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80-502: Placoderms (from Greek πλάξ ( plax , plakos ) ' plate ' and δέρμα ( derma ) 'skin') are vertebrate animals of the class Placodermi , an extinct group of prehistoric fish known from Paleozoic fossils during the Silurian and the Devonian periods . While their endoskeletons are mainly cartilaginous , their head and thorax were covered by articulated armoured plates (hence

160-697: A bodyplan superficially similar to primitive holocephalians , the two groups have little else in common anatomically. The following cladogram shows the interrelationships of placoderms according to Carr et al. (2009): Stensioella Pseudopetalichthys Brindabellaspis Acanthothoraci Rhenanida Yunnanolepis Euantiarcha Petalichthyida Ptyctodontida Wuttagoonaspis Actinolepidae Phyllolepida Phlyctaeniida Holonema Antineosteus Buchanosteidae Pholidosteus Tapinosteus Plate (animal anatomy) A plate in animal anatomy may refer to several things: This animal anatomy –related article

240-452: A Silurian placoderm, Wangolepis of Silurian China and possibly Vietnam, is known only from a few fragments that currently defy attempts to place them in any of the recognized placoderm orders. So far, only three officially described Silurian placoderms are known from more than scraps: The first officially described Silurian placoderm is an antiarch, Shimenolepis , which is known from distinctively ornamented plates from Hunan , China. It

320-500: A carious attack or wear. Primary dentin , the most prominent dentin in the tooth, lies between the enamel and the pulp chamber (near dentinoenamel junction). The outer layer closest to enamel is known as mantle dentin . This layer is unique to the rest of primary dentin. Mantle dentin is formed by newly differentiated odontoblasts and forms a layer consistently 15-20 micrometers (μm) wide. Unlike primary dentin, mantle dentin lacks phosphorylation, has loosely packed collagen fibrils and

400-409: A decrease in the size of the pulp chamber with age. This is clinically known as pulp recession; cavity preparation in young patients, therefore, carries a greater risk of exposing the pulp. If this occurs, the pulp can be treated by different therapies such as direct pulp capping. Previously it was thought that Pulp capping was most successful if followed by a stainless steel crown, however this procedure

480-538: A feature shared by birds and some ichthyosaurs . Early arthrodires, such as the genus Arctolepis , were well-armoured fishes with flattened bodies. The largest member of this group, Dunkleosteus , was a true "superpredator" of the latest Devonian period, reaching 3 to as much as 8 metres in length. In contrast, the long-nosed Rolfosteus measured just 15 cm. Fossils of Incisoscutum have been found containing unborn fetuses, indicating that arthrodires gave birth to live young. Antiarchi ("opposite anus") were

560-468: A group of chimaera-like placoderms closely related to the rhenanid placoderms. Superficially, acanthoracids resembled scaly chimaeras or small, scaly arthrodires with blunt rostrums . They were distinguished from chimaeras by a pair of large spines that emanate from their chests, the presence of large scales and plates, tooth-like beak plates, and the typical bone-enhanced placoderm eyeball. They were distinguished from other placoderms due to differences in

640-511: A long clasper into the female. Elongated basipterygia are also found on the phyllolepid placoderms, such as Austrophyllolepis and Cowralepis , both from the Middle Devonian of Australia, suggesting that the basipterygia were used in copulation. The placoderm claspers are not homologous with the claspers in cartilaginous fishes . The similarities between the structures has been revealed to be an example of convergent evolution . While

720-507: A mosaic of tubercles. Like Stensioella heintzi , the pseudopetalichthids' placement within Placodermi is suspect. The matter is not easy to resolve because there are no complete, undamaged and articulated specimens. The anatomical studies done on the crushed specimens that have been found indicate that if they are placoderms, they may be a group more advanced than the ptyctodonts . As such, placoderm experts consider Pseudopetalichthyida to be

800-407: A movable joint between armour surrounding the head and body. As the lower jaw moved down, the head shield moved, allowing for a larger opening. All arthrodires, save for Compagopiscis , lacked teeth, and used instead the sharpened edges of a bony plate, termed a "tooth plate," as a biting surface ( Compagopiscis had true teeth in addition to tooth plates). The eye sockets are protected by a bony ring,

880-492: A primitive placoderm, though some paleontologists believe the rationale for the placement is inadequate. The paleontologist Philippe Janvier , as well as other paleontologists, has suggested that Stensioella is not a placoderm, but instead is a holocephalian . If this is true, then the holocephalians diverged from sharks before the Chondrichthyan Devonian radiation. Critics of Janvier's position say that aside from

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960-407: A reaction to external stimulation such as cavities and wear. It is of two types, either reactionary, where dentin is formed from a pre-existing odontoblast, or reparative, where newly differentiated odontoblast-like cells are formed due to the death of the original odontoblasts, from a pulpal progenitor cell . Tertiary dentin is only formed by an odontoblast directly affected by a stimulus; therefore,

1040-402: A result of injury to dentin by caries or abrasion, or as part of the normal aging process. Elephant ivory is solid dentin. The structure of the dentinal tubules contributes to both its porosity and its elasticity . Elephant tusks are formed with a thin cap of enamel, which soon wears away, leaving the dentin exposed. Exposed dentin in humans causes the symptom of sensitive teeth . Dentin

1120-418: A similar structure to primary dentin, although its deposition is not always even around the pulp chamber. It appears greater in amounts on the roof and floor of the coronal pulp chamber, where it protects the pulp from exposure in older teeth. The secondary dentin formed is not in response to any external stimuli, and it appears very much similar to the primary dentine. It is the growth of this dentin that causes

1200-479: A skinny Gemuendina with thin, strap-like pectoral fins. Similar to those of the Rhenanida, its armour was a complex mosaic of small, scale-like tubercles. The shoulder joints of its armour are similar to other placoderms, and there are superficial similarities in skull plates, and even more superficial similarities between its tubercles and the tubercles of the rhenanids . It is tentatively placed within Placodermi as

1280-446: A small female placoderm, about 25 cm (10 in) in length, which died in the process of giving birth to a 6 cm ( 2 + 1 ⁄ 2  in) offspring and was fossilized with the umbilical cord intact. The fossil, named Materpiscis attenboroughi (after scientist David Attenborough ), had eggs which were fertilized internally, the mother providing nourishment to the embryo and giving birth to live young. With this discovery,

1360-584: Is 59,000 to 76,000 per square millimeter near the pulp, whereas the density is only half as much near the enamel. Within the tubules, there is an odontoblast process , which is an extension of an odontoblast, and dentinal fluid, which contains a mixture of albumin , transferrin , tenascin and proteoglycans . In addition, there are branching canalicular systems that connect to each other. These branches have been categorized by size, with major being 500–1000 nm in diameter, fine being 300–700 nm, and micro being less than 300 nm. The major branches are

1440-409: Is a stub . You can help Misplaced Pages by expanding it . Dentine Prior to enamel formation, dentine formation begins through a process known as dentinogenesis , and this process continues throughout a person's life even after the tooth has fully developed. Events such as tooth decay and tooth wear can also initiate dentine formation. Dentinogenesis is initiated by the odontoblasts of

1520-424: Is best known for its occurrence in teeth, but in early vertebrates, it was an important part of the dermal skeleton that covered most of the body, and it persists today in a few taxa such as the coelacanth . Because dentin is softer than enamel, it wears away more quickly than enamel. Some mammalian teeth exploit this phenomenon, especially herbivores such as horses , deer or elephants . In many herbivores,

1600-433: Is less mineralized. Below it lies the circumpulpal dentin, more mineralized dentin which makes up most of the dentin layer and is secreted after the mantle dentin by the odontoblasts. Circumpulpal dentin is formed before the root formation is completed. Newly secreted dentin is unmineralized and is called predentin. It is easily identified in hematoxylin and eosin stained sections since it stains less intensely than dentin. It

1680-431: Is made up, by weight, of 70–72% inorganic materials (mainly hydroxylapatite and some non-crystalline amorphous calcium phosphate ), 20% organic materials (90% of which is collagen type 1 and the remaining 10% ground substance, which includes dentin-specific proteins ), and 8–10% water (which is adsorbed on the surface of the minerals or between the crystals). Because it is softer than enamel, it decays more rapidly and

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1760-403: Is most of the times unnecessary in children. it requires the unnecessary removal of enamel which is key to the life of the tooth. Adhesive dentistry allows for conservative restoration techniques that minimize the loss of tooth structure and should be used. In order to maintain space in the primary dentition, attempts are made not to extract a pulpal exposure. Tertiary dentin is dentin formed as

1840-581: Is not clear, as none of the fossil specimens found have preserved mouth parts. Pseudopetalichthyida ("false petalichthyids") is a group of elongated, possibly flattened fishes comprising three, poorly preserved and poorly studied genera. It is known only from rare fossils in Lower Devonian strata in Hunsrück , Germany. Like Stensioella heintzi , and the Rhenanida , the pseudopetalichthids had armour made up of

1920-444: Is possibly due to differences in the rates of formation of coronal and root dentin. The hyaline layer, which has an obscure origin, is a clear layer, unlike the granular layer, with a width of up to 20μm. It can have clinical significance during periodontal regeneration. Circumpulpal dentin forms the majority of the dentin and is generally constant in structure. Peripherally, mineralization can be seen to be incomplete, whereas centrally

2000-508: Is seen in Vit.A deficiency during development. However, if the stimulus is less active, it is laid down less rapidly with a more regular tubular pattern and hardly any cellular inclusions. The speed at which tertiary dentin forms also varies substantially among primate species. Dentinal sclerosis or transparent dentin sclerosis of primary dentin is a change in the structure of teeth characterized by calcification of dentinal tubules. It can occur as

2080-401: Is subject to severe cavities if not properly treated, but due to its elastic properties, it is good support for enamel. Its flexibility prevents the brittle enamel fracturing. In areas where both primary and secondary mineralization have occurred with complete crystalline fusion, these appear as lighter rounded areas on a stained section of dentin and are considered globular dentin. In contrast,

2160-470: Is usually 10-47μm and lines the innermost region of the dentin. It is unmineralized and consists of collagen, glycoproteins, and proteoglycans. It is similar to osteoid in bone and is thickest when dentinogenesis is occurring. Secondary dentin (adventitious dentin) is formed after root formation is complete, normally after the tooth has erupted and is functional. It grows much more slowly than primary dentin but maintains its incremental aspect of growth. It has

2240-411: The occlusal (biting) surface of the tooth is composed of alternating areas of dentin and enamel. Differential wearing causes sharp ridges of enamel to be formed on the surface of the tooth (typically a molar ), and to remain during the working life of the tooth. Herbivores grind their molars together as they chew ( masticate ), and the ridges help to shred tough plant material. In xenarthrans , enamel

2320-646: The Middle to Late Devonian genus , Bothriolepis , known from over 100 valid species. The vast majority of placoderms were predators , many of which lived at or near the substrate . Many, primarily the arthrodires , were active, nektonic predators that dwelled in the middle to upper portions of the water column. A study of the arthrodire Compagopiscis published in 2012 concluded that placoderms (at least this particular genus) likely possessed true teeth contrary to some early studies. The teeth had well defined pulp cavities and were made of both bone and dentine . However,

2400-418: The anatomy of their skulls, and due to patterns on the skull plates and thoracic plates that are unique to this order. From what can be inferred from the mouthplates of fossil specimens, acanthothoracids were shellfish hunters ecologically similar to modern-day chimaeras. Competition with their relatives, the ptyctodont placoderms, may have been one of the main reasons for the acanthothoracids' extinction prior to

2480-436: The architecture and structure depend on the intensity and duration of the stimulus, e.g., if the stimulus is a carious lesion, there is extensive destruction of dentin and damage to the pulp, due to the differentiation of bacterial metabolites and toxins. Thus, tertiary dentin is deposited rapidly, with a sparse and irregular tubular pattern and some cellular inclusions; in this case, it is referred to as "osteodentin". Osteodentin

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2560-482: The claspers in cartilaginous fishes are specialized parts of their paired pelvic fins that have been modified for copulation due to changes in the hox genes hoxd13, the origin of the mating organs in placoderms most likely relied on different sets of hox genes and were structures that developed further down the body as an extra and independent pair of appendages, but which during development turned into body parts used for reproduction only. Because they were not attached to

2640-589: The collagen fibers experience a significant increase in compressive stress of around 90 MPa and, for crack formation to occur, tensile stresses must first overcome this residual compressive stress. Since typical mastication stresses do not exceed 40 MPa, the ITD prevents cracks from forming during normal daily use and helps deflect cracks perpendicular to the dentin tubule and away from the pulp. Inelastic deformation of dentin primarily happens through microcracking. Crack propagation within dentin travels preferentially along

2720-415: The composition of dentine. Unlike enamel, dentin may be demineralized and stained for histological study. Dentin consists of microscopic channels, called dentinal tubules, which radiate outward through the dentin from the pulp to the exterior cementum or enamel border. The dentinal tubules extend from the dentinoenamel junction (DEJ) in the crown area, or dentinocemental junction (DCJ) in the root area, to

2800-595: The darker arc-like areas in a stained section of dentin are considered interglobular dentin. In these areas, only primary mineralization has occurred within the predentin, and the globules of dentin do not fuse completely. Thus, interglobular dentin is slightly less mineralized than globular dentin. Interglobular dentin is especially evident in coronal dentin, near the dentinoenamel junction (DEJ), and in certain dental anomalies, such as in dentinogenesis imperfecta . The different regions in dentin can be recognized due to their structural differences. The outermost layer, known as

2880-518: The dentinogenesis process, the odontoblast cells retreat from the DEJ to the outer lining of the pulp, leaving behind microtubules filled with cytoplasmic extensions and depositing intertubular dentin (ITD) in its place. ITD comprises the bulk of the dentin and, similarly to bone , is a matrix composite of tablet-shaped hydroxyapatite nanoparticles wrapped around collagen fibers. The mineralized collagen fibers are arranged in layers oriented perpendicular to

2960-419: The direction of the dentin microtubules which are lined with peritubular dentin (PTD), a 1-2 μm thick layer of hydroxyapatite tablets with no preferred orientation and lacks any supporting collagen fibers. The hydroxyapatite tablets within the ITD were found to be compressed along the crystallographic c-axis due to tight interaction between the tablets and the collagen fiber. Tablets aligned parallel with

3040-477: The early Silurian. They eventually outcompeted the previously dominant marine arthropods (e.g. eurypterids ) and cephalopod molluscs (e.g. orthocones ), producing some of the first and most infamous vertebrate apex predators such as Eastmanosteus , Dinichthys and the massive Dunkleosteus . Various groups of placoderms were diverse and abundant during the Devonian, but all placoderms became extinct at

3120-563: The end-Devonian Hangenberg event 358.9 million years ago, leaving the niches open for the osteichthyan and chondrichthyan survivors who subsequently radiated during the Carboniferous . Many placoderms, particularly the Rhenanida , Petalichthyida , Phyllolepida , and Antiarchi , were bottom-dwellers. In particular, the antiarchs, with their highly modified, jointed bony pectoral fins, were highly successful inhabitants of Middle-Late Devonian freshwater and shallow marine habitats, with

3200-516: The equally enigmatic Pseudopetalichthyida . These orders are considered to be basal or primitive groups within Placodermi, though their precise placement within the class remains unsure. Fossils of both are currently known only from the Hunsruck lagerstatten . Arthrodira ("jointed neck") were the most diverse and numerically successful of the placoderm orders, occupying roles from giant apex predators to detritus -nibbling bottom dwellers . They had

3280-447: The exquisitely preserved placoderm fossils from Gogo reef changed the picture again. They showed that placoderms shared anatomical features not only with chondrichthyans but with other gnathostome groups as well. For example, Gogo placoderms show separate bones for the nasal capsules as in gnathostomes; in both sharks and bony fish those bones are incorporated into the braincase. Placoderms also share certain anatomical features only with

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3360-482: The extinct and related acanthothoracids , and the living and unrelated holocephalians, most of the ptyctodontids are thought to have lived near the sea bottom and preyed on shellfish . On account of their lack of armour, some paleontologists have suggested that the Ptyctodontida were not placoderms, but holocephalians or the ancestors of holocephalians. Anatomical examinations of whole fossil specimens have shown that

3440-682: The extinction event at the Frasnian – Famennian boundary, the Late Devonian extinctions. The remaining species then died out during the end-Devonian extinction; not a single placoderm species has been confirmed to have survived into the Carboniferous . The earliest studies of placoderms were published by Louis Agassiz , in his five volumes on fossil fishes, 1833–1843. In those days, placoderms were thought to be shelled jawless fish akin to ostracoderms . Some naturalists even suggested that they were shelled invertebrates or even turtle -like vertebrates. In

3520-496: The interfaces of the ITD layers. Since the PTD, the hydroxyapatite tablets are not preferentially orientated; they are under less compressive residual stress, causing the microtubules to act as crack initiation sites. This manifests as cross-hatched shear microcracks forming at the microtubules in compression and as ring-shaped microcracks in tension. The tip of a larger crack creates a stress concentration that helps initiate microcracks around

3600-400: The jawless osteostracans ; because of this, the theory that placoderms are the sister group of chondrichthyans has been replaced by the theory that placoderms are a group of basal gnathostomes. Currently, Placodermi are divided into eight recognized orders . There are two further controversial orders: One is the monotypic Stensioellida, containing the enigmatic Stensioella ; the other is

3680-508: The late 1920s, Dr. Erik Stensiö , at the Swedish Museum of Natural History in Stockholm , established the details of placoderm anatomy and identified them as true jawed fishes related to sharks . He took fossil specimens with well-preserved skulls and ground them away, one tenth of a millimeter at a time. After each layer had been removed, he made an imprint of the next surface in wax . Once

3760-507: The limbs is still not perfectly understood, but most hypothesize that they helped their owners pull themselves across the substrate, as well as allowing their owners to bury themselves into the substrate. Brindabellaspis (" Brindabella's shield") was a long-snouted placoderm from the Early Devonian . When it was first discovered in 1980, it was originally regarded as a weejasperaspid acanthothoracid due to anatomical similarities with

3840-421: The males having pelvic claspers and possibly claspers on the head as well. Rhenanida (" Rhine fish") were flattened, ray-like , bottom-dwelling predators with large, upturned mouths that lived in marine environments. The rhenanids were once presumed to be the most primitive, or at least the closest to the ancestral placoderm, as their armour was made of unfused components—a mosaic of tubercles—as opposed to

3920-516: The mantle dentin layer, is found in the crown of the tooth. It can be identified by the presence of various characteristics, including collagen fibres found perpendicular to the enamel-dentin junction and it is slightly less mineralized (by approximately 5%, compared to the enamel. The dentin undergoes mineralization in the presence of matrix vesicles ("hydroxyapatite-containing, membrane-enclosed vesicles secreted by odontoblasts, osteoblasts, and some chondrocytes; believed to serve as nucleation centers for

4000-400: The microtubules ahead of it, consuming energy and resisting further damage. The imperfect linking of the microcrack to a larger crack also induces 'uncracked ligaments', which help arrest the larger crack. In comparison, enamel does not display the same fracture resistance, and fractures traveling across the DEJ are usually stopped within ~10  μm. The combination of the residual stress and

4080-401: The mid-Devonian extinction event. Petalichthyida ("thin-plated fish") were small, flattened placoderms, typified by their splayed fins and numerous tubercles that decorated all of the plates and scales of their armour. They reached a peak in diversity during the Early Devonian and were found throughout the world. The petalichthids Lunaspis and Wijdeaspis are among the best known. There

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4160-483: The mineralization process in dentin, bone, and calcified cartilage.") The dentinal tubules in this region branch profusely. In the root of the tooth there are two morphologically distinguishable outer layers: the hyaline layer on the periphery of dentin and the granular layer of Tomes beneath this. The granular layer has a dark, granular appearance which occurs due to the branching and looping back of dentinal tubules in this region. This appearance, specific to root dentin,

4240-436: The mineralizing front shows ongoing mineralizing. The innermost layer of dentin is known as predentin, and is the initial dentin matrix that is laid down prior to mineralization. It can be distinguished by its pale color when stained with haematoxylin and eosin. The presence of odontoblastic processes here allows the secretion of matrix components. Predentin can be 10-40μm in width, depending on its rate of deposition. During

4320-536: The name), and the rest of the body was scaled or naked depending on the species . Placoderms were among the first jawed fish (their jaws likely evolved from the first pair of gill arches ), as well as the first vertebrates to have true teeth . They were also the first fish clade to develop pelvic fins , the second set of paired fins and the homologous precursor to hindlimbs in tetrapods . 380-million-year-old fossils of three other genera, Incisoscutum , Materpiscis and Austroptyctodus , represent

4400-407: The oldest known examples of live birth . Placoderms are thought to be paraphyletic , consisting of several distinct outgroups or sister taxa to all living jawed vertebrates , which originated among their ranks. In contrast, one 2016 analysis concluded that Placodermi is likely monophyletic . The first identifiable placoderms appear in the fossil record during the late Llandovery epoch of

4480-425: The other species found at the same locality. According to Philippe Janvier , anatomical similarities in the brain of Brindabellaspis stensioi and the brain of a jawless fish suggest it is a basal placoderm closest to the ancestral placoderm. Various Early to Middle Devonian placoderm incertae sedis have also been inserted in the order. Phyllolepida ("leaf scales") were flattened placoderms found throughout

4560-410: The outer wall of the pulp. From the outer surface of the dentin to the area nearest the pulp, these tubules follow an S-shaped path. The diameter and density of the tubules are greatest near the pulp. Tapering from the inner to the outermost surface, they have a diameter of 2.5 μm near the pulp, 1.2 μm in the middle of the dentin, and 0.9 μm at the dentino-enamel junction . Their density

4640-426: The pelvic fins, as are the claspers in fish like sharks, they were much more flexible and could probably be rotated forward. A study on Kolymaspis showcases that the vertebrate shoulder girdle evolved from gill arches. It was thought for a time that placoderms became extinct due to competition from the first bony fish and early sharks , given a combination of the supposed inherent superiority of bony fish and

4720-437: The peripheral boundary of the dental pulp Because of dentinal tubules, dentin has a degree of permeability , which can increase the sensation of pain and the rate of tooth decay . The strongest held theory of dentinal hypersensitivity suggests that it is due to changes in the dentinal fluid associated with the processes, a type of hydrodynamic mechanism. Dentin is a bone-like matrix that is porous and yellow-hued material. It

4800-404: The perpendicular orientation of the ITD mineralized collagen fibers significantly increases the fracture toughness and fatigue endurance limit along the microtubule direction. Dentin is classified into three types: primary, secondary, and tertiary. Secondary dentin is a layer of dentin formed after the tooth's root has fully formed. Tertiary dentin develops as a result of a stimulus, such as

4880-474: The placoderm became the oldest vertebrate known to have given birth to live young (" viviparous "), pushing the date of first viviparity back some 200 million years earlier than had been previously known. Specimens of the arthrodire Incisoscutum ritchei , also from the Gogo Formation, have been found with embryos inside them indicating this group also had live bearing ability. The males reproduced by inserting

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4960-434: The placoderms is both literally and figuratively fragmented. Until the discovery of Silurolepis (and then, the discoveries of Entelognathus and Qilinyu ), Silurian-aged placoderm specimens consisted of fragments. Some of them have been tentatively identified as antiarch or arthrodire due to histological similarities; and many of them have not yet been formally described or even named. The most commonly cited example of

5040-456: The point of literally resembling a box with eyes, with the sometimes scaled, sometimes naked rear portions often becoming sinuous , particularly with later forms. The pair of pectoral fins were modified into a pair of caliper -like, or arthropod -like limbs. In primitive forms, such as Yunnanolepis , the limbs were thick and short, while in advanced forms, such as Bothriolepis , the limbs were long and had elbow-like joints. The function of

5120-664: The presumed sluggishness of placoderms. With more accurate summaries of prehistoric organisms, it is now thought that they systematically died out as marine and freshwater ecologies suffered from the environmental catastrophes of the Late Devonian and end-Devonian extinctions . The earliest identifiable placoderm fossils are of Chinese origin and date to the early Silurian . At that time, they were already differentiated into antiarchs and arthrodires , as well as other, more primitive, groups. Earlier fossils of basal placoderms have not yet been discovered. The Silurian fossil record of

5200-494: The pulp, along its outer wall, and project into tiny tubules in the dentine. Pre-dentine is composed of 90% type I collagen and 10% non-collagenous proteins (including phosphoproteins , proteoglycans , growth factors, phosphatases such as alkaline phosphatase , and matrix metalloproteinases (MMPs) ), and this composition is significantly altered when it is mineralised into dentine. See the Structure section for information about

5280-407: The pulp. Odontoblasts are specialised cells that lay down an organic matrix known as pre-dentine. This pre-dentine is subsequently mineralised into dentine. Mineralisation of pre-dentine begins at the dentino-enamel junction during tooth development and progresses towards the pulp of the tooth. After growth of pre-dentine and maturation into dentine, the cell bodies of the odontoblasts remain in

5360-559: The scarcity of placoderms in the Silurian fossil record is due to placoderms' living in environments unconducive to fossil preservation, rather than a genuine scarcity. This hypothesis helps to explain the placoderms' seemingly instantaneous appearance and diversity at the very beginning of the Devonian . During the Devonian, placoderms went on to inhabit and dominate almost all known aquatic ecosystems, both freshwater and saltwater . But this diversity ultimately suffered many casualties during

5440-423: The sea floor. Some placoderms were herbivorous, such as the Middle to Late Devonian arthrodire Holonema , and some were planktivores , such as the gigantic arthrodire Titanichthys , various members of Homostiidae , and Heterosteus . Extraordinary evidence of internal fertilization in a placoderm was afforded by the discovery in the Gogo Formation, near Fitzroy Crossing , Kimberley , Western Australia, of

5520-527: The second most successful order of placoderms known, after the Arthrodira . The order's name was coined by Edward Drinker Cope , who, after incorrectly identifying the first fossils as being those of an armored tunicate , mistakenly thought the eye-hole was the mouth, and the opening for the anal siphon was on the other side of the body, as opposed to having both oral and anal siphons together at one end. The front portions of their bodies were heavily armoured, to

5600-561: The similarities between these two groups are superficial. The major differences were that holocephalians have shagreen on their skin, while ptyctodontids do not; the armoured plates and scales of holocephalians are made of dentine , while those of ptyctodontids are made of bone; the craniums of holocephalians are similar to sharks, while those of ptyctodontids are similar to those of other placoderms; and, most importantly, that holocephalians have true teeth, while ptyctodonts have beak-like tooth plates. Ptyctodontids were sexually dimorphic , with

5680-581: The sister group of the Arthrodires + Phyllolepida + Antiarchi trichotomy and the Acanthothoraci + Rhenanida dichotomy . Stensioellida ("[Heintz's] little Stensio ") contains another problematic placoderm of uncertain affinity, known only from the Lower Devonian Hunsrück slates of Germany. Stensioella was a thin fish that, when alive, looked vaguely like an elongated ratfish , or

5760-450: The solidified plates of "advanced" placoderms, such as antiarchs and arthrodires . However, through comparisons of skull anatomies, rhenanids are now considered to be the sister group of the antiarchs. When rhenanids die, their "mosaics" come apart, and it has been suggested that the rarity of rhenanids in the fossil record reflects postmortem disassociation, and is not an actual rarity of the species. Acanthothoraci ("spine chests") were

5840-412: The specimens had been completely ground away (and so destroyed), he made enlarged, three-dimensional models of the skulls to examine the anatomical details more thoroughly. Many other placoderm specialists thought that Stensiö was trying to shoehorn placoderms into a relationship with sharks ; however, as more fossils were found, placoderms were accepted as a sister group of chondrichthyans . Much later,

5920-429: The terminal ends of the tubules. About every 1-2 μm, there are fine branches diverging from dentinal tubules at 45 degree angles. The microtubules diverge at 90 degree angles. The dentinal tubules contain the cytoplasmic extensions of odontoblasts that once formed the dentin and maintain it. The cell bodies of the odontoblasts are aligned along the inner aspect of dentin against a layer of predentin where they also form

6000-477: The tooth and jaw development were not as closely integrated as in modern gnathostomes. These teeth were likely homologous to the teeth of other gnathostomes. One of the largest known arthrodires, Dunkleosteus terrelli , was 3.5–4.1 metres (11–13 ft) long, and is presumed to have had a large distribution, as its remains have been found in Europe, North America and possibly Morocco. Some paleontologists regard it as

6080-419: The top of their head. The orbits for the eyes were extremely small, suggesting the eyes were vestigial and that the phyllolepids may have been blind. Ptyctodontida ("folded teeth") were lightly armoured placoderms with big heads, big eyes and long bodies. They have a strong but superficial resemblance to modern day chimaeras . Their armour was reduced to a pattern of small plates around the head and neck. Like

6160-562: The world's first vertebrate "superpredator", preying upon other predators. Other, smaller arthrodires, such as Fallacosteus and Rolfosteus , both of the Gogo Formation of Western Australia, had streamlined, bullet-shaped head armor, and Amazichthys , with morphology like that of other fast-swimming pelagic organisms , strongly supporting the idea that many, if not most, arthrodires were active swimmers, rather than passive ambush-hunters whose armor practically anchored them to

6240-400: The world. Like other flattened placoderms they were bottom-dwelling predators that ambushed prey. Unlike other flattened placoderms, they were freshwater fish. Their armour was made of whole plates, rather than the numerous tubercles and scales of Petalichthyida. The eyes were on the sides of the head, unlike visual bottom-dwelling predators, such as stargazers or flatfish , which have eyes on

6320-472: Was an independent diversification event that occurred in what is now Southern China, producing a handful of unique genera that were once placed in their own order, "Quasipetalichthyida", named after the first discovered species there, Quasipetalichthys haikouensis . Soon after the petalichthids' diversification, they went into decline. Because they had compressed body forms, it is supposed they were bottom-dwellers that pursued or ambushed smaller fish. Their diet

6400-436: Was originally considered to be from the late Llandovery , although later study reconsidered its age at Ludfordian . Shimenolepis plates are very similar to the early Devonian yunnanolepid Zhanjilepis , also known from distinctively ornamented plates. In 2022, Xiushanosteus is described from complete fossils from Telychian , late Llandovery of Chongqing , China. Paleontologists and placoderm specialists suspect that

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