EVOLUTION OF THE AMPHIBIANS 



177 



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Form Evolution of the Amphibians 



A single impression of a three-toed footprint (Thinopus 

 antiques) in the Upper Devonian shales of Pennsylvania con- 

 stitutes at present the sole palaeontologic proof of the long 

 period of transition of the vertebrates from the fish type to 

 the amphibian type. This transition was a matter of thousands 

 of years. It took place in Lower Devonian if not in Upper 

 Silurian time. Under the 

 influence of the heredity- 

 chromatin it is now re- 

 hearsed or recapitulated in 

 a few days in the metamor- 

 phosis from the tadpole to 

 the frog. 



As compared with 

 fishes, the significant prin- 

 ciple of the evolution of 

 amphibians, as the earliest terrestrial vertebrates, is their reac- 

 tion to marked environmental change. Their entire life re- 

 sponds to the changes of the seasons. They also respond to 

 secular changes of environment in the evolution of types 

 adapted to extremely arid conditions. 



The adaptive radiation of the primordial Amphibia prob- 

 ably began in Middle Devonian time and extended through 

 the great swamp, coal-forming period of the Carboniferous, 

 which afforded over vast areas of the earth's surface ideal con- 

 ditions for amphibian evolution, the stages of which are best 

 preserved in the Coal Measures of Scotland, Saxony, Bohemia, 

 Ohio, and Pennsylvania, and have been revealed through the 

 studies of von Meyer, Owen, Fritsch, Cope, Credner, and 

 Moodie. The earliest of these terrestrio-aquatic types have 



Fig. 58. A Primitive Amphibian. 



Theoretic reconstruction of a primitive sala- 

 mander-like type with large, solidly roofed 

 skull, four limbs, and five fingers on each of 

 the fore and hind feet, such as may have ex- 

 isted in Upper Devonian time. After Fritsch. 



