204 



THE ORIGIN AND EVOLUTION OF LIFE 



anatomy of which land-living ancestry is clearly written, while 

 reversed adaptation for marine pelagic life has resulted in a 

 superficial type of body which presents close analogies to that 

 of the sharks, porpoises, and shark-dolphins (Fig. 41). Integu- 

 mentary median and tail fins precisely similar to those of the 



Fig. 81. Extreme Adaptation of the Ichthyosaurs to Marine Pelagic Life. 



Although primarily of terrestrial origin the ichthyosaurs become quite independent of 

 the shores through the viviparous birth of the young as evidenced by a fossil female 

 ichthyosaur (upper figures) with the foetal skeletons of seven young ichthyosaurs 

 within or near the abdominal cavity. 



A fossil ichthyosaur (lower figure) with preserved body integument and fin outlines re- 

 sembling those of the sharks and dolphins (see Fig. 41). 



Both specimens in the American Museum of Natural History from Holzmaden, Wiirtem- 

 berg. 



sharks evolve, the anterior lateral limbs are secondarily con- 

 verted into fin-paddles, which are externally similar to those 

 of sharks and dolphins, while the posterior limbs are reduced. 

 As in the shark, the tail fin is vertical, while in the dolphin the 

 tail fin is horizontal. In the early history of their marine 

 pelagic existence the ichthyosaurs undoubtedly returned to 

 shore to deposit their eggs, but a climax of imitation of the dol- 

 phins and of certain of the sharks is reached in the develop- 

 ment of the power of viviparity, the growth of the young within 



