Mandibular symphysis

In human anatomy, the facial skeleton of the skull the external surface of the mandible is marked in the median line by a faint ridge, indicating the mandibular symphysis (Latin: symphysis menti) or line of junction where the two lateral halves of the mandible typically fuse at an early period of life. It is not a true symphysis as there is no cartilage between the two sides of the mandible.

This ridge divides below and encloses a triangular eminence, the mental protuberance, the base of which is depressed in the center but raised on either side to form the mental tubercle. The lowest (most inferior) end of the mandibular symphysis — the point of the chin — is called the "menton".[1][2]

It serves as the origin for the geniohyoid and the genioglossus muscles.

Mandibular symphysis
Symphysis menti (Gray190 edit)
Anterior view of mandible, showing mandibular symphysis (red broken line)
Gray177
Medial surface of the left half of the mandible, dis-articulated from the right side at the mandibular symphysis
Details
Identifiers
Latinsymphysis mandibulae
TAA02.1.15.004
FMA75779
Anatomical terms of bone

Other animals

Humpback Whale skeleton
Humpback skeleton showing the flexible "slingshot" symphysis present in baleen whales

Solitary mammalian carnivores that rely on a powerful canine bite to subdue their prey have a strong mandibular symphysis, while pack hunters delivering shallow bites have a weaker one.[3] When filter feeding, the baleen whales, of the suborder Mysticeti, can dynamically expand their oral cavity in order to accommodate enormous volumes of sea water. This is made possible thanks to its mandibular skull joints, especially the elastic mandibular symphysis which permits both dentaries to be rotated independently in two planes. This flexible jaw, which made the titanic body sizes of baleen whales possible, is not present in early whales and most likely evolved within Mysticeti.[4]

References

This article incorporates text in the public domain from page 172 of the 20th edition of Gray's Anatomy (1918)

Notes

  1. ^ "Menton". The Free Dictionary. Retrieved 1 November 2016.
  2. ^ Phulari, Basavaraj Subhashchandra (2013). An atlas on cephalometric landmarks (1st ed.). New Delhi: Jaypee Brothers Medical Publishers. p. 174. ISBN 9789350903247.
  3. ^ Therrien, François (2005). "Mandibular force profiles of extant carnivorans and implications for the feeding behaviour of extinct predators". Journal of Zoology. 267 (3): 249. doi:10.1017/S0952836905007430.
  4. ^ Fitzgerald 2012

Sources

Albertosuchus

Albertosuchus is an extinct genus of crocodyloid crocodylian from the Late Cretaceous of Canada. The type species Albertosuchus knudsenii was named in 2015 from the Scollard Formation in Alberta. Albertosuchus is the northernmost-known Late Cretaceous crocodylian in North America. Albertosuchus lacks the notch in the upper jaw between the maxilla and premaxilla bones that is characteristic of most crocodyloids, and it also has a very short mandibular symphysis (the connection between the two halves of the lower jaw). Phylogenetic analysis indicates that it is one of the most basal members of Crocodyloidea and a close relative of Arenysuchus from the Late Cretaceous of Spain, although the incomplete nature of known material makes these findings uncertain.

Arambourgisuchus

Arambourgisuchus ("[Prof. Camille] Arambourg's crocodile") was a dyrosaurid crocodylomorph from the late Palaeocene of Morocco, found in the region of Sidi Chenane in 2000, following collaboration by French and Moroccan institutions, and described in 2005 by a team led by palaeontologist Stéphane Jouve.

Its type and only species is A. khouribgaensis, after the town of Khouribga, near which the holotype was found.

Aussiedraco

Aussiedraco is a genus of basal ornithocheiroid pterodactyloid pterosaur from the early Cretaceous of Australia.

Aussiedraco is known from holotype QM F10613, a partial mandibular symphysis housed at the Queensland Museum, recovered from rocks of the Toolebuc Formation, about 70 km east of Boulia, western Queensland, dating to Albian stage. It was named by Alexander W.A. Kellner, Taissa Rodrigues and Fabiana R. Costa in 2011 and the type species is Aussiedraco molnari. The generic name is derived from "Aussie", a shortened form of Australian, and "draco", from Latin meaning dragon. The specific epithet honours Ralph E. Molnar, who first described the specimen in 1980.The symphysis fragment is 88 millimetres long and very straight and narrow, with a lanceolate not-expanded tip and triangular cross-section. It lacks a keel or crest and is convex on top, with a median narrow deep groove not reaching the tip, but flat at the bottom. As far as can be judged from the empty elliptical tooth-sockets, the lower jaws carry at least five pairs of teeth, which are rather large and become more outwards inclining and procumbent towards the front. Aussiedraco is estimated to have been smaller in size than Mythunga, a pterosaur from the same formation.

Kellner et al. assigned Aussiedraco to the Pteranodontoidea, a clade roughly containing the same species as the Ornithocheiroidea sensu Unwin. Aussiedraco would be closely related to the Anhangueridae.

Babiacetus

Babiacetus is an extinct genus of early cetacean that lived during the late Lutetian middle Eocene of India (48.6 to 40.4 million years ago).

It was named after its type locality, the Harudi Formation in the Babia Hills (23.5°N 68.8°E / 23.5; 68.8: paleocoordinates 5.9°N 61.8°E / 5.9; 61.8), Kutch District, Gujarat, India.Babiacetus was named by Trivedy & Satsangi 1984 in an abstract based on the specimen's type (GSI 19647, left and right dentaries with cheek teeth). Gingerich and colleagues found a skull (GSP-UM 3005, much of a skull and lower jaws) while collecting a skeleton of a new species of Protosiren (Protosiren sattaensis) in the Drazinda Formation (30.8°N 70.5°E / 30.8; 70.5, paleocoordinates 16.9°N 67.1°E / 16.9; 67.1) in the Sulaiman Range of Punjab, Pakistan. Gingerich et al. 1995 described both the original find and their new specimen.Bajpai & Thewissen 1998 described B. mishrai from the specimen (RUSB 2512, a partial skull) collected in the Harudi Formation.Babiacetus is one of the larger protocetids. Its hydrodynamic skull and pointed, anteroposteriorly (front-back) oriented incisors are typical of archaeocetes. A densely ossified auditory bulla and large mandibular canal indicate it was adapted for hearing in water. Babiacetus differs from pakicetids and ambulocetids (more primitive families) by the large mandibular foramen and a medially concave ascending ramus; distinct from remingtonocetids and basilosauroids (more derived families) by the single-cusped trigonid and talonid on the lower molars. Its long synostotic (fused) mandibular symphysis, which reaches as far back as P2, distinguishes it from Pappocetus and Georgiacetus (other protocetids). Its auditory bulla is more narrow than Rodhocetus'. Babiacetus lacks the prominent molar protocone present in Indocetus.

The anterior premolars are large.Its large size as well as robust teeth suggest that it fed on larger fishes or aquatic vertebrates, or both. To date, only cranial remains have been found, hence nothing is known of Babicetus' mode of locomotion or degree of aquatic adaptation.The mandible is longer in B. indicus than in B. mishrai, and P1 is single-rooted in the former but double-rooted in the latter. The diastemata between P1 and P4 in B. indicus is absent in B. mishrai. B. indicus has larger cheek teeth and a larger M3.

Cephalometric analysis

Cephalometric analysis is the clinical application of cephalometry. It is analysis of the dental and skeletal relationships of a human skull. It is frequently used by dentists, orthodontists, and oral and maxillofacial surgeons as a treatment planning tool. Two of the more popular methods of analysis used in orthodontology are the Steiner analysis (named after Cecil C. Steiner) and the Downs analysis (named after William B. Downs). There are other methods as well which are listed below.

Dalanistes

Dalanistes is an extinct genus of remingtonocetid early whale known from the late early Eocene (Lutetian, 48.6 to 40.4 million years ago) of Kutch, India and Punjab and Balochistan, Pakistan. Dalanistes is closely related to Remingtonocetus (the type genus of Remingtonocetidae, a slightly more derived family of early whales), but also shares several characters with Ambulocetus (the type of Ambulocetidae, earlier more primitive whales), and, with its combination of terrestrial and amphibious adaptations, Dalanistes apparently is an intermediate form between these two groups. Isotopic evidence suggest that Dalanistes had a marine diet.Dalanistes is known from several localities and collections. The holotype is a skull and a postcranial skeleton. Additional fossils referred to Dalanistes include crania, several vertebrae and sacra, possible caudals, one side of the pelvis, and a distal femur. The alveoli is all that is left of the dentition, but the dental formula apparently was 3.1.4.33.1.4.3.The vertebral elements of the sacrum are solidly fused and form a well-developed articular surface for the pelvis. The ilium is robust and long, and has a large acetabulum similar to that in Remingtonocetus. The femur had a spherical head, a medial condyle considerably larger than the lateral, and a shallow patellar groove. Taken together this morphology suggests the presence of well-developed hind limbs.Dalanistes is similar to but 20% larger than Remingtonocetus; the external nares are located more anteriorly (above C1); the sagittal crest is much higher; the rostrum is angled down 20° relative to the main axis of the braincase; the mandibular symphysis is relatively open (ends at P3) and the mandibular canals fail to unite at the symphysis. This mandibular morphology is also different from that of Andrewsiphius (another remingtonocetid).Dalanistes was named by contracting "Dalana" and "-istes" to allude to the Greek name platanistes used for (unrelated) South Asian river dolphin. Both the genus and species name refer to local place names near the type locality: Dalana Nala and Basti Ahmed respectively.

Genasauria

Genasauria is a clade of extinct beaked, primarily herbivorous dinosaurs. Paleontologist Paul Sereno first named Genasauria in 1986. The name Genasauria is derived from the Latin word gena meaning ‘cheek’ and the Greek word saúra (σαύρα) meaning ‘lizard.’ Genasauria is the most inclusive clade within the order Ornithischia. According to Sereno (1986), Genasauria represents all ornithischians except for the most primitive ornithischian, Lesothosaurus. Sereno’s formal definition is, “Ankylosaurus, Triceratops, their most recent common ancestor and all descendants.” It is hypothesized that Genasauria had diverged from Lesothosaurus by the Early Jurassic. Cranial features that characterize Genasauria include a medial offset of the maxillary dentition, a sprout-shaped mandibular symphysis, moderately sized coronoid process, and an edentulous (without teeth) anterior portion of the premaxilla. A distinguishing postcranial feature of Genasauria is a pubic peduncle of the ilium that is less robust than the ischial peduncle.

Genasauria is commonly divided into Neornithischia and Thyreophora. Neornithischia is characterized by asymmetrical distributions of enamel covering the crowns of the cheek teeth, an open acetabulum, and a laterally protruding ischial peduncle of the ilium. Neornithischia includes ornithopods, pachycephalosaurs, and ceratopsians. Thyreophora is characterized by body armor and includes stegosaurs, ankylosaurs, Scelidosaurus, and Scutellosaurus.

Mammalodon

Mammalodon is an extinct genus of archaic baleen whale belonging to the family Mammalodontidae.

Mammalodontidae

Mammalodontidae is a family of extinct whales known from the Oligocene of Australia and New Zealand.There are currently two genera is this family: Janjucetus and Mammalodon. After a new cladistic analysis by Fitzgerald (2010), Janjucetus was transferred into Mammalodontidae, thereby making Janjucetidae a junior synonym of Mammalodontidae.Analysing the jaw morphology of the toothed mysticetes, Fitzgerald 2012 found eight mandibular characters unique to the members of Mammalodontidae:

the mandibular symphysis is short, has a rugose joint surface, but lacks a symphyseal groove. In archaeocetes the symphysis is long. In modern mysticetes, in contrast, the symphysis is very small, its joint surfaces are smooth, and there is a groove on the interior side of the mandible that accommodates the symphyseal ligament which enables them to open their mouth wide.

the external foramina on the mandible are relatively large (smaller or absent in later mysticetes)

the postcanines sit in a longitudinal groove flanked by a lateral edge (the "alveolar margin")

the alveolar (upper) margin forms an angle with the ventral (lower) margin (like in archaeocetes)

the ventral margin is straight in the posterior half of the mandible

teeth have longitudinal ridges

posterior postcanines have two roots joined below the crown base

postcanines are densely packed without long diastemataFrom these mandibular features, Fitzgerald 2006 concluded that in mysticetes enlarge oral cavities adapted for suction feeding evolved before mandibular adaptations for bulk filter feeding, like for example kinetic jaw joints.

Mandible

The mandible, lower jaw or jawbone is the largest, strongest and lowest bone in the human face. It forms the lower jaw and holds the lower teeth in place. The mandible sits beneath the maxilla. It is the only movable bone of the skull (discounting the ossicles of the middle ear).The bone is formed in the fetus from a fusion of the left and right mandibular prominences, and the point where these sides join, the mandibular symphysis, is still visible as a faint ridge in the midline. Like other symphyses in the body, this is a midline articulation where the bones are joined by fibrocartilage, but this articulation fuses together in early childhood.The word "mandible" derives from the Latin word mandibula, "jawbone" (literally "one used for chewing"), from mandere "to chew" and -bula (instrumental suffix).

Mental tubercle

The mandibular symphysis divides below and encloses a triangular eminence, the mental protuberance, the base of which is depressed in the center but raised on either side to form the mental tubercle. The two mental tubercles along with the medial mental protuberance are collectively called the mental trigone.

Mongolestes

Mongolestes ("Mongolian robber") is an extinct genus of mesonychid known from the 'Ulan Gochu' formation of Inner Mongolia, and likely originated in Asia. It was the last surviving representative of Mesonychia, and became extinct in the early Oligocene.

Mongolestes is distinct from other mesonychids in several dental features, including very large teeth and the loss of the third molar, and a mandibular symphysis that is steeper.

Nankangia

Nankangia is an extinct genus of caenagnathoid oviraptorosaurian dinosaur known from the Late Cretaceous Nanxiong Formation of Nankang County, Ganzhou City of Jiangxi Province, southeastern China. It contains a single species, Nankangia jiangxiensis. N. jiangxiensis coexisted with at least four other caenagnathoids, including an unnamed oviraptorid, Banji long, Ganzhousaurus nankangensis and Jiangxisaurus ganzhouensis. The relatively short dentary and non-downturned mandibular symphysis of Nankangia suggest that it may have been more herbivorous than carnivorous.

Platyognathus

Platyognathus is an extinct genus of protosuchian crocodylomorph. Fossils are known from the Early Jurassic Lower Lufeng Formation in Yunnan, China and belong to the type and only species, P. hsui.

Symphysis

A symphysis is a fibrocartilaginous fusion between two bones. It is a type of cartilaginous joint, specifically a secondary cartilaginous joint.

A symphysis is an amphiarthrosis, a slightly movable joint.

A growing together of parts or structuresUnlike synchondroses, symphyses are permanent.

Tazoudasaurus

Tazoudasaurus is a genus of vulcanodontid sauropod dinosaurs hailing from the Early Jurassic Toundoute overthrust beds located in the High Atlas Mountains of Morocco in North Africa. The remains, consisting of a partial adult skeleton and associated partial juvenile skeleton found in continental detrital sediments of the Toarcian aged Toundoute Continental Series, were described by Ronan Allain et al. in early 2004. The generic name derives from one of the localities, Tazouda, while the specific descriptor is a latinization of the Arabic term for "slender" due to the animal's small size for a sauropod. Its fossil was found alongside that of Berberosaurus.

Tazoudasaurus, a small sauropod at 9 meters (30 feet) long, is characterized by rather primitive features such as the prosauropod-like mandible with spatulate and denticle-bearing teeth, lack of a U-shaped mandibular symphysis as other more derived sauropods. Teeth wear in V-shaped marks indicates tooth occlusion, suggesting that vulcanodontids processed food orally when feeding. The neck is flexible with elongate vertebrae that lack true pleurocoels while dorsal and caudal vertebrae series tend to be more rigid. T. naimi bears the most complete fossil skeleton for Early Jurassic sauropod remains found to date due to the scarcity of exposed strata of that age. This sauropod is most closely related to Vulcanodon, differing only in caudal vertebrae features while it also possesses characters that place it outside Eusauropoda.

Turanosuchus

Turanosuchus is an extinct genus of paralligatorid crocodyliform. It is based on PIN 2229/507, a partial lower jaw consisting of the area where the two halves of the lower jaw meet (mandibular symphysis). This specimen was found in rocks of the lower Santonian-age Upper Cretaceous Bostobe Svita of Shakh-Shakh, southern Kazakhstan. Turanosuchus was described in 1988 by Mikhail Efimov. The type species is T. aralensis. Halliday et al. (2015) revised the material attributed to T. aralensis and concluded that it represents non-diagnostic neosuchian material, and as such the genus was considered to be a nomen dubium.Skutschas, Rezvyi & Efimov (2015) considered Turanosuchus aralensis to be a junior synonym of Kansajsuchus extensus. Kuzmin et al. (2019) considered genus Turanosuchus to be a junior synonym of the genus Kansajsuchus; however, the authors were uncertain whether T. aralensis is a junior synonym of K. extensus or a distinct paralligatorid taxon (though closely related to K. extensus). The authors noted that the fossil material attributed to T. aralensis demonstrates a single autapomorphy of K. extensus among paralligatorids (restriction of the frontal participation in the orbital margin), but this feature varies among specimens of T. aralensis. According to the authors, this variability could either reflect ontogenetic/individual variation in this feature or indicate the presence of a taxon distinct from K. extensus.

Zapatadon

Zapatadon is an extinct genus of sphenodontid reptile from the end of the Early Jurassic in the lower part of La Boca Formation of Tamaulipas, Mexico. 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.Zapatadon was first described and named by Víctor-Hugo Reynoso and James M. Clark in 1998, and the name of the genus is a homage to the Mexican revolutionary leader Emiliano Zapata, added to the Greek sufix -odon, "tooth", common in other sphenodontian taxa; the name of the type species, ejidoensis is in gratitude to the people of the ejido (communal land area) of El Huizachal, that allow the investigation of the fossils.In the La Boca Formation, where the fossils of Zapatadon were collected, also have been found fossils of another sphenodontian taxa like Cynosphenodon huizachalensis and the possibly venomous Sphenovipera jimmysjoyi, the primitive diapsid Tamaulipasaurus morenoi, the primitive pterosaur Dimorphodon weintraubi, the tritylodont Bocatherium mexicanum and the mammaliaforms Bocanodon tamaulipensis, Victoriaconocodon inaequalis and Huestaconocodon wiblei, along with fragmentary cranial and postcranial remains of crocodyliforms, and teeth of theropod and ornithischian dinosaurs.

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