The Tetrapoda: Amphibia, Reptiles, Birds 615 



cessful groups of animals they have become adapted to all habitats. 

 Fundamentally, however, a careful examination of the characteristics 

 of the members of this class shows that all their distinctive features 

 are really adaptations to flight and basically they are still reptiles. 



Unfortunately birds with their delicate bones make very poor 

 fossils, and the record of their development is very sketchy. There are 

 a few fossil remains, however, which indicate the line of development 

 from the ancient thecodonts to the modern representatives. Two of 

 these, Archaeopteryx and Archaeornis, which lived in the upper Jurassic 

 show many features transitional between the early ruling reptiles and 

 the birds. 



Both these fossils were found in limestone deposits in Bavaria. 

 Each was approximately 20 inches long, and most of the skeletal features 

 were quite reptilian-like. They had elongate jaws equipped with teeth 

 in sockets ; each wing had clawed digits at the tip ; the metacarpals were 

 separate; the lizardlike tail had 13 vertebrae from each of which long 

 flight feathers originated ; and the head, neck, and part of the body were 

 bare. Both forms are included in the class Aves, however, due to 

 the fact that they did have feathers. These feathers incidentally may 

 be indicative of a homoiothermic condition, for they are excellent in- 

 sulating material. Flight in both was probably quite unlike that of 

 many of our modern birds, more than likely they simply leaped from 

 branch to branch and used the wings for balancing and gliding. 



At the same time that these primitive birds were hopping and gliding 

 about the coniferous forests of the Jurassic, another experiment in flight 

 was taking place among the reptiles. This experiment resulted in the 

 bat-winged pterosaurs. They, like the birds, were taking to the air 

 and developing structures for conquering this medium. While the birds 

 developed feathers, the pterosaurs utilized a lateral extension, a skim- 

 ming membrane, which was attached to the fourth digit of each hand. 

 For a period, the pterosaurs were highly successful, but this line of de- 

 velopment became extinct while the birds continued to diversify and be- 

 come a successful form. 



Characteristics of the Class Aves. — Birds are distinguished from 

 all other living animals by their possession of feathers. Fundamentally 

 these are but modified scales, but are so modified as to present a com- 

 pletely new structure. The feathers are ideal flight structures, for they 

 present large surfaces for support combined with minimum weight. In 



