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69-465: Cyclostomi , often referred to as Cyclostomata / s ɪ k l oʊ ˈ s t ɒ m ə t ə / , is a group of vertebrates that comprises the living jawless fishes : the lampreys and hagfishes . Both groups have jawless mouths with horny epidermal structures that function as teeth called ceratodontes , and branchial arches that are internally positioned instead of external as in the related jawed fishes . The name Cyclostomi means "round mouths". It

138-549: 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 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

207-488: A clade . The "vertebrate hypothesis" held that lampreys are more closely related to gnathostomes than they are to the hagfish. The "cyclostome hypothesis", on the other hand, holds that lampreys and hagfishes are more closely related, making cyclostomi monophyletic . Most studies based on anatomy have supported the vertebrate hypothesis, while most molecular phylogenies have supported the cyclostome hypothesis. There are exceptions in both cases, however. Similarities in

276-427: 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 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

345-403: A typhlosole that increases the inner surface like the spiral valve does in some jawed vertebrates. The spiral valve in the latter develops by twisting the whole gut, while the lamprey typhlosole is confined to the mucous membrane of the intestines. The mucous membranes of hagfishes have a primitive typhlosole in the form of permanent zigzag ridges. This trait could be a primitive one, since it

414-417: A central nervous system arising from a single nerve cord dorsal to the gut tube , headed by a series of (typically paired) brain vesicles, 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

483-459: A circle. Its lateral extremity is ampullated, and opens into the upper part of the vestibule; the opposite end joins with the upper part of the posterior semicircular canal to form the crus osseum commune , which opens into the upper and medial part of the vestibule. The posterior semicircular canal contains the part of the vestibular system that detects rotation of the head around the antero-posterior (sagittal) axis, or in other words, rotation in

552-404: A different duct to be stimulated by movement of the head in different planes, and more than one duct is stimulated at once if the movement is off those planes. The lateral semicircular duct detects angular acceleration of the head when the head is turned and the anterior and posterior semicircular ducts detect vertical head movements when the head is moved up or down. When the head changes position,

621-525: A hollow neural tube ) running along the dorsal aspect of the notochord . Of particular importance and unique to vertebrates 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

690-465: A larval stage followed by metamorphosis into a juvenile form (or adult form in the non-parasitic species). Lamprey larvae live in freshwater and are called ammocoetes, and are the only vertebrates with an endostyle , an organ used for filter feeding that is otherwise found only in tunicates and lancelets . During metamorphosis the lamprey endostyle develops into the thyroid gland. The cyclostomi evolved oxygen transport hemoglobins independently from

759-407: A pilot enters a turn, hair cells in the semicircular ducts are stimulated, telling the brain that the aircraft, and the pilot, are no longer moving in a straight line but rather making a banked turn. If the pilot were to sustain a constant rate turn, the endolymph would eventually catch up with the ducts and cease to deflect the cupula. The pilot would no longer feel as if the aircraft was in a turn. As

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828-589: 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 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

897-458: A second class of hair cells, apparently a derived trait, whereas lampreys and other vertebrates have stereocilia only. Because the inner ear of hagfishes has two forms of sensory ampullae, their single semicircular canal is assumed to be a result of two semicircular canals that have merged into just one. The hagfish blood is isotonic with seawater, while lampreys appears to use the same gill-based mechanisms of osmoregulation as marine teleosts . Yet

966-606: 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 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 ,

1035-424: A vertical axis (i.e. the neck), or in other words, rotation in the transverse plane . This occurs, for example, when one turns the head from side to side (yaw axis). It measures from 12 to 15 mm (0.47 to 0.59 in), and its arch is directed horizontally backward and laterally; thus each semicircular canal stands at right angles to the other two. Its ampullated end corresponds to the upper and lateral angle of

1104-1296: Is also found in some sea squirts such as Ciona . The intestinal epithelia of lampreys also have ciliated cells, which have not been detected in hagfishes. Because ciliated intestines are also found in Chondrostei , lungfishes and the early stages of some teleosts, it is considered a primitive condition that has been lost in hagfishes. After Miyashita et al. 2019. † Haikouella † Haikouichthys † Myllokunmingia † Metaspriggina Gnathostomata (jawed fish) † Cornovichthys † Achanarella † Ciderius † Birkeniida † Lasanius † Euphanerops † Jamoytius † Pipiscius † Euconodonta (conodonts) † Myxinikela † Tethymyxine tapirostrum Rubicundus eos Rubicundus lopheliae Myxine glutinosa Neomyxine biniplicata Eptatretus stoutii Eptatretus burgeri " Paramyxine " spp. † Gilpichthys † Hardistiella † Mayomyzon † Myxineidus † Priscomyzon † Mesomyzon Geotria australis Mordacia mordax Petromyzon marinus Lampetra fluviatilis Vertebrates Ossea Batsch, 1788 Vertebrates ( / ˈ v ɜːr t ə b r ɪ t s , - ˌ b r eɪ t s / ) are animals with

1173-418: Is attached to the cranium. The common ancestor of both cyclostomes and gnathostomes went through a genome duplication before their split, and while a second genome duplication occurred in the stem-gnathostomes, the stem-cyclostomes experienced an independent genome triplication. The mouth apparatus in hagfishes and adult lampreys has some similarities, but differ from one another. Lampreys have tooth plates on

1242-410: Is correlated with their type of locomotion. Specifically, species that are agile and have fast, jerky locomotion have larger canals relative to their body size than those that move more cautiously. The lateral semicircular canal (also known as horizontal or external semicircular canal ) is the shortest of the three canals. Movement of fluid within its duct corresponds to rotation of the head around

1311-440: Is found in invertebrate chordates such as lancelets (a sister subphylum known as 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 :

1380-414: Is more than twice the diameter of the ducts. Each ampulla contains an ampullary crest, the crista ampullaris which consists of a thick gelatinous cap called a cupula and many hair cells . The superior and posterior semicircular ducts are oriented vertically at right angles to each other. The lateral semicircular duct is about a 30-degree angle from the horizontal plane. The orientations of the ducts cause

1449-472: Is oriented in the pitch axis, and the posterior semicircular canal is oriented in the roll axis. Each duct is filled with a fluid called endolymph and contains motion sensors within the fluids. The base of each duct is enlarged, opening into the utricle , and has a dilated sac at one end called the membranous ampulla. Within the ampulla is a mound of hair cells and supporting cells called crista ampullaris . These hair cells have many cytoplasmic projections on

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1518-426: Is used by the vestibular system to detect rotations of the head in the coronal plane. It is the longest of the three semicircular canals, measuring from 18 to 22 mm (0.71 to 0.87 in). Its lower or ampullated end opens into the lower and back part of the vestibule, its upper into the crus osseum commune . Findings from a 2009 study demonstrated a critical late role for bone morphogenetic protein 2 (BMP-2) in

1587-635: The Cephalochordata is the sister taxon to Craniata (Vertebrata). This group, called 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

1656-559: The Chengjiang biota and lived about 518 million years ago. These include Haikouichthys , Myllokunmingia , Zhongjianichthys , and probably Haikouella . 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 jaws in the common sense and relied on filter feeding close to

1725-474: 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

1794-633: 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

1863-464: The Latin vertebratus ("jointed"), from vertebra meaning "joint", in turn from Latin vertere to turn. All vertebrates are built along 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

1932-574: 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

2001-909: 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]   Semicircular canal Each semicircular canal contains its respective semicircular duct , i.e.

2070-399: 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 is replaced by a segmented series of mineralized elements called vertebrae separated by fibrocartilaginous intervertebral discs , which are embryonic and evolutionary remnants of

2139-419: The coronal plane . This occurs, for example, when one moves the head to touch the shoulders, or when doing a cartwheel (roll axis). It is directed superiorly and posteriorly, as per its nomenclature, nearly parallel to the posterior surface of the petrous part of the temporal bone. The vestibular aqueduct is immediately medial to it. The posterior semicircular canal is part of the bony labyrinth and its duct

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2208-404: The endolymph in the ducts lags behind due to inertia and this acts on the cupula which bends the cilia of the hair cells. The stimulation of the hair cells sends the message to the brain that acceleration is taking place. The semicircular canals open into the vestibule by five orifices, one of the apertures being common to two of them. Among species of mammals, the size of the semicircular canals

2277-565: 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,

2346-418: The otic vesicle during early development. These depressions first form in lampreys between the 11 and 42 millimeter larval stages and form in zebrafish 57 hours post-fertilization The semicircular ducts provide sensory input for experiences of rotary movements. They are oriented along the pitch, roll, and yaw axes . The lateral semicircular canal is oriented in the yaw axis, the anterior semicircular canal

2415-410: The prosencephalon ( forebrain ), mesencephalon ( midbrain ) and rhombencephalon ( hindbrain ), which are further differentiated in 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

2484-432: The telencephalon and diencephalon , while the midbrain dominates in fish and some salamanders . In vertebrates with paired appendages, especially tetrapods, 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

2553-430: The vestibule , just above the oval window , where it opens close to the ampullated end of the anterior semicircular canal; its opposite end opens at the upper and back part of the vestibule. The lateral canal of one ear is very nearly in the same plane as that of the other. The anterior semicircular canal (also known as superior semicircular canal ) contains the part of the vestibular system that detects rotations of

2622-406: The apical surface called stereocilia which are embedded in a gelatinous structure called the cupula . As the head rotates, the duct moves, but the endolymph lags behind owing to inertia . This deflects the cupula and bends the stereocilia within. The bending of these stereocilia alters an electric signal that is transmitted to the brain. Within approximately 10 seconds of achieving constant motion,

2691-418: 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 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

2760-427: 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 the vertebrate subphylum. Specifically, 5 CSIs in the following proteins: protein synthesis elongation factor-2 , eukaryotic translation initiation factor 3 , adenosine kinase and

2829-441: The cartilage and muscles of the tongue apparatus also provide evidence of sister-group relationship between lampreys and hagfishes. And at least one molecular phylogeny has supported the vertebrate hypothesis. The embryonic development of hagfishes was once held to be drastically different from that of lampreys and gnathostomes, but recent evidence suggests that it is more similar than previously thought, which may remove an obstacle to

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2898-409: 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

2967-420: The cyclostome hypothesis. Several groups of Paleozoic jawless fish have been suggested to be more closely related to cyclostomes than to jawed fish, including conodonts and anaspids . The presence of mineralised elements in these jawless fish, like the oral conodont elements and the armoured body covering of anaspids and scutes on other species like Lasanius suggests that mineralised tissues were present in

3036-556: The demise of virtually all jawless fishes save for lampreys and hagfishes, as well as 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

3105-465: The direction of the original turn, they may actually re-enter the turn. As the endolymph stabilizes, the semicircular ducts stop registering the gradual turn and the aircraft slowly loses altitude until impact with the ground. Jean Pierre Flourens , by destroying the horizontal semicircular canal of pigeons , noted that they continue to fly in a circle, showing the purpose of the semicircular canals. [REDACTED] This article incorporates text in

3174-408: The dorsal nerve cord and migrate together with 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

3243-422: The endolymph catches up with the movement of the duct and the cupula is no longer affected, stopping the sensation of acceleration. The specific gravity of the cupula is comparable to that of the surrounding endolymph. Consequently, the cupula is not displaced by gravity, unlike the otolithic membranes of the utricle and saccule . As with macular hair cells, hair cells of the crista ampullaris will depolarise when

3312-483: The first gill arch pair evolved into the jointed jaws and form an additional oral cavity ahead of 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

3381-421: The gill arches form during fetal development , and form 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 . 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

3450-400: The gut tube, with a split brain stem circumventing the foregut around each side to form a brain on the dorsal side of 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

3519-419: The head in around the lateral axis, that is, rotation in the sagittal plane . This occurs, for example, when nodding one's head (pitch axis). It is 15 to 20 mm (0.59 to 0.79 in) in length, is vertical in direction, and is placed transversely to the long axis of the petrous part of the temporal bone , on the anterior surface of which its arch forms a round projection. It describes about two-thirds of

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3588-443: The internal gills proper in fishes and by cutaneous respiration in most amphibians. While some amphibians such as axolotl retain 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,

3657-409: The jawed vertebrates. Hagfishes and lampreys lack a thymus , spleen , myelin and sympathetic chain ganglia . Neither species has internal eye muscles and hagfishes also lack external eye muscles. Both groups have only a single olfactory organ with a single nostril. The nasal duct ends blindly in a pouch in lampreys but opens into the pharynx in hagfishes. The branchial basket (reduced in hagfishes)

3726-407: The jawless Agnatha , and the jawed Gnathostomata . The jawed fish include both the cartilaginous fish and the bony fish . Bony fish include 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

3795-425: The last common ancestor of all vertebrates, but were secondarily lost in hagfish and lampreys. Both hagfishes and lampreys have a single gonad, but for different reasons. In hagfishes the left gonad degenerates during their ontogeny and only the right gonad develops, whereas in lampreys the left and right gonads fuse into one. There are no gonoducts present. Hagfishes have direct development, but lamprey go through

3864-445: The lateral, anterior and posterior semicircular ducts, which provide the sensation of angular acceleration and are part of the membranous labyrinth —therefore filled with endolymph . The semicircular canals are a component of the bony labyrinth that are at right angles from each other and contain their respective semicircular duct. At one end of each of the semicircular ducts is a dilated sac called an membranous ampulla , which

3933-432: 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 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

4002-460: 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

4071-457: The morphogenesis of semicircular canals in the zebrafish inner ear. It is suspected that the role of BMP-2 in semicircular canal duct outgrowth is likely to be conserved between different vertebrate species. Additionally, it has been found that the two semicircular canals found in the lamprey inner ear are developmentally similar to the superior and posterior canals found in humans, as the canals of both organisms arise from two depressions in

4140-411: 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 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

4209-403: The pilot exits the turn, the semicircular ducts are stimulated to make the pilot think that they are now turning in the opposite direction rather than flying straight and level. In response to this, the pilot will often lean in the direction of the original turn in an attempt to compensate for this illusion. A more serious form of this is called a graveyard spiral . Rather than the pilot leaning in

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4278-609: The relationships between the Olfactores (vertebrates and tunicates) and 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

4347-484: The same mechanisms are apparent in the mitochondria-rich cells in the gill epithelia of hagfishes, but never develops the ability to regulate the blood's salinity, even if they are capable of regulating the ionic concentration of Ca and Mg ions. It has been suggested that the hagfish ancestors evolved from an anadromous or freshwater species that has since adapted to saltwater over a very long time, resulting in higher electrolyte levels in its blood. The lamprey intestine has

4416-661: 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. 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

4485-430: The stereocilia deflect towards the kinocilium . Deflection in the opposite direction results in hyperpolarisation and inhibition. In the lateral semicircular duct, ampullopetal flow is necessary for hair-cell stimulation, whereas ampullofugal flow is necessary for the anterior and posterior semicircular ducts. This adjustment period is in part the cause of an illusion known as " the leans " often experienced by pilots. As

4554-650: 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

4623-558: The top of a tongue-like piston cartilage, and the hagfish have a fixed cartilaginous plate on the floor of its mouth with groves that allows tooth plates to slide backwards and forwards over it like a conveyor belt, and are everted as they move over the edge of the plate. Hagfishes also have a keratinous palatine tooth hanging from the roof of the mouth. Unlike jawed vertebrates, which have three semicircular canals in each inner ear, lampreys have only two and hagfishes just one. The semicircular canal of hagfishes contains both stereocilia and

4692-402: 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,

4761-434: Was named by Joan Crockford-Beattie . This taxon is often included in the paraphyletic superclass Agnatha , which also includes several groups of extinct armored fishes called ostracoderms . Most fossil agnathans, such as galeaspids , thelodonts, and osteostracans, are more closely related to vertebrates with jaws (called gnathostomes ) than to cyclostomes. Biologists historically disagreed on whether cyclostomes are

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