Os quadratojugale

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Skull drawings of the Devonian bony fish Mimipiscis (= " Mimia ") bartrami ("Palaeonisciformes", Actinopterygii) and Eusthenopteron foordi ( Tetrapodomorpha , Sarcopterygii) each with the position of the quadratojugale (colored red).

The quadratojugal bone (just a Quadratojugale ) is a paired cranial bones of Osteognathostomata (bony fish, including terrestrial vertebrates). It is a dermal cover bone of the postorbital, lower side wall of the skull, which, if present, is always in close spatial relationship with the quadratum , a replacement bone of the palatoquadrum and part of the primary temporomandibular joint . The Quadratojugale is usually identified in zoological and palaeontological skull drawings with the abbreviation Qj or qj .

The presence of a quadratojugale is a feature in the basic blueprint of the osteognathostomata and this bone is found both in the original representatives of the ray fins (Actinopterygii) and in original representatives of the meat fins (Sarcopterygii).

Ray fins

In many original ray fins, such as the " Palaeonisciformes " Cheirolepis , Mimipiscis (formerly Mimia ) or Myothomasia , the quadratojugale is a small bone plate that sits on the quadratum laterally and, together with the posterior part of the maxillary and the preoperculum, a stiff covering the side wall of the skull.

In the sturgeon (Acipenseridae) with their strongly derived and mostly cartilaginous skulls, the quadratojugale is a small bone above the posterior end of the maxillary. However, this bone has also been interpreted as a supramaxillary.

In some basal neopterygians , such as the extinct Macrosemiidae or the pike (Lepisosteidae), the quadratojugale is a narrow bone splint, sometimes partly fused with the quadratum, which is involved in the suspension of the jaw ( jockstrap ). In the Teleosts , the quadratojugale is no longer present as a separate bone and is presumably completely fused with the rear outer edge of the quadratum. However, ontogenetic studies have not always confirmed this statement.

As a general rule, it has not yet been fully clarified whether the bones identified as quadratojugals in the various radiation fin taxa are really all homologous to one another .

Meat finisher

Fish-like representatives

The quadratojugale lies between the maxillary and the preoperculum in many early fish-like flesh fins, similar to the early ray fins, but is a relatively large, flat bone. Its upper (dorsal) edge is in contact with the squamosum . The lungfish (Dipnoi), however, have heavily modified skulls, for which a completely different nomenclature is in use due to the difficult homologation of the bones. In early representatives of the group, such as Dipterus from the middle Devonian, the circumorbital bone No. 10 is considered as an element homologous to the quadratojugale. The cranial anomy of Latimeria , the only living representative of the coelacanth (Actinistia), differs significantly from that of its early relatives. He also no longer has a square jugal.

Terrestrial vertebrates

Skull of Scutosaurus karpinskii (Pareiasauria) with clearly recognizable “horns” pointing downwards and downwards, formed by the quadratojugale.

Due to the complete reduction of the opercular series (bones of the gill cover ) and a general shortening of the postorbital skull, the square jugal is involved in the basic construction plan of the tetrapod at the rear edge of the side wall of the skull and is also in contact with the jugal at its anterior (rostral) end . However, due to the involvement of the jugale at the lower edge of the cranium (skull without the lower jaw), contact with the maxillary is lost. B. Ichthyostega but still present. The skulls of the more original tetrapods (including the labyrinthodontic or stegocephalic ) always have a square jugal in principle. With the modern amphibians (Lissamphibia) and with the amniotes it is partly reduced.

In the frogs (anura), the quadratic jugal is in contact with the maxillary again due to the reduction of the jugal. In most tail amphibians (Urodela) it is no longer present in the course of the dissolution of the cheek region. The very compact skull of the caecilians (Gymnophiona) does not have a separate square jugal.

With the synapsids , in the history of which there has been a general trend towards simplification and compaction of the skull, the quadratojugale is completely reduced at an early stage. It is only present in some “ pelycosaur ” groups and is involved in the delimitation of the temporal window. All therapsids - and with them the mammals - no longer have a square jugal.

In the Pareiasauriern and some Procolophonoiden , perm or permo- Triassic parareptilia that Quadratojugale forms a characteristic, the side facing downward, and sometimes spiny outgrowth.

In the basic plan of the Diapsids it is involved in the delimitation of the lower temporal window. Here, however, there is a difference between early and more modern representatives with a "real" diapsid skull, i. H. with closed lower temporal arch: while in early diapsids the quadratojugal is about half involved in the lower temporal arch, the lower temporal arch is z. B. with the archosaurs formed to a large extent by the Jugale. This is one reason to believe that the lower temporal arch u. a. the archosaur is a secondary neoplasm.

The Schuppenkriechtieren , the formation of a special form of Schädelkinetik is (mesokinetischer skull in connection with (Squamata) probably in the course Streptostylie ), the lower Temporalbogen completely reduced and no separate Quadratojugale longer present.

Skull of a goose . 1–4: Quadratum (marked in green), 5: Articulare (a bone of the lower jaw), 6: Quadratojugale , seamlessly merging into the Jugale towards the beak.

The birds (Aves) have heavily modified the original “ triapsid ” archosaur skull and also developed a special form of skull kinetics (prokinetic skull). In contrast to the squamats, however, the quadratojugale is involved here. It is fused with the jugal to form a rod-shaped bone, which, together with a rearward extension of the maxillary ( processus jugalis ), forms a bone web called the arcus jugalis (also arcus zygomaticus ). This is part of the “push rod system” of the bird skull: it is at its rear end, i.e. H. via the quadratojugale, articulated with the pendulum-like movable quadratum. When the beak opens, this causes an active upward movement of the upper beak, in that the movement of the square, which swings forward when the lower beak is lowered, is passed on like a connecting rod to the lower rear edge of the upper beak.

literature

  • Robert L. Carroll: Vertebrate Paleontology and Evolution. WH Freeman and Co., New York 1988.
  • Milton Hildebrand, George E. Goslow: Comparative and functional anatomy of the vertebrates. Springer, 2004, ISBN 3-540-00757-1 .
  • Ulrich Lehmann: Paleontological Dictionary. 4th edition. Ferdinand Enke Verlag, Stuttgart 1996, ISBN 3-432-83574-4 .
  • Gerhard Mickoleit: Phylogenetic systematics of vertebrates. Publishing house Dr. Friedrich Pfeil, Munich 2004, ISBN 3-89937-044-9 .
  • Franz-Viktor Salomon, Maria-Elisabeth Krautwald-Junghanns: Anatomy of the birds. In: Salomon et al. (Hrsg.): Anatomie für die Tiermedizin . 2nd ext. Edition. Enke Verlag, Stuttgart 2008, ISBN 978-3-8304-1075-1 , pp. 754-814.

Individual evidence

  1. ↑ a b c d Gloria Arratia, Hans-Peter Schultze: Palatoquadrate and its Ossifications: Development and Homology within Osteichthyans. In: Journal of Morphology. Vol. 208, No. 1, 1991, pp. 1-81, doi: 10.1002 / jmor.1052080102 (see especially p. 2 and p. 64 ff.)
  2. ^ A b Adriana López-Arbarello: Phylogenetic Interrelationships of Ginglymodian Fishes (Actinopterygii: Neopterygii). In: PLoS ONE. Volume 7, No. 7, 2012, p. E39370. doi: 10.1371 / journal.pone.0039370
  3. ^ Johannes Müller: Early loss and multiple return of the lower temporal arcade in diapsid reptiles. In: Natural Sciences. Volume 90, No. 10, 2003, pp. 473-476. doi: 10.1007 / s00114-003-0461-0