

ANIMALS OF THE PAST 333 



was frequented by large plesiosaurs and ichthiosaurs; the 

 air served as a highway for the ''flying lizards," or ptero- 

 dactyls (Fig. 101). In later Jurassic times primitive birds 

 (Fig. 102) appeared, and for a time shared the ah- with the 

 flying reptiles. Mammals continued to be small in size, 

 but had differentiated into types suggesting moles, opos- 

 sums, and rodents. Fishes began to have more and more 

 the structures associated with those of modern times: the 

 endoskeleton became less cartilaginous and more bony; the 

 heavy armour on the outside of the body decreased; the 

 fins became more effective organs for swimming. 



Cretaceous Period. Dinosaurs and other great reptiles 

 still flourished at the beginning of this period but before 

 the close many whole groups had died out completely. 

 The gigantic size of some of these animals is indicated by 

 Shimer's estimates concerning Brontosaurus, an herbi- 

 vorous dinosaur, which is believed to have weighed twenty 

 tons and to have consumed about 4000 pounds of leaves 

 and twigs per day. The birds of this time were not lizard 

 like. Their tails were short and the wings did not generally 

 possess free digits, but the jaws were armed with teeth. 

 In some types there were signs of extreme specialization. 

 Hesperornis, for example, had lost its power of flight and 

 the wings were degenerate. The mammals were still 

 small and for the most part not highly specialized. 



During the Mesozoic Age the reptiles were the most im- 

 portant animals; though birds and mammals came into 

 existence they did not play a prominent part in the life of 

 the time. Toward the close of the age many orders of 

 reptiles became extinct. Pterodactyls no longer sailed 

 through the air; ichiosaurs and plesiosaurs were absent 

 from the ocean; dinosaurs had disappeared forever. 

 Among invertebrate animals there had also been many 

 changes. In the ocean true shrimps, lobsters, and crabs 

 had come into existence; the archaic cephalopods, so 

 characteristic of ancient seas, were largely replaced by more 



