Scaly leg

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Skull drawing of Eusthenopteron , a line representative of the terrestrial vertebrates, with identification of the scaly bone (red).

The Scale Leg ( squamosal, Os squamosum of lat . Squama "scale") is a large paired covering bones of the cranial vault of Osteognathostomata (bony fish, including terrestrial vertebrates), who always in close spatial relationship to the otic capsule is. In zoological and palaeontological skull drawings, it is usually identified with the abbreviation Sq or sq .

Skull of a young Nile crocodile ( Crocodylus niloticus ), a representative of the diapsid reptiles, seen from above. The scaly bones (colored red), clearly recognizable, form the posterior lateral part of the posterior dorsal cranial roof with the upper temporal windows, which is typical for crocodiles and is designed like a table.

In terms of developmental history, the scaly leg has been proven for the first time in the trunk group of terrestrial vertebrates (Tetrapoda). In these fish-like trunk tetrapods (or also basal tetrapodomorphs ), known only in fossil form, it forms part of the side covering of the skull between the eye socket (orbit) and the preopercular . In the case of radiation fins , the cover bones sometimes referred to as “squamosum” or “squamosoid” in the literature are not homologous to the tetrapodomorphic scaly bone in representatives of this group . It is also absent from today's lungfish .

In terrestrial vertebrates (Tetrapoda), as a result of the complete reduction of the opercular series (bones of the gill cover ) and a general shortening of the postorbital skull, it now forms part of the posterior side wall of the skull. In more basal, extinct tetrapods (" labyrinthodontic animals"), the scale leg is often on the front and z. Partly also at the lower edge of the so-called otic slit , an indentation in the rear edge of the side wall of the skull.

In the basic construction plan of the amniotic skull , it is in contact with the parietal bone . In the basic blueprint of the skull of diapsid reptiles , the scale bone is involved together with the postorbital on the upper temporal arch , the bone bridge between the lower and upper temporal window .

In the basal synapsids , it is involved together with the jugal in the lower temporal arch (corresponds to the zygomatic arch of mammals ). The mammals that shed leg over the fuses ontogenesis relatively early with the otic bones (Prooticum and Opisthoticum together the temporal bone form, and tympanic). The corresponding composite bone is called the temporal bone in humans and its largest section, which is essentially homologous to the scaly bone , is called the pars squamosa (part of the scale) (in fact, the name squamosum is even derived from the name pars squamosa ). The concave articular surface of the (secondary) temporomandibular joint in mammals is located on the lower side of the zygomatic arch part of the scale bone or the scale part of the temporal bone .

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  • Lexicon of Biology , Volume 7, Verlag Herder, Freiburg, 1986, ISBN 3-451-19647-6 .
  • Robert Lynn Carroll : Vertebrate Paleontology and Evolution. WH Freeman and Co., New York 1988, ISBN 0-7167-1822-7 .
  • 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 .