186 INTRODUCTION TO EVOLUTION 



agile swimmers of the two, occupying the niche in Mesozoic marine hfe 

 held by porpoises and dolphins in modern seas. 



Conquest of the Air 



Another group of Mesozoic reptiles became adapted for flight. These 

 were the pterosaurs (pterodactyls), descended like the dinosaurs from the 

 thecodonts (Fig. 9.3). Pterosaurs developed membranous wings supported 

 by a modification of the pentadactyl limb unlike that of either birds or bats 

 (Fig. 3.1, p. 22). The fourth finger of the hand became greatly elongated, 

 forming support for the front margin of the wing (Fig. 9.10). The first three 



^^^^ 



FIG. 9.10. The most specialized pterosaur, Pferanodon. (From Eaton, Memoirs of the 

 Connecticut Academy of Arts and Sciences, 1910, Vol. 2, pp. 1-38.) 



fingers bore claws by which the creature could cling to rocks or to the limbs 

 of trees. Pterosaurs had small bodies relative to wingspread (Fig. 9.10); 

 this spread reached 27 feet in the largest species. The bones were hollow, 

 and consequently light. The sternum or breastbone was relatively large, 

 furnishing attachment for breast muscles connected to the wings. This mo- 

 tor mechanism was probably not of sufficiently great development to pro- 

 vide for strong and sustained flight. It is thought that pterosaurs used their 

 wings largely in gliding and soaring, launching themselves, perhaps, 

 from the tops of cliffs. The pterosaur brain was large for a reptile, the 

 sense of sight being strongly developed, as in birds. Possibly pterosaurs 



