The splanchnocranium (or visceral skeleton) is the portion of the cranium that is derived from pharyngeal arches. Splanchno indicates to the gut because the face forms around the mouth, which is an end of the gut. The splanchnocranium consists of cartilage and endochondral bone. In mammals, the splanchnocranium comprises the three ear ossicles (i.e., incus, malleus, and stapes), as well as the alisphenoid, the styloid process, the hyoid apparatus, and the thyroid cartilage. In other tetrapods, such as amphibians and reptiles, homologous bones to those of mammals, such as the quadrate, articular, columella, and entoglossus are part of the splanchnocranium.
Evolution Gill slits are thought to have been present in the most recent common ancestor of living deuterostomes; they are present in modern hemichordates and lancelets. In lancelets, numerou gill slits are present; they are used for filter feeding as well as respiration. Haikouichthys, among the earliest known vertebrates, is thought to have been similar. However, Haikouichthys is believed to have had nine cartilaginous pharyngeal arches. The visceral skeleton evolved from structures like these.
Among early gnathostomes, the first or first two pharyngeal arches came to make up the jaws and the second (and to a lesser degree, the third) came to make up the hyoid apparatus. Among fishes, the remaining arches remained as the gills, while among tetrapods they were lost or highly reduced.
Divisions
Mandibular arch The mandibular arch comprises the ancestral portions of the upper and lower jaws. Ancestrally, these elements are the palatoquadrate and Meckel's cartilage, which are only rarely fully ossified among vertebrates. Developmental evidence suggests that the palatoquadrate and Meckel's cartilage represent serial homologues of the epibranchial and the ceratobranchial, respectively. The joint between the two cartilages ancestrally represents the jaw joint; in mammals, it has been modified and serves as a portion of the middle ear.
Meckel's cartilage Articular/Malleus Retroarticular Palatoquadrate Quadrate/Incus Epipterygoid/Alisphenoid Metapterygoid Autopalatine
Hyoid arch Ancestrally, the hyoid arch helps to support the floor of the mouth. It has undergone significant modification in different clades. In teleosts, the arch allows for cranial kinesis; in basal tetrapods, the arch helped to support the tongue (as part of the hyoid apparatus) as well as bracing the skull, later forming part of the middle ear. In mammals, the hyoid arch contains Reichert's cartilage.
Epihyal/Hyomandibula Columella/Stapes Extracolumella/Tympanohyal Interhyal Symplectic Ceratohyal Stylohyal Hypohyal Basihyal
Branchial arch I-V In chondrichthyans and teleosts, elements of these arches form distinct elements, divided into the pharyngobranchials, epibranchials, ceratobranchials, hypobranchials, and basibranchials. In tetrapods, elements of the first and second branchial arch form part of the hyoid apparatus. Elements of the second and third branchial arches may make up the laryngeal and thyroid cartilages; the fourth and fifth arches are absent in tetrapods.
See also Chondrocranium Dermatocranium Endocranium Neurocranium
References