THE SKELETAL SYSTEM 



183 



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

 a gill arch into biting jaws, 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 (Fig. 173). From the evolutionary standpoint this evidence is most 

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



CLAVICLE 



Fig. 173. — 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.) 



adopted the arrangement of the proximal bony elements in the extremity 

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

 Eusthenopteron help to bridge over the structural gulf separating the 

 ichthyopterygium and the cheiropterygium. 



The emergence of animals from the water to the land was accompanied 

 by momentous changes in their skeletons. Somehow or other fins were 

 changed into fingered appendages capable of supporting the weight of the 

 body. This transition was probably accomplished by amphibians which, 

 because they had lungs and could breathe air, were able to meet this 

 crisis in animal life. Legs and not tails became the locomotor organs. 

 Even the vertebral column was affected by these developments. The 

 pelvic girdle became attached to the vertebral column and sacral vertebrae 

 were differentiated. Speed was of life-saving value and legs elongated. 

 All organs — heart, kidneys, brains, and all the others — were improved. 



Profound changes in the visceral skeleton also occurred. With the 

 replacement of gills by lungs as organs of respiration, the skeletal supports 



