Quadratojugal bone

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The quadratojugal is a skull bone present in many vertebrates, including some living reptiles and amphibians. [1]

Contents

Anatomy and function

In animals with a quadratojugal bone, it is typically found connected to the jugal (cheek) bone from the front and the squamosal bone from above. It is usually positioned at the rear lower corner of the cranium. [2] Many modern tetrapods lack a quadratojugal bone as it has been lost or fused to other bones. Modern examples of tetrapods without a quadratojugal include salamanders, mammals, birds, and squamates (lizards and snakes). [3] In tetrapods with a quadratojugal bone, it often forms a portion of the jaw joint.

Developmentally, the quadratojugal bone is a dermal bone in the temporal series, forming the original braincase. The squamosal and quadratojugal bones together form the cheek region [4] and may provide muscular attachments for facial muscles. [5]

In reptiles and amphibians

Skull diagram of the rhynchocephalian Sphenotitan, with the quadratojugal coloured lilac Sphenotitan skull.svg
Skull diagram of the rhynchocephalian Sphenotitan, with the quadratojugal coloured lilac

In most modern reptiles and amphibians, the quadratojugal is a prominent, straplike bone in the skull and provides structural integrity in the postorbital region of the skull. [6] In many reptiles, the inner face of the quadratojugal also connects to the quadrate bone which forms the cranium's contribution to the jaw joint. Early in their evolution, diapsid reptiles evolved a lower temporal bar which was composed of the quadratojugal and jugal. The lower temporal bar forms the lower border of the infratemporal fenestra, one of two holes in the side of the head and a hallmark of a diapsidan skull. However, many diapsids, including modern squamates (lizards and snakes), have lost the lower temporal bar. [7] Crocodilians and rhynchocephalians (the latter represented solely by the tuatara, Sphenodon) retain a quadratojugal. Turtles also seem to possess a quadratojugal. [7] Among living amphibians, a quadratojugal is known to be present in some frogs and caecilians. However, it is notably absent in salamanders. [8]

In birds

In modern birds, the quadratojugal bone is a thin and rodlike element of the skull. Upon ossification, the jugal and quadratojugal bones fuse to form the jugal bar, which is homologous to the lower temporal bar of other diapsids. The sections of the jugal bar derived from the jugal and quadratojugal articulate with the postorbital and squamosal bones, respectively. This facilitates cranial kinesis, by allowing the quadrate bone to rotate during opening of the upper jaw. [9] [5]

In mammals

Advanced cynodonts, including the mammaliaforms, have lost the quadratojugal, with the diminutive quadrate connecting to the stapes to function as a hearing structure. In modern mammals, the quadrate bone evolves to become the incus, one of the ossicles of the middle ear. [10] This is an apomorphy of the mammalian clade, and is used to identify the fossil transition to mammals. [5]

Evolution

Origin

The skull of the tetrapodomorph fish Eusthenopteron, with the quadratojugal bone labelled in pink. Eusthenopteron quadratojugal.png
The skull of the tetrapodomorph fish Eusthenopteron , with the quadratojugal bone labelled in pink.

The quadratojugal likely originated within the clade Sarcopterygii, which includes tetrapods and lobe-finned fish. Although a tiny bone similar in position to the quadratojugal has been observed in the placoderm Entelognathus and some early actinopterygiians ( Mimipiscis , Cheirolepis ), it is unclear whether this bone was homologous to the quadratojugal. A quadratojugal is absent in actinians (coelacanths) and onychodonts, but it was clearly present in Porolepiformes, distant relatives of modern dipnoans (lungfish). Many paleontologists argue that the quadratojugal was formed by a division of the preoperculum, although a few believe that it was present before the preoperculum formed. All tetrapodomorph fish had a quadratojugal, retained by their tetrapod descendants. Elpistostegalians such as Panderichthys , Tiktaalik , and other very tetrapod-like fish were the first vertebrates to have contact between the quadratojugal and jugal. Before the elpistostegalians, the jugal was small and isolated from the quadratojugal by the squamosal and maxilla. [11]

Amphibians (in the broad sense) typically had long, roughly rectangular quadratojugals that contacted the maxilla, jugal, squamosal, and quadrate. In several lineages, most of them traditionally considered "Reptiliomorpha", the jugal expands downwards to reduce the amount of contact between the quadratojugal and maxilla. This is exemplified in reptiles, which have completely lost the contact. Most urodelans (salamanders) lack quadratojugals, except the Miocene genus Chelotriton . [12] A quadratojugal is also missing in the caecilian-like Triassic stereospondyl Chinlestegophis [8] as well as the lysorophians, a group of long-bodied Paleozoic microsaurs. Many other microsaurs had heavily reduced quadratojugals. [13]

Synapsids

The skull of Gorynychus, a therocephalian synapsid. The tiny quadrate-quadratojugal complex is labelled with q-qj. Gorynychus masyutinae.png
The skull of Gorynychus , a therocephalian synapsid. The tiny quadrate-quadratojugal complex is labelled with q-qj.

In synapsids (mammals and their extinct relatives), the quadratojugal undergoes significant transformation during the evolution of the group. Early synapsids such as eothyridids and caseids retained long quadratojugals and in some cases even reacquire quadratojugal-maxilla contact. [14] In most therapsids, including gorgonopsians, therocephalians, and dicynodonts, the quadratojugal is tiny, having lost its contact with the jugal. It usually fuses with the equally small quadrate to form the quadrate-quadratojugal complex. [15] Oddly enough, the cynodont Thrinaxodon retains a separate quadratojugal. In other cynodonts such as Cynognathus , the quadrate-quadratojugal complex remains hidden within the skull, obscured from the side by the large squamosal bone which loosely articulates with it. [16]

Sauropsids

Sauropsids, the group containing reptiles and birds, had completely lost the contact between the quadratojugal and maxilla. In diapsids, the quadratojugal and jugal form the lower temporal bar, which defines the lower border of the infratemporal fenestra, one of two holes in the side of the head. In early diapsids such as Petrolacosaurus and Youngina , the quadratojugal is long as in amphibians, early synapsids, and "anapsid" reptiles. It forms most of the length of the lower temporal bar. However, significant transformation of the temporal region of the skull occurs in many more "advanced" members of Diapsida, with implications for the structure of the quadratojugal. [7]

The skull of Prolacerta, a relative of the Archosauriformes with an incomplete lower temporal bar. The small, crescent-shaped quadratojugal is labelled with 153-0. Prolacerta skull diagram.png
The skull of Prolacerta , a relative of the Archosauriformes with an incomplete lower temporal bar. The small, crescent-shaped quadratojugal is labelled with 153-0.

Numerous diapsids have an incomplete lower temporal bar, where the quadratojugal and jugal fail to contact each other. This leaves the infratemporal fenestra with an arch-like structure, open from below. An incomplete (or absent) lower temporal bar is first seen in the Permian genus Claudiosaurus , and is retained by most other Permian and Triassic diapsids. In many cases, the quadratojugal is lost completely. This loss occurs in several Triassic marine reptiles such as tanystropheids, thalattosaurs, pistosaurs, and plesiosaurs. Squamates, the group containing modern lizards and snakes, also lack a quadratojugal, but early squamate relatives such as Marmoretta do retain the bone. Ichthyosaurs, a group without a lower temporal bar, have a quadratojugal that is taller than it is long, stretching above (rather than below) the open infratemporal fenestra to contact the postorbital bone (rather than the jugal). Early turtles such as Proganochelys also have a tall quadratojugal, which contacts the jugal without any trace of the infratemporal fenestra. [7]

The skull of the dromaeosaurid dinosaur Dromaeosaurus, with the quadratojugal (light blue) labelled. Dromaeosaurus skull en.svg
The skull of the dromaeosaurid dinosaur Dromaeosaurus , with the quadratojugal (light blue) labelled.

Several Triassic reptiles reacquire the lower temporal bar, albeit with the jugal forming most of the bar's length. In these reptiles, the quadratojugal is a small L- or T-shaped bone at the rear edge of the skull. Although early rhynchocephalians such as Gephyrosaurus have an incomplete lower temporal bar and a quadratojugal fused to the quadrate, later members of the group such as the modern tuatara (Sphenodon) do have a complete lower temporal bar, albeit with the quadratojugal still fused to the quadrate. All members of the group Archosauriformes, which contains archosaurs such as crocodilians and dinosaurs, have a complete lower temporal bar. This is also the case in placodonts, Trilophosaurus , some rhynchosaurs, and choristoderes. [7]

Modern birds have a quadratojugal which is assimilated into the thin, splint-like jugal. However, a separate quadratojugal is retained by several Mesozoic avialans, such as Archaeopteryx and Pterygornis . Non-avialan dinosaurs also have a separate quadratojugal. [17]

Related Research Articles

<span class="mw-page-title-main">Synapsida</span> Clade of tetrapods

Synapsida is one of the two major clades of vertebrate animals in the group Amniota, the other being the Sauropsida. The synapsids were the dominant land animals in the late Paleozoic and early Mesozoic, but the only group that survived into the Cenozoic are mammals. Unlike other amniotes, synapsids have a single temporal fenestra, an opening low in the skull roof behind each eye orbit, leaving a bony arch beneath each; this accounts for their name. The distinctive temporal fenestra developed about 318 million years ago during the Late Carboniferous period, when synapsids and sauropsids diverged, but was subsequently merged with the orbit in early mammals.

<span class="mw-page-title-main">Quadrate bone</span> Skull bone

The quadrate bone is a skull bone in most tetrapods, including amphibians, sauropsids, and early synapsids.

<span class="mw-page-title-main">Jugal bone</span>

The jugal is a skull bone found in most reptiles, amphibians and birds. In mammals, the jugal is often called the malar or zygomatic. It is connected to the quadratojugal and maxilla, as well as other bones, which may vary by species.

<span class="mw-page-title-main">Archosauromorpha</span> Infraclass of reptiles

Archosauromorpha is a clade of diapsid reptiles containing all reptiles more closely related to archosaurs rather than lepidosaurs. Archosauromorphs first appeared during the late Middle Permian or Late Permian, though they became much more common and diverse during the Triassic period.

<span class="mw-page-title-main">Euryapsida</span>

Euryapsida is a polyphyletic group of sauropsids that are distinguished by a single temporal fenestra, an opening behind the orbit, under which the post-orbital and squamosal bones articulate. They are different from Synapsida, which also have a single opening behind the orbit, by the placement of the fenestra. In synapsids, this opening is below the articulation of the post-orbital and squamosal bones. It is now commonly believed that euryapsids are in fact diapsids that lost the lower temporal fenestra. Euryapsids are usually considered entirely extinct, although turtles might be part of the sauropterygian clade while other authors disagree. Euryapsida may also be a synonym of Sauropterygia sensu lato.

<span class="mw-page-title-main">Parareptilia</span> Extinct subclass of reptiles (306–201Ma ago)

Parareptilia ("near-reptiles") is an extinct subclass or clade of basal sauropsids/reptiles, typically considered the sister taxon to Eureptilia. Parareptiles first arose near the end of the Carboniferous period and achieved their highest diversity during the Permian period. Several ecological innovations were first accomplished by parareptiles among reptiles. These include the first reptiles to return to marine ecosystems (mesosaurs), the first bipedal reptiles, the first reptiles with advanced hearing systems, and the first large herbivorous reptiles. The only parareptiles to survive into the Triassic period were the procolophonoids, a group of small generalists, omnivores, and herbivores. The largest family of procolophonoids, the procolophonids, rediversified in the Triassic, but subsequently declined and became extinct by the end of the period.

<span class="mw-page-title-main">Squamosal bone</span> Skull bone in most reptiles, amphibians and birds

The squamosal is a skull bone found in most reptiles, amphibians, and birds. In fishes, it is also called the pterotic bone.

<span class="mw-page-title-main">Temporal fenestra</span> Opening in the skull behind the orbit in some animals

Temporal fenestrae are openings in the temporal region of the skull of some amniotes, behind the orbit. These openings have historically been used to track the evolution and affinities of reptiles. Temporal fenestrae are commonly seen in the fossilized skulls of dinosaurs and other sauropsids. The major reptile group Diapsida, for example, is defined by the presence of two temporal fenestrae on each side of the skull. The infratemporal fenestra, also called the lateral temporal fenestra or lower temporal fenestra, is the lower of the two and is exposed primarily in lateral (side) view.

<i>Acerosodontosaurus</i> Extinct genus of reptiles

Acerosodontosaurus is an extinct genus of neodiapsid reptiles that lived during the Late Permian of Madagascar. The only species of Acerosodontosaurus, A. piveteaui, is known from a natural mold of a single partial skeleton including a crushed skull and part of the body and limbs. The fossil was discovered in deposits of the Lower Sakamena Formation. Based on skeletal characteristics, it has been suggested that Acerosodontosaurus individuals were at least partially aquatic.

<i>Heleosaurus</i> Extinct genus of tetrapods

Heleosaurus scholtzi is an extinct species of basal synapsids, known as pelycosaurs, in the family of Varanopidae during the middle Permian. At first H. scholtzi was mistakenly classified as a diapsid. Members of this family were carnivorous and had dermal armor, and somewhat resembled monitor lizards. This family was the most geologically long lived, widespread, and diverse group of early amniotes. To date only two fossils have been found in the rocks of South Africa. One of these fossils is an aggregation of five individuals.

<i>Polonosuchus</i> Extinct genus of reptiles

Polonosuchus is a genus of rauisuchid known from the late Triassic of Poland. It was a huge predator about 5–6 metres in length and, like all rauisuchians, was equipped with a large head of long sharp teeth. The legs were placed almost underneath the body, unlike most reptiles, which would have made it quite fast and a powerful runner. The appearance was very similar to that of the more known Postosuchus, of North America, and shared with the latter the ecological niche of the apex predator.

Australothyris is an extinct genus of basal procolophonomorph parareptile known from the Middle Permian of Tapinocephalus Assemblage Zone, South Africa. The type and only known species is Australothyris smithi. As the most basal member of Procolophonomorpha, Australothyris helped to contextualize the origin of this major parareptile subgroup. It has been used to support the hypotheses that procolophonomorphs originated in Gondwana and ancestrally possess temporal fenestrae, due to its large and fully enclosed temporal fenestra and South African heritage. It also possessed several unique features, including a high tooth number, long postfrontal, small interpterygoid vacuity, and a specialized interaction between the stapes and quadrate.

<i>Jesairosaurus</i> Extinct genus of reptiles

Jesairosaurus is an extinct genus of early archosauromorph reptile known from the Illizi Province of Algeria. It is known from a single species, Jesairosaurus lehmani. Although a potential relative of the long-necked tanystropheids, this lightly-built reptile could instead be characterized by its relatively short neck as well as various skull features.

<i>Palatodonta</i> Extinct genus of reptiles

Palatodonta is an extinct genus of neodiapsid reptile known from the early Middle Triassic of the Netherlands. It was initially described in 2013 as a basal placodontiform closely related to a group of marine reptiles called placodonts, characterized by their crushing teeth and shell-like body armor. Under this interpretation, Palatodonta is transitional between placodonts and less specialized reptiles. Like placodonts, it has a row of large teeth on its palate, but while these teeth are thick and blunt in placodonts, Palatodonta has palatal teeth that are thin and pointed. A 2023 study instead classified it as a sauropterygomorph and the sister taxon to Eusaurosphargis. In other words, it is close to, but not within, Sauropterygia.

Eohyosaurus is an extinct genus of basal rhynchosaur known from the early Middle Triassic Burgersdorp Formation of Free State, South Africa. It contains a single species, Eohyosaurus wolvaardti.

Shuangbaisaurus is genus of theropod dinosaur, possibly a junior synonym of Sinosaurus. It lived in the Early Jurassic of Yunnan Province, China, and is represented by a single species, S. anlongbaoensis, known from a partial skull. Like the theropods Dilophosaurus and Sinosaurus,Shuangbaisaurus bore a pair of thin, midline crests on its skull. Unusually, these crests extended backwards over the level of the eyes, which, along with the unusual orientation of the jugal bone, led the describers to name it as a new genus. However, Shuangbaisaurus also possesses a groove between its premaxilla and maxilla, a characteristic which has been used to characterize Sinosaurus as a genus. Among the two morphotypes present within the genus Sinosaurus, Shuangbaisaurus more closely resembles the morphotype that is variably treated as a distinct species, S. sinensis, in its relatively tall skull.

<i>Kadimakara australiensis</i> Extinct species of reptile

Kadimakara is an extinct genus of early archosauromorph reptile from the Arcadia Formation of Queensland, Australia. It was seemingly a very close relative of Prolacerta, a carnivorous reptile which possessed a moderately long neck. The generic name Kadimakara references prehistoric creatures from Aboriginal myths which may have been inspired by ice-age megafauna. The specific name K. australiensis relates to the fact that it was found in Australia. Prolacerta and Kadimakara were closely related to the Archosauriformes, a successful group which includes archosaurs such as crocodilians, pterosaurs, and dinosaurs.

Boreopricea is an extinct genus of archosauromorph reptile from the Early Triassic of arctic Russia. It is known from a fairly complete skeleton discovered in a borehole on Kolguyev Island, though damage to the specimen and loss of certain bones has complicated study of the genus. Boreopricea shared many similarities with various other archosauromorphs, making its classification controversial. Various studies have considered it a close relative of Prolacerta, tanystropheids, both, or neither. Boreopricea is unique among early archosauromorphs due to possessing contact between the jugal and squamosal bones at the rear half of the skull.

<i>Rugarhynchos</i> Extinct genus of reptiles

Rugarhynchos is an extinct genus of doswelliid archosauriform from the Late Triassic of New Mexico. The only known species is Rugarhynchos sixmilensis. It was originally described as a species of Doswellia in 2012, before receiving its own genus in 2020. Rugarhynchos was a close relative of Doswellia and shared several features with it, such as the absence of an infratemporal fenestra and heavily textured skull bones. However, it could also be distinguished by many unique characteristics, such as a thick diagonal ridge on the side of the snout, blunt spikes on its osteoderms, and a complex suture between the quadratojugal, squamosal, and jugal. Non-metric multidimensional scaling and tooth morphology suggest that Rugarhynchos had a general skull anatomy convergent with some crocodyliforms, spinosaurids, and phytosaurs. However, its snout was somewhat less elongated than those other reptiles.

<i>Taytalura</i> Extinct genus of reptiles

Taytalura is an extinct genus of lepidosauromorph reptile from the Late Triassic of Argentina. It contains a single species, Taytalura alcoberi, which is based on a well-preserved skull from the fossiliferous Ischigualasto Formation. As a lepidosauromorph, Taytalura is a distant relative of modern lepidosaurs such as sphenodontians and squamates. Taytalura did not belong to any group of modern lepidosaurs, since it bears unique features, such as unfused bones in the skull roof and teeth which all sit loosely in a deep groove without sockets. Regardless, Micro-CT scanning reveals features of the skull previously only seen in rhynchocephalians. This suggests that the ancestral condition of the skull in lepidosaurs was more similar to sphenodonts than to squamates.

References

  1. ↑ Lecointre, Guillaume; Le Guyader, Hervé (2006). The tree of life: a phylogenetic classification . Harvard University Press, 2006. pp.  380. ISBN   9780674021839 . Retrieved December 10, 2011.
  2. ↑ Homberger, Dominique G. (2004). Vertebrate dissection. Walker, Warren F. (Warren Franklin), Walker, Warren F. (Warren Franklin). (9th ed.). Belmont, CA: Thomson Brooks/Cole. ISBN   0-03-022522-1. OCLC   53074665.
  3. ↑ Schwenk, Kurt (2000). Feeding: Form, Function and Evolution in Tetrapod Vertebrates (1st ed.). Academic Press. p. 537. ISBN   9780080531632 . Retrieved December 10, 2011.
  4. ↑ Romer, Alfred Sherwood, 1894-1973. (1978). The vertebrate body : shorter version. Parsons, Thomas S. (Thomas Sturges), 1930- (5th ed.). Philadelphia: Saunders. ISBN   0-7216-7682-0. OCLC   3345587.{{cite book}}: CS1 maint: multiple names: authors list (link) CS1 maint: numeric names: authors list (link)
  5. 1 2 3 Kardong, Kenneth V. (2012). Vertebrates : comparative anatomy, function, evolution (6th ed.). New York: McGraw-Hill. ISBN   978-0-07-352423-8. OCLC   664665896.
  6. ↑ Wang, Min; Hu, Han (January 2017). "A Comparative Morphological Study of the Jugal and Quadratojugal in Early Birds and Their Dinosaurian Relatives: The ZYGOMA OF EARLY BIRDS". The Anatomical Record. 300 (1): 62–75. doi: 10.1002/ar.23446 . PMID   28000410. S2CID   3649504.
  7. 1 2 3 4 5 Müller, Johannes (2003-10-01). "Early loss and multiple return of the lower temporal arcade in diapsid reptiles". Naturwissenschaften. 90 (10): 473–476. Bibcode:2003NW.....90..473M. doi:10.1007/s00114-003-0461-0. ISSN   1432-1904. PMID   14564408. S2CID   21007773.
  8. 1 2 Huttenlocker, Adam K.; Small, Bryan J.; Pardo, Jason D. (2017-07-03). "Stem caecilian from the Triassic of Colorado sheds light on the origins of Lissamphibia". Proceedings of the National Academy of Sciences. 114 (27): E5389–E5395. Bibcode:2017PNAS..114E5389P. doi: 10.1073/pnas.1706752114 . ISSN   1091-6490. PMC   5502650 . PMID   28630337.
  9. ↑ Wang, Min; Hu, Han (2017). "A Comparative Morphological Study of the Jugal and Quadratojugal in Early Birds and Their Dinosaurian Relatives". The Anatomical Record. 300 (1): 62–75. doi: 10.1002/ar.23446 . ISSN   1932-8494. PMID   28000410. S2CID   3649504.
  10. ↑ Luo, Zhe-Xi (13 December 2007). "Transformation and diversification in early mammal evolution" (PDF). Nature. 450 (7172): 1011–1019. Bibcode:2007Natur.450.1011L. doi:10.1038/nature06277. ISSN   1476-4687. PMID   18075580. S2CID   4317817.
  11. ↑ Gai, Zhikun; Yu, Xiaobo; Zhu, Min (January 2017). "The Evolution of the Zygomatic Bone From Agnatha to Tetrapoda". The Anatomical Record. 300 (1): 16–29. doi: 10.1002/ar.23512 . ISSN   1932-8494. PMID   28000409. S2CID   3661931.
  12. ↑ Kupfer, Alexander; Poschmann, Markus; Schoch, Rainer R. (2015-03-01). "The salamandrid Chelotriton paradoxus from Enspel and Randeck Maars (Oligocene–Miocene, Germany)". Palaeobiodiversity and Palaeoenvironments. 95 (1): 77–86. doi:10.1007/s12549-014-0182-8. ISSN   1867-1608. S2CID   140561804.
  13. ↑ Marcello Ruta, Michael I. Coates and Donald L. J. Quicke (2003). "Early tetrapod relationships revisited" (PDF). Biological Reviews. 78 (2): 251–345. doi:10.1017/S1464793102006103. PMID   12803423. S2CID   31298396.
  14. ↑ Reisz, Robert R.; Godfrey, Stephen J.; Scott, Diane (2009). "Eothyris and Oedaleops: do these Early Permian synapsids from Texas and New Mexico form a clade?". Journal of Vertebrate Paleontology. 29 (1): 39–47. doi:10.1671/039.029.0112. S2CID   85602931.
  15. ↑ Parrington, F.R. (1946). "LXXIV.—On the quadratojugal bone of synapsid reptiles". Annals and Magazine of Natural History. 13 (107): 780–786. doi:10.1080/00222934608654599.
  16. ↑ Crompton, A.W. (1972). "The evolution of the jaw articulation of cynodonts". Studies in Vertebrate Evolution. Edinburgh: Oliver & Boyd. pp. 231–251.
  17. ↑ Wang, Min; Hu, Han (2017-01-01). "A Comparative Morphological Study of the Jugal and Quadratojugal in Early Birds and Their Dinosaurian Relatives". The Anatomical Record. 300 (1): 62–75. doi: 10.1002/ar.23446 . ISSN   1932-8494. PMID   28000410. S2CID   3649504.
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