The postorbital bar (or postorbital bone) is a bony arched structure that connects the

frontal bone The frontal bone is a bone in the human skull. The bone consists of two portions.''Gray's Anatomy'' (1918) These are the vertically oriented squamous part, and the horizontally oriented orbital part, making up the bony part of the forehead, par ...

of the

skull The skull is a bone protective cavity for the brain. The skull is composed of four types of bone i.e., cranial bones, facial bones, ear ossicles and hyoid bone. However two parts are more prominent: the cranium and the mandible. In humans, the ...

to the

zygomatic arch In anatomy, the zygomatic arch, or cheek bone, is a part of the skull formed by the zygomatic process of the temporal bone (a bone extending forward from the side of the skull, over the opening of the ear) and the temporal process of the zygomati ...

, which runs laterally around the

eye socket In anatomy, the orbit is the cavity or socket of the skull in which the eye and its appendages are situated. "Orbit" can refer to the bony socket, or it can also be used to imply the contents. In the adult human, the volume of the orbit is , of ...

. It is a trait that only occurs in

mammal 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), fur or ...

ian taxa, such as most

strepsirrhine Strepsirrhini or Strepsirhini (; ) is a suborder of primates that includes the lemuriform primates, which consist of the lemurs of Madagascar, galagos ("bushbabies") and pottos from Africa, and the lorises from India and southeast Asia. Collec ...

primate Primates are a diverse order of mammals. They are divided into the strepsirrhines, which include the lemurs, galagos, and lorisids, and the haplorhines, which include the tarsiers and the simians (monkeys and apes, the latter including huma ...

s and the

hyrax Hyraxes (), also called dassies, are small, thickset, herbivorous mammals in the order Hyracoidea. Hyraxes are well-furred, rotund animals with short tails. Typically, they measure between long and weigh between . They are superficially simil ...

, while

haplorhine Haplorhini (), the haplorhines (Greek for "simple-nosed") or the "dry-nosed" primates, is a suborder of primates containing the tarsiers and the simians (Simiiformes or anthropoids), as sister of the Strepsirrhini ("moist-nosed"). The name is so ...

primates have evolved fully enclosed sockets. One theory for this evolutionary difference is the relative importance of vision to both orders. As haplorrhines (

tarsier Tarsiers ( ) are haplorhine primates of the family Tarsiidae, which is itself the lone extant family within the infraorder Tarsiiformes. Although the group was once more widespread, all of its species living today are found in Maritime Southeast ...

s and

simian The simians, anthropoids, or higher primates are an infraorder (Simiiformes ) of primates containing all animals traditionally called monkeys and apes. More precisely, they consist of the parvorders New World monkeys (Platyrrhini) and Catarrhi ...

s) tend to be diurnal, and rely heavily on visual input, many strepsirrhines are

nocturnal Nocturnality is an animal behavior characterized by being active during the night and sleeping during the day. The common adjective is "nocturnal", versus diurnal meaning the opposite. Nocturnal creatures generally have highly developed sens ...

and have a decreased reliance on visual input. Postorbital bars evolved several times independently during mammalian evolution and the evolutionary histories of several other clades. Some species, such as

Tarsier Tarsiers ( ) are haplorhine primates of the family Tarsiidae, which is itself the lone extant family within the infraorder Tarsiiformes. Although the group was once more widespread, all of its species living today are found in Maritime Southeast ...

s, have a postorbital septum. This septum can be considered as joined processes with a small articulation between the frontal bone, the

zygomatic bone In the human skull, the zygomatic bone (from grc, ζῠγόν, zugón, yoke), also called cheekbone or malar bone, is a paired irregular bone which articulates with the maxilla, the temporal bone, the sphenoid bone and the frontal bone. It is si ...

and the

alisphenoid bone The greater wing of the sphenoid bone, or alisphenoid, is a bony process of the sphenoid bone; there is one on each side, extending from the side of the body of the sphenoid and curving upward, laterally, and backward. Structure The greater wi ...

and is therefore different from the postorbital bar, while it forms a composite structure together with the postorbital bar. Other species such as dermopterans have postorbital processes, which is a more primitive incomplete stage of the postorbital bar.

Function

In the past decades, many different hypothesis were made on the possible function of the postorbital bar. Three of them are commonly cited.

External trauma hypothesis

Prince and Simons offered the external trauma hypothesis, where the postorbital bar protects the orbital contents from external trauma. However, a few years later Cartmill showed otherwise. He was convinced that the postorbital bar was not adequate enough to offer protection against sharp objects such as the teeth of other species. He was therefore convinced that the postorbital bar must have a different function.

Mastication hypothesis

Greaves offered a new view on this bone and came up with the

mastication Chewing or mastication is the process by which food is crushed and ground by teeth. It is the first step of digestion, and it increases the surface area of foods to allow a more efficient break down by enzymes. During the mastication process, th ...

hypothesis. Greaves suggests that the bar strengthens the relatively weak orbital area against torsional loading, imposed by bite force in species with large

masseter In human anatomy, the masseter is one of the muscles of mastication. Found only in mammals, it is particularly powerful in herbivores to facilitate chewing of plant matter. The most obvious muscle of mastication is the masseter muscle, since it ...

and

temporalis In anatomy, the temporalis muscle, also known as the temporal muscle, is one of the muscles of mastication (chewing). It is a broad, fan-shaped convergent muscle on each side of the head that fills the temporal fossa, superior to the zygomatic a ...

muscles. However the orientation of the

postorbital process The postorbital process is a projection on the frontal bone near the rear upper edge of the eye socket. In many mammals, it reaches down to the zygomatic arch, forming the postorbital bar. References See also * Orbital process In the human ...

does not match the direction of the forces mentioned by Greaves.

Position hypothesis

Cartmill suggests that in small mammals with large eyes and relatively small temporal fossae, where the

anterior temporal muscle Standard anatomical terms of location are used to unambiguously describe the anatomy of animals, including humans. The terms, typically derived from Latin or Greek roots, describe something in its standard anatomical position. This position pro ...

and the

temporalis fascia The temporal fascia covers the temporalis muscle. It is a strong, fibrous investment, covered, laterally, by the auricularis anterior and superior, by the galea aponeurotica, and by part of the orbicularis oculi. The superficial temporal vessels ...

are pulled to a more lateral position with increasing orbital convergence (front-facing eyes), the tension caused by the contraction of these muscles would distort the orbital margins and disrupt

oculomotor The oculomotor nerve, also known as the third cranial nerve, cranial nerve III, or simply CN III, is a cranial nerve that enters the orbit (anatomy), orbit through the superior orbital fissure and innervates extraocular muscles that enable mo ...

precision. Heesy shows that the postorbital bar stiffens the lateral orbit. Without a stiffened lateral orbit, deformation would displace soft tissues, when contraction of the anterior temporalis muscle takes place, thus impeding eye movement.

Occurrence

A complete postorbital bar has evolved at least eleven times as a convergent adaptation in nine mammalian orders. Postorbital bars are characteristic to the following clades: * Order

Primates Primates are a diverse order of mammals. They are divided into the strepsirrhines, which include the lemurs, galagos, and lorisids, and the haplorhines, which include the tarsiers and the simians (monkeys and apes, the latter including huma ...

* Order Scandentia * Order Perissodactyla ** Subfamily

Equinae Equinae is a subfamily of the family Equidae, which have lived worldwide (except Indonesia and Australia) from the Hemingfordian stage of the Early Miocene (16 million years ago) onwards. They are thought to be a monophyletic grouping.B. J. Ma ...

* Order

Artiodactyla The even-toed ungulates (Artiodactyla , ) are ungulates—hoofed animals—which bear weight equally on two (an even number) of their five toes: the third and fourth. The other three toes are either present, absent, vestigial, or pointing poster ...

** Suborder Neoselenodontia Postorbital bars have furthermore developed individually in the following taxa: * Clade

Metatheria Metatheria is a mammalian clade that includes all mammals more closely related to marsupials than to placentals. First proposed by Thomas Henry Huxley in 1880, it is a more inclusive group than the marsupials; it contains all marsupials as well ...

** Order †

Sparassodonta Sparassodonta (from Ancient Greek, Greek to tear, rend; and , gen.

, ' The comma is a punctuation mark that appears in several variants in different languages. It has the same shape as an apostrophe or single closing quotation mark () in many typefaces, but it differs from them in being placed on the baseline ...

tooth) is an extinct order (biology), order of carnivore, carnivorous metatherian mammals native to South America, related to modern marsupials. They were once con ...

*** †''

Thylacosmilus ''Thylacosmilus'' is an extinct genus of Saber-toothed predator, saber-toothed metatherian mammals that inhabited South America from the Miocene, Late Miocene to Pliocene Epoch (geology), epochs. Though ''Thylacosmilus'' looks similar to the "Mac ...

'' ** Order

Diprotodontia Diprotodontia (, from Greek language, Greek "two forward teeth") is the largest extant order (biology), order of marsupials, with about 155 species, including the kangaroos, Wallaby, wallabies, Phalangeriformes, possums, koala, wombats, and many ...

*** †''

Thylacoleo ''Thylacoleo'' ("pouch lion") is an extinct genus of carnivorous marsupials that lived in Australia from the late Pliocene to the late Pleistocene (2 million to 46 thousand years ago). Some of these marsupial lions were the largest mammalian pred ...

'' * Clade

Eutheria Eutheria (; from Greek , 'good, right' and , 'beast'; ) is the clade consisting of all therian mammals that are more closely related to placentals than to marsupials. Eutherians are distinguished from noneutherians by various phenotypic tra ...

** Order

Carnivora Carnivora is a Clade, monophyletic order of Placentalia, placental mammals consisting of the most recent common ancestor of all felidae, cat-like and canidae, dog-like animals, and all descendants of that ancestor. Members of this group are f ...

*** ''

Barbourofelis ''Barbourofelis'' is an extinct genus of large, predatory, feliform carnivoran mammals of the family Barbourofelidae (false saber-tooth cats). The genus was endemic to North America and Eurasia during the Miocene until its extinction during t ...

'' ** Order †

Litopterna Litopterna (from grc, λῑτή πτέρνα "smooth heel") is an extinct order of fossil hoofed mammals from the Cenozoic era. The order is one of the five great orders of South American ungulates that were endemic to the continent, until the G ...

*** †''

Thoatherium ''Thoatherium'' (meaning "active swift-beast") is an extinct genus of litoptern mammals from the Early Miocene of Argentina. Fossils of the genus have been found in the Santa Cruz Formation in Argentina.Diadiaphorus'' ** Order

Sirenia The Sirenia (), commonly referred to as sea-cows or sirenians, are an order of fully aquatic, herbivorous mammals that inhabit swamps, rivers, estuaries, marine wetlands, and coastal marine waters. The Sirenia currently comprise two distinct f ...

*** ''

Trichechus senegalensis The African manatee (''Trichechus senegalensis''), also known as the West African manatee, is a species of manatee that inhabits much of the western region of Africa – from Senegal to Angola. It is the only manatee species to be found in the ...

'' ** Order

*** ''

Balaenoptera acutorostrata ''Balaenoptera'' () is a genus of rorquals containing eight extant species. ''Balaenoptera'' comprises all but two of the extant species in its family (the humpback whale and gray whale); the genus is currently polyphyletic, with the two afore ...

'' The presence of a postorbital bar in the extinct

Oviraptorosauria Oviraptorosaurs ("egg thief lizards") are a group of feathered maniraptoran dinosaurs from the Cretaceous Period of what are now Asia and North America. They are distinct for their characteristically short, beaked, parrot-like skulls, with or wit ...

species ''

Avimimus portentosus ''Avimimus'' ( ), meaning "bird mimic" (Latin ''avis'' = bird + ''mimus'' = mimic), is a genus of oviraptorosaurian theropod dinosaur, named for its bird-like characteristics, that lived in the late Cretaceous in what is now Mongolia, around 85 t ...

'' was one of several defining characteristics that suggested to

paleontologists Paleontology (), also spelled palaeontology or palæontology, is the scientific study of life that existed prior to, and sometimes including, the start of the Holocene epoch (roughly 11,700 years before present). It includes the study of fossi ...

that the species was more morphologically different from

avian Avian may refer to: *Birds or Aves, winged animals *Avian (given name) (russian: Авиа́н, link=no), a male forename Aviation *Avro Avian, a series of light aircraft made by Avro in the 1920s and 1930s *Avian Limited, a hang glider manufacture ...

species than previously thought, affecting interpretation of the rate of evolution from dinosaurs to birds.

Postorbital process

Postorbital bars are likely derived from well-developed

es, an intermediate condition where a small gap retains between the process and the zygomatic arch. Well-developed postorbital processes have evolved separately within the orders of the Dermoptera and

Hyracoidae Hyraxes (), also called dassies, are small, thickset, herbivorous mammals in the order Hyracoidea. Hyraxes are well-furred, rotund animals with short tails. Typically, they measure between long and weigh between . They are superficially simil ...

and the Chiropteran families of

Emballonuridae Emballonuridae is a family of microbats, many of which are referred to as sac-winged or sheath-tailed bats. They are widely distributed in tropical and subtropical regions around the world. The earliest fossil records are from the Eocene. Descr ...

and Pteropodidae and to varying degrees within many carnivorian taxa. Complete postorbital bars and well-developed postorbital processes, retaining gaps of mere centimetres, spanned by the postorbital ligament, occur as polymorphisms within a number of pteropodid and hyracoid taxa.

References

{{reflist Primate anatomy Skull Mammal anatomy