Post orbitals

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Skull drawing of Eusthenopteron , a line representative of the terrestrial vertebrates, with marking of the postorbital (red).

The postorbital is a pair of skull bones of the osteognathostomata (bony fish including terrestrial vertebrates). It is a dermal cover bone of the side wall of the skull at the transition to the dorsal roof of the skull, which is involved in the posterior border of the eye socket (orbit). As such, it is part of the circumorbital series of dermal skull bones. In zoological and palaeontological skull drawings, the postorbital is usually identified with the abbreviation Po or po .

The postorbitalia of all tetrapodomorpha ( land vertebrates - crown group + closest fossil relatives) can be safely homologated with one another . On the other hand, it is questionable whether the bone element known as post-orbitals of some ray fins, for example of the pike , to which the tetrapodomorpha is homologous.

Amphibians

In the basic construction plan of the tetrapods, the postorbital is in contact ventrally and caudoventrally with the jugale and squamosum and caudodorsally with the supratemporal. Dorsally it can border on the parietal, but it is not infrequently separated from it by an intertemporal. Rostrodorsal it is in contact with the postfrontal.

While fossil amphibians ( Temnospondyls etc.) usually have a postorbital, it is completely reduced in modern amphibians ( Lissamphibia ). In the case of the frog and tail amphibians (Batrachia), the reduction is accompanied by an enlargement of the eye socket. The sneak amphibians (Gymnophiona) have a very compact skull, but studies on ontogenetic series show that in this group, too, the postorbital is not fused with the squamosum or another neighboring skull bone, but that this bone is not created in the first place.

Amniotes

Left postorbital of the tyrannosaurid Daspletosaurus in lateral (A) and medial (B) view.
Rear part of the skull of a crocodile in left side view with optical highlighting of the post orbitals. Note the typical crocodile morphology of the bone with a cylindrical, unsculptured “stem” (as part of the postorbital arch) and a sculptured “head”.

Both in the basic construction plan of the amniotes and in the probably secondary anapsid skull of the turtle there is a postorbital in constant contact with the parietal. In the diapsids with a fenestrated skull, it is usually approximately three-rayed (tri-radial), with the three rays or branches being perpendicular to one another: the medial and the ventral branch together with the parietal or jugal form the anterior borders of the two temporal windows and the caudal branch, together with a rostral ray of the squamosum, forms the upper temporal arch. The bone bridge, which is formed by the ventral branch of the postorbital and the dorsal branch of the jugale, is also the posterior border of the eye socket and is called the postorbital arch . In archosaurs , the postorbital is often merged with the postfrontal . The ventral part of the medial branch is then typically in contact with the side wall of the brain capsule (called laterosphenoid or pleurosphenoid * ). The contact of the medial branch to the parietal can be reduced. The latter is often the case with crocodiles , in which the postorbital is mushroom-shaped, with a cylindrical, unsculptured shaft (ventral ray) and an umbrella-like, sculptured dorsal part, which is involved in the table-like rear dorsal cranial roof, and their medial and caudal branch relative are short.

Also in representatives with a euryapsid skull and in the basal synapsids , the postorbital is at least partially involved in the anterior margin of the temporal window or in the formation of a postorbital arch.

Neither modern birds ( neornithes ) nor mammals have an independent postorbital. In many Mesozoic birds, however, the bone is still there. In the line that leads to the mammals, it is reduced in the "higher" non-mammal cynodontics . The (very probably secondary) postorbital arch of some groups of mammals ( hyrax , shrews , primates , horses ) is formed in varying proportions by the frontal, parietal and jugal.

* The laterosphenoid is a typical feature of the Archosauria and is already present in the representatives of their parent groups and is in contact with the dorsal cranial roof, but usually not with the postorbital, but only with the frontal and the still independent postfrontal.

literature

  • Robert Lynn 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 .
  • Gerhard Mickoleit: Phylogenetic systematics of vertebrates. Publishing house Dr. Friedrich Pfeil, Munich 2004, ISBN 3-89937-044-9 .

Individual evidence

  1. ↑ Hendrik Muller, Oommen V. Oommen, Peter Bartsch: Skeletal development of the direct-developing caecilian Gegeneophis ramaswamii (Amphibia: Gymnophiona: Caeciliidae). Zoomorphology. Vol. 124, No. 4, 2005, pp. 171–188, doi : 10.1007 / s00435-005-0005-6 (alternative full text access : ResearchGate )
  2. ↑ Luis M. Chiappe, Lawrence M. Witmer (Eds.): Mesozoic Birds: Above the Heads of Dinosaurs. University of California Press, 2002, ISBN 0-520-20094-2
  3. ↑ Kenneth D. Rose: The Beginning of the Age of Mammals. The Johns Hopkins University Press, Baltimore 2006, ISBN 0-8018-8472-1 (Chapter 3: The Evolutionary Transition to Mammals)
  4. ↑ James M. Clark, Johann Welman, Jacquas A. Gauthier, J. Michael Parrish: The laterosphenoid bone of early archosauriforms. Journal of Vertebrate Paleontology. Vol. 13, No. 1, 1993, pp. 48-57, doi: 10.1080 / 02724634.1993.10011487 (alternative full text access: ResearchGate )