XIII 



EVOLUTION AND ADAPTIVE RADIATION OF 



AMPHIBIA 



1 . The earliest Amphibia 



There are such close resemblances between the skulls of the earliest 

 amphibians and those of the Devonian crossopterygian fishes that 

 there can be no doubt of the relationship (Fig. 194). At present there 

 is, however, no detailed fossil evidence of the stages of transition 

 from the one type to the other. The fossils that appear to be closest 

 to the possible tetrapod ancestor are the osteolepids of the Lower and 

 Middle Devonian periods, about 375 million years ago. These were 

 definitely fishes, though they may have breathed air. *Elpistostege is 

 a single Upper Devonian skull intermediate between such fishes and 

 the earliest undoubted tetrapods, *Ichthyostega and similar forms, 

 found recently in freshwater beds of Greenland. These are dated as 

 very late Devonian or early Carboniferous, that is to say about 350 

 million years ago. They are the oldest members yet found of the 

 great group of Stegocephalia, which, throughout the succeeding 100 

 million years of the Carboniferous and Permian periods, flourished 

 and developed many different lines, one giving rise to the reptiles and 

 others to the modern amphibia. The term Stegocephalia is convenient 

 to cover the whole group of palaeozoic amphibia, all probably of 

 common descent. At least seven types can be recognized (p. 296), but 

 attempts to group these have not been altogether successful; the 

 nomenclature remains confused. The Labyrinthodontia were the central 

 stock and were in the main terrestrial forms, giving off at intervals lines 

 that returned to the water. A characteristic labyrinthodont feature is 

 a folded pattern of the teeth, similar to that of their crossopterygian 

 ancestors. 



The earliest Stegocephalia were definitely tetrapods and already 

 showed sharp changes from the fish type. *Ichthyostega is known 

 chiefly from the skull (Fig. 208), which shows all the characteristic 

 amphibian features, but retained traces of fish ancestry in its shape, 

 with a short, wide snout and long posterior region (table), and pre- 

 sence of a preopercular bone. The nostril lies on the very edge of the 

 upper lip, apparently partly divided by a flange of the maxilla into 

 internal and external openings. 



