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Postparietals are cranial bones present in fish and many
tetrapods Tetrapods (; ) are four-limb (anatomy), limbed vertebrate animals constituting the superclass Tetrapoda (). It includes extant taxon, extant and extinct amphibians, sauropsids (reptiles, including dinosaurs and therefore birds) and synapsids (p ...
. Although initially a pair of bones, many lineages possess postparietals which were fused into a single bone. The postparietals were
dermal bones A dermal bone or investing bone or membrane bone is a bony structure derived from intramembranous ossification forming components of the vertebrate skeleton including much of the skull, jaws, gill covers, shoulder girdle and fin spines rays ( lepi ...
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 Amniotes are a clade of tetrapod vertebrates that comprises sauropsids (including all reptiles and birds, and extinct parareptiles and non-avian dinosaurs) and synapsids (including pelycosaurs and therapsids such as mammals). They are dis ...
. At several points in
synapsid Synapsids + (, 'arch') > () "having a fused arch"; synonymous with ''theropsids'' (Greek, "beast-face") are one of the two major groups of animals that evolved from basal amniotes, the other being the sauropsids, the group that includes rep ...
evolution, the postparietals fused to each other and the tabulars during
embryological Embryology (from Greek ἔμβρυον, ''embryon'', "the unborn, embryo"; and -λογία, ''-logia'') is the branch of animal biology that studies the prenatal development of gametes (sex cells), fertilization, and development of embryos and ...
development. This fusion produces the interparietal bone, which is inherited by
mammals Mammals () are a group of vertebrate animals constituting the class Mammalia (), characterized by the presence of mammary glands which in females produce milk for feeding (nursing) their young, a neocortex (a region of the brain), fu ...
. Postparietals are common in extinct
amphibians Amphibians are four-limbed and ectothermic vertebrates of the class Amphibia. All living amphibians belong to the group Lissamphibia. They inhabit a wide variety of habitats, with most species living within terrestrial, fossorial, arb ...
and early
reptiles Reptiles, as most commonly defined are the animals in the class Reptilia ( ), a paraphyletic grouping comprising all sauropsids except birds. Living reptiles comprise turtles, crocodilians, squamates (lizards and snakes) and rhynchocephali ...
. However, most living amphibians (of the group
Lissamphibia The Lissamphibia is a group of tetrapods that includes all modern amphibians. Lissamphibians consist of three living groups: the Salientia (frogs, toads, and their extinct relatives), the Caudata (salamanders, newts, and their extinct relative ...
) and living reptiles (of the group
Sauria Sauria is the clade containing the most recent common ancestor of archosaurs (such as crocodilians, dinosaurs, etc.) and lepidosaurs (lizards and kin), and all its descendants. Since most molecular phylogenies recover turtles as more closely r ...
) lack postparietal bones, with a few exceptions.


Evolution


Fish and amphibians

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 Tetrapods (; ) are four-limb (anatomy), limbed vertebrate animals constituting the superclass Tetrapoda (). It includes extant taxon, extant and extinct amphibians, sauropsids (reptiles, including dinosaurs and therefore birds) and synapsids (p ...
and their
sarcopterygian Sarcopterygii (; ) — sometimes considered synonymous with Crossopterygii () — is a taxon (traditionally a class or subclass) of the bony fishes known as the lobe-finned fishes. The group Tetrapoda, a mostly terrestrial superclass includ ...
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 ''Ichthyostega'' (from el, ἰχθῦς , 'fish' and el, στέγη , 'roof') is an extinct genus of limbed tetrapodomorphs from the Late Devonian of Greenland. It was among the earliest four-limbed vertebrates in the fossil record, and was o ...
'', '' 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. 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). 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. Closer to the base of
amniotes Amniotes are a clade of tetrapod vertebrates that comprises sauropsids (including all reptiles and birds, and extinct parareptiles and non-avian dinosaurs) and synapsids (including pelycosaurs and therapsids such as mammals). They are dis ...
, the postparietal shifts from the dorsal portion of the skull to the occipital (braincase) portion, sloping downwards in the process.


Synapsids

Early
synapsids Synapsids + (, 'arch') > () "having a fused arch"; synonymous with ''theropsids'' (Greek, "beast-face") are one of the two major groups of animals that evolved from basal amniotes, the other being the sauropsids, the group that includes reptil ...
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 Neural crest cells are a temporary group of cells unique to vertebrates that arise from the embryonic ectoderm germ layer, and in turn give rise to a diverse cell lineage—including melanocytes, craniofacial cartilage and bone, smooth muscle, ...
elements edged by lateral
mesoderm The mesoderm is the middle layer of the three germ layers that develops during gastrulation in the very early development of the embryo of most animals. The outer layer is the ectoderm, and the inner layer is the endoderm.Langman's Medical Emb ...
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.


Reptiles

The postparietals continue to shrink and move further back in the skull in
reptiles Reptiles, as most commonly defined are the animals in the class Reptilia ( ), a paraphyletic grouping comprising all sauropsids except birds. Living reptiles comprise turtles, crocodilians, squamates (lizards and snakes) and rhynchocephali ...
, no longer forming any contribution to the ceiling of the brain cavity. Small paired or fused postparietals are common in Permian
parareptiles Parareptilia ("at the side of reptiles") is a subclass or clade of basal sauropsids (reptiles), typically considered the sister taxon to Eureptilia (the group that likely contains all living reptiles and birds). Parareptiles first arose near t ...
and eureptiles, including early diapsids such as ''
Petrolacosaurus ''Petrolacosaurus'' ("rock lake lizard") is an extinct genus of diapsid reptile from the late Carboniferous period. It was a small, long reptile, and the earliest known reptile with two temporal fenestrae (holes at the rear part of the skull) ...
'' and '' Youngina''. Postparietals were subsequently lost at the base of
Sauria Sauria is the clade containing the most recent common ancestor of archosaurs (such as crocodilians, dinosaurs, etc.) and lepidosaurs (lizards and kin), and all its descendants. Since most molecular phylogenies recover turtles as more closely r ...
, 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 ''Litorosuchus'' is a genus of armored, semiaquatic archosauriform reptile from the Middle Triassic of China, closely related to the morphologically similar '' Vancleavea''. It contains one species, ''L. somnii''. Description For an archosau ...
'', most or all phytosaurs, and
archosaurs Archosauria () is a clade of diapsids, with birds and crocodilians as the only living representatives. Archosaurs are broadly classified as reptiles, in the cladistic sense of the term which includes birds. Extinct archosaurs include non-avia ...
. The only true archosaur generally considered to possess an interparietal as an adult is a single specimen of '' Gracilisuchus''. Paired postparietals have also been observed in ''Alligator mississippiensis'' embryos, although they are incorporated into the supraoccipital by the time of hatching. One proposal used developmental data to argue that the "parietal" of birds was actually derived from postparietals. 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.


References

{{Tetrapod osteology Vertebrate anatomy