458 VERTEBRATE LIFE AND ORGANIZATION 



Figure 23.9. Aquatic reptiles of the Mesozoic era. Plesiosaurs on the left; ichthyo- 

 saurs on the right. Plesiosaurs reached a length of forty feet: ichthyosaurs, a length of 

 about ten feet. (Courtesy of the Chicago Museum of Natural History.) 



with the evolution and increase of the teleosts (Fig. 22.2). Possibly they 

 could not compete successfully with these fishes. 



Lizard-Like Reptiles. The most abundant of our present-day rep- 

 tiles are the lizard-like ones of which lizards and snakes are the most 

 familiar examples. The most primitive living member of this group is 

 the tuatara (Sphenodon, Fig. 23.10)— the only surviving representative of 

 the order Rhynchocephalia. Rhynchocephalians are lizard-like in general 

 appearance, but have a more primitive skidl structure than any true 

 lizard. At one time the group was very widespread, but now it is 

 limited to a few small islands off the coast of New Zealand. Sphenodon 

 is a surviving "fossil," for it has not changed greatly from species that 

 were living 150 million years ago. 



Figure 23.10. The tuatara. Sphenodon, is one of the most primitive of living 

 reptiles. (Courtesy of the New York Zoological Society.) 



