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Encyclopedia Britannica - Main :: RHY-RON |
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RHYNCHOCEPHALIA . II. The quadrate bone loosely articulated to the cranium and at the proximal end only (Streptostylica). No distinct supramastoid, nor opisthotic; one or no post-orbital bar; scapular arch, when present, external to ribs; ribs one-headed. Order 12, SQUAMATA. While this classification was being considered and prepared, both Cope and G. Baur made a special
series of discussions which ended in the idea, that the class could be most naturally divided into two great subclasses, the one culminating intortoises and mammals, the other in crocodiles, lizards, snakes
Subclass SYNAPSIDA. Primarily with single or undivided temporal arches. Giving rise to the mammals through some unknown member of the Anomodontia. Orders Cotylosauria, Anomodontia, Testudinata and Sauropterygia. Subclass DIAPSIDA. Primarily with double
Orders Dinptosauria (=Protorosauria, Pelycosauria and Rhynchocephalia), Phytosauria (=Belodon, &c.), Ichthyosauria, Crocodilia, Dinosauria, Squamata and Pterosauria. The most exhaustive and modern general work
HOFFMANN
buted by Dr H. Gadow to the Cambridge Natural History mane.(London, 1902). (A. C. G.; A. S. Wo.) II. GENERAL CHARACTERS OF THE CLASS REPTILIA Reptiles, as known in the existing world, are the modified, and in many respects degenerate, representatives of a group of lung-breathing vertebrate animals which attained its maximum development in the Mesozoic period. So far as can be judged from the skeleton, some of the members of this group then living might have become mammals by very slight change , while others might as readily have evolved into birds. It is therefore probable that the class Reptilia, as now understood, comprises the direct ancestors both of the Mammalia and Aves. Assuming that its extinct members, which are known only by skeletons, were organized essentially like its existing representatives, the class ranks higher than that of the lowest five-toed vertebrates (class Batrachia) in the investment of the foetus by two membranous envelopes (the amnion and allantois), and in the total absence of gills even in the earliest embryos. It ranks below both the Mammalia and Aves in the partial mixture of the arterial blood with the venous blood as it leaves the heart, thus causing the organism to be cold-blooded; it also differs both from Mammalia and Aves in retaining a -pair of aortic arches, of which only the left remains in the former, while the right one is retained in the latter. No feature in the endoskeleton is absolutely distinctive, except possibly the degeneration of the parasphenoid bone, which separates the Reptilia from the Amphibia. In the exoskeleton, however, the epidermis forms horny scales, such as never occur in Amphibia, while there are no traces of any structures resembling either hairs or feathers, which respectively characterize Mammalia and Avds.There is little doubt that true reptiles date back to the latter part of the Palaeozoic period, but at that epoch the Amphibia approached them so closely in the characters of the skeleton that it is difficult to distinguish the members of the two classes among the fossils. Some of the Palaeozoic Amphibiaa few of the so-called Labyrinthodontsare proved to have had well-developed gill-arches in their immature state, while there are conspicuous marks of slime-canals on their skulls. Others are , Mein. American Mus. Nat. Hist. (November 1903), vol. i. art. viii. merely regarded as Amphibia because they closely resemble the genera which are proved to have been gill-breathers when immature. All these genera, however, so far as known, agree with the existing Amphibia in the production of their large parasphenoid bone as far forwards as the vomers to form a rigid and complete basicranial axis
which less resemble the typical Labyrinthodonts are characterized by the reduction of the parasphenoid bone so that it no longer reaches the vomers; in these animals the weakened skull exhibits a secondary basicranial axis
In the structure of the skull Palaeohatteria is much like the existing Sphenodon, the cheek-plates which cover the temporal and masseter muscles on each side being pierced by two great vacuities, one superior-temporal, the other lateral-temporal. The majority of the earliest reptiles, however, either resemble the Labyrinthodonts in having the biting muscles completely covered with a roof of bony plates, or exhibit a slight shrinkage of this investment so that a superior-temporal vacuity appears. As the various groups or orders become differentiated, this shrinkage or reduction continues, while the shape of the ossifying ear-capsule changes, and the squamosal bone, which covers the organ of hearing in the fishes, and presumably also in the Palaeozoic Batrachia, is gradually thrust outwards from all connexion with this capsule except at its hinder angle. The resultant modifications are diagrammatically represented in fig 2. In one series of orders, comprising the Anomodontia, Chelonia, Sauropterygia and Ichthyopterygia (fig. 2, B, C), the superior-temporal vacuity (s) first appears, and the cheek-plates in the broad temporal arch thus formed may be variously fused together, sometimes even irregularly perforatedshowing at first, indeed, the usual inconstancy of a new and not completely established feature. From the earliest members of this series of reptiles, palaeontology seems to demonstrate that the Mammalia (with one robust temporal arcade or zygomatic arch) arose. In a second series, comprising the orders Rhynchocephalia, Dinosauria, Crocodilia and Ornithosauria (fig. 2, D), the broad arch of cheek-plates is regularly pierced by a lateral-temporal vacuity, which leaves a narrow bar above, another narrow bar below, and uncovers the middle part of the quadrate bone. By the constant loss of the lower, and the frequent lossof the upper, bar, some members of this series eventually pass into the order Squamata (Lacertilia+Ophidia), in which the quadrate bone is completely exposed and loosely attached to the skull (fig. 2, E); other reptiles exhibiting a similar modification may readily have acquired the typical Avian skull (fig. 2, F) by the loss of the upper and the retention of the lowertemporal bar in question. In view of these and other palaeontological considerations, the Reptilia may be classified into orders as follows: End of Article: RHYNCHOCEPHALIA If you wish, you can link directly to this article.
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