In discussing dinosaurs from a stratigraphic point of view, only those groups are of value which are of a relatively high degree of specialization and whose evolutionary stages are so sharply marked that a comparison of those remotely removed geographically can be made with some degree of accuracy. On this account the conservative carnivorous forms, whose chief evolutionary change is increase of stature, are of little value. The Sauropoda, on the other hand, and especially the armored dinosaurs, which are highly specialized types, are horizon markers of importance.
The Sauropod dinosaurs include some of the greatest of the world’s creatures, exceeded in size only by the largest of existing whales. The huge size, while in itself a high specialization, can only be attained by members of a relatively primitive stock. Thus we see in the attainment of size the development of remarkable specialization on the part of skeletal . . .