silurian , and the anomalocarids . By the middle of the Devonian, several droughts, anoxic events and oceanic competition led a lineage of sarcopterygii to leave water, eventually establishing themselves as terrestrial tetrapods in

4902-878: The Tree of Life Web Project and Delsuc et al., and complemented (based on, and ). A dagger (†) denotes an extinct clade , whereas all other clades have living descendants . Hyperoartia ( lampreys ) [REDACTED] Myxini ( hagfish ) [REDACTED] † Euconodonta [REDACTED] † Myllokunmingiida [REDACTED]   † Pteraspidomorphi [REDACTED] † Thelodonti [REDACTED] † Anaspida [REDACTED] † Galeaspida [REDACTED] † Pituriaspida [REDACTED] † Osteostraci [REDACTED]   † Antiarchi [REDACTED] † Petalichthyida [REDACTED] † Arthrodira [REDACTED] † Ptyctodontida [REDACTED] † Entelognathus   [REDACTED]   Animal Animals are multicellular , eukaryotic organisms in

5031-485: The Wnt and TGF-beta signalling pathways which may have enabled animals to become multicellular by providing a pattern for the body's system of axes (in three dimensions), and another 7 are for transcription factors including homeodomain proteins involved in the control of development . Giribet and Edgecombe (2020) provide what they consider to be a consensus internal phylogeny of the animals, embodying uncertainty about

5160-409: The anterior and posterior longitudinal ligaments at the front and back of the vertebral bodies , the ligamentum flavum in deep to the laminae , the interspinous and supraspinous ligaments between spinous processes , and the intertransverse ligaments between the transverse processes . The vertebrae in the human vertebral column is divided into different body regions , which correspond to

5289-893: The biological kingdom Animalia ( / ˌ æ n ɪ ˈ m eɪ l i ə / ). With few exceptions, animals consume organic material , breathe oxygen , have myocytes and are able to move , can reproduce sexually , and grow from a hollow sphere of cells, the blastula , during embryonic development . Animals form a clade , meaning that they arose from a single common ancestor . Over 1.5 million living animal species have been described , of which around 1.05 million are insects , over 85,000 are molluscs , and around 65,000 are vertebrates . It has been estimated there are as many as 7.77 million animal species on Earth. Animal body lengths range from 8.5 μm (0.00033 in) to 33.6 m (110 ft). They have complex ecologies and interactions with each other and their environments, forming intricate food webs . The scientific study of animals

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5418-493: The cephalochordates ), though it lacks eyes and other complex special sense organs comparable to those of vertebrates. Other chordates do not show any trends towards cephalization. The rostral end of the neural 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 ), which are further differentiated in

5547-665: The evolutionary relationships between taxa . Humans make use of many other animal species for food (including meat , eggs , and dairy products ), for materials (such as leather , fur , and wool ), as pets and as working animals for transportation , and services . Dogs , the first domesticated animal, have been used in hunting , in security and in warfare , as have horses , pigeons and birds of prey ; while other terrestrial and aquatic animals are hunted for sports, trophies or profits. Non-human animals are also an important cultural element of human evolution , having appeared in cave arts and totems since

5676-473: The fossil record during the Cambrian explosion , starting about 539 million years ago, in beds such as the Burgess shale . Extant phyla in these rocks include molluscs , brachiopods , onychophorans , tardigrades , arthropods , echinoderms and hemichordates , along with numerous now-extinct forms such as the predatory Anomalocaris . The apparent suddenness of the event may however be an artifact of

5805-562: The genetics of organisms. 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. Conventional classification has extant vertebrates grouped into seven classes based on traditional interpretations of gross anatomical and physiological traits. The commonly held classification lists three classes of fish and four of tetrapods . In addition to these, there are two classes of extinct armoured fishes,

5934-433: The intervertebral discs . The notochord disappears in the sclerotome (vertebral body) segments but persists in the region of the intervertebral discs as the nucleus pulposus . The nucleus pulposus and the fibers of the anulus fibrosus make up the intervertebral disc. The primary curves (thoracic and sacral curvatures) form during fetal development. The secondary curves develop after birth. The cervical curvature forms as

6063-497: The lobe-finned fish Tiktaalik started to move on to land in the late Devonian , about 375 million years ago. Animals occupy virtually all of earth's habitats and microhabitats, with faunas adapted to salt water, hydrothermal vents, fresh water, hot springs, swamps, forests, pastures, deserts, air, and the interiors of other organisms. Animals are however not particularly heat tolerant ; very few of them can survive at constant temperatures above 50 °C (122 °F) or in

6192-464: The lobe-finned fish , which gave rise to the tetrapods , the animals with four limbs. Vertebrates make up less than five percent of all described animal species ; the rest are all invertebrates , that lack a backbone. The word 'vertebrate' derives from the Latin vertebratus ("jointed"), from vertebra meaning "joint", in turn from Latin vertere to turn. All vertebrates are built along

6321-443: The mesodermal somites to innervate the various different structures that develop in the body. The vertebrates are the only chordate group with neural cephalization , and their neural functions are centralized towards a series of enlarged clusters in the head , which give rise to a brain . A slight swelling of the anterior end of the nerve cord is found in invertebrate chordates such as lancelets (a sister subphylum known as

6450-518: The mouth . The higher functions of the vertebrate CNS are highly centralized towards the brain (particularly the forebrain), while the invertebrate CNS is significantly more decentralized with the segmental ganglia having substantial neural autonomy independent of the brain (which itself is a fused cluster of segmental ganglia from the rostral metameres ). Molecular markers known as conserved signature indels (CSIs) in protein sequences have been identified and provide distinguishing criteria for

6579-412: The paraxial mesoderm that lies at the sides of the neural tube and they contain the precursors of spinal bone, the vertebrae ribs and some of the skull, as well as muscle, ligaments and skin. Somitogenesis and the subsequent distribution of somites is controlled by a clock and wavefront model acting in cells of the paraxial mesoderm. Soon after their formation, sclerotomes , which give rise to some of

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6708-512: The pharynx . Research also suggests that the sixth branchial arch contributed to the formation of the vertebrate shoulder, which separated the head as a distinct part of the body. In amphibians and some primitive bony fishes, the larvae bear external gills , branching off from the gill arches. These are reduced in adulthood, their respiratory function taken over by the internal gills proper in fishes and by cutaneous respiration in most amphibians. While some amphibians such as axolotl retain

6837-500: The protostomes , which includes organisms such as arthropods , molluscs , flatworms , annelids and nematodes ; and the deuterostomes , which include echinoderms , hemichordates and chordates , the latter of which contains the vertebrates . The much smaller basal phylum Xenacoelomorpha have an uncertain position within Bilateria. Animals first appear in the fossil record in the late Cryogenian period and diversified in

6966-436: The sacrum and coccyx are fused without a central foramen. The vertebral arch is formed by a ventral pair of pedicles and a dorsal pair of laminae , and supports seven processes , four articular , two transverse and one spinous , the latter also being known as the neural spine. The transverse and spinous processes and their associated ligaments serve as important attachment sites for back and paraspinal muscles and

7095-435: The spinal canal , a body cavity that contains the spinal cord . Because the vertebral column will outgrow the spinal cord during child development , by adulthood the spinal cord often ends at the upper lumbar spine (at around L1/L2 level), the lower ( caudal ) end of the spinal canal is occupied by a ponytail -like bundle of spinal nerves descriptively called cauda equina (from Latin " horse's tail " ), and

7224-411: The spinal cord and the notochord . This column of tissue has a segmented appearance, with alternating areas of dense and less dense areas. As the sclerotome develops, it condenses further eventually developing into the vertebral body . Development of the appropriate shapes of the vertebral bodies is regulated by HOX genes . The less dense tissue that separates the sclerotome segments develop into

7353-454: The sturgeon . Most vertebrates are aquatic and carry out gas exchange via gills . The gills are carried right behind the head, bordering the posterior margins of a series of crescentic openings from the pharynx to the outside. Each gill is supported by a cartilaginous or bony gill arch , which develop embryonically from pharyngeal arches . Bony fish have three pairs of gill arches, cartilaginous fish have five to seven pairs, while

7482-438: The thoracolumbar fasciae . The spinous processes of the cervical and lumbar regions can be felt through the skin, and are important surface landmarks in clinical medicine . The four articular processes for two pairs of plane facet joints above and below each vertebra, articulating with those of the adjacent vertebrae and are joined by a thin portion of the neural arch called the pars interarticularis . The orientation of

7611-442: The thyroid in most vertebrates. Vertebrates vary in body length ranging from the frog species Brachycephalus pulex , a Brazilian flea toad, with a minimum adult snout–vent length of 6.45 millimetres (0.254 in) to the blue whale , at up to 33 m (108 ft). With only one exception, the defining characteristic of all vertebrates is the vertebral column , in which the embryonic notochord found in all chordates

7740-439: The thyroid gland , the larynx , the columella (corresponding to the stapes in mammals ) and, in mammals, the malleus and incus . The central nervous system of vertebrates is based on the embryonic dorsal nerve cord (which then flattens into a neural plate before folding and fusing over into a hollow neural tube ) running along the dorsal aspect of the notochord . Of particular importance and unique to vertebrates

7869-409: The vertebral body (or centrum ), which is ventral (or anterior , in the standard anatomical position ) and withstands axial structural load ; and the vertebral arch (also known as neural arch ), which is dorsal (or posterior ) and provides articulations and anchorages for ribs and core skeletal muscles . Together, these enclose the vertebral foramen , the series of which align to form

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7998-715: The Cambrian explosion, possibly as early as 1 billion years ago. Early fossils that might represent animals appear for example in the 665-million-year-old rocks of the Trezona Formation of South Australia . These fossils are interpreted as most probably being early sponges . Trace fossils such as tracks and burrows found in the Tonian period (from 1 gya) may indicate the presence of triploblastic worm-like animals, roughly as large (about 5 mm wide) and complex as earthworms. However, similar tracks are produced by

8127-651: The Cephalochordata.   Amphioxiformes (lancelets)  [REDACTED]   Tunicata /Urochordata ( sea squirts , salps , larvaceans )  [REDACTED]   Vertebrata   [REDACTED] Vertebrates originated during the Cambrian explosion , which saw a rise in organism diversity. The earliest known vertebrates belongs to the Chengjiang biota and lived about 518 million years ago. These include Haikouichthys , Myllokunmingia , Zhongjianichthys , and probably Haikouella . Unlike

8256-717: The Cnidaria) never grow larger than 20 μm , and one of the smallest species ( Myxobolus shekel ) is no more than 8.5 μm when fully grown. The following table lists estimated numbers of described extant species for the major animal phyla, along with their principal habitats (terrestrial, fresh water, and marine), and free-living or parasitic ways of life. Species estimates shown here are based on numbers described scientifically; much larger estimates have been calculated based on various means of prediction, and these can vary wildly. For instance, around 25,000–27,000 species of nematodes have been described, while published estimates of

8385-608: The Notochordata, was placed as sister group to the Tunicata (Urochordata). Studies since 2006 analyzing large sequencing datasets however strongly support Olfactores (tunicates + vertebrates) as a clade, and hence the placement of Cephalochordata as sister-group to Olfactores (known as the " Olfactores hypothesis "). The following cladogram summarizes the relationships between the Olfactores (vertebrates and tunicates) and

8514-425: The adjacent vertebra to a degree less than a dislocation. Spondylolysis , also known as a pars defect, is a defect or fracture at the pars interarticularis of the vertebral arch. Spinal disc herniation , more commonly called a "slipped disc", is the result of a tear in the outer ring ( anulus fibrosus ) of the intervertebral disc , which lets some of the soft gel-like material, the nucleus pulposus , bulge out in

8643-420: The alternative name for vertebrates, the craniates ). All vertebrate embryos develop transient pharyngeal arches , which in fish develop into the branchial arches that support the gills . Other vertebrate features are a jaw , hyoid and/or the middle ear ossicles . An iodine-concentrating organ called the endostyle , which functions as a filter feeding organ in aquatic animals has evolved into

8772-475: The basic chordate body plan of five synapomorphies . These are a rigid axial skeleton that includes a vertebral column developed around an elastic notochord . The notochord becomes the intervertebral discs , and runs dorsally to the gut tube along the length of an animal, hence the common name of backbone. The axial endoskeleton typically continues beyond the anus / cloaca to form an elongated tail . Some vertebrates evolved to become tailless with only

8901-416: The bone of the skull, the vertebrae and ribs, migrate, leaving the remainder of the somite now termed a dermamyotome behind. This then splits to give the myotomes which will form the muscles and dermatomes which will form the skin of the back. Sclerotomes become subdivided into an anterior and a posterior compartment. This subdivision plays a key role in the definitive patterning of vertebrae that form when

9030-435: The bony vertebrae and the intervertebral discs, with kyphosis / scoliosis , ankylosing spondylitis , degenerative discs and spina bifida being recognizable examples. The number of vertebrae in a region can vary but overall the number remains the same. In a human vertebral column, there are normally 33 vertebrae. The upper 24 pre-sacral vertebrae are articulating and separated from each other by intervertebral discs , and

9159-427: The bony vertebral body. In most ray-finned fishes , including all teleosts , these two structures are fused with, and embedded within, a solid piece of bone superficially resembling the vertebral body of mammals. In living amphibians , there is simply a cylindrical piece of bone below the vertebral arch, with no trace of the separate elements present in the early tetrapods. In cartilaginous fish , such as sharks ,

9288-506: The centrum. Centra with flat ends are acoelous , like those in mammals. These flat ends of the centra are especially good at supporting and distributing compressive forces. Amphicoelous vertebra have centra with both ends concave. This shape is common in fish, where most motion is limited. Amphicoelous centra often are integrated with a full notochord . Procoelous vertebrae are anteriorly concave and posteriorly convex. They are found in frogs and modern reptiles. Opisthocoelous vertebrae are

9417-423: The cervical and lumbar regions, and is small and rounded in the thoracic region, where motion is more limited. The spinal cord terminates in the conus medullaris and cauda equina . Spina bifida is a congenital disorder in which there is a defective closure of the vertebral arch. Sometimes the spinal meninges and also the spinal cord can protrude through this, and this is called spina bifida cystica . Where

9546-464: The cervical and upper part of the thoracic regions and gradually increasing in size to the last lumbar. They transmit the special spinal nerves and are situated between the transverse processes in the cervical region and in front of them, in the thoracic and lumbar regions. There are different ligaments involved in the holding together of the vertebrae in the column, and in the column's movement. The anterior and posterior longitudinal ligaments extend

9675-430: The cervical region, the transverse processes are placed in front of the articular processes, lateral to the pedicles and between the intervertebral foramina. In the thoracic region they are posterior to the pedicles, intervertebral foramina, and articular processes. In the lumbar region they are in front of the articular processes, but behind the intervertebral foramina. The sides of the vertebral column are separated from

9804-421: The child begins to walk. When viewed from in front, the width of the bodies of the vertebrae is seen to increase from the second cervical to the first thoracic; there is then a slight diminution in the next three vertebrae. Below this, there is again a gradual and progressive increase in width as low as the sacrovertebral angle. From this point there is a rapid diminution, to the apex of the coccyx. From behind,

9933-408: The class's common ancestor. For instance, descendants of the first reptiles include modern reptiles, mammals and birds; the agnathans have given rise to the jawed vertebrates ; the bony fishes have given rise to the land vertebrates ; the traditional " amphibians " have given rise to the reptiles (traditionally including the synapsids or mammal-like "reptiles"), which in turn have given rise to

10062-429: The condition does not involve this protrusion it is known as spina bifida occulta . Sometimes all of the vertebral arches may remain incomplete. Another, though rare, congenital disease is Klippel–Feil syndrome , which is the fusion of any two of the cervical vertebrae. Spondylolisthesis is the forward displacement of a vertebra and retrolisthesis is a posterior displacement of one vertebral body with respect to

10191-402: The cranial zygapophyses and the caudal zygapophyses). The centrum of the vertebra can be classified based on the fusion of its elements. In temnospondyls , bones such as the spinous process , the pleurocentrum and the intercentrum are separate ossifications. Fused elements, however, classify a vertebra as having holospondyly. A vertebra can also be described in terms of the shape of the ends of

10320-403: The curvatures of the vertebral column. The articulating vertebrae are named according to their region of the spine. Vertebrae in these regions are essentially alike, with minor variation. These regions are called the cervical spine, thoracic spine, lumbar spine, sacrum, and coccyx. There are seven cervical vertebrae, twelve thoracic vertebrae, and five lumbar vertebrae. The number of vertebrae in

10449-463: The dark sea floor consume organic matter produced through chemosynthesis (via oxidizing inorganic compounds such as hydrogen sulfide ) by archaea and bacteria . Animals evolved in the sea. Lineages of arthropods colonised land around the same time as land plants , probably between 510 and 471 million years ago during the Late Cambrian or Early Ordovician . Vertebrates such as

10578-454: The earliest times, and are frequently featured in mythology , religion , arts , literature , heraldry , politics , and sports . The word animal comes from the Latin noun animal of the same meaning, which is itself derived from Latin animalis 'having breath or soul'. The biological definition includes all members of the kingdom Animalia. In colloquial usage, the term animal

10707-414: The external gills into adulthood, the complex internal gill system as seen in fish apparently being irrevocably lost very early in the evolution of tetrapods , who evolved lungs (which are homologous to swim bladders ) to breathe air. While the more specialized terrestrial vertebrates lack gills, the gill arches form during fetal development , and form the basis of essential structures such as jaws ,

10836-447: The facet joints restricts the range of motion between the vertebrae. Underneath each pedicle is a small hole (enclosed by the pedicle of the vertebral below) called intervertebral foramen , which transmit the corresponding spinal nerve and dorsal root ganglion that exit the spinal canal. From top to bottom, the vertebrae are: For some medical purposes, adjacent vertebral regions may be considered together: The vertebral column

10965-570: The first hierarchical biological classification for animals in 1758 with his Systema Naturae , which Jean-Baptiste Lamarck expanded into 14 phyla by 1809. In 1874, Ernst Haeckel divided the animal kingdom into the multicellular Metazoa (now synonymous with Animalia) and the Protozoa , single-celled organisms no longer considered animals. In modern times, the biological classification of animals relies on advanced techniques, such as molecular phylogenetics , which are effective at demonstrating

11094-448: The formation of complex structures possible. This may be calcified, forming structures such as shells , bones , and spicules . In contrast, the cells of other multicellular organisms (primarily algae, plants, and fungi ) are held in place by cell walls, and so develop by progressive growth. Animal cells uniquely possess the cell junctions called tight junctions , gap junctions , and desmosomes . With few exceptions—in particular,

11223-520: The fossil record, rather than showing that all these animals appeared simultaneously. That view is supported by the discovery of Auroralumina attenboroughii , the earliest known Ediacaran crown-group cnidarian (557–562 mya, some 20 million years before the Cambrian explosion) from Charnwood Forest , England. It is thought to be one of the earliest predators , catching small prey with its nematocysts as modern cnidarians do. Some palaeontologists have suggested that animals appeared much earlier than

11352-737: The giant single-celled protist Gromia sphaerica , so the Tonian trace fossils may not indicate early animal evolution. Around the same time, the layered mats of microorganisms called stromatolites decreased in diversity, perhaps due to grazing by newly evolved animals. Objects such as sediment-filled tubes that resemble trace fossils of the burrows of wormlike animals have been found in 1.2 gya rocks in North America, in 1.5 gya rocks in Australia and North America, and in 1.7 gya rocks in Australia. Their interpretation as having an animal origin

11481-441: The larvae feed on the hosts' living tissues, killing them in the process, but the adults primarily consume nectar from flowers. Other animals may have very specific feeding behaviours , such as hawksbill sea turtles which mainly eat sponges . Most animals rely on biomass and bioenergy produced by plants and phytoplanktons (collectively called producers ) through photosynthesis . Herbivores, as primary consumers , eat

11610-596: The late Cambrian to the end of the Triassic. The first jawed vertebrates may have appeared in the late Ordovician (~445 mya) and became common in the Devonian period , 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 hagfishes, as well as

11739-432: The length of the vertebral column along the front and back of the vertebral bodies. The interspinous ligaments connect the adjoining spinous processes of the vertebrae. The supraspinous ligament extends the length of the spine running along the back of the spinous processes, from the sacrum to the seventh cervical vertebra . From there it is continuous with the nuchal ligament . The striking segmented pattern of

11868-403: The locations of other parts of human anatomy, such as the positions of organs . The general structure of vertebrae in other animals is largely the same as in humans. Individual vertebrae are composed of a centrum (body), arches protruding from the top and bottom of the centrum, and various processes projecting from the centrum and/or arches. An arch extending from the top of the centrum is called

11997-433: The lower nine are fused in adults, five in the sacrum and four in the coccyx , or tailbone . The articulating vertebrae are named according to their region of the spine. From top to bottom, there are 7 cervical vertebrae , 12 thoracic vertebrae and 5 lumbar vertebrae . The number of those in the cervical region, however, is only rarely changed, while that in the coccygeal region varies most. Excluding rare deviations,

12126-408: The lumbar region, by narrower intervals in the neck, and are closely approximated in the middle of the thoracic region. Occasionally one of these processes deviates a little from the median line — which can sometimes be indicative of a fracture or a displacement of the spine. On either side of the spinous processes is the vertebral groove formed by the laminae in the cervical and lumbar regions, where it

12255-459: 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. In phylogenetics , the relationships between animals are illustrated as a hierarchy known as a phylogenetic tree . The cladogram below is based on studies compiled by Philippe Janvier and others for

12384-405: The mechanisms involved in vertebral segmentation are conserved across vertebrates. In humans the first four somites are incorporated in the base of the occipital bone of the skull and the next 33 somites will form the vertebrae, ribs, muscles, ligaments and skin. The remaining posterior somites degenerate. During the fourth week of embryogenesis , the sclerotomes shift their position to surround

12513-753: The most extreme cold deserts of continental Antarctica . The blue whale ( Balaenoptera musculus ) is the largest animal that has ever lived, weighing up to 190 tonnes and measuring up to 33.6 metres (110 ft) long. The largest extant terrestrial animal is the African bush elephant ( Loxodonta africana ), weighing up to 12.25 tonnes and measuring up to 10.67 metres (35.0 ft) long. The largest terrestrial animals that ever lived were titanosaur sauropod dinosaurs such as Argentinosaurus , which may have weighed as much as 73 tonnes, and Supersaurus which may have reached 39 meters. Several animals are microscopic; some Myxozoa ( obligate parasites within

12642-423: The next vertebral body fits. Even these patterns are only generalisations, however, and there may be variation in form of the vertebrae along the length of the spine even within a single species. Some unusual variations include the saddle-shaped sockets between the cervical vertebrae of birds and the presence of a narrow hollow canal running down the centre of the vertebral bodies of geckos and tuataras , containing

12771-405: The number of cervical vertebrae can range from a single vertebra in amphibians to as many as 25 in swans or 76 in the extinct plesiosaur Elasmosaurus . The dorsal vertebrae range from the bottom of the neck to the top of the pelvis . Dorsal vertebrae attached to the ribs are called thoracic vertebrae, while those without ribs are called lumbar vertebrae. The sacral vertebrae are those in

12900-620: The oldest animal phylum and forming a sister clade to all other animals. Despite their morphological dissimilarity with all other animals, genetic evidence suggests sponges may be more closely related to other animals than the comb jellies are. Sponges lack the complex organization found in most other animal phyla; their cells are differentiated, but in most cases not organised into distinct tissues, unlike all other animals. They typically feed by drawing in water through pores, filtering out small particles of food. The comb jellies and Cnidaria are radially symmetric and have digestive chambers with

13029-446: The opposite, possessing anterior convexity and posterior concavity. They are found in salamanders, and in some non-avian dinosaurs. Heterocoelous vertebrae have saddle -shaped articular surfaces. This type of configuration is seen in turtles that retract their necks, and birds, because it permits extensive lateral and vertical flexion motion without stretching the nerve cord too extensively or wringing it about its long axis. In horses,

13158-525: The origins of animals to unicellular ancestors, providing the external phylogeny shown in the cladogram. Uncertainty of relationships is indicated with dashed lines. Holomycota (inc. fungi) [REDACTED] Ichthyosporea [REDACTED] Pluriformea [REDACTED] Filasterea [REDACTED] [REDACTED] [REDACTED] The most basal animals, the Porifera , Ctenophora , Cnidaria , and Placozoa , have body plans that lack bilateral symmetry . Their relationships are still disputed;

13287-413: 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 jaws 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

13416-423: The pelvic region, and range from one in amphibians, to two in most birds and modern reptiles, or up to three to five in mammals. When multiple sacral vertebrae are fused into a single structure, it is called the sacrum. The synsacrum is a similar fused structure found in birds that is composed of the sacral, lumbar, and some of the thoracic and caudal vertebra, as well as the pelvic girdle . Caudal vertebrae compose

13545-519: The plant material directly to digest and absorb the nutrients, while carnivores and other animals on higher trophic levels indirectly acquire the nutrients by eating the herbivores or other animals that have eaten the herbivores. Animals oxidize carbohydrates , lipids , proteins and other biomolecules, which allows the animal to grow and to sustain basal metabolism and fuel other biological processes such as locomotion . Some benthic animals living close to hydrothermal vents and cold seeps on

13674-418: The posterior part of one somite fuses to the anterior part of the consecutive somite during a process termed resegmentation. Disruption of the somitogenesis process in humans results in diseases such as congenital scoliosis. So far, the human homologues of three genes associated to the mouse segmentation clock, (MESP2, DLL3 and LFNG), have been shown to be mutated in cases of congenital scoliosis, suggesting that

13803-417: The posterior surface by the articular processes in the cervical and thoracic regions and by the transverse processes in the lumbar region. In the thoracic region, the sides of the bodies of the vertebrae are marked in the back by the facets for articulation with the heads of the ribs. More posteriorly are the intervertebral foramina, formed by the juxtaposition of the vertebral notches, oval in shape, smallest in

13932-420: The presence of a cranium , or skull. The vertebrates make up the subphylum Vertebrata with some 65,000 species in the phylum Chordata . The vertebrates include mammals , birds , amphibians , and various classes of reptiles and fish . Classes of fish include the jawless Agnatha , and the jawed Gnathostomata . The jawed fish include both the cartilaginous fish and the bony fish . Bony fish include

14061-425: The primitive jawless fish have seven pairs. The ancestral vertebrates likely had more arches than seven, as some of their chordate relatives have more than 50 pairs of gill opens, although most, if not all, of these openings are actually involved in filter feeding rather than respiration . In jawed vertebrates , the first gill arch pair evolved into the jointed jaws and form an additional oral cavity ahead of

14190-542: The sacrovertebral articulation, and ends at the point of the coccyx ; its concavity is directed downward and forward as a kyphotic curve. The thoracic and sacral kyphotic curves are termed primary curves, because they are present in the fetus . The cervical and lumbar curves are compensatory , or secondary , and are developed after birth. The cervical curve forms when the infant is able to hold up its head (at three or four months) and sit upright (at nine months). The lumbar curve forms later from twelve to eighteen months, when

14319-753: The sister group to all other animals could be the Porifera or the Ctenophora, both of which lack hox genes , which are important for body plan development . Hox genes are found in the Placozoa, Cnidaria, and Bilateria. 6,331 groups of genes common to all living animals have been identified; these may have arisen from a single common ancestor that lived 650 million years ago in the Precambrian . 25 of these are novel core gene groups, found only in animals; of those, 8 are for essential components of

14448-407: The smaller, motile gametes are spermatozoa and the larger, non-motile gametes are ova . These fuse to form zygotes , which develop via mitosis into a hollow sphere, called a blastula. In sponges, blastula larvae swim to a new location, attach to the seabed, and develop into a new sponge. In most other groups, the blastula undergoes more complicated rearrangement. It first invaginates to form

14577-401: The spine is established during embryogenesis when somites are rhythmically added to the posterior of the embryo. Somite formation begins around the third week when the embryo begins gastrulation and continues until all somites are formed. Their number varies between species: there are 42 to 44 somites in the human embryo and around 52 in the chick embryo. The somites are spheres, formed from

14706-446: The spinous process of the seventh thoracic vertebra. This curve is known as a kyphotic curve. The lumbar curve is more marked in the female than in the male; it begins at the middle of the last thoracic vertebra, and ends at the sacrovertebral angle. It is convex anteriorly, the convexity of the lower three vertebrae being much greater than that of the upper two. This curve is described as a lordotic curve. The sacral curve begins at

14835-474: The sponges and placozoans —animal bodies are differentiated into tissues . These include muscles , which enable locomotion, and nerve tissues , which transmit signals and coordinate the body. Typically, there is also an internal digestive chamber with either one opening (in Ctenophora, Cnidaria, and flatworms) or two openings (in most bilaterians). Nearly all animals make use of some form of sexual reproduction. They produce haploid gametes by meiosis ;

14964-411: The structure at the base of the tree (dashed lines). Porifera [REDACTED] Ctenophora [REDACTED] Placozoa [REDACTED] Cnidaria [REDACTED] Xenacoelomorpha [REDACTED] Ambulacraria [REDACTED] Chordata [REDACTED] Ecdysozoa [REDACTED] Spiralia [REDACTED] An alternative phylogeny, from Kapli and colleagues (2021), proposes

15093-522: The subsequent Ediacaran period in what is known as the Avalon explosion . Earlier evidence of animals is still controversial; the sponge -like organism Otavia has been dated back to the Tonian period at the start of the Neoproterozoic , but its identity as an animal is heavily contested. Nearly all modern animal phyla became clearly established in the fossil record as marine species during

15222-649: The succeeding Carboniferous . Amniotes branched from amphibious tetrapods early in the Carboniferous period. The synapsid amniotes were dominant during the late Paleozoic, the Permian , while diapsid amniotes became dominant during the Mesozoic . In the sea, the teleosts and sharks became dominant. Mesothermic synapsids called cynodonts gave rise to endothermic mammals and diapsids called dinosaurs eventually gave rise to endothermic birds , both in

15351-416: The tail, and the final few can be fused into the pygostyle in birds, or into the coccygeal or tail bone in chimpanzees (and humans ). The vertebrae of lobe-finned fishes consist of three discrete bony elements. The vertebral arch surrounds the spinal cord, and is of broadly similar form to that found in most other vertebrates. Just beneath the arch lies a small plate-like pleurocentrum, which protects

15480-415: The tail. The general structure of human vertebrae is fairly typical of that found in other mammals , reptiles , and birds ( amniotes ). The shape of the vertebral body does, however, vary somewhat between different groups. In humans and other mammals, it typically has flat upper and lower surfaces, while in reptiles the anterior surface commonly has a concave socket into which the expanded convex face of

15609-467: The total number of nematode species include 10,000–20,000; 500,000; 10 million; and 100 million. Using patterns within the taxonomic hierarchy, the total number of animal species—including those not yet described—was calculated to be about 7.77 million in 2011. 3,000–6,500 4,000–25,000 Evidence of animals is found as long ago as the Cryogenian period. 24-Isopropylcholestane (24-ipc) has been found in rocks from roughly 650 million years ago; it

15738-419: The total number of vertebrae ranges from 32 to 35. In about 10% of people, both the total number of pre-sacral vertebrae and the number of vertebrae in individual parts of the spine can vary. The most frequent deviations are: 11 (rarely 13) thoracic vertebrae, 4 or 6 lumbar vertebrae, 3 or 5 coccygeal vertebrae (rarely up to 7). There are numerous ligaments extending the length of the column, which include

15867-401: 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). 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,

15996-463: The upper surface of the notochord , and below that, a larger arch-shaped intercentrum to protect the lower border. Both of these structures are embedded within a single cylindrical mass of cartilage. A similar arrangement was found in the primitive Labyrinthodonts , but in the evolutionary line that led to reptiles (and hence, also to mammals and birds), the intercentrum became partially or wholly replaced by an enlarged pleurocentrum, which in turn became

16125-433: The various vertebrate groups. Two laterally placed retinas and optical nerves form around outgrowths from the midbrain, except in hagfish which may have secondarily lost the structures. The forebrain is more well-developed in most tetrapods and subdivided into the telencephalon and diencephalon , while the midbrain dominates in fish and some salamanders . In vertebrates with paired appendages, especially tetrapods,

16254-449: The vertebrae consist of two cartilaginous tubes. The upper tube is formed from the vertebral arches, but also includes additional cartilaginous structures filling in the gaps between the vertebrae, and so enclosing the spinal cord in an essentially continuous sheath. The lower tube surrounds the notochord, and has a complex structure, often including multiple layers of calcification . Lampreys have vertebral arches, but nothing resembling

16383-409: The vertebral bodies found in all higher vertebrates . Even the arches are discontinuous, consisting of separate pieces of arch-shaped cartilage around the spinal cord in most parts of the body, changing to long strips of cartilage above and below in the tail region. Hagfishes lack a true vertebral column, and are therefore not properly considered vertebrates, but a few tiny neural arches are present in

16512-475: The vertebral column presents in the median line the spinous processes. In the cervical region (with the exception of the second and seventh vertebrae), these are short, horizontal, and bifid. In the upper part of the thoracic region they are directed obliquely downward; in the middle they are almost vertical, and in the lower part they are nearly horizontal. In the lumbar region they are nearly horizontal. The spinous processes are separated by considerable intervals in

16641-399: The vertebrate subphylum. Specifically, 5 CSIs in the following proteins: protein synthesis elongation factor-2 , eukaryotic translation initiation factor 3 , adenosine kinase and a protein related to ubiquitin carboxyl-terminal hydrolase are exclusively shared by all vertebrates and reliably distinguish them from all other animals. A specific relationship between vertebrates and tunicates

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