The rise of our cranium came in steps. First, a chondrocranium formed in cartilaginous fishes like lampreys and hagfishes. A chondrocranium is essentially a brain pan; a sheet of cartilage on which the brain and associated cranial nerves rests and branches out throughout the body. As can be expected, there was not much protection of the vital organs in these being and that didn’t come until the formation of a dermatocranium. A dermatocranium was the first bony skull and is also referred to as a neurocranium. Early dermatocrania consisted of just six different bones known as the parietal, post-orbital, squamosal, quadrate, jugal, and quadratojugal. The fusion of these bones articulated with jaws, vertebrate, and other bony structures to protect the vital aspects of the central nervous system like the brain and the spinal cord. A third crania arose in fishes and it is referred to as a splanchnocranium. This is the bony (or cartilaginous depending on fish) structure that supports the gills and other thoracic structures. The splanchnocranium evolved into our axial skeleton over time and is now only prevalent in fishes and marine mammals.
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