The skull roof, or the roofing bones 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, t ...

, are a set of

bone A bone is a rigid organ that constitutes part of the skeleton in most vertebrate animals. Bones protect the various other organs of the body, produce red and white blood cells, store minerals, provide structure and support for the body, an ...

s covering the

brain The brain is an organ that serves as the center of the nervous system in all vertebrate and most invertebrate animals. It consists of nervous tissue and is typically located in the head ( cephalization), usually near organs for special ...

eye Eyes are organs of the visual system. They provide living organisms with vision, the ability to receive and process visual detail, as well as enabling several photo response functions that are independent of vision. Eyes detect light and conv ...

s and

nostril A nostril (or naris , plural ''nares'' ) is either of the two orifices of the nose. They enable the entry and exit of air and other gasses through the nasal cavities. In birds and mammals, they contain branched bones or cartilages called t ...

s in

bony fishes Osteichthyes (), popularly referred to as the bony fish, is a diverse superclass of fish that have skeletons primarily composed of bone tissue. They can be contrasted with the Chondrichthyes, which have skeletons primarily composed of cartila ...

and all land-living vertebrates. The bones are derived from

dermal bone 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 ( l ...

and are part of the dermatocranium. In

comparative anatomy Comparative anatomy is the study of similarities and differences in the anatomy of different species. It is closely related to evolutionary biology and phylogeny (the evolution of species). The science began in the classical era, continuing in ...

the term is used on the full dermatocranium. Romer, A.S. & T.S. Parsons. 1977. ''The Vertebrate Body.'' 5th ed. Saunders, Philadelphia. (6th ed. 1985) In general anatomy, the roofing bones may refer specifically to the bones that form above and alongside the brain and neurocranium (i.e., excluding the marginal upper jaw bones such as the maxilla and premaxilla), and in

human anatomy The human body is the structure of a human being. It is composed of many different types of cells that together create tissues and subsequently organ systems. They ensure homeostasis and the viability of the human body. It comprises a head ...

, the skull roof often refers specifically to the skullcap.

Origin

Early

armoured fish Placodermi (from Greek πλάξ 'plate' and δέρμα 'skin', literally 'plate-skinned') is a class of armoured prehistoric fish, known from fossils, which lived from the Silurian to the end of the Devonian period. Their head and thorax were cov ...

did not have a skull in the common understanding of the word, but had an

endocranium The endocranium in comparative anatomy is a part of the skull base in vertebrates and it represents the basal, inner part of the cranium. The term is also applied to the outer layer of the dura mater in human anatomy. Structure Structurally, t ...

that was partially open above, topped by

s forming

armour Armour (British English British English (BrE, en-GB, or BE) is, according to Lexico, Oxford Dictionaries, "English language, English as used in Great Britain, as distinct from that used elsewhere". More narrowly, it can refer specificall ...

. The dermal bones gradually

evolved Evolution is change in the heritable characteristics of biological populations over successive generations. These characteristics are the expressions of genes, which are passed on from parent to offspring during reproduction. Variation te ...

into a fixed unit overlaying the endocranium like a heavy "lid", protecting the animal's head and brain from above.

Cartilaginous fish Chondrichthyes (; ) is a class that contains the cartilaginous fishes that have skeletons primarily composed of cartilage. They can be contrasted with the Osteichthyes or ''bony fishes'', which have skeletons primarily composed of bone tissue. ...

whose skeleton is formed from cartilage lack a continuous dermal armour and thus have no proper skull roof. A more or less full shield of fused

s was common in early

of the

Devonian The Devonian ( ) is a geologic period and system of the Paleozoic era, spanning 60.3 million years from the end of the Silurian, million years ago (Mya), to the beginning of the Carboniferous, Mya. It is named after Devon, England, w ...

, and particularly well developed in shallow water species.

Bony fishes and amphibians

In early

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 ...

s the skull roof was composed of numerous bony plates, particularly around the nostrils and behind each eye. The skull proper was joined by the bones of the operculum. The skull itself was composed rather loosely, with a joint between the bones covering the brain (the

parietal bone The parietal bones () are two bones in the skull which, when joined at a fibrous joint, form the sides and roof of the cranium. In humans, each bone is roughly quadrilateral in form, and has two surfaces, four borders, and four angles. It is named ...

s and the bones behind them) and the snout (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, pa ...

nasal bone The nasal bones are two small oblong bones, varying in size and form in different individuals; they are placed side by side at the middle and upper part of the face and by their junction, form the bridge of the upper one third of the nose. Ea ...

and the bones in front and to the side of them). This joint disappeared in the evolving labyrinthodonts, at the same time the number of bones were reduced and the operculum disappeared. In

frogs A frog is any member of a diverse and largely carnivorous group of short-bodied, tailless amphibians composing the order Anura (ανοὐρά, literally ''without tail'' in Ancient Greek). The oldest fossil "proto-frog" ''Triadobatrachus'' is ...

and

salamander Salamanders are a group of amphibians typically characterized by their lizard-like appearance, with slender bodies, blunt snouts, short limbs projecting at right angles to the body, and the presence of a tail in both larvae and adults. All ten ...

s the skull roof is further reduced and has large openings. Only in

caecilian Caecilians (; ) are a group of limbless, vermiform or serpentine amphibians. They mostly live hidden in the ground and in stream substrates, making them the least familiar order of amphibians. Caecilians are mostly distributed in the tropics ...

s can a full covering skull roof be found, an adaption for burrowing. The skull roof in

lungfish Lungfish are freshwater vertebrates belonging to the order Dipnoi. Lungfish are best known for retaining ancestral characteristics within the Osteichthyes, including the ability to breathe air, and ancestral structures within Sarcopterygii, i ...

is composed of a number of bony plates that are not readily compared to those found in early amphibians. Carroll, R.L. (1988): Vertebrate Paleontology and Evolution,'' W.H. Freeman & Co'', New York NY, p.148 In most ray-finned fishes the skull is often reduced to a series of loose elements, and a skull roof as such is not found.

Labyrinthodonts and early reptiles

The pattern of plates of the labyrinthodonts formed that basis for that seen in all land-living vertebrates. The roof itself formed a continuous cover over the whole of the head, leaving only openings for nostrils, eyes and a

parietal eye A parietal eye, also known as a third eye or pineal eye, is a part of the epithalamus present in some vertebrates. The eye is located at the top of the head, is photoreceptive and is associated with the pineal gland, regulating circadian rhyth ...

between the

s. This type of skull was inherited by the first reptiles as they evolved from labyrinthodont stock in the Carboniferous. This type of skull roof without any above openings behind the eyes is called '' anapsid''. Today anapsid skulls are only found in

turtle Turtles are an order of reptiles known as Testudines, characterized by a special shell developed mainly from their ribs. Modern turtles are divided into two major groups, the Pleurodira (side necked turtles) and Cryptodira (hidden necked ...

s, though this may be a case of secondary loss of the post orbital openings.deBraga, M. and Rieppel, O. (1997). "Reptile phylogeny and the interrelationships of turtles." ''Zoological Journal of the Linnean Society'', 120: 281-354.

Diapsids and synapsids

In two groups of early reptiles the skull roof evolved post orbital openings to allow for greater movement of the jaw muscles. The two groups evolved the openings independently: * The

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 ...

s having one opening on each side, fairly low on the side of the skull, between 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 ...

and the elements above. * The

Diapsid 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 ...

s having two openings on each side, the two openings separated by an arch formed from processes of the

postorbital The ''postorbital'' is one of the bones in vertebrate skulls which forms a portion of the dermal skull roof and, sometimes, a ring about the orbit. Generally, it is located behind the postfrontal and posteriorly to the orbital fenestra. In some v ...

and

squamosal The squamosal is a skull bone found in most reptiles, amphibians, and birds. In fishes, it is also called the pterotic bone. In most tetrapods, the squamosal and quadratojugal bones form the cheek series of the skull. The bone forms an ancestral co ...

bones. The synapsids are the

mammal-like reptiles Pelycosaur ( ) is an older term for basal or primitive Late Paleozoic synapsids, excluding the therapsids and their descendants. Previously, the term ''mammal-like reptile'' had been used, and pelycosaur was considered an order, but this is now ...

and the mammals. In mammals, the side opening is closed by the sphenoid bone, so that the skull roof appear whole, despite the temporal opening. All other reptiles (with the possible exception of turtles) and the

bird Birds are a group of warm-blooded vertebrates constituting the class Aves (), characterised by feathers, toothless beaked jaws, the laying of hard-shelled eggs, a high metabolic rate, a four-chambered heart, and a strong yet lightweig ...

s are diapsids.

References

{{Reflist Vertebrate anatomy Human head and neck Flat bones