REPTILES, BIRDS, AND MAMMALS 441 



short-tailed body. Cotylosaurs disappeared before the end of the Triassic, 

 but one of their offshoots, the turtles, has survived to modern times. The 

 turtles retained the primitive anapsid skull and sprawling legs of the 

 stem reptiles but in other respects became highly specialized. They 

 developed one of the most remarkable armors ever evolved by a terrestrial 

 vertebrate, consisting of horny skin scales fused to the broadened ribs 

 and sternum. The group first appeared during the Permian period; by 

 Cretaceous times marine forms with reduced armor and paddlelike legs 

 had appeared, from which the 

 modern sea turtles have descended. 

 The land tortoises and fresh-water 

 turtles are the most primitive of 

 surviving reptiles. 



Marine reptiles of the Mesozoic. 

 In addition to the Cretaceous sea 

 turtles a number of other groups of 

 Mesozoic reptiles became adapted 

 to marine existence, some of them 



with marked success. Especially jjj^ jjjjfc l\\\ j|jj 



noteworthy were the fishlike par- 



apsid reptiles called ichthyosaurs. Fig. 28.4. The contrast in leg position be- 

 Thp^Prlpvplnnpdj^nprfWtlvstrPflm- tween the earlier land vertebrates ( ste &- 



inese developed as penectiy stream- ocephalians and cotylosaurs, left) and the 



lined a body as any fish or porpoise— higher reptiles and mammals (right) . (From 

 i j • it i i Romer, Alan and the Vertebrates, by vermis- 



deep, narrow, and spindle-shaped, sion Universtty of chicago Pre88m) 

 with the legs modified into finlike 



steering blades and the back furnished with a cartilaginous stabilizing fin 

 resembling that of a shark. The jaws were beaked, with many sharp teeth; 

 the eyes were very large, the nostrils set just in front of them, and the head 

 was joined to the body with no perceptible neck. The propelling organ 

 was a bilobed fishlike tail, though the vertebral axis turned down into its 

 lower lobe instead of into the upper lobe as in fishes. The ichthyosaurs 

 were fish feeders, occupying the ecological niche now filled by the dolphins 

 and porpoises. They did not even need to go ashore for breeding, for as 

 proved by fossil remains they were ovoviviparous, carrying the developing 

 eggs in the abdomen until they were ready to hatch. These "fish reptiles" 

 appeared in the Triassic period, became abundant in the Jurassic, and 

 survived until late in the Cretaceous. 



Another remarkable group of marine reptiles comprised the euryapsid 

 plesiosaurs, which have been described as having the body of a turtle 

 strung on a snake. They had a broad, flattened trunk plated with bony 

 armor; the legs were powerful swimming paddles; and they had long 

 necks and shorter tails. In some the head was small and the neck very 

 long; others had a shorter neck and a large, often beaked head. Small 



