Quadratojugal bone

The os quadratojugale ( just a Quadratojugale ) is a paired bone of the skull Osteognathostomata ( bony fish, including terrestrial vertebrates). It is a dermal bone covering the post- orbital, the lower side of the skull wall, which, if present, always in close spatial relationship with the quadrate, a replacement of the bone Palatoquadratums and part of the primary jaw joint, is. The Quadratojugale is featured in anatomical drawings usually with the symbol Qj or qj.

The presence of a Quadratojugale is a characteristic of the basic blueprint of Osteognathostomata and these bones can be found in both the original representatives of the ray-finned fishes ( Actinopterygii ) as well as representatives of the original meat -finned fishes ( Sarcopterygii ).

Ray-finned fishes

With many original Strahlenflossern as the " Palaeonisciformes " Cheirolepis, Mimipiscis (formerly Mimia ) or Myothomasia, the Quadratojugale is a small bone plate which rests the quadrate laterally ( lateral) and together with the rear portion of the maxilla and the Präoperculum a stiff lining the cranial side wall forms.

In the sturgeons ( Acipenseridae ) with their highly derived and largely cartilaginous skulls the Quadratojugale is a small bone above the rear end of the maxilla. This bone but has been interpreted as Supramaxillare.

In some basal Neopterygiern as the extinct Macrosemiiden or the gars ( Lepisosteidae ) is the Quadratojugale a narrow, sometimes partially intergrown with the quadrate Knochensplint that to the suspension of the jaw ( jockstrap ) is involved. In teleosts the Quadratojugale is no longer present as a separate bone and probably completely fused to the rear outer edge of the Quadratums. However, ontogenetic studies have not confirmed this statement in every case.

In general, is not yet completely clear whether the identified as Quadratojugale in the various Strahlenflossertaxa bones really are all homologous.

Meat -finned

Fishy representatives

The Quadratojugale is many early fishy Fleischflossern, similar to the early Strahlenflossern, between maxilla and Präoperculum, but is a relatively large, flat bone. His upper ( dorsal ) edge is in contact with the squamosal. However, the lungfish ( Dipnoi ) have highly modified skull, for that is an entirely different nomenclature in use due to the difficult Homologisierbarkeit the bone. In the early members of the group as Dipterus from the Middle Devonian, the Circumorbitalknochen No. 10 is considered as the Quadratojugale homologous element into consideration. The Schädelanaomie of Latimeria, the sole living representative of the coelacanth ( Actinistia ), differs significantly from that of his early relatives. Even with him no more Quadratojugale is available.

Land Vertebrates

Due to the complete reduction of the Opercularserie ( bones of the operculum ) and a general shortening of postorbital skull the Quadratojugale is involved in the basic plan of tetrapods at the rear edge of the side of the skull wall and also stands on its front ( rostral ) end with the Jugale in contact. However, going through the participation of Jugale at the bottom of the cranium ( skull excluding the mandible ) the contact with the maxilla lost - but is still present at very early forms such as Ichthyostega. The skull of primitive tetrapods (among labyrinthodonts or Stegocephalen ) in principle all have always been a Quadratojugale on. In modern amphibians ( Lissamphiba ) and in amniotes, it is reduced to some extent.

In the anurans ( Anura ) the Quadratojugale is due to the reduction of Jugale again with the maxilla in contact. In most urodeles ( Urodela ), it is no longer available as part of the resolution of the cheek region. The very compact skull of caecilians ( Gymnophiona ) has no separate Quadratojugale.

The synapsids, in their development history there is a general trend towards simplification and compaction of the skull, the Quadratojugale is early completely reduced. Just look at some " Pelycosaurier " groups it is present and involved in the boundary of the pace Ralf Sters. All therapsids - and with them the mammals - have no more Quadratojugale.

The Pareiasauriern and some Procolophonoideen, Permian or Permo - Triassic reptiles Para, the Quadratojugale forms a characteristic laterally downward, sometimes spiny outgrowth.

In the basic blueprint of Diapsiden it is involved in the boundary of the lower pace Ralf Sters. Here, however, there is a difference between early and modern representatives of "true " diapsiden skull, ie with closed lower Temporalbogen: While early Diapsiden the Quadratojugale is involved in about half of the lower Temporalbogen, the lower Temporalbogen for example, is formed from Jugale in archosaurs to a great extent. This is one reason for the assumption that the lower Temporalbogen the archosaurs is a secondary neoplasm.

The Schuppenkriechtieren ( Squamata ) probably during the formation of a special form of Schädelkinetik ( mesokinetischer skull ), the lower Temporalbogen is completely reduced, and no separate Quadratojugale longer available.

The Birds ( Aves) have their original " triapsiden " Archosaurierschädel heavily modified and also a special form of Schädelkinetik developed ( prokinetic skull). In contrast to the squamates here, however, the Quadratojugale is involved. It is fused with the Jugale to a rod-shaped bone, which together with a rearward extension of the maxilla (Processus jugalis ) forming a bone bridge, the (also zygomatic arch ) is called Arcus jugalis. This is part of the " push-rod system 'of the bird skull: It is connected at its rear end pivotally connected to the pendulum-like movable quadrate. This causes opening of the spout, a upward movement of the active upper beak, by the movement of the lowering of the lower mandible to the front swinging Quadratums is passed a connecting rod to the rear lower edge of the upper beak.