CHAPTER 



y^ 



THE RE-ENTRANTS 



Life spread from the place of its origin in the sea up the rivers and to the 

 marshes and lakes of fresh water. From fresh water, the land became populated, 

 and reptiles, birds, and mammals came into being. Since each group of animals 

 tends to evolve by adaptive radiation into all the niches available to it, within the 

 limits of its own body plan, these higher vertebrate groups have returned to the 

 sea, each retaining the basic characteristics of its group, however. The sea turtles 

 and the birds lav eggs on land, and the mammals suckle their young. All breathe 

 air. But each has also become molded to its watery environment, having paddles, 

 flippers, or fins for propulsion, and pursuing fish or other harvest of the sea 

 for food. 



The animals included here are those that live and feed in the sea, not those 

 that spend only a small part of their lives there. 



REPTILES: Glass Reptilia 



The age of reptiles is over. For 150 million years the earth— air, land, and sea- 

 was ruled by this scaly clan. Now a remnant, a fascinating remnant to be sure, 

 remains. These are the snakes, lizards, crocodiles. New Zealand sphenodons, and 

 turtles. Of these, two groups in North America have marine representatives, 

 •turtles and snakes. (The American crocodile is not a truly marine representative 

 since it lives in brackish waters.) 



Turtles: Order Testudinata or Chelonia 



About 225 million years ago turtlelike reptiles first appeared, having evolved 

 from a primitive reptile stock. Even at that time turtles were marked by the 

 beginnings of a shell, consisting of an upper part, the carapace, and a lower one, 

 the plastron. They lost teeth and replaced these with a horny beak. By the 

 Cretaceous Period, over 100 million years ago, modern turtles were present. 

 Shelled and toothless, the turtles have conservatively remained to this day, 

 spreading over the earth to the ponds and rivers, the deserts, and to the sea. 

 But while their habits vary greatly, their body form has been remarkably un- 

 changed. Clumsy as this body may appear, it is persevering. No other air-breath- 



303 



