AMPHIBIA 181 



The Amphibia apparently did not find the climatic conditions of the 

 Lower Carboniferous especially adapted to them and did not really 

 become a successful and dominant race till during the Upper Carbon- 

 iferous and Permian times, when their great deployment and first 

 adaptive radiation ccurred. 



The first Amphibia (Fig. 105, A) were probably small-headed, long- 

 bodied forms with fish-like appearance, resembfing, doubtless, our 

 modern newts and salamanders. During the Upper Carboniferous, 

 however, there was an adaptive radiation resulting in the develop- 

 ment of large-headed, short-bodied types, more or less resembling 

 our frogs and toads (Fig. 105, B), but without jumping legs. There 

 also appeared some broad, fiat types (Fig. 105, E) with reduced limbs 

 that must have been bottom-feeders (e. g., Diplocaulus). 



Three orders of Stegocephali are distinguished: 



Order I. Stegocephali Leptospondyli. — This group is characterized 

 by pseudocentrous vertebrae, by which is meant that a thin shell of 

 bone surrounds the notochord. Two types of these animals are 

 distinguished, one in which the form was evidently much like our 

 modem newts. They were broad-headed, had several pairs of gills, 

 at least in the young (Fig. 106). As an example of the skull structure 

 of the group, that of Branchiosaurus (Fig. 107), one of the most gen- 

 eralized of vertebrate skulls, is shown. The other type of this order 

 was a snake-like form, without limbs, evidently a precociously senes- 

 cent type in which the ribs reach about halfway round the body. 



Order II. Stegocephali Temnospondyli. — The vertebrae are composed 

 of three separate pieces, two dorsal and one ventral. These animals 

 had rather long ribs and their armor was chiefly ventral. Some of the 

 types were: Chelydosaurus, a turtle-like form; Dissorophus, a sort of 

 "Batrachian Armadillo;" Archegosaurus, a thoroughly terrestrial 

 form about five feet in length. 



Order III. Stegocephali Stereospondyli. — The three components of 

 the vertebra miite into one sohd amphiccelous vertebra. This group 

 has been given the name of Labyrinthodonta on occount of their much 

 folded teeth. The vertebrae are sometimes one-headed, sometimes 

 two-headed. The limb-girdles are very primitive and strikingly re- 

 semble those of crossopterygian fishes. The foot skeleton is extremely 

 generalized. It now seems likely that this is the group of early Am- 

 phibia that is most nearly related to the crossopterygian fishes. Wat- 

 son has studied the skulls of the Carboniferous labyrinthodonta 



