dividual adaptive adjustment. The return to a relatively 

 yolk-free egg in the mammals is a reflection of the devel- 

 opment of a placenta, a new way of supplying nutritive 

 materials to the developing embryo and foetus. Nutritive 

 needs have called forth a variety of kinds of placentation in 

 several kinds of animals. 



Ontogeny may occasionally give clews to phylogeny; for 

 example, the appearance of a dorsal diverticulum in some 

 sharks suggests a rudimentary lung. In birds the pterygoid 

 bone is primitively a single unit which, in many species, 

 later becomes divided into two articulating parts, the more 

 anterior one of which fuses with the palatine. This assumed 

 phylogenetic sequence is revealed by the development of 

 the bone. The appearance of rudimentary structures, some 

 of which later degenerate and disappear, suggests retention 

 of phylogenetically revealing pathways of development. 



On the basis of this review of morphological problems 

 and theories, the actual structuring of a phylogeny can now 

 be approached. 



VERTEBRATE PHYLOGENIES 



The delineation of phylogenies has occupied the attention 

 of many comparative morphologists working on the theory 

 that an understanding of the actual pathways followed in 

 evolution will contribute to our understanding of the evo- 

 lutionary process. The student will find many diagrams and 

 discussions of this matter in the literature of morphology. 

 Table 15-1 is a summary of opinions held up to about 1930. 

 This pattern does not represent any individual's opinion, 

 but rather is a composite or average view. The position of 

 the ostracoderms is peculiar, but it was not until this time 

 that it was realized that they were agnath fishes. A more 

 recent tree of the vertebrates. Table 15-2 (Romer, 1955) 

 shows a slight modification of earlier views, a replacement 

 of the idea that the dipnoans are along the line of decent 

 of the tetrapod organisms. Here the dipnoans become a 

 side issue, and the osteolepiforms lie on the main line of 

 descent. In this scheme the actinopterygian evolved through 

 two series, the chondrosteans and holosteans. 



Table 15-3 is a current European view which is peculiar 

 only in that the coelacanths are removed from the choanates, 

 and the Dipnoi are set aside as a dead-end group distinct 

 from the choanates. The term choanate is here restricted to 

 crossopterygian fishes and their derivatives, the various 

 kinds of tetrapods. This particular scheme is somewhat 

 unique in that it derives the urodeles from the porolepi- 

 forms and the labyrinthodonts from the osteolepiforms. 

 Further, the sauropsid and therapsid, the reptiles, are de- 

 rived directly from osteolepiforms rather than indirectly 

 through a common ancestor as the Anura also are. 



Table 15-4 is a summary of the basic views presented in 

 this book. It varies from the others in that the holocephalans 

 and sharks are not placed together any more closely than 

 either is to the arthrodires and acanthodians. This phylogeny 

 does not include primitive groups which have not been 



discussed; there are other branches, then, among those 

 shown. The branch of the gnathostomes which leads to the 

 Osteichthyes produced two main lines, the actinopterygians 

 and the choanates. The actinopterygian line gave rise 



TABLE 15- ? A DEP/CT/ON OF THE VtBWS, GENERALLY HELD 

 PRIOR TO 1930, OF THE PHYLOGENY OF THE VER- 

 TEBRATES (AFTER JARVIK, J 960) 



Other Tetrapoda 



Urodela 



Holostei Teleostei 



(internal skeleton ossified) 



Chondrostei 

 (cartilaginous internal skeleton) 



Grossopterygii .Actinopterygii 



Dipnoi 



(with internal nares, lungs) 



Teleostomi 



"Osteichthyes 



Chondrichthyes 



(cartilaginous internal skeleton, 



jawed, placoid scales, paired fins) 



Ostracodenni ^ 

 ( jawless, bony e.\o- 

 skeieton and some 

 bone in endocranium) 



Cyclostomi 



■~ (no bony skeleton, -' 



jawless, without fins) 



Placodermi 



TABLE 15-2 THE BASIC PHYLOGENY OF THE VERTEBRATES AS 

 CONCEIVED BY ROMER (1955) 



Chondrichthyes .^ves Mammalia 



Reptilia 



Amphibia 



Teleostei 



Holostei 



Chondrostei 



CHOANICHTHYES ACTINOPTERYGII 



, OSTEICHTHYES 



PLACODERMI 



AGNATHA 



446 • A SUMAAATION OF CHORDATE MORPHOLOGY 



