460 



Comparative Morphology of Chordates 



Fig. 356. Cacops, a small labyrinthodont amphibian from the Lower Permian. 

 Original about 16 inches long. (After Williston. Courtesy, Romer: "Vertebrate 

 Paleontology," Chicago, University of Chicago Press.) 



amphibians were more or less extensively covered by dermal bony 

 scales, especially on the ventral surface of the body. The teeth had a 

 peculiarly complex internal structure (Fig. 41), because of which the 

 group is named Labyrinthodontia. Because of the cephalic covering 

 by extensive plates of dermal bone (enlarged scales), it is also called 

 Stegocephalia (Fig. 356). In size, many of the old amphibians greatly 

 exceeded even our modern 5-foot salamander of eastern Asia, some 

 being probably as much as 15 feet long. Others, however, were of the 

 size of modern salamanders and newts. 



In the course of the later history of the Class, the pectoral girdle 

 lost connection with the skull and the pelvic girdle acquired connection 

 with a sacral vertebra, thereby increasing the efficiency of the hind- 

 legs in pushing the body forward. The scaly armor of the body was 

 all but lost, the labyrinthine pattern of the teeth vanished, the aver- 

 age size of the animals was much reduced, and the group differentiated 

 into the types represented by the three existing orders. This later his- 

 tory is known in only a very fragmentary way because of scarcity of 

 fossil evidence. No fossil caecilians (Apoda) have ever been found. 



The early amphibians were contemporary with crossopterygian 

 ("lobe-fin") fishes whose beginnings were considerably before (early 

 Devonian) those of amphibians. In structure of skull and in other 

 skeletal features, and especially in the possession of an armor of cal- 

 careous scales, there is much resemblance between the early amphib- 

 ians and the fishes. The skulls of the fishes give evidence of connection 

 between nasal cavities and mouth (internal nares), which implies 

 possession of lungs. All of the facts indicate that the fishes and amphib- 

 ians lived in similar fresh-water environments where, as in the case of 

 our modern lungfishes (Dipnoi), there was a seasonal necessity for 

 breathing by lungs instead of by gills. While there are obvious similar- 

 ities between urodele amphibians and modern lungfishes, the early 

 amphibians were more similar to the crossopterygians than to the early 



