4. Skull Evolution
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4.3. Tetrapods
4.3. Tetrapods
The skulls of the earliest tetrapods closely resembled those of their ancestors amongst the lobe-finned fishes. The skull roof is formed of a series of plate-like bones, including the maxilla, frontals, parietals, and lacrimals, among others. It is overlaying the endocranium, corresponding to the cartilaginous skull in sharks and rays. The various separate bones that compose the temporal bone of humans are also part of the skull roof series. A further plate composed of four pairs of bones forms the roof of the mouth; these include the vomer and palatine bones. The base of the cranium is formed from a ring of bones surrounding the foramen magnum and a median bone lying further forward; these are homologous with the occipital bone and parts of the sphenoid in mammals. Finally, the lower jaw is composed of multiple bones, only the most anterior of which (the dentary) is homologous with the mammalian mandible. In living tetrapods, a great many of the original bones have either disappeared or fused into one another in various arrangements.
The structure of the tetrapod skull
The structure of the tetrapod skull
The skull can be divided into neurocranium (braincase, calvaria) which is formed by the bones surrounding the brain, and the viscerocranium (fascial skeleton) which is formed by the bones of the face.
Figure 1. The eight bones that form the human neurocranium (colored). The viscerocranium is shown in white. More details.
The neurocranium is further divided into dermatocranium (skull roof), which is the squamous shell formed through intramembranous ossification, and the endocranium (chondrocranium), which is formed from a cartilage template, by endochondral ossification. The dermatocranium is a set of bones covering the brain, eyes and nostrils in bony fishes and all land-living vertebrates.
Figure 2. The full complement of bones of the tetrapod skull roof, as seen in the labyrinthodont Xenotosuchus. More details.
The early armoured fish did not have a skull in the common understanding of the word, but had an endocranium that was partially open above, 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. Cartilaginous fish 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 dermal bones was common in early bony fishes of the Devonian, and particularly well developed in shallow water species.
The endocranium has a boxlike shape, open at the top. The posterior margin exhibit the foramen magnum, an opening for the spinal cord. The floor of the endocranium has several paired openings for the cranial nerves, and the anterior margin holds a spongy construction, allowing for the external nasal nerves to pass through. All bones of the structure derive from the cranial neural crest during fetal development and form by endochondral ossification.
In humans and other mammals, the endocranium forms during fetal development as a cartilaginous neurocranium, that ossifies from several centers. Several of these bones merge, and in the adult primates (including humans), the endocranium is composed of only five bony elements (from front to back):
The Ethmoid bone, lying behind the nose.
The Sphenoid bone, underlying the forward portion of the brain
Paired petrous part of the temporal bones, containing the inner ear structures
The Occipital bone, surrounding the foramen magnum
Notice that several of these bones also have a squamous region that is formed by dermal bone. Therefore only a portion of them is really part of the endocranium.
Figure 3. The ossified endocranium of a human skull. Endocranial elements colored in pink. More details.
Summary
Summary
As vertebrates colonized the ground, they evolved stronger and simpler skulls. These have basically two parts: the neurocranium protecting the brain, and the splanchnocranium forming the face. The neurocranium is further divided into dermatocranium and endocranium based on the developmental history of its bones. Several of the skull bones fuse during development, resulting in fewer bones in adults.
Key terms
Key terms
Neurocranium, endocranium, dematocranium, chondrocranium, splanchnocranium
Figure credits
Figure credits
Figure 1 by Original svg image is by Edoarado, re-allocated text by Was a bee. - Own work based on: File:Cranial bones en.svg., CC0, https://commons.wikimedia.org/w/index.php?curid=37598752
Figure 2 by Smokeybjb - Own work, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=15741396
Figure 3 by Petter Bøckman, based on photo File:Schädelbasis1.jpg by Welleschik - Own work, Public Domain, https://commons.wikimedia.org/w/index.php?curid=8758860
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