456 VERTEBRATE LIFE AND ORGANIZATION 



206. Evolution and Adaptations of Reptiles 



Stem Reptiles. Having solved the essential problems of terrestrial 

 life at a time when there were few competitors upon the land, the rep- 

 tiles multiplied rapidly, spread into all of the ecologic niches available 

 to them, and became specialized accordingly. The earliest reptiles, which 

 separated from the labyrinthodonts during the late Carboniferous 

 period, were the cotylosaurs (order Cotylosauria). This stem group was 

 soon replaced by other lines of reptilian evolution that arose directly or 



indirectlv from it. 



Turtles. Turtles (order Chelonia) are believed to be direct de- 

 scendants of cotylosaurs (Fig. 22.2), but they are specialized by being 

 encased in a protective shell composed of bony plates overlaid by horny 

 scales. The bony plates have ossified in the dermis of the skin, but they 

 have also fused with the ribs and some other deeper parts of the skele- 

 ton. The portion of the shell covering the back is known as the cara- 

 pace; the ventral portion, the plastron. 



Ancestral turtles were stiff-necked creatures, unable to retract their 

 heads, but modern species can withdraw theirs into the shell. This is 

 accomplished by bending the neck in an S-shaped loop in either the 

 vertical plane (North American species such as the red-eared turtle, 

 Pseudemys scripta elegans) or in the horizontal plane (Australian side- 

 necked turtle, Chelodma longicollis, Fig. 23.8 C). Sea turtles belong to 

 the former group. They have also adapted to an aquatic mode of life, 

 swimming about by means of oarlike flippers. They come ashore only 

 to lay their cleidoic eggs in holes which they dig on the beaches. 



Niarine Blind Alleys. Sea turtles are not the only reptiles that have 

 returned to the ocean. In the Mesozoic, two lines of reptilian evolution 

 adapted to marine conditions. Plesiosaurs (order Sauropterygia, Fig. 

 23.9) were superficially turtle-shaped (though they lacked the shell), 

 with squat, heavy bodies and long necks. Some species reached a length 

 of 40 feet. They propelled themselves by means of large paddle-shaped 

 appendages. Members of the other line, the ichthyosaurs (order Ichthy- 

 osauria, Fig. 23.9), were porpoise-like in size and probably in habits. 

 They moved with fishlike undulations of the trunk. 



Plesiosaurs could probably get onto the beaches to lay their eggs, 

 but the extreme aquatic adaptation of the ichthyosaurs would preclude 

 their doing so. How then did they reproduce, for cleidoic eggs cannot 

 develop submerged in water? In an unusual fossil, several small ichthy- 

 osaurs are lodged in the posterior part of the mother's abdominal cavity, 

 and one individual is part way out the cloaca. These must have been 

 offspring about to be born, for the skeletons of specimens that had been 

 eaten would not remain intact during a passage through the digestive 

 tract. Apparently these reptiles, like some modern lizards and snakes, 

 were viviparous, the eggs being retained in the oviduct until embryonic 

 development was complete. 



These marine reptiles flourished during the Mesozoic, competing 

 with the more primitive kinds of fishes. Just why they became extinct 

 near the close of this era is uncertain, but their extinction coincides 



