Phylogeny 



In North America to-day, the Carboniferous strata made in the 

 Coal Period are best represented in the region of the Alleghenies. 

 It is in this region, in the upper Coal Measures of Pennsylvania, 

 Ohio, and adjoining states that the fossils of the Stegocephali 

 are found. They have also come to light in the Coal Measures 

 of Nova Scotia. 



These earliest four-footed, air-breathing vertebrates were 

 often of considerable size (several feet in length). This is proved 

 not only by their skeletons, but also by footprints left by them 

 on slabs of sandstone both in Europe and in America. Usually 

 they resembled the Salamanders in shape, sometimes possessing 

 very long tails. The body carried a partial armour of bony plates 

 or scales and the head was heavily armoured. The skeleton 

 is in a relatively generalized condition such as might have 

 developed into the relatively more specialized types found in the 

 lower Batrachians (Apoda and Urodela) and in some extinct Rep- 

 tiles. The presence of pentadactyle limbs of course proves the 

 Stegocephali higher than all Fishes, but the shoulder-girdle is prim- 

 itive, resembling that in the Dipnoian Fishes, and the pelvis 

 and cranium present features very different from those of the 

 typical Fishes, but in some respects like those in the Dipnoi and 

 Crossoptergii and some extinct Reptiles. That the Stegocephali 

 are not Reptiles is proved perhaps by the presence of two 

 occipital condyles, whereas Reptiles have but one; it is proved to 

 a certainty by the structure of the vertebrae. (They are never 

 gastrocentrous, as in Reptiles.) 



The path of evolution from these extinct Stegocephali to 

 the Urodela and the specialized Salientia is obscure. It is not 

 known whether the intermediate ancestors were forms breathing 

 by mean of gills throughout their lives (Perennibranchiata) or 

 were already air-breathers. In fact, it is not even known whether 

 the Urodela and Salientia arose separately from the Stegocephali 

 or whether they had intermediate common ancestors. The oldest 

 form of the Salientia known is represented by fossils from the 

 Jurassic beds of the Rocky Mountains (representing the middle 

 period of the Age of Reptiles). So little is known of this form, 

 however, that it has little weight. The earliest reliable evidence 

 of anything approaching modern types is given by a small skele- 

 ton found in the Cretaceous of Belgium (the latest period of the 

 Reptilian Age, just previous to the Tertiary) ; this is thought to 



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