THE PHYLUM CHORDATA 



Figure 57. Cross-section of a 

 Part of a Labyrinthodont 

 Tooth. The sinuous lines repre- 

 sent the complex infolding of 

 the enamel layer of the tooth. 

 (From Colbert, "Evolution of 

 the Vertebrates," John Wiley & 

 Sons, Inc., 1955. ) 



of diplospondylous vertebrae based upon a hypocentrum and a pleuro- 

 centrum. The relative sizes and spatial relations of the parts of the verte- 

 brae were characteristic for each order. Taking their origin in the De- 

 vonian, they were a major part of the fauna of the Carboniferous swamps. 



A second type of amphibian, the Lepospondyli, also appears among the 

 Devonian fossils. They lack labyrinthodont teeth, and their vertebrae were 

 formed by direct ossification around the notochord rather than from arcu- 

 alia. These vertebrae are perforated longitudinally, permitting passage of 

 the notochord. Because they are structurally more remote from the cros- 

 sopterygians, it is believed that the Lepospondyli must have been derived 

 from labyrinthodont ancestors, but their separation must have occurred 

 before either group actually appears in the fossil record. And at least one 

 competent anatomist has come to the conclusion that they arose inde- 

 pendently, with the Lepospondyli coming from Dipnoan ancestors. The 

 Lepospondyli were fairly prominent during the Pennsylvanian, but they 

 disappear from the record in mid-Permian times. 



By the end of the Triassic, the last of the ancient orders of Amphibia 

 had disappeared. As early as the Pennsylvanian, some labyrinthodonts 

 showed reductions of the skull and vertebrae similar to those of the 

 Anura (frogs and toads). The hypocentrum developed at the expense of 

 the pleurocentrum. A more advanced anuran-like labyrinthodont is known 

 from the Triassic of Madagascar, and scattered remains af anurans have 

 been found in Jurassic deposits. Numerous amphibian fossils, both of 

 Anura and of Urodela (tailed Amphibia, newts and salamanders) occur 

 in the Cretaceous, and these orders have continued up to the present as 

 minor groups. Intermediates in the formation of the urodeles are un- 

 known, but they have lepospondylous vertebrae, and so may have been 

 derived from the Lepospondyli. The obscure, worm-like Apoda do not 

 appear in the record until the beginning of the Cenozoic. Like the Uro- 

 dela, they have lepospondylous vertebrae, so it is plausible that they have 

 a similar origin. 



Class Reptilia. The Amphibia play a minor role in the vertebrate 

 fauna of today, and perhaps their greatest importance lies in their role 

 as the source of the class Reptilia. The origin of the reptiles from primi- 

 tive labyrinthodonts is unusually well attested, for there are many transi- 



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