PHYLOGENY 



CROCODILIA- 

 CROCODILES+ ALLIGATORS 



STEM OF RULING REPTILES 



Figure 59. Ruling Reptiles. (From Romer, "Man and the Vertebrates," University 

 of Chicago Press, 1946. ) 



But as yet no final decision on this problem is possible, and other factors, 

 such as drought and scarcity of food, deserve consideration. 



While the reptiles dominated the entire Mesozoic era, only four of the 

 sixteen orders have survived into Cenozoic times. One of these, the order 

 Rhyncocephalia, is represented today only by a single species, Sphenodon 

 punctatum, a lizard-like reptile which is restricted to a few small islands 

 off the coast of New Zealand. Although this animal superficially resembles 

 the lizards, its skeleton is very much more primitive. For example, in the 

 earliest reptiles, there was a progressive reduction of the hypocentrum, 

 with the pleurocentrum finally forming the entire body of the vertebra. 

 In most other living reptiles, this process is complete, but in Sphenodon 

 the hypocentra still form small wedges between the successive vertebrae. 

 It possesses many such archaic characters, and is often referred to as a 

 "living fossil," because its morphological affinities are with long extinct 

 types rather than with living types. 



A second living order of reptiles is the Chelonia, the turtles, an ancient 



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