THE REPTILES AND THE BIRDS 12/ 



plesiosaurs, both probably having invaded salt w^ater by 

 way of the marshes. 



The Jurassic saw the tendency to increased size cul- 

 minate in the truly gigantic dinosaurs. The American 

 remains of these animals have all come from the so-called 

 'Morrison formation' of the Upper Jurassic, named for its 

 excellent exposure near that Colorado town, but the for- 

 mation extends over an area of a hundred thousand 

 square miles of the Rocky Mountain region. This is a 

 typical terrestrial formation representing a great alluvial 

 plain crossed by sluggish streams flowing eastward and 

 interrupted by swamps and small lakes. Though, as in 

 all other times, the climate varied considerably from one 

 locality to another, the Morrison beds reveal a warm 

 temperature and high humidity, supporting lush vegeta- 

 tion of scouring rushes, tree ferns, cycads, ginkgos, and 

 conifers— all evergreens, for the deciduous hardwood 

 trees were as yet unknown. The Jurassic flora was much 

 the same all over the world, in North America, Siberia, 

 arctic Alaska, Spitsbergen, Scandinavia, England, and 

 even in Louis Philippe Peninsula below Cape Horn. In 

 this favorable mflieu the reptiles developed to their fan- 

 tastic and now familiar extremes. The pterodactyls had 

 extended the little finger into a long arc from which a 

 leathery wing stretched along the arm to the body; they 

 certainly were capable of soaring and some may have 

 been capable of sustained flight, though on the ground 

 they walked on all four feet. They were not birds and 

 not related to birds, but cold-blooded flying reptiles and 

 more properly likened to winged dragons. 



Among the dinosaurs, one of the best-knownn Ameri- 

 can forms, Brontosaurus, reached a length of 65 feet 

 and a weight of some 30 tons; the more slender Diplo- 

 docus, a length of nearly 80 feet. Brachiosaurus, with 

 giraffelike proportions and long forelegs, carried its head 

 34 feet above the ground. Stegosaurus was covered with 

 armor fabricated of massive plates and weighed an esti- 

 mated 10 tons. These were primarily herbivorous ani- 



