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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 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 Table may refer to: * Table (furniture), a piece of furniture with a flat surface and one or more legs * Table (landform), a flat area of land * Table (information), a data arrangement with rows and columns * Table (database), how the table data ...
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 human anatomy, the neurocranium, also known as the braincase, brainpan, or brain-pan is the upper and back part of the skull, which forms a protective case around the brain. In the human skull, the neurocranium includes the calvaria or skul ...
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 distingu ...
. At several points in synapsid evolution, the postparietals fused to each other and the tabulars during embryological development. This fusion produces the
interparietal bone An interparietal bone (os interparietale or Inca bone or ''os inca var.'') is a dermal bone situated between the parietal and supraoccipital. It is homologous to the postparietal bones of other animals. In humans, it corresponds to the upper p ...
, which is inherited by mammals. 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, arbore ...
and early
reptiles Reptiles, as most commonly defined are the animals in the Class (biology), class Reptilia ( ), a paraphyletic grouping comprising all sauropsid, sauropsids except birds. Living reptiles comprise turtles, crocodilians, Squamata, squamates (lizar ...
. 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 relatives), ...
) and living reptiles (of the group Sauria) 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 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 ''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 on ...
'', '' Elpistostege'', and ''
Edops ''Edops'' ('swollen face') is an extinct genus of temnospondyl amphibian from the Early Permian Period. Unlike more advanced temnospondyls of the time, such as ''Eryops'', ''Edops'' exhibited an archaic pattern of palatal bones, and still possess ...
'' 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 The lateral line, also called the lateral line organ (LLO), is a system of sensory organs found in fish, used to detect movement, vibration, and pressure gradients in the surrounding water. The sensory ability is achieved via modified epithelial ...
, 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 Diadectomorpha is a clade of large tetrapods that lived in Euramerica during the Carboniferous and Early Permian periods and in Asia during Late Permian (Wuchiapingian), They have typically been classified as advanced reptiliomorphs (transitional ...
, the postparietals fuse to each other. The
seymouriamorph Seymouriamorpha were a small but widespread group of limbed vertebrates (tetrapods). They have long been considered reptiliomorphs, and most paleontologists may still accept this point of view, but some analyses suggest that seymouriamorphs are s ...
''
Discosauriscus ''Discosauriscus'' was a small seymouriamorph which lived in what is now Central and Western Europe in the Early Permian Period. Its best fossils have been found in the Broumov and Bačov Formations of Boskovice Furrow, in the Czech Republic. ...
'' 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 distingu ...
, 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 Sauropsida, sauropsids, the group that inc ...
inherited postparietals (sometimes paired) from their non-amniote ancestors. Embryological data indicates that the
interparietal bone An interparietal bone (os interparietale or Inca bone or ''os inca var.'') is a dermal bone situated between the parietal and supraoccipital. It is homologous to the postparietal bones of other animals. In humans, it corresponds to the upper p ...
of mammals forms from the fusion of four bones during early development: a pair of medial neural crest 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 E ...
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 (biology), class Reptilia ( ), a paraphyletic grouping comprising all sauropsid, sauropsids except birds. Living reptiles comprise turtles, crocodilians, Squamata, squamates (lizar ...
, 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 Eureptilia ("true reptiles") is one of the two major subgroups of the clade Sauropsida, the other one being Parareptilia. Eureptilia includes Diapsida (the clade containing all modern reptiles and birds), as well as a number of primitive Permo-C ...
, including early
diapsids Diapsids ("two arches") are a clade of sauropsids, distinguished from more primitive eureptiles by the presence of two holes, known as temporal fenestrae, in each side of their skulls. The group first appeared about three hundred million years ago ...
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 Archosauriformes (Greek for 'ruling lizards', and Latin for 'form') is a clade of diapsid reptiles that developed from archosauromorph ancestors some time in the Latest Permian (roughly 252 million years ago). It was defined by Jacques Gauthi ...
(and their sister taxon ''
Tasmaniosaurus ''Tasmaniosaurus'' ('lizard from Tasmania', although this genus is not a true lizard) is an extinct genus of archosauromorph reptile known from the Knocklofty Formation (Early Triassic) of West Hobart, Tasmania, Australia. The type species is ...
'') 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 ''Vancleavea'' is a genus of extinct, armoured, non- archosaurian archosauriforms from the Late Triassic of western North America. The type and only known species is ''V. campi'', named by Robert Long & Phillip A Murry in 1995. At that time, ...
'', ''
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. 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