A HISTORY Of VERTEBRATES: AMPHIBIANS AND REPTILES 



447 



Figure 23.1. A restoration of life in a Carboniferous swamp 250 million years ago. 

 The labyrinthodont amphibians were the first terrestrial vertebrates. (Courtesy of the 

 American Museum of Natural History.) 



different environment. That is, they became preadapted to certain 

 terrestrial conditions. Given this preadaptation, an abundance of food 

 (various invertebrates, stranded fishes, plants) upon the land or the 

 shores of swamps, little competition upon the land, and overcrowding 

 and intense population pressure in the swamps, it is not hard to imagine 

 some of the crossopterygians making the adaptive shift from water to 

 land and becoming the amphibians (Fig. 23.1). No one knows how long 

 the transition from crossopterygians to amphibians took, but the first 

 amphibian fossils are found in strata that were formed nearly 50 million 

 years later than those containing the first crossopterygians. 



Amphibians, in turn, acquired additional terrestrial features, and 

 reptiles still more. But the pinnacle of terrestrial adaptation is achieved 

 only by the reptiles' descendants— the birds and mammals. 



203. Evolution and Characteristics of Amphibians 



The ancestral amphibians, which are known as the labyrinthodonts, 

 finally diverged from the crossopterygians during the late Devonian 

 period (Fig. 22.2). An interesting detail they shared with the crossop- 

 terygians was a peculiar, labyrinthine infolding of the enamel in their 

 teeth. The name labyrinthodont is derived from this feature. All were 

 fairly clumsy, salamander-shaped creatures with rudimentary necks and 

 heavy, muscular tails inherited from their piscine ancestors (Fig. 23.1). 

 Their rather heavy limbs were sprawled out at right angles to the body, 

 and probably served only as aids to fishlike, lateral undulations of the 

 trunk and tail in progressing along the land. All became extinct during 



