CHAPTER XXXVIII 



THE EVOLUTION OF THE AMPHIBIA .* THE FIRST LAND CHORDATES 



That the amphibia arose from fish there is no doubt, and their 

 ancestor must have been one of the primitive Osteichthyes, 

 related to the stock which also gave rise to Osteolepis and 

 Dipterus. For purposes of comparison Osteolepis may be 

 taken as approaching the structure of this ancestor. 



The resemblances between Osteolepis, on the one hand, 

 and one of the earliest Stegocephalian amphibia such as 

 Loxomma on the other extend to the following features. In 

 both, the skull is a complete bony box, the dermal bones of 

 which can in most cases be identified with certainty because 

 the amphibia also had lateral-line canals which occupied 

 grooves in the bones. The bones of the palate are similar, 

 and both had nostrils which lead through into the cavity of the 

 mouth. The amphibian Eogyrinus had a shoulder girdle the 

 dermal bones of which were attached to the post-temporal 

 bone of the skull by the supra- cleithrum, as in the fish. Also, 

 these early amphibia had no sacrum, for the ilium was not 

 attached to the ribs. The walls of the teeth were folded, in 

 the Labyrinthodont pattern. The amphibia are autostylic, 

 as are the Dipnoi including Dipterus. The otic process in 

 Osteolepis did not reach the auditory capsule, however, and 

 it is a question as to whether the common ancestor of Osteo- 

 lepids, Dipnoi, and Tetrapods was autostylic or not. A lung 

 was almost certainly present in Osteolepis. 



The ancestor of the Tetrapods must, however, have had 

 pectoral and pelvic fins equally developed and similar in 

 structure, and this condition has not yet been found in any 

 Osteolepid (or other) fish. The really distinctive feature of 



431 



