PHYLOGENY 



hypocentrum. There are other parallels between these groups, but this 

 may be sufficient to indicate the probability that the Crossopterygii were 

 the vertebrates which led to the conquest of the land. 



The discussion above is based upon only one of the two suborders of 

 the Crossopterygii, namely the Rhipidistia. These were the dominant 

 fishes of the Devonian, but they became extinct early in the Permian. At 

 the height of their development in the Devonian, they gave rise to a 

 marine suborder, the Coelacanthini, which appears in the fossil record 

 as a minor group until the Cretaceous. But, by the end of the Cretaceous, 

 some 75,000,000 years ago they had disappeared from the known fossil 

 record, and all paleontologists regarded them as completely extinct. It 

 was, therefore, an event of great scientific importance when a strange fish, 

 brought into a South African port by commercial fishermen in 1939, was 

 identified as an extant coelacanth crossopterygian. Unfortunately, the soft 

 parts of the body were already extensively deteriorated, and little new 

 knowledge was gained from this fish beyond the important fact that the 

 coelacanths are not extinct. The species was described under the name of 

 Latimeria chalumnae. Since 1952, many specimens have been taken near 

 the Comoro Islands, north of the Mozambique Channel. As this is French 

 territory, they are being studied in Paris. 



Class Amphibia. The first amphibians which crawled out upon the 

 mud banks of late Devonian streams and lakes were little more than fishes 

 with fins sufficiently modified to support the weight of the body. These 

 limbs were longer than typical crossopterygian fins, and it is probable 

 that the muscular lobes were more highly developed. The radials were 

 simplified to form a five-fingered— pentadactyl— hand which could be 

 turned palm down to support the body. These early amphibians appear 

 to have been aquatic animals in competition with their near relatives, the 

 crossopterygians. As long as they remained in water, they probably were 

 the poorer competitors, for legs are less efficient swimming organs than 

 are fins. But seasonal droughts were the rule, and hence those animals 

 which could leave a stagnating pond and go over land to a more favorable 

 one had a selective advantage. In the long run, it was these which sur- 

 vived and gave rise to the land vertebrates, while those crossopterygians 

 which failed to make adaptations to land living became extinct. The single 

 known exception has been noted above. 



The Amphibia quickly broke up into several orders. Three more orders 

 are living today. The problem of how these orders are related to one an- 

 other, and how they should be classified is a very difficult one, and the 

 scheme to be outlined here must be regarded as tentative. All of the 

 primitive, extinct Amphibia resembled our present-day Urodela (newts 

 and salamanders) in general bodily form, but differed from them greatly 

 in the details of anatomy. The most primitive group, including those forms 

 most similar to the crossopterygians, comprises the superorder Labyrinth- 

 odontia. This name is based upon the fact that, in common with the 

 crossopterygians, they possessed labyrinthodont teeth (Figure 57). They 

 continued into the Triassic, but were extinct by the end of that period. 

 The labyrinthodont orders were generally characterized by the possession 



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