Dentaries Assignment of two dentaries to a caseid is based on the shape of the symphyseal area. As in other caseids, the dorsal edge of the dentary bone curves ventrally near the symphysis, and forms an acute angle with the ventral edge of the bone. This results in a substantially more slender dentary bone near the symphysis than in the rest of the bone. As in other caseids, this is related to the presence of a well-developed anterior process of the splenial bone, one that contributes to a large portion of the symphysis medially.
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Large holes and cavities in the skull called adductor chambers and temporal openings would have provided room for large jaw-closing muscles. A ridge on the dentary bone of the lower jaw may have provided a surface for chewing or even supported a beak.
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In addition, the surface characteristics of the labial side of the dentaries are similar to those of the maxillae, showing little sculpturing, and occasional small foramina. A well-developed, anteriorly extending Meckelian canal is formed by the dentary bone, below which it would be attached to the splenial by sutures.
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The dentary bone is pneumatised. The lower rear branch of the dentary is twisted, causing its outer side to be somewhat directed to below. The first metacarpal is long and slender with its transverse width equal to a fifth of its length. The hand claws have prominent lips on their upper rears.
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The largest found so far is estimated to have been long including the root when the animal was alive, making it the largest tooth of any carnivorous dinosaur yet found. The lower jaw was robust. Its front dentary bone bore thirteen teeth. Behind the tooth row, the lower jaw became notably taller.
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In other words, jaw joints and ears do not define any except the most recent groups of mammals. Mammalian and non-mammalian jaws. In the mammal configuration, the quadrate and articular bones are much smaller and form part of the middle ear. Note that in mammals the lower jaw consists of only the dentary bone.
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Baeotherates is known from the holotype OMNH 55758, a dentary bone from the right mandible. It was collected within the Dolese Brothers Limestone Quarry of Richard's Spur in Comanche County, Oklahoma and found in the Garber Formation of the Sumner Group, which dates to the middle Sakmarian stage of the Early Permian, about 289 ± 0.68 million years ago.
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The referral of the additional femur and tibia has been questioned by later researchers. In 2003 Jaime Eduardo Powell tentatively referred them to cf. Argyrosaurus and in 2012 Philip Mannion and Alejandro Otero considered it an indeterminate titanosaur. The incomplete mandible attributed to A. wichmannianus is squared off at the front with each dentary bone being L shaped.
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The mandible (lower jaw) was tipped by the predentary, a bone unique to ornithischians. This bone also supported a beak similar to the one found on the premaxilla. All the teeth in the lower jaw were found on the dentary bone. Snouts of Heterodontosaurus (A), Abrictosaurus (B), and Tianyulong (C) Heterodontosaurids are named for their strongly heterodont dentition.
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Mouth is protractible, upwardly directed, with small teeth. Lower jaw has an upper expansion within mouth (high dentary bone). There are two separate dorsal fins, with all rays of first and 1-2 anterior rays of second dorsal fin being unsegmented. The anal fin is similar to the second dorsal fin, while the caudal fin is forked.
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Atherina hepsetusThe body is rather long, slender and moderately flattened. The mouth is protrusible, directed upward with small teeth; the head and body are scaly. The lower jaw has an upper expansion within the mouth (high dentary bone). Two separate dorsal fins, all rays of first and 1-2 anterior rays of second dorsal fin are unsegmented, with the remaining rays segmented.
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They appear to have been flat and rectangular. The only skull fragment that is certainly Calyptosuchus is a dentary bone - no other cranial bones have been assigned to it with certainty. Only a middle part of the dentary is present, with an edentulous patch to the anterior and nine dental alveoli posteriorly. No teeth are preserved except a few fragments of root.
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In 2018, the British palaeontologist Thomas M. S. Arden and colleagues found that the Portuguese skeleton did not belong to Baryonyx, since the front of its dentary bone was not strongly upturned. Some additional spinosaurid remains from Iberia may belong to taxa other than Baryonyx, including Vallibonavenatrix from Morella, which appears to be closer to the African genus Spinosaurus and the Asian Ichthyovenator.
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Modern humpback whales ram feeding. Unlike living rorquals, Plesiobalaenoptera was probably not capable of ram feeding. During ram feeding, modern whales swim toward their prey with open mouths and engulf them in an expandable throat. Plesiobalaenoptera has a postcoronoid fossa, or hole in the dentary bone of the lower jaw, which would have made this method of feeding difficult to perform.
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Members of the genus Atractaspis share the following characters. Venom fangs enormously developed; a few teeth on the palatines, none on the pterygoids; mandibles edentulous anteriorly, with 2 or 3 very small teeth in the middle of the dentary bone. Postfrontal bone absent. Head small, not distinct from neck, covered with large symmetrical shields; nostril between 2 nasals; no loreal; eye minute, with round pupil.
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These relatively small snakes rarely exceed 30 cm in length; only Trilepida macrolepis and Leptotyphlops occidentalis grow larger. The cranium and upper jaws are immobile and no teeth are in the upper jaw. The lower jaw consists of a much elongated quadrate bone, a tiny compound bone, and a relatively larger dentary bone. The body is cylindrical with a blunt head and a short tail.
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The type species comes from Park County in northwestern Wyoming, and is composed of teeth, cranial, and postcranial bones. The bone fragments were broken and scattered suggesting a taphonomic pathway that includes trampling before burial. The holotype specimen (UM 83874) from Wyoming, USA consists of upper and lower dentition with dentary bone. Post cranial remains include the scapula, ulna, radius, ankle bones, and others.
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Fusion of sutures (joints) in the braincase indicates the holotype specimen was a mature individual. A second specimen, consisting of a dentary bone from a supposedly larger individual, has been used to extrapolate a length of , a skull long, and a weight of . Some writers have considered the largest size estimates for both specimens exaggerated. Giganotosaurus has been compared to an oversized version of the well-known genus Allosaurus.
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In Megaconus, these bones are still associated with the lower jaw, placed within a groove behind the dentary bone of the lower jaw. Megaconus is estimated to have weighed about . It probably had an outwardly similar appearance to multituberculates, a major group of Mesozoic mammals. However, its body is longer than those of multituberculates and most other Mesozoic mammaliaforms, having more back vertebrae (24) than other early mammals.
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The lower jaw curved downwards towards its tip, and the jaw joint was located well below the level of the tooth row. There is no evidence for a beak. Each side of the upper jaw was lined with 20 teeth – four in the premaxilla, the front bone of the upper jaw, and 16 in the maxilla which followed behind. The dentary bone of the lower jaw likewise had 20 teeth.
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The fishes of the genus Filimanus have a body which is oblong to moderately deep and compressed. They have a well developed adipose eyelid and the diameter of the eye is longer than the length of the snout. They have a well developed lip on their lower jaws and the teeth on the dentary bone are restricted to its dorsal surface. There are narrow bands of villiform teeth on their jaws, palatine bone and ectopterygoids.
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Though incompletely preserved, it is assumed that the occipital plate sloped back and upwards from the occipital condyles (as in some relatives). The orbito-temporal vascular system inside the skull of Catopsbaatar did not differ much from those of related genera. Catopsbaatar mandible was robust and very elongated. The diastema (gap between the front and cheek teeth) was concave, and extended for 20 percent of the dentary bone (the main bone of the lower jaw).
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In Amphisbaenia, the extracolumella is particularly lengthened and firmly connects with a layer of skin over dentary bone of the lower jaw. This connection appears to facilitate detection of airborne vibrations in the facial area. The embedding in the skin often occurs at a specially enlarged labial scale. As a result, the amphisbaenian is able to detect substrate vibrations as it burrows through the ground while protecting the internal ear from damage.
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They are distantly related to the extant Sphenodon (tuatara of New Zealand) with which they shared a number of skeletal features including a large tooth row along the side of the palatine bone (part of the palate) and posterior process of the dentary bone (part of the lower jaw). The type species, G. bridensis, lived during Early Jurassic in Wales, UK.Evans, S.E. 1981. Caudal autonomy in a lower Jurassic Eosuchian. Copeia 1981: 883-884.
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The lower jaws were elongated and met at their tips in a shared epidentary bone, the core of the toothless lower beak. In the dentary bone, the tooth battery curved to the outside to meet the battery of the upper jaw. At the rear of the lower jaw, the articular bone was exceptionally wide, matching the general width of the jaw joint. T. horridus can be distinguished from T. prorsus by having a shallower snout.
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The lower jaw tapered towards the front, and the dentary bone (the main part of the lower jaw) was robust. The front of the jaws were covered by a toothless keratinous beak (or rhamphotheca). The upper beak covered the front of the premaxilla bone and the lower beak covered the predentary, which are, respectively, the foremost bones of the upper and lower jaw in ornithischians. This is evidenced by the rough surfaces on these structures.
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This is illustrated by a 419-million-year-old fossil, Entelognathus, from China, which is the only known placoderm with a type of bony jaw like that found in modern bony fishes. This includes a dentary bone, which is found in humans and other tetrapods. The jaws in other placoderms were simplified and consisted of a single bone. Placoderms were also the first fish to develop pelvic fins, the precursor to hindlimbs in tetrapods, as well as true teeth.
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The toothed dentary bone had two rear prongs surrounding the mandibular fenestra. Unique to Turfanosuchus among gracilisuchids, the lower prong was much longer than the upper prong. This had the added effect of excluding the angular bone (which formed the rear lower edge of the jaw) from forming the lower border of the mandibular fenestra. The surangular, which forms the upper rear portion of the jaw (and the upper border of the mandibular fenestra), was large.
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The dentary bone is long (occupying half of the jaw's length), shallow, and triangular, and it bears a groove that widens towards the back of the bone. A distinct fossa on the top surface of the surangular bone separates Jianianhualong from other troodontids. The angular bone projects upwards behind the dentary, like Sinovenator and various other members of the Deinonychosauria. The maxilla of Jianianhualong bore 21 teeth on each side, while the dentary bore 25 on each side.
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In many mammals, including humans, the squamosal fuses with the periotic bone and the auditory bulla to form the temporal bone, then referred to as the squama temporalis. In mammals, the quadrate bone evolves to form the incus, one of the ossicles of the mammalian ear. Similarly, the articular bone evolves to form the malleus. The squamosal bone migrates and lengthens to become a new point of articulation with the lower jaw (at the dentary bone).
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Unlike other Sternarchogiton species it has long, conical teeth (numbering 5 plus 2-3 replacements) on the premaxillary of the upper jaw; there is one row of teeth (numbering 10 plus 3 replacements) on the dentary bone of the lower jaw. Both upper and lower pharyngeal tooth plates are present, bearing 6 and 4-5 teeth respectively. The long anal fin contains 189-210 soft rays. The pectoral fins are broad and pointed, with 12-14 rays.
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The skull of the Brasilitherium is about in length. The dentary bone in the skull is more slender than in previous ancestors, as well as a slightly smaller skull in general. The genus Brasilodon is different from Brasilitherium due to the lower post canines as well as the pre orbital. Although more studies and more fossils are needed to truly confirm, paleontologists seem to believe that Brasilodon and Brasilitherium are not sister groups and that Brasilitherium is more closely related to mammaliaformes.
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The dentition is the number and type of teeth that an animal possesses. The mammalian jaw is composed of a lower jaw known as the mandible (dentary bone) that houses the lower molars, and an upper jaw commonly referred to as the maxilla that contains the upper molars. The dentition of the Jamaican fig-eating bat is specialized for its frugivorous diet. The first and second upper molars of the maxilla have a broad surface that is used for shearing fruit.
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It closely resembles that of Arizonasaurus, indicating that the two species are close relatives. The dentary bone is partially preserved for both left and right, and would have held thirteen teeth on each side, although only the nutrient foramina remain. Like that of most archosaurs from the Triassic, the dentary is unspecialised. The cervical vertebrae have large, flattened neural spines which make up most of their height and would have formed a sail-like structure in life, similar to other ctenosauriscids.
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In addition, the original remains of Labrosaurus did not include teeth, and an additional species, Labrosaurus ferox (now considered synonymous with Allosaurus fragillis), was based on a dentary bone bearing teeth of different morphology to those of L. sulcatus. Thus, Rauhut concluded there was no basis for attributing the Tendaguru teeth to Labrosaurus. He tentatively referred all of them except MB R 1084 to Ceratosaurus (under the name Ceratosaurus? stechowi), on the basis of anatomical similarities with teeth from that genus.
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As the dentary bone of the lower jaw continued to enlarge during the Triassic, the older quadrate-articular joint fell out of use. Some of the bones were lost, but the quadrate, the articular, and the angular bones became free-floating and associated with the stapes. This occurred at least twice in the mammaliformes. The multituberculates had jaw joints that consisted of only the dentary and squamosal bones, and the quadrate and articular bones were part of the middle ear.
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E. platyops skull Unlike the sea turtles Toxochelys and Eochelone, Euclastes has a secondary palate. However, the secondary palate of Euclastes is not as extensive as it is in Ctenochelys and Angolachelys. The genus can be distinguished by later sea turtles based on its broad, low skull; broad, flat palate; wide, flat dentary bone with an elongated symphysis; and low tomial ridge on the beak. The widened palate and dentaries give Eochelone wide, flat jaws suitable for crushing hard-shelled organisms.
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The genus attracted much interest and became part of a scientific debate about the maximum sizes of theropod dinosaurs. Giganotosaurus was one of the largest known terrestrial carnivores, but the exact size has been hard to determine due to the incompleteness of the remains found so far. Estimates for the most complete specimen range from a length of , a skull in length, and a weight of . The dentary bone that belonged to a supposedly larger individual has been used to extrapolate a length of .
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A part of the jugal is expanded, forming a shelf projecting almost across the tooth row vertically, perhaps protecting the teeth. The mandibles have no teeth preserved, and only one dental alveolus, right at the posterior end of the dentary bone. It is estimated that the whole mandible was about 135-140 mm long. A ridge extends along the external side of the mandible, which may have been for the attachment of soft cheek tissue to prevent plant matter escaping while being chewed as in Notosuchus.
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The front branch of the pterygoid bone makes a right angle to the outer side. The ectopterygoid bone is positioned behind the pterygoid and has a recurved outer side branch in the direction of the jugal as well as a rear branch. Teeth in the maxilla and the dentary bone possess multiple cusps, sometimes as high as six or seven, while erupted teeth with three cusps are absent. The first, second and third maxillary teeth are recurved, with the curvature gradually diminishing along the series.
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Magosternarchus species are immediately recognizable by their large jaws, allowing for a wide gape, and their numerous, large, conical teeth. Both the premaxilla and the dentary bone are elongated, the former bearing two to three rows of teeth (numbering 9-14 in the outermost row) and the latter bearing three rows of teeth (numbering 15-19 in the outermost row). The deeply concave dorsal margin of the opercle also serves to distinguish this genus. The eyes are small and placed towards the top of the head.
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The name Brachygnathosuchus (Greek for "short jawed crocodile") refers to its characteristically broad, stout mandible, unusual among crocodilians. The holotype specimen was referred to Crocodilia on the basis of a partial right ramus of the lower jaw consisting of a portion of a very large dentary bone. The bone differs from those of other related fossil and modern crocodilians and was originally said to resemble that of a dinosaur. An eroded osteoderm also associated with the specimen provides further evidence for its classification as a crocodilian.
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As a result of its small size and heightened vulnerability to predation, this species of crocodile has a heavily armoured neck, back, and tail and also has osteoderms on its belly and underside of neck. Osteolaemus has a blunt short snout, as long as it is wide, similar to that of a Cuvier's dwarf caiman, probably a result of occupying a similar ecological niche. The dentition consists of four premaxillary teeth, 12 to 13 on the maxilla, and 14 to 15 on the dentary bone. O. t.
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The jaws of early synapsids, including the ancestors of mammals, were similar to those of other tetrapods of the time, with a lower jaw consisting of a tooth-bearing dentary bone and several smaller posterior bones. The jaw joint consisted of the articular bone in the lower jaw and the quadrate in the upper jaw. The early pelycosaurs (late Carboniferous and early Permian) likely did not have tympanic membranes (external eardrums). Additionally, their massive stapes bones supported the braincase, with the lower ends resting on the quadrates.
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The dentary bone of the lower jaw has ten teeth on either side, with two diastemata separating them. The forward-most teeth are suited for capturing food, while the back teeth are adapted for food processing. The palatine bone in the roof of the mouth connects to the maxilla bone of the snout by a region of bone called the cuneiform process. Caipirasuchus also has large pterygoid and ectopterygoid bones and a well-developed hole in front of the eye sockets called the antorbital fenestra.
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The largest specimen of T. marauna measures long. Like other members of its family, this fish is knife- shaped with a long anal fin extending the length of its body, an electroreceptive appendage on its back, and no dorsal or pelvic fins. The head is long and low, with a rounded snout and a straight dorsal profile. The teeth are elongated and conical, numbering around 15 on the premaxillary bone (at the front of the upper jaw) and around 30 on the dentary bone (the lower jaw).
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Giganotosaurus ( ) is a genus of theropod dinosaur that lived in what is now Argentina, during the early Cenomanian age of the Late Cretaceous period, approximately 98 to 97 million years ago. The holotype specimen was discovered in the Candeleros Formation of Patagonia in 1993, and is almost 70% complete. The animal was named Giganotosaurus carolinii in 1995; the genus name translates as "giant southern lizard" and the specific name honors the discoverer, Rubén D. Carolini. A dentary bone, a tooth and some tracks, discovered before the holotype, were later assigned to this animal.
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The outer (marginal) tooth row was present solely on the dentary bone, which was narrow and had alternating regions of light and absent texturing. It had at most 33 teeth, including a symphysial fang (an enlarged tooth near the chin) which was only slightly larger than the other dentary teeth. The inner tooth row stretched along four plate-like bones: the parasymphysial plate and three coronoid bones. The parasymphysial plate has a large tooth at its front edge, followed by a smaller tooth and a diastema (toothless area), similar to Ichthyostega.
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This is another distinguishing characteristic of Yonghesuchus, as other early archosauriforms have shorter and wider basisphenoid bones. Moreover, the entrance of the internal carotid artery, which passes through a foramen in the basisphenoid to supply blood to the brain, is in a different position than related genera. Its position is more similar to that of Dorosuchus (a euparkeriid) and more derived archosauriforms. In the lower jaw, the dentary bone has two projections at its posterior end where it attaches to the mandible, the higher one being markedly longer than the lower one.
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In the lower jaw, a ridge on the outer surface of the angular bone articulated with the rear of the dentary bone, creating a locking mechanism unique to Tarbosaurus and Alioramus. Other tyrannosaurids lacked this ridge and had more flexibility in the lower jaw. Restoration of an adult and subadult Tarbosaurus next to a human Tyrannosaurids varied little in body form, and Tarbosaurus was no exception. The head was supported by an S-shaped neck, while the rest of the vertebral column, including the long tail, was held horizontally.
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Taxonomy, phylogeny, biostratigraphy, biochronology, paleobiogeography, and evolution of the Late Triassic Aetosauria (Archosauria: Crurotarsi) Zentralblatt für Geologie und Paläontologie Teil I 1998 Heft 11–12:1539-1587Long RA, Murry PA. 1995. Late Triassic (Carnian and Norian) tetrapods from the southwestern United States. New Mexico Museum of Natural History and Science Bulletin 4:1-254 recent work has found it generically distinct from Stagonolepis proper. This is because more material from the same species has been discovered, including a dentary bone, a possible maxilla, and further vertebrae from the neck and trunk.
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The teeth on the outer row, also known as "marginal teeth", are attached to the dentary bone and are sharp and curved. They are also slightly flattened in an anteroposterior (front-to-back) direction, although not to the extent seen in capitosaurs such as Mastodonsaurus. At the front of the jaw, the dentary tooth row also twists slightly outwards. The front of the jaw also possesses an enlarged tooth known as a parasymphyseal tusk, although this tooth is less than twice as wide as the rest of the marginal teeth.
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Sereno countered that it would be difficult to determine a size range for a species based on few, incomplete specimens, and both paleontologists agreed that other aspects of these dinosaurs were more important than settling the "size contest". In 1998, the paleontologist Jorge O. Calvo and Coria assigned a partial left dentary bone (part of the lower jaw) containing some teeth (MUCPv-95) to Giganotosaurus. It had been collected by Calvo near Los Candeleros in 1988 (found in 1987), who described it briefly in 1989, while noting it may have belonged to a new theropod taxon.
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The retroarticular process of the mandible (a backwards projection) was long, and the surangular shelf was strongly horizontal. The dentary bone (the front part of the mandible where most of the teeth there were attached) had an up-curved rather than pointed chin. The chin had a large foramen at the tip, and a row of small foramina ran in rough parallel with the upper edge of the dentary. On the inner side, the mandibular symphysis (where the two halves of the lower jaw connected) was flat and smooth, and showed no sign of being fused with its opposite half.
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The species name multidentata alludes to the many teeth preserved in the jaw. The jaw, which is housed in the Field Museum and cataloged as FM PR 1820, curves strongly downward but was probably straight to begin with, having been deformed by the process of fossilization after the individual died. Rooted in the dentary bone along the outermost edge of the jaw are 88 small, pointed marginal teeth. An additional row of even smaller teeth runs along the coronoids, three bones positioned lengthwise along the lower boundary of the dentary on the inner surface of the lower jaw.
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As in other oviraptorids, it had a pair of tooth-like projections on the palate that were directed downwards (a feature that has been called "pseudo- teeth"). Nemegtomaia had small foramina (openings) on the sides of the suture (joint) between the premaxillae at the front of the snout, which may have been nutrient openings (and which indicate the presence of a keratinous bill). The lower jaw was short and deep, with a convex lower surface, and reached 153 mm (6 in) in length. The dentary bone of the lower jaw reached 50 mm (2 in) at its highest point.
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Morganucodontidae and other transitional forms had both types of jaw joint: dentary-squamosal (front) and articular-quadrate (rear). During the Permian and early Triassic the dentary of therapsids, including the ancestors of mammals, continually enlarged while other jaw bones were reduced. Eventually, the dentary bone evolved to make contact with the squamosal, a bone in the upper jaw located anterior to the quadrate, allowing two simultaneous jaw joints: an anterior "mammalian" joint between the dentary and squamosal and a posterior "reptilian" joint between the quadrate and articular. This "twin- jointed jaw" can be seen in late cynodonts and early mammaliforms.
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The postcanines of the upper and lower jaw fit tightly together as an adaptation to processing plant material. An autapomorphy or unique feature of Bauriidae is the expansion of the dentary bone of the lower jaw inward toward the jaw midline, forming a wide shelf of bone to either side of the tooth row. A similar expansion of the maxilla in the upper jaw is also seen in bauriids, although it is not unique to the group as the feature has also evolved in a group of cynodont therapsids called gomphodonts (which, like bauriids, were herbivorous).
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Like the rest of the skull, the lower jaw of Alioramus was long and slender, another possible juvenile characteristic. As in Tarbosaurus, a ridge on the outer surface of the angular bone of the lower jaw articulated with the rear of the dentary bone, locking the two bones together and removing much of the flexibility seen in other tyrannosaurids. Other tyrannosaurids had four premaxillary teeth, D-shaped in cross section, on each side. Including 16 or 17 in each maxilla, and 18 in each dentary, Alioramus had 76 or 78 teeth, more than any other tyrannosaurid.
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Bones in a skeleton of C. bauri at the American Museum of Natural History, now interpreted as those of a crocodylomorph The teeth of Coelophysis were typical of predatory dinosaurs, blade-like, recurved, sharp and jagged with fine serrations on both the anterior and posterior edges. Its dentition shows that it was carnivorous, probably preying on the small, lizard-like animals that were discovered with it. It may also have hunted in packs to tackle larger prey.Coelophysis bauri has approximately 26 teeth on the maxillary bone of the upper jaw and 27 teeth on the dentary bone of the lower jaw.
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292 even though the dentary bone itself was exceptionally long at the back, reaching a point below the middle of the eye socket.Cristiano dal Sasso & Simone Maganuco, 2011, Scipionyx samniticus (Theropoda: Compsognathidae) from the Lower Cretaceous of Italy — Osteology, ontogenetic assessment, phylogeny, soft tissue anatomy, taphonomy and palaeobiology, Memorie della Società Italiana de Scienze Naturali e del Museo Civico di Storia Naturale di Milano XXXVII(I): 1-281 Teeth did not extend as far back as the orbits, and neither tooth row spanned much more than one-third of the skull.Osborn (1903), p. 460; Norman (1990), p.
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They may have resembled basal cynodonts such as the Procynosuchidae, and they may have descended from a procynosuchid-like ancestor, but the galesaurids were more advanced than the basal Cynodontia. It is clear that, like many other epicynodontians, many galesaurids had a complete secondary palate, which allowed them to swallow food while breathing, and the dentary bone was enlarged relative to those of their ancestors. Their temporal fenestrae are much larger than those of the procynosuchids, but not as large as in more advanced epicynodontians. Their snouts are broad, rather than tall, and they may have walked erect, with the legs beneath the body like most other cynodontians.
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Oral jaw from side and above of Piaractus brachypomus, a close relative of piranhas In vertebrates, the lower jaw (mandible or jawbone)The mandible is also in some sources still referred to as the inferior maxillary bone, though this is an outdated term which goes back to at least the 1858 first edition of Gray's Anatomy, if not earlier. is a bone forming the skull with the cranium. In lobe-finned fishes and the early fossil tetrapods, the bone homologous to the mandible of mammals is merely the largest of several bones in the lower jaw. It is referred to as the dentary bone, and forms the body of the outer surface of the jaw.
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The holotype's smaller size, smoother surface, and lack of co-ossified (fused) sutures all indicate that it belongs to a juvenile individual; while MNHN GDF 365 probably represents an adult. The tip of Cristatusauruss premaxilla was short and expanded, while the rear end was narrowed near the suture with the maxilla; this rosette-like snout shape was characteristic of spinosaurids. The front of the upper jaw was concave on the bottom, shaped to interlock with what would have been the convex and also enlarged tip of the mandible's dentary bone. A thin sagittal crest ran lengthwise on top of the premaxillae, a condition present in Baryonyx and Suchomimus, and very prominent in Angaturama (a possible synonym of Irritator).
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The basal New Egypt is a massive clayey, glauconitic marl that closely resembles the Navasink Formation into which it grades below. Ammonites and other invertebrates found at the Spheno Run site correlate well with the middle Severn Formation of Maryland. Spheno Run has so far produced a remarkable number of vertebrate specimens, especially from marine reptiles, including: carapace elements from at least two species of turtles, Peritresius ornatus and Taphrosphys sulcatus; various bone elements from at least two species of mosasaurs including a sizable fragment of dentary bone from Prognathodon rapax and numerous shed teeth from Mosasaurus maximus. Vertebrate remains also include material from sharks, particularly teeth and unusually large vertebral centra from an individual lamniform shark Squalicorax pristodontus, bony fish, and, rarely, dinosaurs.
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By the Late Permian many other groups of tetrapods had entered that niche, and increased competition among herbivores likely resulted in the eventual extinction of diadectids. Alveusdectes may have been able to persist because the fauna of north China seems to have been isolated from other Laurasian faunas during the Late Permian, meaning that fewer herbivores were competing for the same ecological space. Alveusdectes differs from other diadectids in having a pair of large holes at the back of its skull called suborbital fenestrae, which would have been attachment points for large jaw muscles. Other defining features include a large fourth tooth in the dentary bone of the lower jaw and an elongate Meckelian fenestra positioned near the back of the jaw.
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Both species of Laganosuchus are known only from their lower jaws, those of L. thaumastos almost complete save for the left retroarticular process and those of L. maghrebensis only from a fragment of the dentary bone. L. thaumastos had a total jaw length of 838 mm and a jaw length from tip to articular facet of 750 mm, of which 490 mm actually bore teeth. Across the jaws, the total width of the lower jaw ranged from around 140 mm at the symphysis to 240 mm at the articular facets, widening fairly evenly all the way along. All the teeth were simple straight spikes, with the first pair the largest and the rest of the teeth decreasing in size towards the back of the mouth.
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Further back, there were at least 22 teeth per upper jaw side in the maxilla, while the entire lower jaw side carried 32 teeth in the dentary bone. Closeup of front of the snout and dentition The upper jaw had a prominent kink just behind the rosette, protruding downwards; this convexly curved part of the maxilla had the longest teeth of the entire skull. The internal bone shelves of the maxillae met each other in the midline of the skull over a long distance, forming a closed secondary palate that stiffened the snout, and setting off the internal nostrils and palatal complex (including the pterygoid, palatine and ectopterygoid) towards the back of the skull. The nostrils, unlike in most theropods, were retracted further back on the skull and behind the premaxillary teeth.
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Libognathus is known from a single holotype specimen including the left dentary bone and coronoid process of the lower jaw, as well as six teeth implanted in the jaw. Libognathus can be identified as a procolophonid by its wide molar-like teeth, each of which has a ridge running along its width and several projections or cusps around it. It is considered a member of the procolophonid subfamily Leptopleuroninae based on the presence of more conically-shaped teeth at the front of the jaw. Libognathus is distinguished from other procolophonids by the steep angle of the lower margin of the dentary (indicating a deeper lower jaw than in related procolophonids such as Hypsognathus and Leptopleuron) and a small hole or foramen on the jaw's outer surface below the fifth tooth.
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Obamadon is known from two lower jaw fragments, each less than a centimeter in length. One was found in the collections of the University of California Museum of Paleontology after having been collected from the Hell Creek Formation in Montana, and the other collected from the Lance Formation in Wyoming. When it was first named in 2012, Obamadon was identified as a member of the extinct group Polyglyphanodontia on the basis of a V-shaped connection between the two halves of the lower jaw, a slot-and-ridge type connection between the dentary bone of the lower jaw and another missing bone called the splenial bone, and teeth that are implanted within the jaw bone. Its jaw is thin and straight, unlike the curved jaws of most other polyglyphanodontians.
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Restoration of two specimens The American paleontologist O. C. Marsh named Sphenacodon (from Greek ' "wedge" + ' "point" + ' (-odon) "tooth") in 1878, based on part of a lower jaw (dentary) bone found in the redbeds of northern New Mexico by fossil collector David Baldwin. In his very short description of the jaw, Marsh cited the back teeth as characteristic ("crowns are much compressed, and have very sharp cutting edges without crenulations") and assessed the animal as "about six feet in length, and carnivorous in habit," although the rest of the skeleton was not known. He did not provide an illustration of the specimen. Marsh gave the genus the Latin specific name ferox "fierce" and erected the new family Sphenacodontidae, placed under the primitive reptilian order "Rhynchocephala" (=Rhynchocephalia), then including nearly all groups of early reptiles in addition to the living tuatara.
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28, No. 4 (Dec. 12, 2008), pp. 1130-1143. Is known from a nearly complete skull with mandible of a post- hatchling individual (the specimen IGM 3497, in the Instituto de Geologia, of the Universidad Nacional Autónoma de Mexico), and is one of the smallest skulls between the sphenodontians, with an estimated total length of 11.3 millimetres, a bit smaller than the hatchling individuals observed in the modern tuatara (Sphenodon); features like the oblique mandibular symphysis suggests that the holotype is from an individual in a relatively mature stage of ontogenic development. Zapatadon is diagnosed by their hatchling tooth series located in a depression in the anterior part of the dentary bone, the prefrontal bone surrounding the dorsal process of the maxilla and the broad jugal that extends over the maxillary suborbital process, been almost excluded of the orbit.
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Mammals are unique in having evolved a three-ossicle middle-ear independently of the various single-ossicle middle ears of other land vertebrates, all during the Triassic period of geological history. Functionally, the mammalian middle ear is very similar to the single-ossicle ear of non-mammals, except that it responds to sounds of higher frequency, because these are better taken up by the inner ear (which also responds to higher frequencies than those of non-mammals). The malleus, or "hammer", evolved from the articular bone of the lower jaw, and the incus, or "anvil", from the quadrate. In other vertebrates, these bones form the primary jaw joint, but the expansion of the dentary bone in mammals led to the evolution of an entirely new jaw joint, freeing up the old joint to become part of the ear.
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In the lineage most closely related to mammals, the jaws of Hadrocodium (about 195M years ago in the very early Jurassic) suggest that it may have been the first to have a nearly fully mammalian middle ear: it lacks the trough at the rear of the lower jaw, over which the eardrum stretched in therapsids and earlier mammaliformes. The absence of this trough suggests that Hadrocodium’s ear was part of the cranium, as it is in mammals, and that the former articular and quadrate had migrated to the middle ear and become the malleus and incus. Hadrocodium’s dentary has a "bay" at the rear which mammals lack, a hint that the dentary bone retained the same shape as if the articular and quadrate had remained part of the jaw joint. However, several studies have cast doubt on whether Hadrocodium did indeed possess a definitive mammalian middle ear; Hadrocodium likely had an ossified connection between the middle ear and the jaw, which is not visible in the fossil evidence due to limited preservation.
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