|Bones: Dermal Bones|
|The Vertebrates||Mandibular Series: Surangular|
Anatomy: The surangular is a dermal bone of the lower jaw (fig. 1). It typically covers much of the dorsal and lateral surface of the posterior jaw and variably extends onto the medial (inside) surface. The surangular articulates with and supports the articular, which forms the jaw joint with the quadrate in most vertebrates, and is in turn supported by the angular. More distally, the surangular bridges the mandibular fenestra, if one is present. It may also bear a raised dorsal coronoid process, an important attachment site for the mandibular adductors. At its distal (or rostral) end, the surangular meets the dentary, the main tooth-bearing bone of the lower jaw in most vertebrates. In certain fishes, an "infradentary" separates the two. On the medial face of the jaw, the surangular broadly articulates with the prearticular which covers much of the posterior medial face of the lower jaw. Across the jaw line, the surangular faces the quadratojugal and/or the jugal bones of the cheek region. The surangular is highly variable in shape, but most typically is slightly arched, either upward (as in fig. 1) or downward (as in fig.2).
Functions: Functionally, then, the surangular is associated with the following processes.
1) Support and orientation of the articular. In many cases, for example dinosaurs, the articular is reduced to a small, specialized articular surface, and the entire structural role is taken over by the surangular.
2) Attachment of jaw adductors. The surangular normally lies over the center of mass of the lower jaw, and thus occupies the most mechanically advantageous site for the attachment of the muscles closing the jaw. This is presumably why the mandibular fenestra is located there. The fenestra and the coronoid process provide additional surface area for muscle attachment at this critical site. In lepidosaurs and some other forms, the coronoid may be a separate element, and the mandibular fenestra is unique to the archosaurs.
3) Lengthening the jaw. Many vertebrates rely heavily on the speed of jaw closure for prey capture. Strength is less important than speed in capturing small prey. This gives a selective (and mechanical!) advantage to jaw which is long, particularly distal to the adductors. One important "function" of the surangular is simply to make a longer lever arm between the hinge and the business end with teeth.
4) Jaw kinesis. This function takes no standard vertebrate form. However, the surangular may work with other jaw elements to provide a mid-jaw "hinge" allowing the jaw to bend outward (laterally) or up-and-down dorso-ventrally).
Phylogeny: A surangular, sometimes labeled "supraangular," appears as soon as the dermal bones of the cranium begin to stabilize with the earliest osteichthyans. Here, the surangular is an irregularly-shaped element which caps the lower jaw. In neopterygians, it may develop a coronoid process, as in tetrapods. Nevertheless, it is not completely clear that this surangular is homologous with the surangular in tetrapods. Sarcopterygians tend to have a series of vaguely rib-like bones posterior to the dentary. They are oriented more-or-less diagonally and are often referred to as "infradentaries." In early tetrapods, the opercular (gill cover) series is lost, and these "infradentary" bones come to lie horizontally and extend further posteriorly on the skull. In that case, the most dorsal of the series is referred to as the surangular; and it is this bone that is clearly homologous with the surangular of all later tetrapods.
The later fate of this bone varies in different lineages. In some lepospondyls, and in frogs and salamanders, the surangular is absent. However, it becomes increasingly significant in the anthracosaur lineage. In turtles, it is one of the two principle bones of the lower jaw. In lepidosaurs, it is less important because of the development of a separate coronoid bone. In advanced lizards and pythonomorphs, it may fuse with the articular and perhaps other bones and loses its separate identity. Its function in archosaurs has been discussed above. See also Anatomical Dictionary.
In syanapsids, a secondary jaw joint develops between the surangular and the squamosal, which becomes the unique mammalian jaw articulation. However, the surangular fuses with the dentary and becomes the unitary mammalian "mandible" without a separate identity.
Some arguably relevant links: Anatomical Dictionary (dinosaurs); cranial more dinosaurs); Skull of Einiosaurusprocurvicornis in lateral view (yep, another one).