EVOLUTION OF TEMPOROMANDIBULAR JOINT- A REVIEW
Running title: Evolution of temporomandibular joint.
Type of manuscript: Review article
Saveetha Dental College,
Chennai , India
Mr. K. Yuvaraj. Babu
Department of Anatomy
Saveetha Dental College , Saveetha University, Chennai , India
Mr. K. Yuvaraj Babu
Department of Anatomy
Saveetha Dental College,
162, Poonamallee High Road
Tamil Nadu , India
Word count: 3,370 words
Having three ossicles in the middle ear is one of the defining features of mammals. All reptiles and birds have only one middle ear ossicle, the stapes or columella. How these two additional ossicles came to reside and function in the middle ear of mammals has been studied for the last 200 years and represents one of the classic example of how structures can change during evolution to function in new and novel ways. From fossil data, comparative anatomy and developmental biology it is now clear that the two new bones in the mammalian middle ear, the malleus and incus, are homologous to the quadrate and articular, which form the articulation for the upper and lower jaws in non-mammalian jawed vertebrates. The incorporation of the primary jaw joint into the mammalian middle ear was only possible due to the evolution of a new way to articulate the upper and lower jaws, with the formation of the dentary-squamosal joint, or TMJ in humans. The evolution of the three-ossicle ear in mammals is thus intricately connected with the evolution of a novel jaw joint, the two structures evolving together to create the distinctive mammalian skull.
KEYWORDS: temporomandibular joint, jaw joint, mammals, reptiles,
It has been known for a long time 1 that it is conceivable to see in the warm blooded creature like reptiles, and particularly in the therocephalians and cynodonts, a dynamic increment in the size of the dentary and dynamic decrease in the size of the accessory jawbones. Subsequently, the jaw joint formed by the articular and quadrate turned out to be weaker until in the early well evolved mammals, for example, Morganucodon 2,3and Diarthrognathus’ 4 another mammalian joint was built up between the dentary and the squamosal, and the primitive reptilian jaw joint formed just a little part of the composite jaw joint. The decrease in size and quality of the reptilian jaw joint in the cynodonts, for instance, was joined by an increase in the mass of the jaw-shutting musculature and therefore an increment in the strain to which the lower jaw was subjected. Many endeavors 5,6 have been made to clarify the obvious puzzler of an in-wrinkle in the mass of the jaw closing musculature, from one viewpoint, and a progressive debilitating of the jaw joint on the other. None of these clarifications is completely trustworthy. In the investigation of warm blooded animal like reptiles and well evolved creature jaws an endeavor was made to determine the regions of inception and inclusion of the jaw-closing muscles at various evolutionary levels keeping in mind the end goal to decide the powers to which the jaw joint was subjected.
In cotylosaurs the principle jaw closing muscle, the capiti-mandibularis (C.M.) 7 was likely inadequately differentiated and inserted in the adductor fossa. This was arranged roughly halfway between the glenoid and the posterior teeth. The course of the fibres of the capiti-mandibularis was – most likely on a normal vertical. Some portion of the adductor mass apparently inserted on the ventral surface of the lower jaw beneath the glenoid. This part was presumably homologous with the pterygoideus musculature of more developed structures. The upward thrust (C.M.) caused by these muscles was adjusted by a vertical descending push (R) through the quadrate onto the articular, and a vertical descending push (B) (bite force) through the upper dentition onto the lower dentition. In cotylosaurs, for example, Labidosaurus the descending push through the quadrate (R) was as extraordinary as the descending push through the teeth (R). The direction of pull of the capiti-mandibularis seems to have formed a right angle with a line interfacing the adductor fossa to the glenoid. Subsequently, this muscle was most effective when the jaws were shut, i.e., in this position the use was most prominent. In view of the expansive vertical powers acting through the jaw joint in these structures, a diminishment in the bones framing the jaw joint would not have been conceivable. This records for the vast size of the bones shaping the jaw joint in these structures.
In the pelycosaurs the lower jaw had an early coronoid process anterior to the adductor fossa. The capiti mandibularis mass was most likely differentiated into a particular shallow masseter (S.M.) 8 that inserted on the external surface of the reflected lamina, an external pterygoid (E.P.) that embedded in the adductor fossa (homologous with the mammalian external pterygoid), and a main segment (T + D.M.) homologous with the mammalian temporalis and deep masseter. An internal pterygoid (I.P.) (foremost pterygoid of different creators) presumably wrapped around the ventral surface of the angular behind the attachment of the reflected lamina to the angular. The angle of the strands of this fibres was presumably the same as that of the superficial masseter. That piece of the capiti-mandibularis homologous with the temporalis of later structures apparently embedded on the simple coronoid process, while the profound masseter most likely embedded in the adductor fossa. The course of the strands of these muscles seems to have been marginally in a posterodorsal bearing so regardless they shaped a correct edge with a line associating their zone of inclusion and the glenoid. This is especially valid for those that embedded on the beginning coronoid process. Therefore, the temporalis was most proficient when the jaws were in the shut position. The foremost segment. of the shallow masseter seems to have adjusted the back segment of the temporalis. The broadened transient fossa, and more prominent separation of the capiti-mandibularis, show that the mass of the jaw-shutting musculature in pelycosaurs was marginally more noteworthy than in the cotylosaurs.
On the off chance that it is accepted that: (1) the mass and strength of the capiti-mandibularis in a cotylosaur and the temporalis and deep masseter in pelycosaurs were equal and (2) the superficial masseter represented to an expansion to the mass of the jaw-shutting musculature in pelycosaurs, at that point it can be shown that in pelycosaurs the vertical push through the quadrate (R) would have been not exactly in cotylosaurs regardless of the increment in the mass of the jaw-shutting musculature. It can likewise be exhibited that the vertical push through the upper dentition (B) would have been expanded. The purpose behind this is in pelycosaurs the temporalis pulled posterodorsal and not dorsally as in cotylosaurs. This can undoubtedly be exhibited in a straightforward model of a pelycosaur and cotylosaur jaw in which versatile groups are utilized to mimic muscles.
THEROCEPHALIA AND GORGONOPSIANS
In primitive therocephalians and genuinely advanced gorgonopsians the coronoid process was genuinely very much developed. The temporal opening has increases extraordinarily in measure by extending posteriorly. The direction of the fibres of the temporalis muscle appears to have been nearer to the horizontal level than in pelycosaurs, yet the fibres on an normal still seem to have formed a right angle with a line associating the area of a insertion of this muscle on the coronoid process with the glenoid. As a result of the more horizontal orientation of the fibres of the temporalis muscle, the vertical push through the quadrate (R) was further decreased. In therocephalians and gorgonopsians the decrease in size of the accessory jaw bones, and particularly the quadrate and articular, has all the earmarks of being correlated with this reality.
In any case, the jaw joint of therocephalians, gorgonopsians, and other early therapsids was not just subjected to forces acting descending in the vertical course (R), yet in addition to powers acting through the jaw joint in a horizontal plane in either a anterior or posterior direction.The horizontal part of both the temporalis and superficial masseter were bigger than the comparing vertical parts (s.m.v.). In the event that these level segments are appeared in a ventral view it can be seen that the superficial masseter is inserted on the external surface of the lower jaw (reflected lamina, r.lam.), and the temporalis on the inward surface. At the point when the temporalis and superficial masseter on one side of the skull contracted synchronously, along these lines, they would tend to drive the jaw rami foremost of the jaw joint in an average course. This is imperative just when the jaws are somewhat open; when shut, the transverse procedure of the pterygoid would anticipate average development of the jaw ramus. Constriction of the shallow masseter alone would have pulled the jaw ramus forward, and withdrawal of the temporalis alone would have pulled the jaw ramus backward. These powers, either pulling the jaw in reverse or forward or diverting the jaw rami, would have a tendency to disengage the jaw joint. Be that as it may, the articular and quadrate in gorgonopsians and therocephalians were constructed to anticipate disengagement. The quad-rate condyle and glenoid in the articular in these structures were transversely extended. A dorsal procedure 10 extends upward from the articular behind the horizontal condyle of the quadrate and kept the lower jaw being pulled forward when the shallow masseter contracted. The average quadrate condyle, then again, faces anteroventral and explains with the middle piece of the glenoid in the articular, which faces posterodorsally. The introduction of these articulating surfaces would have kept the jaw ramus being constrained in reverse when the temporalis contracted. The jaw joint of some meat eating mammals is additionally intended to withstand powers following up on the jaw in either a front or back bearing. The adaptations in mammals are, in utilitarian terms, nearly identical to those present in gorgonopsians and therocephalians. In cynodonts, which are most likely the relatives of early therocephalians, a further lessening in the extent of the extra jaw bones occurred. Keeping in mind the end goal to accomplish this, it was fundamental for these structures not exclusively to diminish the vertical powers acting through the jaw joint yet additionally to lessen the powers which constrained the jaw either forward or in reverse. A decreased jaw joint would be not able withstand the powers to which a therocephalian or gor-gonopsian jaw joint was subjected.
In an early cynodont, Thrinaxodon the dentary was, moderately, substantially bigger than in the therocephalians. The coronoid procedure was significantly extended and expanded in reverse genuinely far into the augmented transient opening. Its dorsal edge about achieved an indistinguishable plane from the dorsal surface of the parietal. The filaments of the temporalis were more on a level plane situated than in therocephalians. The vertical powers acting through the jaw joint (R) were, along these lines, additionally diminished. Other imperative changes, be that as it may, had occurred. The profound masseter had augmented, moved forward, and was mostly embedded on the external surface of the extended coronoid process. The reflected lamina of the rakish was lessened in measure and had mi ground forward. A portion of the front filaments of the shallow masseter had exchanged their inclusion from the reflected lamina crnto the external surface of the posteroventral corner of the dentary. The outcome was that the filaments of the shallow masseter were more vertically arranged than in genealogical structures. In therocephalians the strands of the shallow masseter shaped an edge of roughly 200 with the cranial pivot. In Thrinaxodon, this point had expanded to around 40. The inclusion of the interior pterygoid had likewise pushed ahead and was additionally halfway embedded on the internal surface of the posteroventral corner of the dentary. The forward relocation of the addition of the shallow masseter and inward pterygoid brought about a slight diminishment of the vertical push (R) acting through the quadrate, be that as it may, all the more critically, it enormously expanded the vertical push (B) acting through the back post canines, i.e., the chomp drive when the jaws were shut was significantly expanded in cynodonts. The post canines in Thrinaxodon have genuinely complex crowns, and it is sensible to correlate this component with the more supported chomp of which these creatures were fit. Since the shallow masseter and between pterygoid were all the more vertically arranged, they had littler flat segments. Thusly, neither one of the muscles contracting alone would have constrained the jaw forward to an indistinguishable degree from in the therocephalians. Neither would the joined activity of the temporalis and shallow masseter on one side tend to drive the jaw ramus medially to an indistinguishable degree from in therocephalians and gorgonopsians. Subsequently, in Thrinaxodon, it was workable for the bones shaping the jaw joint to be littler than in therocephalians. It is fascinating to take note of that in Thrinaxodon the dorsal procedure of the articular is spoken to just by a vestigial structure. In the Cynognathus zone (figs. 1E and 6) and Middle Triassic cynodonts the dentary had expanded further in estimate and the coronoid process extended further. This was joined by a diminishment in the span of the embellishment jawbones. The majority of the jaw-shutting muscles had transferred their insertions onto the dentary. The filaments of the temporalis were more flat than in prior structures, with the outcome that the vertical push through the quadrate (R) was additionally decreased. The dentary had a very much created plot for the addition of the shallow masseter and inferior pterygoid. Filaments of the shallow masseter never again embedded on the reflected lamina, and that structure is minimal in these structures. These muscles were more vertical than in Thrinaxodon, and thus the anteriorly and posteriorly guided powers to which the jaw joint was subjected were additionally diminished. These realities are in concurrence with the further diminishment in the size and quality of the jaw joint in cutting edge cynodonts. The energy of the chomp (B), then again, was enormously expanded. The perplexing pulverizing or cutting postcanines of these progressed cynodonts were probably related with this reality. In therocephalians the temporalis seems to have shaped a bigger piece of the jaw-shutting musculature than in cutting edge cynodonts. It creates the impression that in the evolution of the cynodonts the shallow and profound masseter and the inside pterygoid expanded in mass to end up noticeably the overwhelming jaw-shutting muscles, so the temporalis framed a littler level of the jaw-shutting musculature than in prior structures.
In the transitional shape, Diarthrognathus (figs. 1F, 7 and 8), a simple dentary condyle that verbalized with the squamosal was available and the reptilian joint was greatly little. The composite condyle seems to have been weaker than in T *S.M. E. v FIG. 7. Diarthrognathus broomi. Outline to indicate resultants of powers of the jaw-shutting muscles. propelled cynodonts. The adaptations that were appeared to diminish the powers following up on the jaw joint in prior structures were developed a phase further. The temporalis was more flat than in prior structures. The dentary had a profound forwardly put point. Therefore, the inclusions of the shallow masseter and inner pterygoid were far forward, and these muscles were almost vertically arranged. In cutting edge cynodonts and Diarthrognathus the development of the jaw joint was with the end goal that it couldn’t have forestalled forward relocation of the jaw amid the compression of the jaw-shutting muscles. It is, thusly, accepted in the grouping of jaws showed in fig. 1 that the muscles could alter their quality with the goal that the powers acting at the jaw joint did not have a level segment. In Diarthrognathus the powers of the temporalis (T.) and the shallow masseter (S.M.) (+ interior pterygoid) would have met a descending pushed (B) through the back post canines at a point. Since the three powers go through a point and are in static balance, there can be no power at R, generally its minute about the purpose of crossing point would not be zero and the framework couldn’t be in harmony. Subsequently, when Diarthrognathus was gnawing with its back post canines, the quality of the individual muscles could have been balanced to the point that no vertical push (R) coordinated descending through the quadrate onto the articular was available. A comparative wonder was portrayed in some meat eating warm blooded creatures by Maynard Smith and Savage (1959). In Diarthrognathus the shallow masseter had a little even component. Thus, this muscle did not to any awesome degree tend to drive the jaw forward or, when contracting synchronously with the temporalis, help to compel the jaw ramus medially. Therefore, an exceptionally powerless jaw joint was conceivable in these structures. An intriguing element of the lower jaw of Diarthrognathus (fig. 8) is the foremost degree of the front edge of the coronoid process. It stretched out along the side to the back post canines. This element was likewise present in tritylodontids (Kiihne, 1956), yet is obviously obscure in Mesozoic well evolved creatures. This element apparently empowered the addition of the profound masseter to be as far expelled as conceivable from the jaw joint. This would expand the chomp over the teeth, yet diminish the vertical push through the quadrate and squamosal to the articular and dentary. The structure of the lower jaw of Diarthrognathus delineates how it was possible to consolidate in one creature effective jaw musculature and a to a great degree frail jaw joint. All together this might be accomplished, in any case, it was fundamental that the dentary be profound behind the dentition, have a significantly extended coronoid process, and have a profound, adjusted, anteriorly put point. These highlights are trademark to a more noteworthy or lesser degree of every single propelled therapsid with powerless jaw joints, i.e., of both meat eating and herbivorous structures. The weaker the jaw joint, the more extreme were the adaptations to lessen the powers to which the jaw joint was subjected. The particular preferred standpoint to Triassic warm blooded creature like reptiles of having expanded jaw musculature empowering a capable nibble, and having these muscles embedded on a solitary bone, seems to have more than made up for the drawbacks of having a frail jaw joint and a profound and cumbersome dentary.
In the Rhaetic warm blooded creature Morganucodon (Kermack and Mussett, 1958), a twofold jaw enunciation was as yet introduce he mammalian explanation was more grounded than in Diarthrognathus, despite the fact that not as solid as in later well evolved creatures. It would, in this manner, be normal that the extreme adaptations vital in Diarthrognathus to limit the powers to which the jaw joint was subjected were never again important in Morganucodon. The lower jaw of Morganucodon contains a furrow for the embellishment jawbone like that present in the propelled therapsids and Diarthrognathus, yet the general state of the jaw is more like that of the jaws of Jurassic well evolved creatures than to that of a propelled therapsid, for example, Oligokyphus and transitional structures, for example, Diarthrognathus. In specific angles the lower jaw of Morganucodon lies between later well evolved creatures and propelled therapsids. Morganucodon has, in compari child with Diarthrognathus, a long, thin dentary. The coronoid procedure is littler and its front outskirt does not lie parallel to the back post canines. Morganuco-wear has a little point, arranged more remote posteriorly than in Diarthrognathus. In Morganucodon and a portion of the Rhaetic and Jurassic well evolved creatures, a portion of the patterns which portray the development of the lower jaw in cutting edge therapsids seem to have been turned around. From early cynodonts to structures, for example, Diarthrognathus, the jaw joint dynamically diminished in estimate while the dentary extended and developed a profound anteriorly put edge and an extended coronoid process. In early warm blooded creatures the recently procured squamosal-dentary jaw joint was reinforced. The edge moved posteriorly and diminished top to bottom and the width of the coronoid procedure was decreased.
Rundown The lower jaws and jaw musculature of a progression of warm blooded creature like reptiles is quickly depicted and talked about. It is exhibited how the inclusion of the jaw-shutting musculature in these structures step by step moved from the adornment jawbones onto the nook tary and how the segment parts of the jaw musculature slowly changed their introduction such that the powers to which the jaw joint was subjected during constriction of the jaw-shutting muscles were continuously diminished. This made conceivable a diminishment of the extra jaw bones and an expansion in the span of the dentary until the point when the last bone set up contact with the squamosal. The decline in the powers to which the jaw joint was subjected was joined by an expansion in the quality of the nibble, particularly in the area of the postcanine teeth. The improvement of “molariform” teeth in advertisement advanced therapsids seems, by all accounts, to be associated with this reality.
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