PHYLOGENY 



changed descendants of the progenitors of the Amphibia, and through 

 them, of the higher tetrapods. The Dipnoi have certain characters which 

 lend themselves to this interpretation. Most important, they are lungfishes, 

 and their lungs are developed in much the fashion described above. In 

 degree of subdivision, their lungs surpass those of the Amphibia. Their 

 fins have an arrangement somewhat like an elm leaf. There is a single row 

 of basals running the length of the fin, and around this the fin rays are 

 arranged like the veins in an elm leaf. A muscular lobe extends along the 

 row of basals. Gegenbaur regarded this type of fin as the probable source 

 of tetrapod appendages, and he called it an archipterygium ( = primitive 

 limb) to indicate this. He supposed that the tetrapod limb was formed 

 from the archipterygium by the suppression of all of the fin rays except 

 the terminal five. Finally, the aortic arches of the Dipnoi are closely simi- 

 lar to those of the tailed Amphibia ( Urodela ) . 



The theory of the dipnoan ancestry of the Ampliibia enjoyed wide 

 acceptance for a time, but it has some serious faults. The bones of the 

 skull of the Dipnoi show a peculiar pattern, and efforts to homologize 

 them with bones of amphibian skulls have been futile. And it is difficult 

 to see how the archipterygium could have given rise to a limb in which 

 two parallel bones form the second segment. In spite of these difficulties, 

 the dipnoan theory might have continued to enjoy favor were it not that 

 the Crossopterygii provide a much more plausible solution to the prob- 

 lems of amphibian ancestry. 



The Crossopterygii, like other Devonian bony fishes, had lungs. The 

 question now arises, why were lungs so general a feature of these fishes? 

 Paradoxically, it appears that lungs were originally an adaptation to per- 

 mit fishes to remain in the water. The fresh-water streams of Devonian 



Figure 55. Dorsal Views of the Skulls of a Crossopterygian, Eiisthcnopteron 

 (A), AND OF A Labyrintiiodont Amphibian, Palaeogtjrinus (B). Homologous bones 

 are indicated by the same letters. (From Romer, "The Vertebrate Body," 2nd Ed., 

 W. B. Saunders Co., 1955. ) 



164 



