The skull roof or the roofing bones of the skull are a set of bones covering the brain, eyes and nostrils in bony fishes, including land-living vertebrates. The bones are derived from dermal bone and are part of the dermatocranium. In comparative anatomy, the term is applied to the whole dermatocranium. 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). In human anatomy, the skull roof often refers specifically to the skullcap.
Origin
Early armoured fish (such as jawless ostracoderms and jawed placoderms) did not have a skull in the common understanding of the word, but instead had a cartilaginous endocranium that was partially open from above. The loose cartilage was topped by dermal bones forming armour. The dermal bones gradually evolved into a fixed unit overlaying the endocranium like a heavy "lid", protecting the animal's head and brain from above. A more or less full shield of fused dermal bones was common in early bony fishes of the Devonian, and particularly well developed in shallow water species. Cartilaginous fish, such as sharks, have a skeleton which is entirely formed from cartilage. They lack a continuous dermal armour and thus have no proper skull roof.
Bony fishes
In early sarcopterygians ("lobe-finned fish"), 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 and the snout. The skull roof in lungfish is composed of a number of bony plates that are not readily compared to those found in early amphibians. 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.
Early tetrapods
As tetrapodomorph fish trended towards greater adaptations for life on land, the skull became more tightly integrated. At the same time, the number of bones were reduced, the skull bones separated from the shoulder girdle, and the operculum disappeared. The earliest limbed tetrapods ("amphibians" in the broad sense) solidified a pattern of plates which formed the basis for that seen in all land-living vertebrates. These early tetrapods (including temnospondyls, embolomeres, and various minor groups) have historically been termed "labyrinthodonts" ("maze teeth") or "stegocephalians" ("roof heads"). Not including marginal or cheek bones (such as the premaxillae, maxillae, jugals, quadratojugals, and squamosals), the skull roof bones established by these early tetrapods include the following:
Nasals (midline) Lacrimals Prefrontals Frontals (midline) Postfrontals Postorbitals Parietals (midline) Intertemporals Supratemporals Postparietals (midline) Tabulars The skull roof itself formed a continuous cover over the whole of the head, leaving only openings for nostrils (nares), eyes (orbits), and a small parietal eye (also known as a pineal foramen) between the parietal bones. This type of skull was inherited by the first amniotes (fully terrestrial tetrapods), which evolved in the Carboniferous. This type of skull roof without any above openings behind the eyes is called anapsid. Today, the only reptiles with anapsid skulls are turtles, though this is likely a case of secondary loss of the temporal fenestrae. In modern amphibians such as frogs and salamanders, the skull roof is further reduced and has large openings. Only in caecilians can a full covering skull roof be found, an adaption for burrowing.
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