332 PRINCIPLES OF ANIMAL BIOLOGY 



guished while all that now remains is from the primitive early members 

 of the stock. In the horse series, we have left today the most highly 

 modified member, while its less specialized ancestors and several less 

 speciahzed lateral branches have been lost. In the elephants and 

 camels, also, the greatest amount of modification of which we gain any 

 knowledge from fossils is preserved in the modern animals, while only 

 the forebears and several sister Hnes of less complexity have met extinc- 

 tion. In the cephalopods, however, the maximum development was 

 lost. The forms with bent, crooked, and foliaceous sutures are known 

 only as fossils, while the living forms of today are almost identical with 

 the nautiloids, whose sutures were only gently curved. If, as is naturally 

 assumed, the Nautilus of today is the practically unaltered direct de- 

 scendant of the nautiloids of Silurian time, relatively primitive forms 

 have been preserved while the highly specialized have perished. 



Among the reptiles, too, the paths of glory led but to the grave. 

 The reptiles rose to a mighty station in the Mesozoic era, which is known 

 as the Age of Reptiles. They spread over most of the earth, and North 

 America was an important scene of their development. Some were 

 small animals, others as large as the largest whales. Their skeletons show 

 the largest ones to be 60 to 80 feet in length; such animals in the flesh 

 must have weighed many tons. But they did not speciahze in size alone; 

 they exhibited a variety of structure. Some were lizard-like and evidently 

 ran low upon the ground, while others stood high on their legs. Some 

 bore their weight on all fours; others, as indicated by two-footed tracks 

 and the size of the bones, ran upon their hind legs. The forelimbs of the 

 latter were used only for grasping. Necks and tails were long and slender 

 in some, short and stout in others. Many of them were armored in strik- 

 ing fashions. Curious armatures had arisen before Mesozoic time, for 

 Edaphosaurus (Figs. 228 and 229), a Paleozoic form, had long spines on 

 its back joined together like a fin. It was in the dinosaurs of the Mesozoic 

 era, however, that bizarre external structures attained their greatest 

 diversity, as exemplified in the rows of flat plates of Stegosaurus (Fig. 

 230), and the curious horns on the head of Triceratops (Fig. 231). 



The group of dinosaurs, dominant among the reptiles, of multitudinous 

 form and high specialization, took its rise in the Triassic period; under- 

 went its evolution in early and middle Mesozoic time; reached the extreme 

 specialization described above in the Cretaceous period; and then 

 perished! The reptiles of today are all much less specialized. The 

 nearest modern reptilian relatives of the dinosaurs appear to be the croco- 

 diles, of which there are but few species and none of them are highly 

 specialized. In this case, as in the cephalopods, the most highly spe- 

 cialized branches of the group were extinguished, while the more primi- 

 tive survived. 



The cause of the extinction of whole groups of animals is unknown. 



