244 



COMPARATIVE ANATOMY 



Summary of Skeletal Evolution. Most animal phyla, even the 

 protozoa, have some sort of skeletal structures. But there seems to be no 

 genetic connexion between the skeletons of invertebrates and those of 

 vertebrates. In the evolution of a skeleton, vertebrates have been given 

 carte blanche. In the process of acquiring a skeleton, chordates first 

 converted the roof of their alimentary canal into a supporting rod, and 

 later used this as a foundation upon which to build a vertebral column 

 of cartilage. The replacement of cartilage by bone in higher vertebrates 

 is an astonishing engineering feat for which our current theories of adapta- 

 tion seem quite inadequate. 



To the notochord, cyclostomes added neural arches and the rough 

 beginnings of brain case and visceral skeleton. Elasmobranchs show a 



CLAVICLE 



Fig. 203. — The pectoral girdle and fin of Sauripterus, an upper Devonian Crossop- 

 terygian fish. Interest in this type of fish fin lies in the similarity of relations of the 

 proximal elements of the extremity to those found in the pectoral extremity of tetrapods. 

 (Redrawn after Broom.) 



marked advance towards a more elaborate skeleton. To the vertebrae 

 they added hemal arches, centra and spinous processes. They converted 

 a gill arch into a biting jaw, and invented paired fins and a scaly dermal 

 skeleton. Ganoids went to extremes in dermal armor, but made a perma- 

 nent contribution to our skulls. In this group, bone began to replace 

 cartilage. 



Crossopterygian ganoids of the Devonian suggest the beginnings of 

 the tetrapod extremity. The extremities of two of these, Sauripterus and 

 Eusthenopteron, are especially significant. The skeletal elements of the 

 pectoral extremity of these forms consisted of a single proximal element, 

 and articulating with this two distal elements. Furthermore, the proximal 

 element, interpreted as a humerus, articulates in a socket of the pectoral 

 girdle. From the evolutionary standpoint this evidence is most important 

 since it shows that in fishes even before locomotion on land was adopted 

 the arrangement of the proximal bony elements in the extremity had 

 already come to resemble that of land animals. Thus Sauripterus and 



