104 KINDS OF INDICATORS. 



405, 420). Throughout the major portion of the Cenophytic, the serai genera 

 as well as the life-f orms were essentially the same as those of to-day, and their 

 indicator value is readily inferred from existing conditions. 



Plant indicators of animals. — The general indicator relations of fossil 

 plants and animals have long been recognized and utilized by paleontologists, 

 but chiefly on the animal side. The correlation between the appearance of a 

 dominant angiospermous flora and the evolution of mammals is the most 

 outstanding example of this, but the rise of the cursorial ungulates in response 

 to an expanding grassland climax is hardly less striking. Such correlations 

 must be superlatively general before the Cenophytic, though the existing 

 relations between serai and climax communities and the great groups of 

 animals must have had analogies at least during the Mesophytic era. Since 

 the larger animals were all totally different, and the dominant genera of 

 plants practically all different likewise, the use of plant and animal indicators 

 as a basic method in paleo-ecology must be confined chiefly to the Cenophytic 

 era for the present. Here, however, it seems to offer great possibilities, some 

 of which must wait upon the further study of communities as biotic units 

 with development and structure. The indicator value of plants in this con- 

 nection is limited only by our knowledge of existing correlations with animals. 

 This is due to the fact that a large number of modern genera of plants have 

 existed since the Cretaceous. The evolution of animals has been much more 

 rapid, and the number of existing genera of mammals, for example, which 

 reach back to the Eocene is very small. However, among the rodents and 

 ungulates, where plant correlations are most important, nearly half the families 

 contain both modern and fossil genera. With respect to the birds and insects, 

 our knowledge is much less complete, but it appears highly probable that 

 many existing families and orders had arisen at least by the Tertiary. As a 

 consequence, it becomes possible to scan the rapidly growing list of plant 

 indicators, and to extend their correlations as far into the past as the recorded 

 existence of the genera or related genera permits. 



Animal indicators of plants. — The reciprocal relation of plants and animals 

 as indicators, whether as communities or species, greatly extends the use of 

 indicators in geological times. In many horizons, animals have been pre- 

 served to a much larger degree than plants, while in some, plant remains are 

 entirely lacking. Fossil animals are especially significant in the reconstruc- 

 tion of upland life, since the cursorial forms of the uplands were preserved in 

 fairly large number, while the record of the associated plants is exceedingly 

 fragmentary. Moreover, animals may serve to indicate the presence of 

 plants in regions or in periods where they are not yet actually found. Outside 

 of the insects, there are few extinct animals in which there is an indicator 

 correlation with a single species of plant. On the other hand, the correlation 

 of herbivores with plant communities, both climax and serai, is practically 

 universal, and they serve to indicate with a high degree of probability the 

 development and extension of sedgeland and grassland from the Cretaceous 

 to the Pliocene. The general correlation of browsing ungulates with forest 

 and scrub, of the earlier types of grazing animals with' sedgeland and meadow, 

 and of the highly specialized upland types with the climax grassland of 

 xerophytic grasses (Osborn, 1910 : 9, 237) is fundamental, and has been used 

 to furnish the basis for the treatment of the development and structure of 

 the biotic communities in the Bad Lands of the West. 



