CHAPTER I. 



THE PROBLEM OF THE AMPHIBIA FROM THE COAL MEASURES. 



The Amphibia from the Coal Measures of North America present the problem 

 of the origin of the land vertebrates, since the air-breathing vertebrates in the Coal 

 Measures of this continent are the earliest known in the western hemisphere. The 

 difference in age between the chief amphibian-bearing deposits of North America 

 and Europe is not great, although it has been asserted that Pholidogaster and its 

 allied fauna, described by Huxley from Scotland (331), is much older, probably 

 Mississippian. It is interesting to note that these earliest representatives of the 

 Amphibia in Scotland are all temnospondyles, of which there are very few represen- 

 tatives in the Coal Measures of North America. 



The forms so far described from the North American Coal Measures present a 

 very high degree of development and differentiation, the earliest known species 

 being already specialized and well adapted for various modes of life. As far back 

 in geological time as the middle Coal Measures, when the first well-defined forms 

 are known, environmental conditions had effected a wide diversity of structure 

 within the group. Thus, early in the geological history of the land vertebrates, we 

 have, among the Coal Measures Amphibia, various forms which had specialized into 

 strictly aquatic, terrestrial, sub terrestrial, and arboreal, or at least partly arboreal. 

 Specialization had extended to the loss of limbs, ribs, and ventral armature in a 

 few species, and to the acquirement of claws, running legs, or a long propelling 

 tail with expanded neural and haemal arches in others. The forms range in size 

 from small creatures less than an inch in length to large species which must have 

 attained a length of several feet. A rather interesting parallel, though of no phy- 

 logenetic significance, can be drawn between the Amphibia of the North American 

 Coal Measures and the reptiles of to-day. The snakes are represented by the limb- 

 less, snake-like forms, such as Ptyonius and Phlegethontia. The lizards find their 

 counterpart in the Hylonomidae and the Tuditanidse. No known characters of 

 these animals tend to ally them directly with any known group of fishes, except 

 in the most general way. These facts all indicate a long antecedent history for the 

 amphibian group or else a preceding period of greatly accelerated development of 

 which we now know nothing. 



The Amphibia whose remains have been brought to light from the Coal Measures 

 have hitherto been regarded as pertaining to a single order, the Stegocephalia, 

 characterized by the' completely roofed-over cranium and a large parasphenoid. 

 The writer (469) had previously assigned 5 suborders to the group : the Branchio- 

 sauria, Microsauria, Aistopoda, Temnospondylia, and Stereospondylia. All of 

 these groups are represented in the Coal Measures of North America. It has seemed 

 inadvisable, in the light of our present knowledge of the Amphibia, to retain these 

 5 groups as suborders, and, in the revised scheme of classification which has been 



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