THE PHYLUM CHORDATA 



times were subject to alternating periods of flooding and of stagnation and 

 drying. As a result, only those fish which could breathe air could survive 

 the periods of stagnation. But the air-breathing habit also made possible 

 excursions over the land to reach larger and more favorable lakes or 

 streams. The lungfishes of today also live in habitats in which seasonal 

 drying gives selective value to the ability to breathe air. 



The crossopterygian skull also has much in common with the skulls of 

 primitive amphibians ( Figure 55 ) . They differ mainly in that a few bones 

 of the crossopterygian skull are no longer present in the amphibian skull, 

 and that certain bones have fused. In both, there is an opening for a pineal 

 eye. In both, the internal choanae or nostrils are present. And in both, 

 labyrinthodont teeth are present. This is a pecuhar type of tooth, known 

 only in the crossopterygians and in a very primitive type of amphibian, in 

 which the enamel of the teeth forms deep ridges extending into the den- 

 tine. The crossopterygian fin is also of a dichotomous type which can be 

 much more readily homologized with the limbs of amphibians than can 

 the archipterygial fin. In fact, the similarity between primitive amphibian 

 limbs and the dichotomous fins of crossopterygians is close, as can be seen 

 in Figure 56. Both are characterized by a single, heavy piece, the hu- 

 merus, which articulates with the shoulder girdle; by two parallel mem- 

 bers, the radius and ulna, distal to the humerus; and by the less exactly 

 homologized radial bones at the distal end of the appendage. 



Finally, the vertebrae of both the Crossopterygii and the primitive Am- 

 phibia were diplospondylous, that is, in each body segment there were 

 two centra, one developed from the pleurocentrum, the other from the 



humerus 



radius 



radi;ilia 



A 



intermedials 



humerus 



ulna 



prepollex 

 ulnaria 



B 



Figure 56. Primitive Fore Limbs. A, archipterygium of Ceratodus; B, dichotomous 

 fin of Sauripterus, a crossopterygian; C, limb of Eryops, a labyrinthodont. Note the 

 similarity of B and C, and the complete dissimilarity of both of these to A. (From 

 Hyman, "Comparative Vertebrate Anatomy," 2nd Ed., University of Chicago Press, 

 1942. ) 



165 



