File:The chordates (1950) (20423565560).jpg

Title: The chordates
Identifier: chordates00rand (find matches)
Year: 1950 (1950s)
Authors: Rand, Herbert W. (Herbert Wilbur), 1872-1960
Subjects: Chordata
Publisher: Philadelphia : Blakiston
Contributing Library: MBLWHOI Library
Digitizing Sponsor: MBLWHOI Library

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Sauropsida: Class Reptilia 501 an outer plate consisting of dermal bones, mainly the squamosal, jugal, and postorbital. In its simplest form this plate is an uninter- rupted arch extending from the lateral border of the parietal above, down to the series of bones forming the upper jaw. The space thus enclosed (temporal cavity) opens externally in front and behind. It is occupied by the bulky and powerful muscle which closes the mouth by pulling the lower jaw upward. In most reptiles, however, the tem- poral arch, or "arcade," is pierced by one or two large "windows" or fenestrae. These openings doubtless ease the working of the muscle by giving opportunity for bulging when it contracts. When there are two fenestrae in each arcade, one is dorsal to the other and they are separated by a bridge formed by extensions of the squamosal and postorbital bones (Fig. 394). In some reptiles there is only an upper temporal fenestra, the squamosal and postorbital meeting below it. In others there is only a lower fenestra, the squamosal and postorbital meeting above it. Skulls of the anapsid type, having no temporal fenestrae, are found in the most ancient reptiles (cotylosaurs) and in Chelonia (more or less modified in many of them). A skull having only the upper fenestra is called "parapsid." The skulls of lizards and ichthyosaurs have been described as being of the parapsid type, but much doubt exists as to the interpretation of the obscure structure of the temporal regions of these skulls. The diapsid skull, having both upper and lower fenestrae, is found in Rhynchocephalia (Fig. 373), dinosaurs, pterosaurs, and crocodilians. With considerable modifica- tion, it appears in birds. This type was most common in ancient rep- tiles. The synapsid skull, having only the lower fenestra, occurred in the mammal-like reptiles (Theromorpha, Therapsida) and is found, with some modification, in mammals. Systematic groups of reptiles (Anapsida, etc.) have been defined on the basis of these differences in the number and position of temporal fenestrae. It is by no means certain, however, that these character- istics, by themselves, are a reliable index to phylogenetic relationships.
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Fig. 393. A Jurassic ichthyosaur, much reduced. (After von Stromer. Courtesy, Romer: "Vertebrate Paleontology," Chicago, University of Chicago Press.)

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