144 DINOSAURS OF NORTH AMERICA. 



The nearer relations of these groups to one another and to allied 

 forms will be treated more fully in the concluding portion of this article. 



The geological range of the Dinosauria, so far as at present known, 

 is confined entirely to the Mesozoic, or the Age of Eeptiles. The first 

 indications of the group are found in the lower Triassic, and during 

 this period these reptiles increased in number and size. In Jurassic 

 time they were especially abundant, and in size and diversity of form 

 far surpassed all other forms of vertebrate life then existing. During 

 the entire Cretaceous they were represented by many strange and 

 highly specialized types, and at the close of this period all apparently 

 became extinct. 



The wide geographical extent of these reptiles is also of interest. 

 While North America seems to have contained the greatest number of 

 different types, some of the larger species are now known to have lived in 

 the southern half of this continent. Europe stands next to America in 

 variety and number of these reptiles, large and small. In Asia, Africa, 

 and Australia, also, characteristic remains have been discovered, and 

 doubtless many more will be found at no distant day. The geological 

 horizons in which the dinosauriau remains of the Old World occur are 

 essentially the same as those in which the corresponding types have 

 been found in America. 



The introduction and succession of the Dinosauria in North America 

 form a most interesting chapter in the life-history of this continent, and 

 one that has an important bearing on geology as well. As these rep- 

 tiles were the dominant types of land animals during the whole of 

 Mesozoic time, and the circumstances under which they lived were 

 especially favorable to the preservation of their remains, the latter 

 mark definite geological horizons, which have proved of great serv- 

 ice in ascertaining the age of large series of strata containing few 

 other characteristic fossils. In this way one important horizon in the 

 Jurassic and another in the Cretaceous have been accurately 

 determined by the remains of the gigantic dinosaurs entombed in 

 them, while still other lines have been approximately drawn by less 

 characteristic fossils from the same group 01 reptiles. 



In describing briefly the various dinosaurs now known to have lived 

 in North America, it will be most instructive to begin with the oldest, 

 in the Triassic, and then treat of their successors as they left their 

 remains in subsequent deposits of Jurassic and Cretaceous age. To 

 make this succession clear to the reader, the diagram on page 145 (tig. 1) 

 has been prepared. This diagram represents the principal geological 

 horizons of vertebrate fossils in North America, as determined by the 

 writer, and if carefully examined will be found in reality to be a synop- 

 sis of the whole subject. The first appearance, so far as known, of 

 each important group of vertebrate animals may be ascertained, 

 approximately, from the data given. Some of the more recent genera of 

 each group are also recorded, with the period in which they lived. 



