THE PHYLUM CHORDATA 



order of which Permian records are known. Although the turtles are less 

 varied than many orders, yet they have been able to occupy habitats 

 ranging from desert to marine. In terms of numbers of genera, they are 

 second only to the Squamata today. 



The order Crocodilia is represented today only by a few genera, rem- 

 nants of a once important line. It includes the crocodiles and alligators. 

 They arose in Triassic times from the order Thecodontia, the same group 

 which gave rise to the great Ruling Reptiles of the Mesozoic Era. They 

 reached a peak in numbers in the Cretaceous, but have since been only 

 a minor part of the reptilian fauna of the world. 



The final order of living reptiles is the Squamata, the snakes and lizards, 

 popularly the best known reptiles today, but by no means the most typi- 

 cal. The order appears to have arisen in the Jurassic, reached a peak in the 

 Cretaceous, and continued to the present on a more restricted scale. Yet 

 they are found in all parts of the world except the arctic and antarctic 

 zones. All are carnivorous, but they are adapted to predation upon animals 

 ranging from insects to large mammals. Because there is a tendency to 

 think of most reptiles as being poisonous, it is worth mentioning that all 

 poisonous reptiles are confined to the order Squamata, and that only a few 

 of the many families in this order have developed poison mechanisms. 



Class Aves. The oldest known bird fossils are of late Jurassic age, and 

 they are most instructive with respect to the probable origin of the Aves. 

 Their skeletal characteristics are largely those of the primitive dinosaurian 

 order Thecodontia. Like these, they had many simple, conical teeth; a 

 skull of similar pattern; vertebrae unfused; a long tail composed of many 

 unmodified vertebrae; and bipedal locomotion. These are all reptilian 

 characters, but the fossils also include feathers, a characteristic known 

 only in birds. Hence they have been assigned to the genus Archeopteryx 

 (Figure 60), the oldest known genus of birds. But, had the feathers not 

 been preserved in the fossils, it is quite probable that the specimens would 

 have been assigned to the Thecodontia on the basis of the skeleton. Thus 

 the origin of the birds from thecodont ancestors is quite probable. It 

 should be noted that the birds are not the only flyers developed from the 

 Ruling Reptiles, for the pterodactyls also belong in this series. But they 

 were remotely related to the birds, and they became extinct without 

 leaving any descendants. 



The features in which birds differ from reptiles are almost all adaptive 

 to flight. At the outset, they do have feathers, which form a planing sur- 

 face for flight. The early embryology of feathers is quite similar to that 

 of reptilian scales, and it is commonly believed that feathers developed 

 as modifications of scales. The legs of birds are still covered with reptilian- 

 type scales. The feathers also provide insulation, thus aiding in the main- 

 tenance of the high body temperature of birds, so necessary for the 

 maintenance of the high metabolic rate required for flight. The light, 

 hollow bones of birds and the air sacs associated with the respiratory 

 system are probably also best interpreted as adaptations to flight, for they 

 reduce the weight of the bird. Other modifications affect practically every 

 organ system. 



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