Postparietal

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The skull of Xenotosuchus, a temnospondyl amphibian. Postparietals are in deep pink, at the back of the skull Xenotosuchus skull roof.svg
The skull of Xenotosuchus , a temnospondyl amphibian. Postparietals are in deep pink, at the back of the skull

Postparietals are cranial bones present in fish and many tetrapods. Although initially a pair of bones, many lineages possess postparietals which were fused into a single bone. The postparietals were dermal bones situated along the midline of the skull, behind the parietal bones. They formed part of the rear edge of the skull roof, and the lateral edge of each postparietal often contacts the tabular and supratemporal bones. In fish, the postparietals are elongated, typically the largest components of the skull roof. Tetrapods possessed shorter postparietals, which were reduced further and shifted towards the braincase in amniotes. [1] At several points in synapsid evolution, the postparietals fused to each other and the tabulars during embryological development. This fusion produces the interparietal bone, which is inherited by mammals. [2] Postparietals are common in extinct amphibians and early reptiles. However, most living amphibians (of the group Lissamphibia) and living reptiles (of the group Sauria) lack postparietal bones, with a few exceptions. [2]

Contents

Evolution

Fish and amphibians

The skull of a bowfin (Amia calva), one of the most basal living actinopterygiians. Skull bones are labelled based on tetrapod homologies. Bowfin.png
The skull of a bowfin (Amia calva), one of the most basal living actinopterygiians. Skull bones are labelled based on tetrapod homologies.

Watson & Day (1916)'s "orthodox" interpretation of fish skulls argued that fish lacked independent postparietals, with the elongated paired midline bones at the back of the skull being interpreted as parietals. On the other hand, Westoll (1938) proposed an alternative interpretation which identified the bones as postparietals based on comparisons between early tetrapods and their sarcopterygian ancestors. This latter interpretation has usurped the "orthodox" interpretation and is currently more widespread among paleontologists Although the generally large size of fish postparietals are inconsistent with the smaller postparietals of tetrapods, there are many factors supporting the identification of the large posterior midline elements as postparietals, rather than parietals. These include their contact with tabulars and supratemporals, the fact that they are positioned behind the bones which surround the parietal foramen (i.e. the parietal bones), and how transitional taxa show apparent homology with tetrapod postparietals and the large posterior midline elements of fish. Studies of Ichthyostega , Elpistostege , and Edops in particular have demonstrated this concept. One objection to this interpretation is that the single midline postparietal of Ichthyostega has a transverse bend of the lateral line, which in fish typically occurs on extrascapular elements (plates at the back of the skull formed from enlarged neck scales). Proponents of the "orthodox" interpretation used this to argue that the unpaired postparietal of Icthyostega is a modified extrascapular element not homologous to what they identify as the "parietals" of fish. However, this is more easily explained by a simple shift in the position of the lateral line, as the postparietals of Icthyostega are otherwise identical in proportion and position (and therefore considered homologous) to the large paired posterior midline elements of fish. [3] [1] [4]

Many sarcopterygian fish (including living coelocanths) possess a large, robust plate at the back of the skull known as a postparietal shield. This plate consists mostly of the large postparietals along its midline, with smaller tabular bones and one or more supratemporal bones along its edge. The postparietal shield often articulates with the rest of the skull through a mobile joint. As sarcopterygians acquire more derived features and eventually evolve into tetrapods, the postparietals gradually shrink, losing their status as the largest midline elements of the skull and allowing the more anteriorly-situated parietal bones (and the newly acquired frontal bones) to acquire that status. The postparietals also becomes more solidly attached to the parietals. Most lissamphibians lack postparietals, with the exception of a few anurans such as Pelates (spadefoot toads) and Bombina (fire-bellied toads). [2] In diadectomorphs, the postparietals fuse to each other. The seymouriamorph Discosauriscus has a high degree of interspecific variation in regards to postparietal fusion; they may either fuse with each other, not at all, or with the adjacent tabulars but not with each other. [5] Closer to the base of amniotes, the postparietal shifts from the dorsal portion of the skull to the occipital (braincase) portion, sloping downwards in the process.

Synapsids

The skull of Dimetrodon (an early synapsid with a single midline postparietal) in occipital view. Dimetrodon skull occipital.svg
The skull of Dimetrodon (an early synapsid with a single midline postparietal) in occipital view.

Early synapsids inherited postparietals (sometimes paired) from their non-amniote ancestors. Embryological data indicates that the interparietal bone of mammals forms from the fusion of four bones during early development: a pair of medial neural crest elements edged by lateral mesoderm elements. The medial neural crest-derived pair are considered homologous to the postparietals of other vertebrates, while the lateral mesoderm bones are considered homologous to the tabular bones. In almost all mammals, all four bones are fused to each other by the time of birth, and in many cases they additionally fuse to the parietal and supraoccipital in adulthood. Many non-mammalian synapsids have three bones in the interparietal region as adults: one midline bone and two lateral bones. In these situations, the midline bone (often also termed an interparietal) is a fused postparietal while the lateral bones are tabulars. Independently-derived fusion between paired postparietals and/or the adjacent tabulars is common among synapsids, meaning that many different lineages have one, three, or four bones in the region which makes up the mammalian interparietal. In rare cases there are two interparietal bones, formed when left and right postparietals each fuse to their corresponding tabular, but not each other. [2]

Reptiles

The postparietals continue to shrink and move further back in the skull in reptiles, no longer forming any contribution to the ceiling of the brain cavity. Small paired or fused postparietals are common in Permian parareptiles and eureptiles, including early diapsids such as Petrolacosaurus and Youngina . Postparietals were subsequently lost at the base of Sauria, the expansive diapsid subgroup containing all living species of reptiles. Archosauriforms (and their sister taxon Tasmaniosaurus ) briefly reacquired postparietals in the form of a single fused bone. As in synapsids, this bone has sometimes been termed an interparietal. Proterosuchids, erythrosuchids, euparkeriids, and Asperoris are all known to have possessed interparietals, while the bone was absent in proterochampsids, Doswellia , Vancleavea , Litorosuchus , most or all phytosaurs, and archosaurs. [6] The only true archosaur generally considered to possess an interparietal as an adult is a single specimen of Gracilisuchus . [7] Paired postparietals have also been observed in Alligator mississippiensis embryos, although they are incorporated into the supraoccipital by the time of hatching. [8] One proposal used developmental data to argue that the "parietal" of birds was actually derived from postparietals. [9] However, subsequent review provided evidence against that hypothesis by demonstrating that birds had inherited a consistent relationship between the skull roof and brain cavity; this relationship excluded the postparietals from the brain cavity (at least in reptiles) and supported the traditional interpretation that the parietal of birds was homologous to that of other reptiles. [7]

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<span class="mw-page-title-main">Adelospondyli</span> Extinct order of amphibians

Adelospondyli is an order of elongated, presumably aquatic, Carboniferous amphibians. They have a robust skull roofed with solid bone, and orbits located towards the front of the skull. The limbs were almost certainly absent, although some historical sources reported them to be present. Despite the likely absence of limbs, adelospondyls retained a large part of the bony shoulder girdle. Adelospondyls have been assigned to a variety of groups in the past. They have traditionally been seen as members of the subclass Lepospondyli, related to other unusual early tetrapods such as "microsaurs", "nectrideans", and aïstopods. Analyses such as Ruta & Coates (2007) have offered an alternate classification scheme, arguing that adelospondyls were actually far removed from other lepospondyls, instead being stem-tetrapod stegocephalians closely related to the family Colosteidae.

<i>Seymouria</i> Extinct genus of tetrapodomorphs

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

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<i>Platyhystrix</i> Genus of amphibians (fossil)

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

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

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<i>Acherontiscus</i> Extinct genus of amphibians

Acherontiscus is an extinct genus of stegocephalians that lived in the Early Carboniferous of Scotland. The type and only species is Acherontiscus caledoniae, named by paleontologist Robert Carroll in 1969. Members of this genus have an unusual combination of features which makes their placement within amphibian-grade tetrapods uncertain. They possess multi-bone vertebrae similar to those of embolomeres, but also a skull similar to lepospondyls. The only known specimen of Acherontiscus possessed an elongated body similar to that of a snake or eel. No limbs were preserved, and evidence for their presence in close relatives of Acherontiscus is dubious at best. Phylogenetic analyses created by Marcello Ruta and other paleontologists in the 2000s indicate that Acherontiscus is part of Adelospondyli, closely related to other snake-like animals such as Adelogyrinus and Dolichopareias. Adelospondyls are traditionally placed within the group Lepospondyli due to their fused vertebrae. Some analyses published since 2007 have argued that adelospondyls such as Acherontiscus may not actually be lepospondyls, instead being close relatives or members of the family Colosteidae. This would indicate that they evolved prior to the split between the tetrapod lineage that leads to reptiles (Reptiliomorpha) and the one that leads to modern amphibians (Batrachomorpha). Members of this genus were probably aquatic animals that were able to swim using snake-like movements.

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

Odonterpeton is an extinct genus of "microsaur" from the Late Carboniferous of Ohio, containing the lone species Odonterpeton triangulare. It is known from a single partial skeleton preserving the skull, forelimbs, and the front part of the torso. The specimen was found in the abandoned Diamond Coal Mine of Linton, Ohio, a fossiliferous coal deposit dated to the late Moscovian stage, about 310 million years ago.

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

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<span class="mw-page-title-main">Skull roof</span> Roofing bones of the skull

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<span class="mw-page-title-main">Interparietal bone</span> Dermal bone situated between the parietal and supraoccipital

An interparietal bone is a dermal bone situated between the parietal and supraoccipital. It is homologous to the postparietal bones of other animals.

This glossary explains technical terms commonly employed in the description of dinosaur body fossils. Besides dinosaur-specific terms, it covers terms with wider usage, when these are of central importance in the study of dinosaurs or when their discussion in the context of dinosaurs is beneficial. The glossary does not cover ichnological and bone histological terms, nor does it cover measurements.

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

The intertemporal bone is a paired cranial bone present in certain sarcopterygians and extinct amphibian-grade tetrapods. It lies in the rear part of the skull, behind the eyes.

The supratemporal bone is a paired cranial bone present in many tetrapods and tetrapodomorph fish. It is part of the temporal region, usually lying medial (inwards) relative to the squamosal and lateral (outwards) relative to the parietal and/or postparietal. It may also contact the postorbital or intertemporal, or tabular, when those bones are present.

References

  1. 1 2 Panchen, A. L.; Smithson, T. R. (1987). "Character Diagnosis, Fossils and the Origin of Tetrapods". Biological Reviews. 62 (4): 341–436. doi:10.1111/j.1469-185X.1987.tb01635.x. ISSN   1469-185X. S2CID   83672017.
  2. 1 2 3 4 Koyabu, Daisuke; Maier, Wolfgang; Sánchez-Villagra, Marcelo R. (2012-08-28). "Paleontological and developmental evidence resolve the homology and dual embryonic origin of a mammalian skull bone, the interparietal" (PDF). Proceedings of the National Academy of Sciences. 109 (35): 14075–14080. Bibcode:2012PNAS..10914075K. doi: 10.1073/pnas.1208693109 . ISSN   0027-8424. PMC   3435230 . PMID   22891324.
  3. ↑ Parrington, F. R. (1967-10-01). "The identification of the dermal bones of the head". Zoological Journal of the Linnean Society. 47 (311): 231–239. doi:10.1111/j.1096-3642.1967.tb01406.x. ISSN   0024-4082.
  4. ↑ Ahlberg, Per Erik (1991-11-01). "A re-examination of sarcopterygian interrelationships, with special reference to the Porolepiformes". Zoological Journal of the Linnean Society. 103 (3): 241–287. doi:10.1111/j.1096-3642.1991.tb00905.x. ISSN   0024-4082.
  5. ↑ Klembara, Jozef (1995-01-01). "Some cases of fused and concrescentexocranial bones in the Lower Permian seymouriamorph Tetrapod Discosauriscus Kuhn; 1933". Geobios. Premiers Vertandébrandés et Vertandébrandés Infandérieurs. 28: 263–267. doi:10.1016/S0016-6995(95)80124-3. ISSN   0016-6995.
  6. ↑ Ezcurra, Martín D. (2016-04-28). "The phylogenetic relationships of basal archosauromorphs, with an emphasis on the systematics of proterosuchian archosauriforms". PeerJ. 4: e1778. doi: 10.7717/peerj.1778 . ISSN   2167-8359. PMC   4860341 . PMID   27162705.
  7. 1 2 Fabbri, Matteo; Mongiardino Koch, Nicolás; Pritchard, Adam C.; Hanson, Michael; Hoffman, Eva; Bever, Gabriel S.; Balanoff, Amy M.; Morris, Zachary S.; Field, Daniel J.; Camacho, Jasmin; Rowe, Timothy B. (11 September 2017). "The skull roof tracks the brain during the evolution and development of reptiles including birds". Nature Ecology & Evolution. 1 (10): 1543–1550. doi:10.1038/s41559-017-0288-2. ISSN   2397-334X. PMID   29185519. S2CID   3326766.
  8. ↑ Klembara, Jozef (2001). "Postparietal and prehatching ontogeny of the supraoccipital in Alligator mississippiensis (Archosauria, Crocodylia)". Journal of Morphology. 249 (2): 147–153. doi:10.1002/jmor.1046. ISSN   1097-4687. PMID   11466742. S2CID   22370661.
  9. ↑ Maddin, Hillary C.; Piekarski, Nadine; Sefton, Elizabeth M.; Hanken, James (2016). "Homology of the cranial vault in birds: new insights based on embryonic fate-mapping and character analysis". Royal Society Open Science. 3 (8): 160356. Bibcode:2016RSOS....360356M. doi:10.1098/rsos.160356. PMC   5108967 . PMID   27853617.
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