Basisphenoid
From Wikipedia, the free encyclopedia
The basisphenoid is a bone in the skull of vertebrates. It is an unpaired midline element that forms part of the posterior and ventral margin of the braincase; it is typically located anterior to the basioccipital and posterior to the parasphenoid. It ossifies endochondrally and makes up a part of the chondrocranium.[1] The parasphenoid contains the sella turcica, which houses the pituitary gland.[2]
Evolution
The basisphenoid originates as a part of the chondrocranium[1]; among placoderms, chondrichthyans, and early acanthodians it is present as a region of the cartilaginous braincase.[3]
In non-tetrapod osteichthyans, the basisphenoid contains grooves for the spiracle.[3] It is typically absent in teleosts.[4]
In many osteichthyans, including early stegocephalians and amniotes, the basisphenoid fuses to the parasphenoid (of dermal origin) very early in ontogeny; it may also fuse to the sphenethmoid.[5] In such cases, the two bones cannot be distinguished, and fused element is referred to as the parabasisphenoid.[6] Fusion of the two elements is thought to have occurred convergently in tetrapods and in actinopterygians.[5]
In non-mammalian synapsids, the basisphenoid is often covered ventrally by the parasphenoid, to which it may fuse; among cynodonts, it articulates with the prootic to form a portion of the bony labyrinth. The portion of the basisphenoid which forms part of the bony labyrinth is known as the basisphenoid wing.[7]
In mammals, the basisphenoid is much smaller and does not contribute to the bony labyrinth, due to an enlargement of the petrosal to form the promontorium.[7] It is typically fused to the presphenoid, orbitosphenoid, and the alisphenoid (of the splanchnocranium) to form the sphenoid.[1] It makes up the posterior part of the sphenoid base.[8]